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A power inverter converts DC power or direct current to standard AC power or alternating current, which allows you to run electrical equipment off your car or marine battery for mobile applications, emergencies or simple convenience.
Power inverters are small rectangular devices that have a trailing wire with a jack that plugs directly into the cigarette lighter on the dashboard. They might also come with jumper-like cables for connecting directly to a battery. The device normally has one or two outlets for standard electrical cords. Your laptop, small-screen television, video game player or portable DVD theater are all examples of devices that will get you through a long ride, assuming you're not the one driving!
Power inverters are great for camping at parks that do not provide electricity. The toaster, blender, and boom box can all still be used. On your boat you can plug in the digital movie camera to capture those great water-skiing videos you might have missed after the camera's battery ran low!
In a utility outage a power inverter can be used for emergency electricity. Just run an extension cord from your car into the house, or if you have a charged spare battery you can connect the power inverter directly. Plug in a radio to tune into important alerts, run essential medical equipment, lights, or whatever else you need that falls within the inverter's power limits.
Power inverters come in many models that vary in watts. The amount of wattage you will require on yours depends on the total draw of the devices you'd like to use. If you have a two-outlet inverter and will be plugging in 2 devices at once, add up the total wattage of both devices then add at least 50% more to account for peaks or spikes in the power draw. For example if your DVD theater draws 100 watts and your laptop another 100 watts, a minimum 300-watt inverter is recommended.
When using your power inverter continuously inside a vehicle that is not running, the engine should be started at least once an hour for 10-15 minutes to keep the battery from discharging. WARNING: Do not start a vehicle in a closed garage as the carbon monoxide in the exhaust is fatal. Power inverters operate on the assumption that the battery is in good condition and fully charged. A weak battery will be drained easily if demands are too high. This could leave you stranded so be sure to check the battery's condition before using a power inverter in a stationary vehicle.
If the power inverter is being used while the vehicle is running as in the case of a road trip, there should be no problem with the extra draw providing the battery is in good condition.
Power inverters produce one of three different types of wave output:
• Square Wave
• Modified Square Wave (Modified Sine Wave)
• Pure Sine Wave (True Sine Wave)
The three different wave signals represent three different qualities of power output and consequently, three different price categories. Square wave inverters result in uneven power delivery that is not efficient for running most devices. Square wave inverters were the first types of inverters made and are obsolete.
Modified square wave (modified sine wave) inverters deliver power that is consistent and efficient enough to run most devices fine. This type of inverter is probably the most popular.
Pure sine wave inverters are the most expensive, but they also deliver the most consistent wave output. Some sensitive equipment requires a sine wave, like certain medical equipment and variable speed or rechargeable tools. If you aren't sure if the device you want to use requires a pure sine wave or not, call the manufacturer to ask. Or if you don't mind the price difference any device will run on a pure sine wave, whether it requires it or not. The only drawback would be in spending more than you need to for your power inverter.
Always use a power inverter that is rated high enough for the device(s) you are running and avoid adapters that would allow more outlets than the unit is designed to accommodate.
Working with car batteries can be dangerous and can result in serious injury, and improper use of a power inverter can lead to electrocution, so for your own safety be sure to read and follow any and all safety precautions that are listed in your owner's manual, which will come with your power inverter.
Electrical generator
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This article is about machines that produce electricity. For other uses, see Generator.
“Dynamo” redirects here. For other uses, see Dynamo (disambiguation).


Early 20th century alternator made in Budapest, Hungary, in the power generating hall of a hydroelectric station
In electricity generation, an electrical generator is a device that converts kinetic energy to electrical energy, generally using electromagnetic induction. The reverse conversion of electrical energy into mechanical energy is done by a motor, and motors and generators have many similarities. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, or any other source of
Thyristor
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Circuit symbol for a thyristor
The thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as a switch, conducting when their gate receives a current pulse, and continue to conduct for as long as they are forward biased.
Some sources define silicon controlled rectifiers and thyristors as synonymous[1]; others define SCRs as a subset of thyristors, along with gate turn-off thyristor (GTO), triode ac switch (triac), static indution transistor (SIT), static induction thyristor (SITH) and MOS-controlled thyristor (MCT). Among the latter, the International Electrotechnical Commission 60747-6 standard stands out.
Non-SCR thyristors include devices with more than four layers, such as triacs and DB-GTOs[2].
Contents
[hide]
• 1 Function
o 1.1 Function of the gate terminal
o 1.2 Switching characteristics
• 2 History
• 3 Applications
o 3.1 Snubber circuits
• 4 Comparisons to other devices
• 5 Failure modes
• 6 Silicon carbide thyristors
• 7 Types of thyristors
• 8 References
o 8.1 Footnotes
o 8.2 Bibliography
o 8.3 External links
o 8.4 See also

[edit] Function

The thyristor is a four-layer semiconducting device, with each layer consisting of an alternately N-type or P-type material, for example P-N-P-N. The main terminals, labeled anode and cathode, are across the full four layers, and the control terminal, called the gate, is attached to p-type material near to the cathode. (A variant called a SCS Silicon Controlled Switch brings all four layers out to terminals.) The operation of a thyristor can be understood in terms of a pair of tightly coupled Bipolar Junction Transistors, arranged to cause the self-latching action.
Thyristors have three states:
1. Reverse blocking mode -- Voltage is applied in the direction that would be blocked by a diode
2. Forward blocking mode -- Voltage is applied in the direction that would cause a diode to conduct, but the thyristor has not yet been triggered into conduction
3. Forward conducting mode -- The thyristor has been triggered into conduction and will remain conducting until the forward current drops below a threshold value known as the "holding current"
[edit] Function of the gate terminal
The thyristor has three p-n junctions (serially named J1, J2, J3 from the anode).


Layer Diagram of Thyristor
When the anode is at a positive potential VAK with respect to the cathode with no voltage applied at the gate, junctions J1 and J3 are forward biased, while junction J2 is reverse biased. As J2 is reverse biased, no conduction takes place (Off state). Now if VAK is increased beyond the breakdown voltage VBO of the thyristor, avalanche breakdown of J2 takes place and the thyristor starts conducting (On state).
If a positive potential VG is applied at the gate terminal with respect to the cathode, the breakdown of the junction J2 occurs at a lower value of VAK. By selecting an appropriate value of VG, the thyristor can be switched into the on state immediately.
It should be noted that once avalanche breakdown has occurred, the thyristor continues to conduct, irrespective of the gate voltage, until either: (a) the potential VG is removed or (b) the current through the device (anode−cathode) is less than the holding current specified by the manufacturer. Hence VG can be a voltage pulse, such as the voltage output from a UJT relaxation oscillator.
These gate pulses are characterized in terms of gate trigger voltage (VGT) and gate trigger current (IGT). Gate trigger current varies inversely with gate pulse width in such a way that it is evident that there is a minimum gate charge required to trigger the thyristor.
[edit] Switching characteristics
In a conventional thyristor, once it has been switched on by the gate terminal, the device remains latched in the on-state (i.e. does not need a continuous supply of gate current to conduct), providing the anode current has exceeded the latching current (IL). As long as the anode remains positively biased, it cannot be switched off until the anode current falls below the holding current (IH).


V - I Characteristics
A thyristor can be switched off if the external circuit causes the anode to become negatively biased. In some applications this is done by switching a second thyristor to discharge a capacitor into the cathode of the first thyristor. This method is called forced commutation.
After a thyristor has been switched off by forced commutation, a finite time delay must have elapsed before the anode can be positively biased in the off-state. This minimum delay is called the circuit commutated turn off time (tQ). Attempting to positively bias the anode within this time causes the thyristor to be self-triggered by the remaining charge carriers (holes and electrons) that have not yet recombined.
For applications with frequencies higher than the domestic AC mains supply (e.g. 50Hz or 60Hz), thyristors with lower values of tQ are required. Such fast thyristors are made by diffusing into the silicon heavy metals ions such as gold or platinum which act as charge combination centres. Alternatively, fast thyristors may be made by neutron irradiation of the silicon.
[edit] History
1956 The Silicon Controlled Rectifier (SCR) or Thyristor proposed by William Shockley in 1950 and championed by Moll and others at Bell Labs was developed first by power engineers at General Electric (G.E.) led by Gordon Hall and commercialised by G.E.'s Frank W. "Bill" Gutzwiller.
[edit] Applications


A bank of six, 2000 A Thyristors (white pucks). The clear tubes are for cooling water


Thyristor voltage regulated by phase control
Thyristors are mainly used where high currents and voltages are involved, and are often used to control alternating currents, where the change of polarity of the current causes the device to automatically switch off; referred to as Zero Cross operation. The device can be said to operate synchronously as, once the device is open, it conducts current in phase with the voltage applied over its cathode to anode junction with no further gate modulation being required to replicate; the device is biased fully on. This is not to be confused with symmetrical operation, as the output is unidirectional, flowing only from cathode to anode, and so is asymmetrical in nature.
Thyristors can be used as the control elements for phase angle triggered controllers, also known as phase fired controllers.
Thyristors can also be found in power supplies for digital circuits, where they can be used as a sort of "circuit breaker" or "crowbar" to prevent a failure in the power supply from damaging downstream components. The thyristor is used in conjunction with a zener diode attached to its gate, and when the output voltage of the supply rises above the zener voltage, the thyristor conducts, shorting the power supply output to ground (and in general blowing an upstream fuse).
The first large scale application of thyristors, with associated triggering diac, in consumer products related to stabilized power supplies within color television receivers in the early 1970s. The stabilized high voltage d.c supply for the receiver was obtained by moving the switching point of the thyristor device up and down the falling slope of the positive going half of the a.c supply input (if the rising slope was used the output voltage would always rise towards the peak input voltage when the device was triggered and thus defeat the aim of regulation). The precise switching point was determined by the load on the output d.c. supply as well fluctuations on the input a.c. supply. They proved to be unpopular with the a.c. grid power supplier companies because the simultaneous switching of many television receivers, all at approximately the same time, introduced asymmetry into the supply waveform and, as a consequence injected d.c. back into the grid with a tendency towards saturation of transformer cores and overheating. Thyristors were largely phased out in this kind of application by the end of the decade.
[edit] Snubber circuits
Because thyristors can be triggered on by a high rate of rise of off-state voltage, in many applications this is prevented by connecting a resistor-capacitor (RC) snubber circuit between the anode and cathode terminals in order to limit the dV/dt (i.e., rate of change of voltage versus time).
[edit] Comparisons to other devices


SCR rated about 100 amperes, 1200 volts
The functional drawback of a thyristor is that, like a diode, it only conducts in one direction. A similar self-latching 5-layer device, called a TRIAC, is able to work in both directions. This added capability, though, also can become a shortfall. Because the TRIAC can conduct in both directions, reactive loads can cause it to fail to turn off during the zero-voltage instants of the ac power cycle. Because of this, use of TRIACs with (for example) heavily-inductive motor loads usually requires the use of a "snubber" circuit around the TRIAC to assure that it will turn off with each half-cycle of mains power. Inverse parallel SCRs can also be used in place of the triac; because each SCR in the pair has an entire half-cycle of reverse polarity applied to it, the SCRs, unlike TRIACs, are sure to turn off. The "price" to be paid for this arrangement, however, is the added complexity of two separate but essentially identical gating circuits.
An earlier gas filled tube device called a Thyratron provided a similar electronic switching capability, where a small control voltage could switch a large current. It is from a combination of "thyratron" and "transistor" that the term "thyristor" is derived.
Modern thyristors can switch large amounts of power (up to megawatts). In the realm of very high power applications, they are still the primary choice. However, in low and medium power (from few tens of watts to few tens of kilowatts) they have almost been replaced by other devices with superior switching characteristics like MOSFETs or IGBTs. One major problem associated with SCRs is that they are not fully controllable switches. The GTO (Gate Turn-off Thyristor) and IGCT are two related devices which address this problem. In high-frequency applications, thyristors are poor candidates due to large switching times arising from bipolar conduction. MOSFETs, on the other hand, have much faster switching capability because of their unipolar conduction (only majority carriers carry the current).
[edit] Failure modes
As well as the usual failure modes due to exceeding voltage, current or power ratings, thyristors have their own particular modes of failure, including:
• Turn on di/dt — in which the rate of rise of on-state current after triggering is higher than can be supported by the spreading speed of the active conduction area (SCRs & triacs).
• Forced commutation — in which the transient peak reverse recovery current causes such a high voltage drop in the sub-cathode region that it exceeds the reverse breakdown voltage of the gate cathode diode junction (SCRs only).
[edit] Silicon carbide thyristors
In recent years, some manufacturers[3] have developed thyristors using Silicon carbide (SiC) as the semiconductor material. These have applications in high temperature environments, being capable of operating at temperatures up to 350 °C.
[edit] Types of thyristors
• SCR — Silicon controlled rectifier
• ASCR — asymmetrical SCR
• RCT — reverse conducting thyristor
• LASCR — light activated SCR, or LTT — light triggered thyristor
• DIAC & SIDAC — both forms of trigger devices
• BOD — breakover diode — a gateless thyristor triggered by avalanche current, used in protection applications
• TRIAC — a bidirectional switching device containing two thyristor structures
• GTO — gate turn-off thyristor
• IGCT — Integrated gate commutated thyristor
o MA-GTO — Modified anode gate turn-off thyristor
o DB-GTO — Distributed buffer gate turn-off thyristor
• MCT — MOSFET controlled thyristor containing two additional FET structures for on/off control.
o BRT — Base Resistance Controlled Thyristor
• SITh — Static induction thyristor, or FCTh — Field controlled thyristor containing a gate structure that can shut down anode current flow.
The GTO is a tri state device. with a 8 function setup. it also has a equation: v=j-o x n/n o
[edit] References
[edit] Footnotes
1. ^ Christiansen, Donald; Alexander, Charles K. (2005); Standard Handbook of Electrical Engineering (5th ed.). McGraw-Hill, ISBN 0-07-138421-9
2. ^ Dorf, Richard C., editor (1997), Electrical Engineering Handbook (2nd ed.). CRC Press, IEEE Press, Ron Powers Publisher, ISBN 0-8493-8574-1
3. ^ Example: Silicon Carbide Inverter Demonstrates Higher Power Output in Power Electronics Technology (2006-02-01)
[edit] Bibliography
• General Electric Corporation, SCR Manual, 6th edition, Prentice-Hall, 1979.
[edit] External links

Look up Thyristor in
Wiktionary, the free dictionary.
• The Early History of the Silicon Controlled Rectifier — by Frank William Gutzwiller (of G.E.)
[edit] See also
• Thyristor tower
• Latchup
Retrieved from "http://en.wikipedia.org/wiki/Thyristor"
MOSFET
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The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is by far the most common field-effect transistor in both digital and analog circuits. The MOSFET is composed of a channel of n-type or p-type semiconductor material (see article on semiconductor devices), and is accordingly called an NMOSFET or a PMOSFET (also commonly nMOSFET, pMOSFET).
The 'metal' in the name is often incorrect as current process technologies usually use polysilicon gates. Aluminium was used as the gate material until the 1980s when polysilicon became dominant owing to its capability to form self-aligned gates. Lately, at the 65nm node and smaller, metal gates are again in use. IGFET is a related, more general term meaning insulated-gate field-effect transistor, and is almost synonymous with MOSFET, though it can refer to FETs with a gate insulator that is not oxide. Some prefer to use "IGFET" when referring to devices with polysilicon gates, but most still call them MOSFETs. With the new generation of high-k technology that Intel and IBM have announced [1], metal gates in conjunction with the high-k dielectric material replacing the silicon dioxide are making a comeback replacing the polysilicon due to leakage problems in processes at about 65nm or smaller.
Usually the semiconductor of choice is silicon, but some chip manufacturers, most notably IBM, have begun to use a mixture of silicon and germanium (SiGe) in MOSFET channels. Unfortunately, many semiconductors with better electrical properties than silicon, such as gallium arsenide, do not form good gate oxides and thus are not suitable for MOSFETs. Yet, there has been constant research to deposit oxides with acceptable electrical characteristics on such material.
The gate terminal in the current generation (65 nanometer node) of MOSFETs is a layer of polysilicon (polycrystalline silicon; why polysilicon is used will be explained below) placed over the channel, but separated from the channel by a thin insulating layer of what was traditionally silicon dioxide, but more advanced technologies used silicon oxynitride. Some companies have started to introduce a high-k + metal gate combination in the 45 nanometer node. When a voltage is applied between the gate and source terminals, the electric field generated penetrates through the oxide and creates a so-called "inversion channel" in the channel underneath. The inversion channel is of the same type – P-type or N-type – as the source and drain, so it provides a conduit through which current can pass. Varying the voltage between the gate and body modulates the conductivity of this layer and makes it possible to control the current flow between drain and source.


Photomicrograph of two MOSFETs in a test pattern. Probe pads for two gates and three source/drain nodes are labeled.
Contents
[hide]
• 1 Circuit symbols
• 2 MOSFET operation
o 2.1 Metal–oxide–semiconductor structure
o 2.2 MOSFET structure
o 2.3 Modes of operation
o 2.4 Body effect
• 3 The primacy of MOSFETs
o 3.1 Digital
o 3.2 Analog
• 4 MOSFET scaling
o 4.1 Reasons for MOSFET scaling
o 4.2 Difficulties arising due to MOSFET scaling
 4.2.1 Subthreshold conduction
 4.2.2 Interconnect capacitance
 4.2.3 Heat production
 4.2.4 Gate oxide leakage
 4.2.5 Process variations
• 5 MOSFET construction
o 5.1 Gate material
• 6 Other MOSFET types
o 6.1 Dual gate MOSFET
o 6.2 Depletion-mode MOSFETs
o 6.3 NMOS logic
o 6.4 Power MOSFET
o 6.5 DMOS
• 7 MOSFET analog switch
o 7.1 Single-type MOSFET switch
o 7.2 Dual-type (CMOS) MOSFET switch
• 8 References and notes
• 9 See also
• 10 External links

[edit] Circuit symbols
A variety of symbols are used for the MOSFET. The basic design is generally a line for the channel with the source and drain leaving it at right angles and then bending back into the same direction as the channel. Sometimes a broken line is used for enhancement mode and a solid one for depletion mode, but the awkwardness of drawing broken lines means this distinction is often ignored. Another line is drawn parallel to the channel for the gate.
The bulk connection, if shown, is shown connected to the back of the channel with an arrow indicating PMOS or NMOS. Arrows always point from P to N, so an NMOS (N-channel in P-well or P-substrate) has the arrow pointing in. If the bulk is connected to the source (as is generally the case with discrete devices) it is angled to meet up with the source leaving the transistor. If the bulk is not shown (as is often the case in IC design as they are generally common bulk) an inversion symbol is sometimes used to indicate PMOS, alternatively an arrow on the drain may be used in the same way as for bipolar transistors (out for NMOS in for PMOS).
Comparison of enhancement-mode and depletion-mode MOSFET symbols, along with JFET symbols:




P-channel




N-channel
JFET MOSFET enh MOSFET dep
For the symbols in which the bulk, or body, terminal is shown, it is here shown internally connected to the source. This is a typical configuration, but by no means the only important configuration. In general, the MOSFET is a four-terminal device, and in integrated circuits many of the MOSFETs share a body connection, not necessarily connected to the source terminals of all the transistors.
[edit] MOSFET operation
[edit] Metal–oxide–semiconductor structure


Metal–oxide–semiconductor structure
A traditional metal–oxide–semiconductor (MOS) structure is obtained by depositing a layer of silicon dioxide (SiO2) and a layer of metal (polycrystalline silicon is commonly used instead of metal) on top of a semiconductor die. As the silicon dioxide is a dielectric material its structure is equivalent to a plane capacitor, with one of the electrodes replaced by a semiconductor.
When a voltage is applied across a MOS structure, it modifies the distribution of charges in the semiconductor. If we consider a P-type semiconductor (with NA the density of holes), a positive VGB (see figure) tends to reduce the concentration of holes and increase the concentration of electrons. If VGB is high enough, the concentration of negative charge carriers near the gate is more than that of positive charges, in what is known as an inversion layer.
This structure with P-type body is the basis of the N-type MOSFET, which requires the addition of an N-type source and drain regions.
[edit] MOSFET structure


Cross Section of an NMOS
A metal–oxide–semiconductor field-effect transistor (MOSFET) is based on the modulation of charge concentration caused by a MOS capacitance. It includes two terminals (source and drain) each connected to separate highly doped regions. These regions can be either P or N type, but they must both be of the same type. The highly doped regions are typically denoted by a '+' following the type of doping (see the image at the right). These two regions are separated by a doped region of opposite type, known as the body. This region is not highly doped, denoted by the lack of a '+' sign. The active region constitutes a MOS capacitance with a third electrode, the gate, which is located above the body and insulated from all of the other regions by an oxide.
If the MOSFET is an N-Channel or nMOS FET, then the source and drain are 'N+' regions and the body is a 'P' region. When a positive gate-source voltage is applied, it creates an N-channel at the surface of the P region, just under the oxide, by depleting this region of holes. This channel extends between the source and the drain, but current is conducted through it only when the gate potential is high enough to attract electrons from the source into the channel. When zero or negative voltage is applied between gate and source, the channel disappears and no current can flow between the source and the drain.
If the MOSFET is a P-Channel or pMOS FET, then the source and drain are 'P+' regions and the body is a 'N' region. When a negative gate-source voltage (positive source-gate) is applied, it creates a P-channel at the surface of the N region, just under the oxide, by depleting this region of electrons. This channel extends between the source and the drain, but current is conducted only when the gate potential is low enough to attract holes from the source into the channel. When a near-zero or positive voltage is applied between gate and body, the channel disappears and no current can flow between the source and the drain.
The source is so named because it is the source of the charge carriers (electrons for N-channel, holes for P-channel) that flow through the channel; similarly, the drain is where the charge carriers leave the channel.
[edit] Modes of operation


MOSFET drain current vs. drain-to-source voltage for several values of VGS − Vth


Cross section of a MOSFET operating in the linear region


Cross section of a MOSFET operating in the saturation region
The operation of a MOSFET can be separated into three different modes, depending on the voltages at the terminals. For an enhancement-mode, n-channel MOSFET the modes are:
Cut-off or sub-threshold mode
When VGS < Vth where Vth is the threshold voltage of the device.
According to the basic threshold model, the transistor is turned off, and there is no conduction between drain and source. In reality, the Boltzmann distribution of electron energies allows some of the more energetic electrons at the source to enter the channel and flow to the drain, resulting in a subthreshold current that is an exponential function of gate–source voltage. While the current between drain and source should ideally be zero when the transistor is being used as a turned-off switch, there is a weak-inversion current, sometimes called subthreshold leakage.
Triode or linear region
When VGS > Vth and VDS < (VGS − Vth)
The transistor is turned on, and a channel has been created which allows current to flow between the drain and source. The MOSFET operates like a resistor, controlled by the gate voltage relative to both the source and drain voltages. The current from drain to source is modeled as:

where μn is the charge-carrier effective mobility, W is the gate width, L is the gate length and Cox is the gate oxide capacitance per unit area. The transition from the exponential subthreshold region to the triode region is not as sharp as the equations suggest.
Saturation
When VGS > Vth and VDS > VGS − Vth
The switch is turned on, and a channel has been created, which allows current to flow between the drain and source. Since the drain voltage is higher than the gate voltage, a portion of the channel is turned off. The onset of this region is also known as pinch-off. The drain current is now relatively independent of the drain voltage (in a first-order approximation) and the current is controlled by only the gate–source voltage, modeled as:

this equation can be multiplied by (1 + λVDS) to take into account the channel length modulation (Early effect).
When the channel length becomes very short, carrier transport will be by quasi-ballistic transport. When short-channel effects dominate, the I-V characteristics are no longer well approximated by the above equations. Rather, the saturation drain current is more nearly linear than quadratic in VGS.
[edit] Body effect


Ohmic contact to body
The body effect describes the changes in the threshold voltage by the change in the source-bulk voltage, approximated by the following equation:
,
where VTO is the zero substrate bias, γ is the body effect parameter, and 2φ is the surface potential parameter.
The body can be operated as a second gate, and is sometimes referred to as the "back gate"; the body effect is sometimes called the "back-gate effect". [1]
[edit] The primacy of MOSFETs
In 1960, Dawon Kahng and Martin Atalla at Bell Labs invented the metal oxide semiconductor field-effect transistor (MOSFET). Theoretically different from Shockley's transistor, the MOSFET was structured by putting an insulating layer on the surface of the semiconductor and then placing a metallic gate electrode on that. It used crystalline silicon for the semiconductor and a thermally oxidized layer of silicon dioxide for the insulator. Not only did it possess such technical attractions as low cost of production and ease of integration, the silicon MOSFET did not generate localized electron traps (interface states) at the interface between the silicon and its native oxide layer, and thus was free of the characteristic that had impeded the performance of earlier transistors. Buoyed by this stroke of good fortune, the MOSFET has achieved electronic hegemony. It is this serendipity that sustains the large-scale integrated circuits (LSIs) underlying today's information society.
[edit] Digital
The growth of digital technologies like the microprocessor has provided the motivation to advance MOSFET technology faster than any other type of silicon-based transistor. The principal reason for the success of the MOSFET was the development of digital CMOS logic, which uses p- and n-channel MOSFETs as building blocks. The great advantage of CMOS logic is that they allow no current to flow (ideally), and thus no power to be consumed, except when the inputs to logic gates are being switched. CMOS accomplishes this by complementing every nMOSFET with a pMOSFET and connecting both gates and both drains together. A high voltage on the gates will cause the nMOSFET to conduct and the pMOSFET not to conduct and a low voltage on the gates causes the reverse. During the switching time the voltage goes from one state to another and both will conduct briefly. This arrangement greatly reduces power consumption and heat generation. Overheating is a major concern in integrated circuits, since ever more transistors are packed into ever smaller chips.
Another advantage of MOSFETs for digital switching is that the oxide layer between the gate and the channel prevents DC current from flowing through the gate, further reducing power consumption and giving a very large input impedance. The insulating oxide between the gate and channel effectively isolates a MOSFET in one logic stage from earlier and consequent stages, which allows to drive a considerable number of MOSFET inputs from a single MOSFET output. Bipolar transistor-based logic (such as TTL) do not have such a high fanout capacity. This isolation also makes it easier for the designers to ignore to some extent loading effects between logic stages independently. That extent is defined by the operating frequency: as frequencies increase, the input impedance of the MOSFETs decreases.
[edit] Analog
The MOSFET's strengths as the workhorse transistor in most digital circuits do not translate into supremacy in all analog circuits. The bipolar junction transistor (BJT) has traditionally been the analog designer's transistor of choice, due largely to its high transconductance and unique properties. Nevertheless, MOSFETs are widely relied upon for analog purposes as well in many types of analog circuits. The characteristics and performance of many analog circuits can be designed by changing the sizes (length and width) of the MOSFETs used. Only in specialized bipolar circuits does the size of the bipolar device used significantly affect the performance. MOSFETs' ideal characteristics regarding gate current (zero) and drain-source offset voltage (zero) also make them nearly ideal switch elements and makes switched capacitor analog circuits practical. In their linear region, MOSFETs can be used as precision resistors, which can have a much higher controlled resistance than BJTs. In high power circuits, MOSFETs sometimes have the advantage of not suffering from thermal runaway as BJTs do. Also, they can be formed into capacitors and specialized circuits allow op-amps made from them to appear as inductors, thereby allowing all of the normal analog devices, except for diodes (which can be made smaller than a MOSFET anyway), to be built entirely out of MOSFETs. This allows for complete analog circuits to be made on a silicon chip in a much smaller space. Some ICs combine analog and digital MOSFET circuitry on a single chip, making the needed board space even smaller. This creates a need to isolate the analog circuits from the digital circuits on a chip level, leading to the use of isolation rings and Silicon-On-Insulator (SOI). The main advantage of BJTs vs MOSFETs in the analog design process is the ability of BJTs to handle a larger current in a smaller space. Fabrication processes exist that incorporate BJTs and MOSFETs into a single device, these mixed-transistor devices are called Bi-FETs (Bipolar-FETs) if they contain just one BJT-FET and BiCMOS (bipolar-CMOS) if they contain complementary BJT-FETs. This device provides for the advantages of both the insulated gate and the higher current density.
The BJT also has some advantages over the MOSFET in certain digital circuits. BJTs are currently better for at least two digital jobs. The first is in high speed switching because they don't have the "larger" capacitance from the gate, which when multiplied by the resistance of the channel gives the intrinsic time constant of the process. The intrinsic time constant places a limit on the speed a MOSFET can operate at because higher frequency signals are filtered out. Widening the channel reduces the resistance of the channel, but increases the capacitance by the exact same amount. Reducing the width of the channel increases the resistance, but reduces the capacitance by the same amount. R*C=Tc1, 0.5R*2C=Tc1, 2R*0.5C=Tc1. There is no way to minimize the intrinsic time constant for a certain process. Different processes using different channel lengths, channel heights, gate thicknesses and materials will have different intrinsic time constants. You can skip most of this problem with a BJT because it doesn't have a gate.
The second job stems from the first. When driving many other gates, called fanout, the resistance of the MOSFET is in series with the gate capacitances of the other FETs, creating a secondary time constant. Delay circuits use this fact to create a set signal delay by using a small CMOS device to send a signal to many other, many times larger CMOS devices. The secondary time constant can be minimized by increasing the driving FETs channel width to decrease its resistance and decreasing the channel width of the FETs being driven, decreasing their capacitance. This does have a drawback because it increases the capacitance of the driving FET and increases the resistance of the FETs being driven, but usually those drawbacks are a minimal problem when compared to the timing problem. BJTs are better to drive the other gates because they can output more current than MOSFETs, allowing for the FETs being driven to charge faster. Many chips will employ MOSFET inputs and BiCMOS (see above) outputs.
[edit] MOSFET scaling
Over the past decades, the MOSFET has continually been scaled down in size; typical MOSFET channel lengths were once several micrometres, but modern integrated circuits are incorporating MOSFETs with channel lengths of less than a tenth of a micrometre. Indeed Intel began production of a process featuring a 65 nm feature size (with the channel being even shorter) in early 2006. Until the late 1990s, this size reduction resulted in great improvement to MOSFET operation with no deleterious consequences. Historically, the difficulties with decreasing the size of the MOSFET have been associated with the semiconductor device fabrication process.
[edit] Reasons for MOSFET scaling
Smaller MOSFETs are desirable for several reasons. The main reason to scale the transistors is to pack more and more devices in a given chip area. This results in either smaller chips or chips with more computing power in the same area. Since fabrication costs for a semiconductor wafer are relatively fixed, the cost per integrated circuits is mainly related to the number of chips that can be produced per wafer. Hence, smaller ICs allow more chips per wafer, reducing the price per chip. In fact, over the past 30 years the number of transistors per chip has been doubled every 2-3 years once a new technology node is itroduced. For example the number of MOSFETs in a microprocessor fabricated in a 45 nm technology is twice as large as in a 65 nm chip. This doubling of the transistor count was first observed by Gordon Moore in 1965 and is comonly referred to as Moore's law.
It is also expected that smaller transistors switch faster. While this has been traditionally the case for the older technologies, for the state-of-the-art MOSFETs scaling of the transistor dimensions does not neccessarily translate to higher speed. Commensurate scaling of the MOSFET requires that all device dimensionas are scaled with the same pace. The main device dimensions are the transistor length, width, and the oxide thickness, each (used to) scale with a factor of 0.7 per node. This way, the transistor channel resistance does not change with scaling, while gate capacitance is cut by a factor of 0.7. Hence, the RC delay of the transistor scales with a factor of 0.7.
[edit] Difficulties arising due to MOSFET scaling
Producing MOSFETs with channel lengths much smaller than a micrometre is a challenge, and the difficulties of semiconductor device fabrication are always a limiting factor in advancing integrated circuit technology. In recent years, the small size of the MOSFET, below a few tenths of a micrometre, has created operational problems.
[edit] Subthreshold conduction
Because of small MOSFET geometries, the voltage that can be applied to the gate must be reduced to maintain reliability. To maintain performance, the threshold voltage of the MOSFET has to be reduced as well. As threshold voltage is reduced, the transistor cannot be completely turned off; that is, the transistor operates in weak-inversion mode, with a subthreshold leakage, or subthreshold conduction, between source and drain. Subthreshold conduction, which was ignored in the past, now can consume upwards of half of the total power consumption of modern high-performance VLSI chips.
Some micropower analog circuits are designed to take advantage of subthreshold conduction; by working in the weak-inversion region, the MOSFETs in these circuits deliver the highest possible transconductance-to-current ratio.
[edit] Interconnect capacitance
Traditionally switching time was roughly proportional to the gate capacitance of gates. However, with transistors becoming smaller and more transistors being placed on the chip, interconnect capacitance (the capacitance of the wires connecting different parts of the chip) is becoming a large percentage of capacitance. Signals have to travel through the interconnect, which leads to increased delay and lower performance.
[edit] Heat production
The ever-increasing density of MOSFETs on an integrated circuit is creating problems of substantial localized heat generation that can impair circuit operation. Circuits operate slower at high temperatures, and have reduced reliability and shorter lifetimes. Heat sinks and other cooling methods are now required for many integrated circuits including microprocessors.
Power MOSFETs are at risk of thermal runaway. As their on-state resistance rises with temperature, if the load is approximately a constant-current load then the power loss rises correspondingly, generating further heat. When the heatsink is not able to keep the temperature low enough, the junction temperature may rise quickly and uncontrollably, resulting in destruction of the device.
[edit] Gate oxide leakage
The gate oxide, which serves as insulator between the gate and channel, should be made as thin as possible to increase the channel conductivity and performance when the transistor is on and to reduce subthreshold leakage when the transistor is off. However, with current gate oxides with a thickness of around 1.2 nm (which in silicon is ~5 atoms thick) the quantum mechanical phenomenon of electron tunneling occurs between the gate and channel, leading to increased power consumption.
Insulators (referred to as high-k dielectrics) that have a larger dielectric constant than silicon dioxide, such as group IVb metal silicates e.g. hafnium and zirconium silicates and oxides are going to be used to reduce the gate leakage from the 45 nanometer technology node onwards. Increasing the dielectric constant of the gate oxide material allows a thicker layer while maintaining a high capacitance. The higher thickness reduces the tunneling current between the gate and the channel. An important consideration is the barrier height of the new gate oxide; the difference in conduction band energy between the semiconductor and the oxide (and the corresponding difference in valence band energy) will also affect the leakage current level. For the traditional gate oxide, silicon dioxide, the former barrier is approximately 8 eV. For many alternative dielectrics the value is significantly lower, somewhat negating the advantage of higher dielectric constant.
[edit] Process variations
With MOSFETS becoming smaller, the number of atoms in the silicon that produce many of the transistor's properties is becoming fewer. During chip manufacturing, random process variation can affect the size of the transistor, which becomes a greater percentage of the overall transistor size as the transistor shrinks. The transistor characteristics become less deterministic, but more statistical. This statistical variation increases design difficulty.
[edit] MOSFET construction
[edit] Gate material
The primary criterion for the gate material is that it is a good conductor. Highly-doped polycrystalline silicon is an acceptable, but certainly not ideal conductor, and it also suffers from some more technical deficiencies in its role as the standard gate material. Nevertheless, there are several reasons favoring use of polysilicon as a gate material:
1. The threshold voltage (and consequently the drain to source on-current) is modified by the work function difference between the gate material and channel material. Because polysilicon is a semiconductor, its work function can be modulated by adjusting the type and level of doping. Furthermore, because polysilicon has the same bandgap as the underlying silicon channel, it is quite straightforward to tune the work function, so as to achieve low threshold voltages for both NMOS and PMOS devices. By contrast the work functions of metals are not easily modulated, so tuning the work function to obtain low threshold voltages becomes a significant challenge. Additionally, obtaining low threshold devices on both PMOS and NMOS devices would likely require the use of different metals for each device type, adding additional complexity to the fabrication process.
2. The Silicon-SiO2 interface has been well studied and is known to have relatively few defects. By contrast many metal–insulator interfaces contain significant levels of defects which can lead to fermi-level pinning, charging, or other phenomena that ultimately degrade device performance.
3. In the MOSFET IC fabrication process, it is preferable to deposit the gate material prior to certain high-temperature steps in order to make better performing transistors. Such high temperature steps would melt some metals, limiting the types of metals that could be used in a metal-gate based process.
While polysilicon gates have been the defacto standard for the last twenty years, they do have some disadvantages, which have led to the announcement of their replacement by metal gates. These disadvantages include:
1. Polysilicon is not a great conductor (approximately 1000 times more resistive than metals) which reduces the signal propagation speed through the material. The resistivity can be lowered by increasing the level of doping, but even highly doped polysilicon is not as conductive as most metals. In order to improve conductivity further, sometimes a high temperature metal such as tungsten, titanium, cobalt, and more recently nickel, is alloyed with the top layers of the polysilicon. Such a blended material is called silicide. The silicide-polysilicon combination has better electrical properties than polysilicon alone and still does not melt in subsequent processing. Also the threshold voltage is not significantly higher than polysilicon alone, because the silicide material is not near the channel. The process in which silicide is formed on both the gate electrode and the source and drain regions is sometimes called salicide, self-aligned silicide.
2. When the transistors are extremely scaled down, it is necessary to make the gate dielectric layer very thin, around 1 nm in state-of-the-art technologies. A phenomenon observed here is the so-called poly depletion, where a depletion layer is formed in the gate polysilicon layer next to the gate dielectric when the transistor is in the inversion. To avoid this problem a metal gate is desired. A variety of metal gates such as tantalum, tungsten, tantalum nitride, and titanium nitride, usually in conjunction with high-k dielectrics. An alternative is to use fully-silicided polysilicon gates, and the process is referred to as FUSI.
[edit] Other MOSFET types
[edit] Dual gate MOSFET
The dual gate MOSFET has a tetrode configuration, where both gates control the current in the device. It is commonly used for small signal devices in radio frequency applications where the second gate is normally used for gain control or mixing and frequency conversion.
[edit] Depletion-mode MOSFETs
There are depletion-mode MOSFET devices, which are less commonly used than the standard enhancement-mode devices already described. These are MOSFET devices that are doped so that a channel exists even with zero voltage from gate to source. In order to control the channel, a negative voltage is applied to the gate (for an n-channel device), depleting the channel, which reduces the current flow through the device. In essence, the depletion-mode device is equivalent to a normally closed (on) switch, while the enhancement-mode device is equivalent to a normally open (off) switch.[2]
Due to their low noise figure in the RF region, and better gain, these devices are often preferred to bipolars in RF front-ends such as in TV sets. Depletion-mode MOSFET families include BF 960 by Siemens and BF 980 by Philips (dated 1980s), whose derivatives still AGC and RF mixers in front ends.
[edit] NMOS logic
n-channel MOSFETs are smaller than p-channel MOSFETs and producing only one type of MOSFET on a silicon substrate is cheaper and technically simpler. These were the driving principles in the design of NMOS logic which uses n-channel MOSFETs exclusively. However, unlike CMOS logic, NMOS logic consumes power even when no switching is taking place. With advances in technology, CMOS logic displaced NMOS logic in the 1980s to become the preferred process for digital chips.
[edit] Power MOSFET


Cross section of a Power MOSFET, with square cells. A typical transistor is constituted of several thousand cells
Main article: Power MOSFET
Power MOSFETs have a different structure than the one presented above.[2] As with all power devices, the structure is vertical and not planar. Using a vertical structure, it is possible for the transistor to sustain both high blocking voltage and high current. The voltage rating of the transistor is a function of the doping and thickness of the N epitaxial layer (see cross section), while the current rating is a function of the channel width (the wider the channel, the higher the current). In a planar structure, the current and breakdown voltage ratings are both function of the channel dimensions (respectively width and length of the channel), resulting in inefficient use of the "silicon estate". With the vertical structure, the component area is roughly proportional to the current it can sustain, and the component thickness (actually the N-epitaxial layer thickness) is proportional to the breakdown voltage.
It is worth noting that power MOSFETs with lateral structure are mainly used in high-end audio amplifiers. Their advantage is a better behaviour in the saturated region (corresponding to the linear region of a bipolar transistor) than the vertical MOSFETs. Vertical MOSFETs are designed for switching applications.
[edit] DMOS
DMOS stands for double-Diffused Metal Oxide Semiconductor. Most of the power MOSFETs are made using this technology.
[edit] MOSFET analog switch
MOSFET analog switches use the MOSFET channel as a low–on-resistance switch to pass analog signals when on, and as a high impedance when off. Signals flow in both directions across a MOSFET switch. In this application the drain and source of a MOSFET exchange places depending on the voltages of each electrode compared to that of the gate. For a simple MOSFET without an integrated diode, the source is the more negative side for an N-MOS or the more positive side for a P-MOS. All of these switches are limited on what signals they can pass or stop by their gate-source, gate-drain and source-drain voltages, and source-to-drain currents; exceeding the voltage limits will potentially damage the switch.
[edit] Single-type MOSFET switch
This analog switch uses a four-terminal simple MOSFET of either P or N type. In the case of an N-type switch, the body is connected to the most negative supply (usually GND) and the gate is used as the switch control. Whenever the gate voltage exceeds the source voltage by at least a threshold voltage, the MOSFET conducts. The higher the voltage, the more the MOSFET can conduct. An N-MOS will pass through all voltages less than (Vgate–Vtn). When the switch is conducting, it typically operates in the saturation region, since the source and drain voltages will typically be nearly equal.
In the case of a P-MOS, the body is connected to the most positive voltage, and the gate is brought to a lower potential to turn the switch on. The P-MOS switch passes all voltages higher than (Vgate+Vtp). Threshold voltage (Vtp) is typically negative in the case of P-MOS.
A P-MOS switch will have about three times the resistance of an N-MOS device of equal dimensions because electrons have three times the mobility of holes in silicon.
[edit] Dual-type (CMOS) MOSFET switch
This "complementary" or CMOS type of switch uses one P-MOS and one N-MOS FET to counteract the limitations of the single-type switch. The FETs have their drains and sources connected in parallel, the body of the P-MOS is connected to the high potential (VDD) and the body of the N-MOS is connected to the low potential (Gnd). To turn the switch on the gate of the P-MOS is driven to the low potential and the gate of the N-MOS is driven to the high potential. For voltages between (VDD–Vtn) and (Gnd+Vtp) both FETs conduct the signal, for voltages less than (Gnd+Vtp) the N-MOS conducts alone and for voltages greater than (VDD–Vtn) the P-MOS conducts alone.
The only limits for this switch are the gate-source, gate-drain and source-drain voltage limits for both FETs. Also, the P-MOS is typically three times the width of the N-MOS so the switch will be balanced.
Tri-state circuitry sometimes incorporates a CMOS MOSFET switch on its output to provide for a low ohmic, full range output when on and a high ohmic, mid level signal when off.
[edit] References and notes
1. ^ http://equars.com/~marco/poli/phd/node20.html
2. ^ Power Semiconductor Devices, B. Jayant Baliga, PWS publishing Company, Boston. ISBN 0-534-94098-6
[edit] See also
• Metal–oxide–semiconductor structure
[edit] External links