Field effect transistor
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The field-effect transistor (FET) is a transistor that relies on an electric field to control the shape and hence the conductivity of a "channel" in a semiconductor material. FETs, like all transistors, can be thought of as voltage-controlled resistors.
Most FETs are made with conventional bulk semiconductor processing techniques, using the single-crystal semiconductor wafer as the active region, or channel. The channel region of TFTs (thin-film transistors), on the other hand, is a thin film that is deposited onto a substrate (often glass, since the primary application of TFTs is in liquid crystal displays). For more on TFTs, see thin-film transistor; the remainder of this article deals with the transistors most commonly used in integrated circuits. In biology, voltage-gated ion channels work in a similar way.
The terminals in FET are called gate, drain and source. (Compare these to the terminology used for BJTs: base, collector and emitter.) The voltage applied between the gate and source terminals modulates the current between source and drain.
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Types of field-effect transistors
The FET is simpler in concept than the bipolar transistor and can be constructed from a wide range of materials. The different types of field-effect transistors can be distinguished by the method of isolation between channel and gate:
- The MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) utilizes an isolator (typically SiO2).
- Power MOSFETs become less conductive with increasing temperature and can therefore be thought of as n-channel devices by default. Silicon devices that use electrons, rather than holes, as the majority carriers are slightly faster and can carry more current than their P-type counterparts. The same is true in GaAs devices.
- The JFET (Junction Field-Effect Transistor) uses a p-n junction.
- Substituting the p-n-junction with a Schottky barrier gives a MESFET (Metal-Semiconductor Field-Effect Transistor), used for GaAs and other III-V semiconductor materials.
- Using bandgap engineering in a ternary semiconductor like AlGaAs gives a HEMT (High Electron Mobility Transistor), also named an HFET (heterostructure FET). The fully depleted wide-band-gap material forms the isolation.
- TFTs (thin-film transistor) use amorphous silicon, polycrystalline silicon or other amorphous semiconductors as body material.
- A subgroup of TFTs are organic field effect transistors that are based on organic semiconductors and often apply organic gate insulators and electrodes.
- The channel region of any FET is either doped to produce n-type semiconductor, giving an "N-channel" device, or with p-type to give a "P-channel" device. The doping determines the polarity of gate operation.
FET Operation
The shape of the conducting channel in a FET is altered when a potential is applied to the Gate terminal (potential relative to either Source or Drain.) In an N-channel device, a negative Gate potential causes an insulating Depletion Zone to expand in size and encroach on the channel from the side, narrowing the channel. If the Depletion Zone pinches the channel closed, the resistance of the channel becomes very large, and the FET is turned off entirely. At low voltages, the channel width remains large, and small changes to the Gate potential will alter the channel resistance. This is the "Variable Resistance" mode of FET operation. This mode has uses but is not employed in conventional amplifier circuits.
If a larger potential difference is applied between the Source and Drain terminals, this creates a significant current in the channel and produces a smooth gradient in potential distributed along the channel. This also causes the shape of the Depletion Zone to become asymmetrical, and one part of the channel becomes narrow while another part widens. If the voltage is large enough, the Depletion Zone begins to close the channel entirely. Something unusual then happens: negative feedback arises, since a closed channel would lead to flat potential gradient and a symmetrical Depletion Zone, which would open the channel. Rather than closing entirely, the Depletion Zone shapes itself to produce an extremely narrow channel of variable length. Any attempted increases to the channel current will change the shape of the Depletion Zone, lengthening the channel. This increases the channel resistance and prevents the value of current from increasing. This mode of operation is called "Pinchoff mode." In this mode of operation, the channel behaves as a constant-current source rather than as a resistor. The value of channel current is relatively independent of the voltage applied between Source and Drain. The value of Gate voltage determines the value of the constant current in the channel.
Uses
The most common use of MOSFET transistors today is the CMOS (complementary metallic oxide semiconductor) integrated circuit which is the basis for most digital electronic devices. These use a totem-pole arrangement where one transistor (either the pull-up or the pull-down) is on while the other is off. Hence, there is no DC drain, except during the transition from one state to the other, which is very short. As mentioned, the gates are capacitive, and the charging and discharging of the gates each time a transistor switches states is the primary cause of power drain.
The C in CMOS stands for 'complementary.' The pull-up is a P-channel device (using holes for the mobile carrier of charge) and the pull-down is N-channel (electron carriers). This allows busing of the control terminals, but limits the speed of the circuit to that of the slower P device (in silicon devices). The bipolar solutions to push-pull include 'cascode' using a current source for the load. Circuits that utilize both unipolar and bipolar transistors are called Bi-Fet. A recent development is called 'vertical P.' Formerly, BiFet chip users had to settle for relatively poor (horizontal) P-type FET devices. This is no longer the case and allows for quieter and faster analog circuits.
FETs can switch signals of either polarity, if their amplitude is significantly less than the gate swing, as the devices (especially the parasitic diode-free DFET) are basically symmetrical. This means that FETs are the most suitable type for analog multiplexing. With this concept, one can construct a solid-state mixing board, for example.
The power MOSFET has a 'parasitic diode' (back-biased) normally shunting the conduction channel that has half the current capacity of the conduction channel. Sometimes this is useful in driving dual-coil magnetic circuits (for spike protection), but in other cases it causes problems.
The high impedance of the FET gate makes it rather vulnerable to electrostatic damage, though this is not usually a problem after the device has been installed.
A more recent device for power control is the insulated-gate bipolar transistor, or IGBT. This has a control structure akin to a MOSFET coupled with a bipolar-like main conduction channel. These have become quite popular.
External links
- PBS The Field Effect Transistor (http://www.pbs.org/transistor/science/info/transmodern.html)
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