| Field Effect Transistors | |||
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Overview Types of FET Tips and tricks Overview: Field Effect Transistors (FETs from here on) are a more modern incarnation of the bipolar transistor. They have a similar function to bipolar transistors, which is to control the flow of current between two points, but do it in a different way. If you don't fully understand bipolar transistors it might be a good idea to have a read of the bipolar transistors article before continuing. Like bipolar transistors, FETs are used to control the flow of current from two points in a circuit. The bipolar transistor does this by allowing a current to flow from the collector to the emitter which is proportional to the current flowing into the base, while the FET allows a current to flow from the "drain" to the "source" which is proportional to the voltage applied to the "gate". It is often said that a bipolar transistor is a "current controlled current source" and field effect transistor is a "voltage controlled current source". For most practical purposes the drain, gate and source can be considered the equivalent of bipolar transistor's collector, base and emitter. (in that order) Because FETs use an electric field to control the flow from drain to source, (this pathway is often called the "channel") they do away with the diode junctions found in bipolar transistors. This gives rise to two major differences between FETs and bipolar transistors. A high resistance gate due to the lack of electrical connection between gate and channel, and low resistance channel because a single semiconductor can be used for this pathway rather than multiple junctions between different semiconductor types. This is an advantage for the likes of CMOS (Complementary Metal Oxide Semiconductor) integrated circuits, where high impedance inputs (the gate of an internal FET) allow reductions in current consumption and low impedance latches allow outputs to drive larger loads more efficiently. However, this means that FETs are not always suitable replacements for bipolar transistors. For example, the application circuit on the bipolar transistors page may not work properly with FETs because it relies on the base-emitter junction of each transistor to charge the capacitors. (the gate of a FET would not pass enough current) Types of FET: There are two main different types of FET, the Junction FET (JFETs) and the Metal Oxide Semiconductor FET. (MOSFET) Each has a different symbol as illustrated below. 
The JFET uses a diode junction to control the flow of current in the channel. (yes, I did just say that FETs don't use diode junctions but this is a little different) The diode junction in a JFET is reverse biased during normal operation which maintains the high impedance gate as reverse biased diodes pass very little current. In order to keep this junction reverse biased the voltage on the gate must always be larger than the voltage on the source, otherwise the diode junction will become forward biased and pass a large current, (due to the low channel impedance) destroying the FET. MOSFETs are constructed with an insulating barrier between the channel and the gate allowing the gate voltage to exceed the source without risk to the device. MOSFETs are more often used than JFETs, even though they are more susceptible to damage by electrostatic discharge. (ESD) The JFET and MOSFET both come in two different varieties, called N-channel and P-channel. While these can be thought of as FET equivalents of NPN and PNP, the names N-channel and P-channel are actually a reflection of the polarity of the electric field used to control the channel. And thats about it. FETs are essentially transistors with a few key differences when compared to bipolar transistors. For more details on transistors in general, see the bipolar transistors page. Below are a few hints worth keeping in mind to avoid potential headaches with FETs. Tips and tricks:
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