By David Herres
The field-effect transistor (FET) resembles the bipolar transistor in some of its behavioral characteristics, but its inner workings are markedly different. Rather than base, emitter and collector, FETs have gates, sources and drains. The big difference is that the FET notices changes in the voltage level on its input. The FET input current is infinitesimal. Consequently, FET input impedance is quite high, on the order of 100 MΩ. Any stage, circuit or device that connects to the input, even if its output impedance is high, will only see a slight amount of load. This is a distinct advantage in many applications. The FET is virtually invisible to the upstream circuitry.
FETs are identified by the material in their charge-carrying channel region. A P-channel FET can be identified by its outward pointing arrow. An N-channel FET can be identified by its inward pointing arrow.
Because of its advantages over bipolar junction transistors, FETs dominated the three-wire transistor scene for a few years in the twentieth century until it in turn was supplanted by a device of still greater input impedance, the metal-oxide semiconducting field-effect transistor (MOSFET). Here, the “metal” refers to the gate material but it has become a misnomer because MOSFET gates now are often a layer of polysilicon (polycrystalline silicon).
Presently there are a great many FETs still in service, and they are useful in some applications where a specific impedance is needed for matching purposes. Unlike a bipolar junction transistor which contains junctions from which the charge carriers either congregate or move away, a FET contains a narrow channel that runs a short distance between the source and drain. An electrostatic charge that is created along the outside of the channel either expands or contracts the channel laterally, varying the amount of current that flows between the source and drain.
As in other transistors, the output section is in essence a resistor. The resistance varies, often at a very high speed. The output circuit is made up of the source and drain. Throughout this circuit including the exterior wiring, in accordance with Kirchoff’s current law, the quantity of electrons passing any point at a given instant is constant.
An FET falls into either of two categories, depletion mode and enhancement mode, depending on whether it is in an on or off state when the voltage measured between gate and source is zero. An enhancement mode device is off when this voltage is zero, whereas the depletion mode version is on at that instant.
More specifically, enhancement-mode MOSFETs are characterized by a voltage drop across the oxide inducing a conducting channel between the source and drain contacts via the field effect. The term “enhancement mode” refers to the rise in conductivity with a rise in field that adds carriers to the channel, also referred to as the inversion layer.
If the channel contains electrons the device is then called an nMOSFET or nMOS transistor. If the channel contains holes it is called a pMOSFET or pMOS transistor. Depletion-mode MOSFETs are less common than enhancement mode devices. Their channel consists of carriers in a surface impurity layer of a type opposite to that of the substrate. Channel conductivity drops when a field is applied that depletes carriers from this surface layer.
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