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Section J2: FET Introductory Comments As with the bipolar junction transistor, the field effect transistor (FET) is a three terminal device (remember that this just means that there are three places to connect the device to the outside world) whose operation is based on the voltage between two of its terminals controlling the current flow in the third. The three terminals of the FET are designated drain (D), source (S), and gate (G). As mentioned in your text, an analogy may be made between the FET and BJT in terms of the device terminals: the drain of the FET to the collector of the BJT, the source to the emitter, and the gate to the base. A major difference between the transistor types (which we will get into more later) has to do with the physical construction of the devices. The FET is generally a symmetric structure – i.e., the terminals defined as source and drain may be interchanged without affecting the operation of the transistor. This attribute may be contrasted with the structure of the BJT, where each region of the device possesses a unique geometric definition and/or doping level to ensure proper device operation (Section C2). In spite of the similarities between the FET and BJT, quite different device characteristics are realized due to distinct differences in the current control mechanisms of the two transistor types. In general, FET devices are considered unipolar, since a single carrier type (electrons for n-type devices and holes for p-type devices) is predominant in the conduction process, as opposed to both carrier types being involved in the bipolar operation of a BJT. FETs may also provide advantages over BJTs such as: ¾ a significantly higher input impedance (remember that an ideal device has infinite input impedance). This property has several potential benefits, including: superior performance when used as the input stage to a multistage amplifier; easier matching to standard microwave systems; and the time constants associated with the large impedance allows the use of the FET as a storage element (in memories, etc). ¾ higher switching speeds and cutoff frequencies due to unipolar operation; ¾ at high current levels, the FET possesses a negative temperature coefficient. This means that as device temperature increases, the current through the device decreases. This property prevents the thermal runaway phenomena that may occur in bipolar transistors. The negative temperature coefficient also ensures that FET devices remain thermally stable under the conditions of numerous parallel-connected transistors or large active device areas. ¾ FETs are generally easier to fabricate than BJTs. This means that more devices may be fabricated on a single chip – the more devices that can be packed onto a chip – the cheaper (and, in many cases, the more reliable) the final product becomes. ¾ FETs are not as sensitive to radiation as BJTs (very important in certain industrial and space electronic applications); and ¾ much lower intermodulation and cross modulation products since FETs are primarily linear or square law devices compared to exponential behaviors for BJTs (if you get into this stuff seriously this is way important, but we probably won’t be doing a lot with it in this class). However, there is no free lunch (big surprise). There are disadvantages to the FET that limit their use in some applications, in particular: ¾ FETs may be damaged due to static electricity; ¾ some types of FETs may exhibit poor linearity; and ¾ the high input capacitance (part of the large impedance) of FET amplifiers results in a limited frequency response when compared with some BJT amplifier configurations. Well, that’s enough preliminary stuff. As we go through this material (and, indeed, all material) keep in mind the basic physics of last semester. No matter what we do – how simple or exotic the device structure – we will always and forever have to obey the physical behaviors we talked about before! In the next section, we will be discussing two of the major classes of the FET – the junction FET (JFET) and the metal-oxide semiconductor FET (MOSFET).