Download Section J2: FET Introductory Comments

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:
ƒ superior performance when used as the input stage to a multistage
amplifier;
ƒ easier matching to standard microwave systems; and
ƒ 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).