Page 207 - Electrical Properties of Materials

Field-effect transistors 189

suddenly increased, that is, the capacitance is decreased. The charge on the
plates can not change instantaneously; so the reduced capacitance must lead
∗ The magic factor 2 in frequency is not
to increased voltage (Q = CU must stay constant). The voltage across the
necessary either. Furthermore, it is not
capacitor therefore jumps abruptly at t 1 , t 3 , t 5 , etc., and it is unaffected at even a good thing, because if it were,
t 2 , t 4 , t 6 , etc., as shown in Fig. 9.31(c). we would have to know the signal fre-
We may argue in a similar manner that the charge must also increase. When quency at all times, together with its
phase. Clearly this would be impossibly
the plate is pushed back at t 2 , the voltage is not affected, but the capacitance difficult for an arbitrary signal. Instead, a
goes back to its original value. Hence, when the capacitor is charged again, Q three-frequency scheme is used, in which
will come to a higher peak value, as shown in Fig. 9.31(d). a strong pump at an unrelated but gener-
ally much higher frequency ω 3 is used
Note that the important thing is to vary the capacitance in the resonant
to amplify a weak signal at a frequency
circuit at twice the resonant frequency. Amplification is achieved then at the ω 1 . The process involves two steps. In
expense of the energy available to vary the capacitance. It may be shown (both the first, the pump and signal are mixed
theoretically and experimentally) that the variation of the capacitance need not together in the varactor to create a so-
be abrupt. Any reasonable variation of the capacitance at twice the rate of the called ‘idler’ at a frequency ω 2 = ω 3 –ω 1 .
Because of the relative strength of the
signal frequency would do. ∗ pump, the idler is much larger than the
It is interesting to note that the possibility of parametric amplification had signal. In the second, the idler is mixed
been known for over fifty years, but it has become practical only after the with the pump to create a new (and
even larger) term at the signal frequency,
advent of the p–n junction.
which adds in phase with the signal to
Now with what sort of properties would we like to endow our p–n junction give the net effect of amplification. To
to make it suitable for this particular application? Well, it will be the integ- make the device practical, resonant cir-
ral part of some sort of tuned circuit, where losses are generally unwelcome. cuits are needed at all three frequencies.
These circuits keep the components at
Hence, we shall use heavy doping to reduce the resistance. We should not dope
each frequency separate, except where
too much, however, because that would lead to narrow depletion regions and they must come together for mixing.
low Zener breakdown. Since the varactor diode must operate under reverse bias
(to get the capacitance) its useful range is between U = 0 and U = U B ;alow
breakdown voltage is obviously undesirable. There must be a compromise
In practice the p-side of the junction is usually doped very heavily, so that it between reducing the resistivity
does not contribute at all to the total series resistance. All the depletion layer is and ensuring a high breakdown
then in the n-type material, whose length is limited to the possible minimum. voltage.
It is equal to the length of the depletion region just below breakdown (when
the depletion region is the longest).
That is roughly how parametric amplifiers work. But is it worth making
a complicated amplifier which needs a high-frequency pump oscillator when
a ‘simple’ transistor will amplify just as well? The limitation of a transistor
is that it will be a source of noise as well as gain. All amplifiers introduce
†
additional noise due to the random part of their electronic motion, so the † Noise due to electric currents was men-
emitter and collector currents in a transistor are fairly copious noise sources. tioned briefly in a previous footnote
As there is almost no standing current in a varactor, it introduces very little (Section 1.2). A fuller discussion is bey-
ond our present scope, but if you wish
noise; so parametric amplifiers are worth their complications in very sensitive
for further reading in this interesting
receivers, for example for satellite communication links, radio astronomy, and topic see F.N.H. Robinson, Noise and
radar. fluctuations in electronic devices and
circuits, O.U.P., 1974.
9.14 Field-effect transistors
Having become acquainted with a number of two-terminal devices, let us now
look at another representative of a three-terminal device, the so-called field-
effect transistor (FET). It is not quite clear why it is regarded as a relative of
the bipolar transistor, which we have discussed in quite some detail in Sec-
tion 9.6. The only common factors are that they are both solid state devices