XVI. Прочитать текст, найти часть, касающуюся типов полевых транзисторов и написать его краткое содержание. The Junction Field-Effect Transistor Action
It was previously emphasized that one of the main properties of the bipolar
transistor is that it is a current-controlled amplifying device; the output current is
controlled by a small input current. In the case of the field-effect transistor (FET) it
is the input voltage which controls the output current. The current drawn by the
input is usually negligible. This is a great advantage where the signal comes from a
device such as capacitor microphone or piezoelectric transducer, which is unable to
supply a significant current. FET’s are basically of two types: the junction fieldeffect
transistor (JFET) and the insulated gate field-effect transistor (IGFET). The
latter is more commonly known by a name metal-oxide semiconductor field-effect
transistor (MOSFET).
At a point along the bar a region of p-type silicon forms a p-n junction. In
normal operation, the junction is reverse-biased. The lower contact on the bar is
called the source and the upper contact the drain. The electron current flows from
source to drain and is controlled by the voltage applied to the p-region called the
gate.
An alternative type of construction is the p-channel device where the gate is
made of n-type material.
The operation of the JFET depends upon variations in the size of the depletion
layer at the reverse-biased gate junction. The p-type gate is much more heavily
doped than the n-type bar, so that the depletion region exists almost entirely in the
bar. The gate carries a negative bias voltage relative to the source which give rise
to the particular shape of the depletion region: this is wider at the top than the
bottom. The wider the depletion layer, the narrower the channel there is available
for the flow of electrons from source to drain, since the depletion region itself
being devoid of current carries, behaves like an insulator.
Unlike the bipolar transistor, the FET employs only majority carriers for its
operation. It is therefore sometimes called the unipolar transistor and is less
susceptible than the bipolar type to temperature changes and nuclear radiation.