Handbook for Sound Engineers

Tubes, Discrete Solid State Devices, and Integrated Circuits 327

tion region causes the material to become practically
nonconductive.
Disregarding the p region and applying a voltage to
the ends of the bar cause a current and create a potential
gradient along the length of the bar material, with the
voltage increasing toward the right, with respect to the
negative end or ground. Connecting the p region to
ground causes varying amounts of reverse-bias voltage
across the pn junction, with the greatest amount devel-
oped toward the right end of the p region. A reverse
voltage across the bar will produce the same depletion
regions. If the resistivity of the p-type material is made
much smaller than that of the n-type material, the deple-
tion region will then extend much farther into the n
material than into the p material. To simplify the
following explanation, the depletion of p material will
be ignored.

The general shape of the depletion is that of a wedge,
increasing the size from left to right. Since the resis-
tivity of the bar material within the depletion area is
increased, the effective thickness of the conducting
portion of the bar becomes less and less, going from the

end of the p region to the right end. The overall resis-

tance of the semiconductor material is greater because

the effective thickness is being reduced. Continuing to

increase the voltage across the ends of the bar, a point is

reached where the depletion region is extended practi-

cally all the way through the bar, reducing the effective

thickness to zero. Increasing the voltage beyond this

point produces little change in current.

The p region controls the action and is termed a gate.

The left end of the bar, being the source of majority

carriers, is termed the source. The right end, being

where the electrons are drained off, is called the drain.

A cross-sectional drawing of a typical FET is shown in

Fig. 12-21C, and three basic circuits are shown in Fig.

12-21F–H.

Insulated-gate transistors (IGT) are also known as

field-effect transistors, metal-oxide silicon or semicon-

ductor field-effect transistors (MOSFET), metal-oxide

silicon or semiconductor transistors (MOST), and insu-

lated-gate field-effect transistors (IGFET). All these

devices are similar and are simply names applied to

them by the different manufacturers.

Figure 12-21. Field-effect transistors (FETs).

Length

Width

n-Type

Gate

S

n-Type Field

D

P-Type

VD ID

Thickness

Source Gate-1 Drain

p-Type

n-Type Channel

Gate-2

A. Plain semiconductor bar. B. Bar with gate added and drain

voltage applied.

C. Cross-sectional view of the

construction for a single- or double-

gate field-effect transistor.

Source

P Silicon

Source

Metallic

Film Drain

P Silicon

Gate insulator

n Silicon silicon dioxide

D. Internal construction of an

insulated-gate transistor (IGT).

n Input

Gate

22 M 7

20 V

2 k 7

Drain

Substrate

Source

E. Typical circuit for an IGT transistor.

+ G D+

RLD

RG RS

F. n-channel field-effect transistor

circuit.

G. p-channel field-effect transistor circuit. H. n-channel double-gate field-effect transistor circuit.

0 0

V

(^) +
S

• G D+
  RLD
  RG RS
  S


• G D+
  RLD
  RG RS
  S