340 SEMICONDUCTOR DEVICES
4. 35 × 10 −^4 S/m. Good electrical characteristics and feasible fabrication technology have been
requisites for the prevalence of silicon technology. Compound semiconductors, such as gallium
arsenide, are being developed for microwave and photonic applications, while germanium is used
for a few special purposes.
When unbound negatively charged electrons move through a crystal, as shown in Figure
7.1.1(a), electrical conduction in semiconductors can take place with a direction of current opposite
to the direction of movement of the electrons. When a bound electron that should be present in the
valence bond is missing, the vacancy that arises is known as ahole.Holes are positively charged
particles with a charge equal in magnitude to that of the electron. Mobile positively charged holes
can also give rise to a current, as shown in Figure 7.1.1(b), with the direction of current in the
same direction as the movement of holes. Both holes and unbound electrons are known as charge
carriers, or simplycarriers.
Pure semiconductors, known asintrinsic semiconductors,have very few charge carriers
and may hence be classified as almost insulators or very poor electrical conductors. However,
by adding (through a process known asdoping) tiny controlled amounts of impurities (such
as boron, gallium, indium, antimony, phosphorus, or arsenic), a semiconductor can be made to
contain a desired number of either holes or free electrons and is then known asextrinsic(impure)
material. Ap-type semiconductorcontains primarily holes, whereas ann-type semiconductor
contains primarily free electrons. While holes are themajority carriersin ap-type material, it
is possible to inject electrons artificially intop-type material, in which case they become excess
minority carriers.Minority carriers do play a vital role in certain devices. The doping substance
is called anacceptorwhen the extrinsic semiconductor is thep-type with holes forming the
majority carriers and electrons forming the minority carriers. The doping substance is known as
thedonorwhen the extrinsic semiconductor is then-type with free electrons forming the majority
carriers and holes forming the minority carriers. Bothp-andn-type semiconductors are vitally
important in solid-state device technology. Diodes, transistors, and other devices depend on the
characteristics of apn-junction formed when the two materials are joined together as a single
crystal.− − −− − −
− − −Direction of
currentDirection of
current
(a)+ + ++ + +
+ + +(b)Figure 7.1.1Electrical conduc-
tion.(a)Electrons moving from left
to right give rise to a current di-
rected from right to left.(b)Holes
moving from left to right give rise
to a current directed from left to
right.7.2 DIODES
A singlepn-junction with appropriate contacts for connecting the junction to external circuits
is called a semiconductorpn-junction diode.The fundamental building block upon which all
semiconductor devices are based is thepn-junction. The most common two-terminal nonlinear
resistor is the semiconductor diode, whose symbol is shown in Figure 7.2.1(a). The terminal
voltage and current are denoted byvDandiD, respectively. The physical structure of thepn-
junction is shown in Figure 7.2.1(b). The junction is made by doping the two sides of the crystal
with different impurities. Figure 7.2.1(c) shows thevolt–ampere curve(orstatic characteristic)
of the ideal (or perfect) diode. Note that whenvDis zero,iDis not and vice versa, a condition