19-04-2014, 11:00 AM
Semiconductor Diode
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INTRODUCTION
It has already been discussed in the previous chapter that a pn junction conducts current easily
when forward biased and practically no current flows when it is reverse biased. This unilateral
conduction characteristic of pn junction (i.e. semiconductor diode) is similar to that of a
vacuum diode. Therefore, like a vacuum diode, a semiconductor diode can also accomplish the
job of rectification i.e. change alternating current to direct current. However, semiconductor
diodes have become more *popular as they are smaller in size, cheaper and robust and usually
operate with greater efficiency. In this chapter, we shall focus our attention on the circuit perfor-
mance and applications of semiconductor diodes.
Crystal Diode as a Rectifier
Fig. 6.3 illustrates the rectifying action of a crystal diode. The a.c. input voltage to be rectified, the
diode and load RL are connected in series. The d.c. output is obtained across the load as explained in
the following discussion. During the positive half-cycle of a.c. input voltage, the arrowhead becomes
positive w.r.t. bar. Therefore, diode is forward biased and conducts current in the circuit. The result
is that positive half-cycle of input voltage appears across RL as shown. However, during the negative
half-cycle of input a.c. voltage, the diode becomes reverse biased because now the arrowhead is
negative w.r.t. bar. Therefore, diode does not conduct and no voltage appears across load RL. The
result is that output consists of positive half-cycles of input a.c. voltage while the negative half-cycles
are suppressed. In this way, crystal diode has been able to do rectification i.e. change a.c. into d.c. It
may be seen that output across RL is pulsating d.c.
Resistance of Crystal Diode
It has already been discussed that a forward biased diode conducts easily whereas a reverse biased
diode practically conducts no current. It means that forward resistance of a diode is quite small as
compared with its reverse resistance.
1. Forward resistance. The resistance offered by the diode to forward bias is known as forward
resistance. This resistance is not the same for the flow of direct current as for the changing current.
Accordingly; this resistance is of two types, namely; d.c. forward resistance and a.c. forward resistance.
(i) d.c. forward resistance. It is the opposition offered by the diode to the direct current. It is
measured by the ratio of d.c. voltage across the diode to the resulting d.c. current through it. Thus,
referring to the forward characteristic in Fig. 6.5, it is clear that when forward voltage is OA, the
forward current is OB.
Centre-Tap Full-Wave Rectifier
The circuit employs two diodes D1 and D2 as shown in Fig. 6.24. A centre tapped secondary winding
AB is used with two diodes connected so that each uses one half-cycle of input a.c. voltage. In other
words, diode D1 utilises the a.c. voltage appearing across the upper half (OA) of secondary winding
for rectification while diode D2 uses the lower half winding OB.
Operation. During the positive half-cycle of secondary voltage, the end A of the secondary winding
becomes positive and end B negative. This makes the diode D1 forward biased and diode D2 reverse
biased. Therefore, diode D1 conducts while diode D2 does not. The conventional current flow is through
diode D1, load resistor RL and the upper half of secondary winding as shown by the dotted arrows. During
the negative half-cycle, end A of the secondary winding becomes negative and end B positive. Therefore,
diode D2 conducts while diode D1 does not. The conventional current flow is through diode D2, load RL
and lower half winding as shown by solid arrows. Referring to Fig. 6.24, it may be seen that current in the
load RL is in the same direction for both half-cycles of input a.c. voltage. Therefore, d.c. is obtained across
the load RL. Also, the polarities of the d.c. output across the load should be noted.