1-23
For example at point A the forward-bias voltage is 1 volt and the forward-bias current is 5
milliamperes. This represents 200 ohms of resistance (1 volt/5mA = 200 ohms). However, at point B the
voltage is 3 volts and the current is 50 milliamperes. This results in 60 ohms of resistance for the diode.
Notice that when the forward-bias voltage was tripled (1 volt to 3 volts), the current increased 10 times
(5mA to 50 mA). At the same time the forward-bias voltage increased, the resistance decreased from 200
ohms to 60 ohms. In other words, when forward bias increases, the junction barrier gets smaller and its
resistance to current flow decreases.
On the other hand, the diode conducts very little when reverse biased. Notice at point C the reverse
bias voltage is 80 volts and the current is only 100 microamperes. This results in 800 k ohms of
resistance, which is considerably larger than the resistance of the junction with forward bias. Because of
these unusual features, the PN junction diode is often used to convert alternating current into direct
current (rectification).
Q24. What is the name of the area in a PN junction that has a shortage of electrons and holes?
Q25. In order to reverse bias in a PN junction, what terminal of a battery is connected to the P
material?
Q26. What type of bias opposes the PN junction barrier?
PN JUNCTION APPLICATION
Until now, we have mentioned only one application for the diode-rectification, but there are many
more applications that we have not yet discussed. Variations in doping agents, semiconductor materials,
and manufacturing techniques have made it possible to produce diodes that can be used in many different
applications. Examples of these types of diodes are signal diodes, rectifying diodes, Zener diodes (voltage
protection diodes for power supplies), varactors (amplifying and switching diodes), and many more. Only
applications for two of the most commonly used diodes, the signal diode and rectifier diode, will be
presented in this chapter. The other diodes will be explained later on in this module.
Half-Wave Rectifier
One of the most important uses of a diode is rectification. The normal PN junction diode is
well-suited for this purpose as it conducts very heavily when forward biased (low-resistance direction)
and only slightly when reverse biased (high-resistance direction). If we place this diode in series with a
source of ac power, the diode will be forward and reverse biased every cycle. Since in this situation
current flows more easily in one direction than the other, rectification is accomplished. The simplest
rectifier circuit is a half-wave rectifier (fig. 1-21 view A and view B) which consists of a diode, an ac
power source, and a load resister.
