Page 996 - Basic Electrical Engineering

Through the voltage divider R and R as in Fig. 14.33 (b) a positive
1
2
voltage when applied, the circuit is switched on. The device can be set to be
on for a desired level of drain current, by determining the V from the
GS
transfer characteristic provided by the manufacturer. To make sure that the
device gets switched off, the V must be brought below the minimum
GS
threshold voltage, i.e., V GST .

14.10 SILICON-CONTROLLED RECTIFIER

A silicon controlled rectifier (SCR) is a four-layer three-junction device.
Thyristor is the general name given to a family of such semiconductor

devices having four layers and three junctions. After semiconductor diodes
and transistors, thyristors were introduced in a big way in almost all control
applications in industry.

A SCR shown in Fig. 14.34 has four alternate layers of p, n, p, and n. It has
three terminals, viz A (anode), K (cathode), and G (gate). The three p–n

junctions are J , J , and J .
3
2
1
The majority charge carriers in the p-type material are the holes and in the
n-type material are the electrons, respectively. Electrons are negatively
charged while the holes are positively charged. The gate terminal is

connected to the p-layer near the cathode. The gate is provided with positive
supply to trigger the SCR. Now, let us see what happens when the SCR is

biased as in Fig. 14.35.

14.10.1 Characteristics of SCR

The anode is connected to the positive terminal of the battery while the

cathode is connected to the negative terminal. Junctions J and J are forward
1
3
biased while junction J is reverse biased.
3
Junction J will have an expanded depletion layer because of its reverse
3
biasing. As such no current will flow from anode to cathode except a very
minute amount of leakage current also called reverse saturation current. The

SCR under the forward-biased condition will not conduct. However, if we