A thyristor is a four layer pnpn semiconductor device consisting of three pn junctions. It has three terminals: an anode a cathode and a gate. Figure 4.1 shows the thyristor symbol and a sectional view of the three pn junctions.
Figure 4.1 Thyristor Symbol & pn Junctions
When the anode voltage is made positive with respect to the cathode, junctions J1 and J3 are forward biased and junction J2 is reverse biased. The thyristor is said to be in the forward blocking or off-state condition. A small leakage current flows from anode to cathode and is called the off-state current.
If the anode voltage VAK is increased to a sufficiently large value, the reverse biased junction J2 would breakdown. This is known as avalanche breakdown and the corresponding voltage is called the forward breakdown voltage VBO. Since the other two junctions J1 and J3 are already forward biased, there will be free movement of carriers across all three junctions. This results in a large forward current. The device is now said to be in a conducting or on-state. The voltage drop across the device in the on-state is due to the ohmic drop in the four layers and is very small (in the region of 1 V). In the on-state the anode current is limited by an external impedance or resistance as shown in figure 4.2(a).
V-I Characteristics of Thyristor
Figure 4.2 shows the V-I characteristics and the circuit used to obtain these characteristics.
Figure 4.2 Thyristor Circuit & V-I Characteristics
The important points on this characteristic are :
Latching Current IL
This is the minimum anode current required to maintain the thyristor in the on-state immediately after a thyristor has been turned on and the gate signal has been removed.
If a gate current greater than the threshold gate current is applied until the anode current is greater than the latching current IL then the thyristor will be turned on or triggered.
Holding Current IH
This is the minimum anode current required to maintain the thyristor in the on-state.
To turn off a thyristor, the forward anode current must be reduced below its holding current for a sufficient time for mobile charge carriers to vacate the junction. If the anode current is not maintained below IH for long enough, the thyristor will not have returned to the fully blocking state by the time the anode-to-cathode voltage rises again. It might then return to the conducting state without an externally-applied gate current.
Reverse Current IR
When the cathode voltage is positive with respect to the anode, the junction J2 is forward biased but junctions J1 and J3 are reverse biased. The thyristor is said to be in the reverse blocking state and a reverse leakage current known as reverse current IR will flow through the device.
Forward Breakover Voltage VBO
If the forward voltage VAK is increased beyond VBO , the thyristor can be turned on. But such a turn-on could be destructive. In practice the forward voltage is maintained below VBO and the thyristor is turned on by applying a positive gate signal between gate and cathode.
- Once the thyristor is turned on by a gate signal and its anode current is greater than the holding current, the device continues to conduct due to positive feedback even if the gate signal is removed. This is because the thyristor is a latching device and it has been latched to the on-state.