A thyristor is a four-layer
p-n-p-n semiconductor device consisting of three p-n junctions. It has three terminals: anode, cathode and a gate. Figure (1)
shows the thyristor symbol and a sectional view of the three-pn junctions
When the anode voltage made positive with respect to the cathode,
junctions J1 and J3 are forward biased and junction J2 is reverse biased. The thyristor said to be in the forward blocking or off-state condition. 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 already forward biased, there will be free movements of the carriers across
all three junctions. This results in a large
forward current. The device now said to be in a conducting or on state. The
voltage drops across the in the on-state is due to ohmic drop in the four
layers and is very small (in the region of 1 V). In the
on-state anode current is limited by an external impedance or resistance as
show in fig (2-a).
V-I Characteristics of Thyristor
Figure (2) shows the V-I characteristics and the circuit used to obtain these characteristics.
The important points on this characteristic are:
1.Latching Current IL
2. 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-tocathode voltage rises again. It might then return to the conducting state without an externally applied gate current.
3. 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.
V-I Characteristics of Thyristor
Figure (2) shows the V-I characteristics and the circuit used to obtain these characteristics.
The important points on this characteristic are:
1.Latching Current IL
2. 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-tocathode voltage rises again. It might then return to the conducting state without an externally applied gate current.
3. 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.
4.Forward Break-over Voltage VBO
If the forward voltage VAK is increased beyond VBO, the thyristor can be turned on. However, 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.
Note
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.
