symbol_of_thyristorThe Thyristors is a semiconductor switch either of the pnpn or npnp type, whose bi-stable action depends on regenerative internal feedback. Figure at the left is symbol of thyristor, where A is anoda terminal, C is cathode terminal and G is gate terminal.

The four layer device is usually silicon although germanium has been used. Devices with the two endmost layers only connected to external terminals (anode and cathode) are called four layer diode, those with three layers accessible (anode, cathode, and p gate) are called thyristors. SCR (Silicon Controlled Rectifier) is other name of thyristors.

The structure is best visualized as consisting of two transistors, a pnp and an npn interconnected to form a regenerative feedback pair as shown in figure bellow.

Current gain around the internal feedback loop G (Gate) is hfe1 x hfe2, where hfe1 and hfe2 are the common emitter current gains of the individual sections. If Ico1 is the collector to base leakage current of the npn section and Ico2 is the collector to base leakage of the pnp section, then:
for the pnp section: Ic1 = hfe1 (Ic2 + Ico1) + Ico1
for the npn section: Ic2 = hfe2 (Ic1 + Ico2) + Ico2
and the total anode to cathode current Ia = (Ic1 + Ic2)

from which Ia = [(1 + hfe1) (1 + hfe2) (Ico1 + Ico2)] / [1 – (hfe1) (hfe2)]

With a proper bias applied, ie positive anode to cathode voltage, the structure is said to be in the forward blocking or high impedance 'off ' state. The switch to the low impedance 'on' state is initiated simply by raising the loop gain G to unity. As this occurs the circuit starts to regenerate, each transistor driving its mate to saturation. Once in saturation all junctions assume a forward bias, and the total potential drop across the device approximates to that of a single junction. Anode current is then only limited by the external circuit.

To turn off thyristor in a minimum time it is necessary to apply a reverse voltage and under this condition the holes and electrons in the vicinity of the two end junctions will diffuse in these junction and result in a reverse current in the external circuit. The voltage across the thyristor will remain at about 0.7 V positive as long as an appreciable reverse current flows.

After the been removed, the reverse current will cease and the junction assume a blocking state. The turn-off time is usually of the order of 10-15 μs. The fundamental difference between the transistor and thyristor is that with the former conduction can be stopped at any point in the cycle because the current gain is less than unity. This is not so far the thyristor, conduction only stopping at a current zero.

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