A thyristor is a four-layer semiconductor device with alternating p-type and n-type materials, usually having three electrodes: an anode, a cathode, and a gate. Its working principle can be understood as follows:
Structure and Basics
- A thyristor consists of four semiconductor regions: p-n-p-n. The outer p region forms the anode, and the outer n region forms the cathode. It has three p-n junctions (serially named J1, J2, J3 from the anode).
- When the anode is at a positive potential with respect to the cathode and no voltage is applied at the gate, junctions J1 and J3 are forward biased, while junction J2 is reverse biased. As J2 is reverse biased, no conduction takes place (off state).
Forward Blocking State
- Voltage is applied in the direction that would cause a diode to conduct, but the thyristor has not been triggered into conduction. A small forward leakage current may flow, but it is not enough to trigger the thyristor, so the thyristor is in an off state similar to an open switch.
Reverse Blocking State
- Voltage is applied in the reverse direction, and the thyristor blocks the current in the same way as a reverse-biased diode. It can only conduct in one direction and blocks in the reverse direction, acting as an open switch.
Forward Conducting State
- Triggering: When a positive potential is applied at the gate terminal with respect to the cathode, the breakdown voltage of junction J2 is reduced. By selecting an appropriate value of the gate voltage, the thyristor can be switched into the on state quickly. This can also happen when the anode-cathode voltage is increased beyond the breakdown voltage of the thyristor, causing avalanche breakdown of J2.
- Latching: Once avalanche breakdown has occurred, the thyristor continues to conduct, irrespective of the gate voltage, until the potential across the anode and cathode is removed or the current through the device (anode-cathode) becomes less than the holding current specified by the manufacturer. This is because the thyristor's structure forms a positive feedback loop within the device, similar to two back-to-back transistors, where the output of one transistor feeds the input of the other, causing the total current gain of the device to exceed one, and quickly building up the current until both transistors are fully turned on or saturated.
In summary, the thyristor acts as a bistable switch, which can be in the off state (forward or reverse blocking) or the on state (forward conducting). It remains in the on state until the forward current drops below the holding current or the anode-cathode voltage is...