The thyristor is a four layer P-N-P-N
device with different levels of doping for each layer. The cathode is
the most heavily doped and the gate and anode are less heavily doped .
The central N type layer is only slightly doped and is also thicker
than the other layers enabling it to support a high blocking voltage
In operation the thyristor may be considered as an NPN and a PNP
transistor connected back to back, forming a positive feedback loop
within the device. The output of one transistor is fed to the input of
the second and the output of the second transistor is in turn fed back
to the input of the first. A small trigger pulse on the gate will turn
the thyristor on and once a current starts to flow, it quickly builds up
until both transistors are fully turned on or saturated and the only
way it can be turned off is by removing the supply voltage
The device is designed to act as a switch and can carry very high currents
