THYRISTOR DEVICE

DESCRIPTION :

                          Thyristor is now defined as a generic as a generic term applicable to the whole range of four layer (PNPN) semiconductor switches. It is also known commercially as silicon controlled rectifier (SCR). The structure of a thyristor with the three terminals and its electrical symbol are device can carry current only in one direction from anode to cathode and the instant of initiation of conduction can be controlled by the gate. 

          The voltage rating of a thyristor (the ability to withstand, when turned off) is now in the range of 5 kv while the current rating has gone up to 3000A. While the current ratings available now are adequate, thus avoiding the ne necessity of parallel connection, the voltage rating is insufficient to make up a high voltage valve. Thus,series connection of devices to make up a thyristor valve is necessary and introduces some problems that have to be considered in the design of valves and protection.

              Increasing the voltage rating of a thyristor is feasible, but it is at the cost of increased losses,turn-off times and reduced peak allowable junction temperatures. The capability of a thyristor to with stand high voltages critically depends on the quality of silicon crystal from which the device is made the more uniform the crystal using low energy neutron or gamma rays results in light, precisely controlled doping.