Static Induction Thyristor (SITH) – Symbol and its Workings:

An Static Induction Thyristor (SITH) or SI-thyristor is a self-controlled GTO-like on-off device commercially introduced in Japan in 1988. Similar device, known as field-controlled thyristors (FCT) or field-controlled diode (FCD), were developed in USA. The device symbol is shown in Fig. 11.10.

It is essentially a P⁺ NN⁺ diode. Similar to SIT, SITH is a normally on device with the N-region saturated with minority carrier. The turn-off behavior of SITH is similar to that of GTO. It is a self-controlled GTO-like high power device (1200 V, 800A).

Its general comparison with GTO is given below.

• Unlike GTO, SITH is a normally on device-asymmetric blocking.
• The conduction drop is higher (4.0 V)
• The turn-off current gain is lower (1 to 3)
• The switching frequency is higher (t_ON = 2 μs, t_OFF = 9 µs) and has lower switching losses.
• The dv/dt and di/dt ratings are higher. (2 kV/μs, 900A/μs.
• The SOA is improved and T_j is limited.