Static Induction Thyristors (SIT)
Construction and output characteristics
- A SIT or SI-thyristor, is a self controlled GTO like ON-OFF device. It is a three terminal device. The three terminals are Anode, Cathode and Gate.
- The characteristics of SIT are very similar to those of a power MOSFET. It is turned on by applying a positive gate voltage and turned off by applying a negative gate voltage.
- It is minority carrier device. Its important characteristics are fast switching speeds, high dv/dt, di/dt capacity and low on state resistance.
- Figure (a) shows the basic structure of SIT and Figure (b) shows its circuit symbol.
- This device does not have a reverse blocking capability that it will start conducting means immediately if a reverse voltage is applied between its anode and cathodes.
- The turn off characteristics of SIT are similar to these of a GTO i.e. to turn off SIT, it is necessary to apply a large negative gate current.
- SIT is a normally ON device i.e. if its anode is positive and gate voltage is zero, then the device will behave like a forward biased diode and the current will flow freely.
- The forward biasing of the P+ N junction will cause a hole injection into the N– region and its conductivity will change.
- When the gate is reverse biased, with respect to cathode, a depletion layer will block the anode current and the device gets OFF.
Features of SIT
- SIT is a normally ON device.
- The ON state voltage drop is high.
- Low turn off gain i.e. large negative IG is required for turning off SIT.
- High switching frequency.
- High dv/dt and di/dt ratings.
- Improved SOA.