Power devices are fundamentally of two types: BJT based devices and MOSFET-based devices. TRIAC and SCR are BJT based power devices while IGBT is MOSFET-based devices.
Diode behave like a one way switch that means diode shall allow the current to flow while it is in forward bias and it shall not allow the current flow while it is in reverse bias. while in forward bias the Si diode shall have 0.7 V drop across the diode and current shall pass through the diode acting as a closed switch while when it is in reverse bias, it shall act like an open switch as illustrated in b Figure (a).
(a) (b) (c)
Figure: (a) Diode Switch; (b) npn and pnp BJT; and (c) Transistor Switch
The BJT switch may be realized by applying a voltage among base and emitter. While the voltage V across base and emitter are zero the switch is open and no current pass between collector and emitter, the BJT is off. While a nonzero V voltage (> 0.7 V) is applied between the base and emitter, the BJT is on, current pass between collector and emitter. Note down the current direction and applied voltage V polarities are opposite for the n-p-n and n-p-n BJT as illustrated in Figure (b) and the Figure (c) draws a transistor switch built up of three terminals of a transistor.