Test: Power Semiconductor Diode & Transistors - Electrical Engineering (EE) MCQ

In a Schottky diode junction is formed between metal and semiconductor

A Schottky diode has very low reverse voltage rating as compared to alloyed and diffused diodes. Maximum voltage rating is about 100 V. Thus, assertion is a false statement.
In a schottky diode, a thin epitaxial layer is grown on n-type base (substrate). On the epitaxial layer metal (gold, chromium or platinum) is deposited to form a thin layer. This metallic layer is surrounded by a ring of insulating material such as SiO₂. Thus reason is a true statement.

• In fast recovery diode, reverse recovery time is reduced by addition of gold or platinum due to which rate of recombination is increased. Hence, statements-2 is not correct.
• MOSFET and transistor are used in low and medium power applications. Thus, statements-3 is not correct.

A power transistor
1. can not be easily paralleled because of V_be matching problems and local current concentration.
2. is susceptible to secondary breakdown.
3. has high transconductance.
4. has positive temperature coefficient on collector current since I_c in creases with temperature.
5. has low switching speed than MOSFET.

TRIAC is a bidirectional device whereas BJT, GTO, SCR  and IGBT are unidirectional devices.

The reverse blocking safe operating area (RBSOA) for a power transistor is shown below.

It is clear from above curve that with increased reverse bias, the RBSOA decreases in size.

Energy loss during turn-off

= 0.1603 Watt-sec
Total energy loss in one cycle
= 0.1067 + 0.1603 
= 0.267 W-sec
Average power loss in transistor
= Switching frequency x energy loss in one cycle
∴ Allowable switching frequency

BJTcan easily turn on and turn off using base current.