ISCET Semiconductor Devices 4

Question 1

What is thermal runaway in a BJT circuit, and what component is commonly used to prevent it?

Accepted Answer

Uncontrolled increase in IC due to temperature rise; prevented by emitter resistor (RE)

Answer

Thermal runaway occurs because rising temperature increases IC, which further heats the device; an emitter resistor provides negative feedback to stabilize the operating point.

Question 2

The SCR (Silicon Controlled Rectifier) will turn OFF when:

Accepted Answer

The anode current drops below the holding current (IH)

Answer

Once triggered, an SCR latches on and only turns off when the anode current falls below the minimum holding current (IH), regardless of the gate signal.

Question 3

In a common-emitter BJT amplifier, what is the phase relationship between the input and output voltages?

Accepted Answer

180° phase inversion

Answer

The common-emitter configuration inverts the signal — a positive-going input produces a negative-going output — resulting in a 180° phase shift.

Question 4

What is the primary difference between a depletion-mode and an enhancement-mode MOSFET?

Accepted Answer

Depletion-mode has a physical channel at VGS = 0; enhancement-mode does not

Answer

Depletion-mode MOSFETs have a physically doped channel that exists at VGS = 0, while enhancement-mode MOSFETs require gate voltage to induce a channel.

Question 5

Which semiconductor device is a four-layer PNPN structure that acts as a bidirectional switch?

Accepted Answer

TRIAC

Answer

A TRIAC is a bidirectional thyristor that can conduct in both directions and is triggered by either polarity of gate signal, making it ideal for AC power control.

(ISCET) International Society of Certified Electronics Technicians Practice Test

(ISCET) International Society of Certified Electronics Technicians Practice Test

ISCET Semiconductor Devices 4