CET Semiconductor Devices & Circuits 5

Question 1

What does the thermal resistance θJA represent in a transistor datasheet?

Accepted Answer

Temperature rise per watt of power dissipated from junction to ambient air

Answer

θJA (junction-to-ambient thermal resistance) quantifies how many degrees Celsius the junction temperature rises for each watt of dissipated power.

Question 2

In a CMOS inverter, when the input is logic HIGH, which transistor is ON?

Accepted Answer

The NMOS transistor

Answer

A HIGH input turns on the NMOS (pulling output to GND) while turning off the PMOS, producing a LOW output.

Question 3

The transition frequency fT of a BJT is the frequency at which:

Accepted Answer

The current gain β drops to unity (1)

Answer

fT (unity-gain bandwidth) is the frequency at which the common-emitter current gain magnitude falls to 1, marking the device's useful amplification limit.

Question 4

Which type of semiconductor device is used as the basis of a UJT (Unijunction Transistor) relaxation oscillator?

Accepted Answer

A device with one p-n junction, one emitter, and two base contacts

Answer

A UJT has a single emitter junction on an N-type silicon bar with two ohmic base contacts (B1 and B2) used to trigger oscillations.

Question 5

In power electronics, a freewheeling (flyback) diode is placed across an inductive load primarily to:

Accepted Answer

Clamp and recirculate inductor current when the switch turns off

Answer

When the switch opens, the inductor's collapsing field generates a back-EMF; the freewheeling diode provides a safe current path to prevent voltage spikes from damaging the switch.

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CET Semiconductor Devices & Circuits 5