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Electronics and Communication Engineering - Electromagnetic Field Theory

Exercise : Electromagnetic Field Theory - Section 3
36.
Electrons can be emitted from a metal surface due to high electric field.
True
False
Answer: Option
Explanation:

This is called field emission.

37.
In an n type semiconductor the fermi level is 0.35 eV below the conduction band, the concentration of donor atoms is increased to three times. The new position of Fermi level will be
0.35 eV below conduction band
about 0.32 eV below conduction band
about 0.32 eV above conduction band
about 0.1 eV below conduction band
Answer: Option
Explanation:

Therefore, conductivity increases.

38.

Assertion (A): In a BJT, the base region is very thick.

Reason (R): In p-n-p transistor most of holes given off by emitter diffuse through the base.

Both A and R are true and R is correct explanation of A
Both A and R are true but R is not a correct explanation of A
A is true but R is false
A is false but R is true
Answer: Option
Explanation:

Base region is thin.

39.

Assertion (A): The behaviour of FET is similar to that of a pentode.

Reason (R): FETs and vacuum triode are voltage controlled devices.

Both A and R are true and R is correct explanation of A
Both A and R are true but R is not a correct explanation of A
A is true but R is false
A is false but R is true
Answer: Option
Explanation:

Low work function permits easy emission.

40.
SCR can be turned on by
  1. applying anode voltage at a sufficient fast rate
  2. applying sufficiently large anode voltage
  3. increasing the temperature of SCR to a sufficiently
  4. applying sufficiently large gate current.
1, 2, 4 only
4 only
1, 2, 3, 4
none
Answer: Option
Explanation:

If we apply the anode voltage above breakover voltage of SCR, SCR can be triggered. Also by sufficiently fast rate of rise of anode voltage and large gate current will trigger SCRon.

During forward blocking most of the applied voltage appears across reverse biased junction J2.

This voltage across J2 associated with leakage current may rise temperature of this junction.

With increase in temperature, leakage current through junction J2 further increases and this cumulative process may turn on the SCR at some high temperature.

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