Q: In an insulator, the forbidden energy gap is typically:

Greater than about 3\text{ eV}. Diamond, for example, has a gap of about 6\text{ eV}, far beyond the reach of thermal energy at room temperature.

Q: In a semiconductor, the forbidden energy gap is typically:

Of the order of 1\text{ eV}. A gap of this size lets a useful number of electrons cross it by thermal excitation at ordinary temperatures.

Q: A semiconductor sample has resistivity 0.5\,\Omega\cdot\text{m}. Its conductivity is:

2\text{ S/m}. \sigma = \dfrac{1}{\rho} = \dfrac{1}{0.5} = 2\text{ S/m}.

Question 1

The energy gap in a solid is the separation between the:

Accepted Answer

Top of the valence band and the bottom of the conduction band. No electron states exist within this forbidden gap, so an electron must acquire at least E_g to become free.

Question 2

In a conductor, the valence band and the conduction band:

Accepted Answer

Overlap each other. With overlapping bands, electrons can move to higher states under even the feeblest field.

Question 3

In an insulator, the forbidden energy gap is typically:

Accepted Answer

Greater than about 3	ext{ eV}. Diamond, for example, has a gap of about 6	ext{ eV}, far beyond the reach of thermal energy at room temperature.

Question 4

In a semiconductor, the forbidden energy gap is typically:

Accepted Answer

Of the order of 1	ext{ eV}. A gap of this size lets a useful number of electrons cross it by thermal excitation at ordinary temperatures.

Question 5

The electrical resistivity of a semiconductor lies:

Accepted Answer

Between that of a conductor and that of an insulator. This intermediate behaviour, controllable by doping, is what makes semiconductors so useful.

Question 6

A semiconductor sample has resistivity 0.5\,\Omega\cdot	ext{m}. Its conductivity is:

Accepted Answer

2	ext{ S/m}. \sigma = \dfrac{1}{ho} = \dfrac{1}{0.5} = 2	ext{ S/m}.

Question 7

In an intrinsic semiconductor, the numbers of free electrons and holes are:

Accepted Answer

Equal. Every electron promoted to the conduction band leaves behind exactly one hole, so n_e = n_h = n_i.

Question 8

An extrinsic semiconductor is one that has been:

Accepted Answer

Deliberately doped with a suitable impurity. Doping in a proportion as small as one part in a million transforms the conductivity.

Question 9

An n-type semiconductor is produced by doping with a:

Accepted Answer

Pentavalent impurity such as phosphorus or arsenic. The fifth valence electron is not needed for bonding and becomes a free conduction electron.

Question 10

A p-type semiconductor is produced by doping with a:

Accepted Answer

Trivalent impurity such as boron or indium. With only three valence electrons, the impurity leaves a vacancy in one bond — a hole.

Question 11

In an n-type semiconductor, the majority and minority carriers are respectively:

Accepted Answer

Electrons and holes. Donor atoms supply plenty of electrons while holes arise only from thermal generation.

Question 12

In a p-type semiconductor, the majority and minority carriers are respectively:

Accepted Answer

Holes and electrons. Acceptor atoms create an abundance of holes, leaving electrons as the scarce minority carriers.

Question 13

Germanium doped with phosphorus becomes:

Accepted Answer

An n-type semiconductor. Phosphorus is pentavalent, so it donates electrons.

Question 14

Silicon doped with boron becomes:

Accepted Answer

A p-type semiconductor. Boron is trivalent, so it accepts electrons and creates holes.

Question 15

In an intrinsic semiconductor the relation between electron, hole and intrinsic concentrations is:

Accepted Answer

n_e = n_h = n_i. Thermal generation always produces electrons and holes in pairs.

Question 16

The conductivity of an extrinsic semiconductor compared with that of the intrinsic material is:

Accepted Answer

Very much greater. Doping supplies carriers far more numerous than those produced by thermal generation alone.

Question 17

When a p-n junction is forward biased:

Accepted Answer

The potential barrier is reduced and a large current flows. The external supply opposes the built-in field, letting majority carriers cross freely.

Question 18

When a p-n junction is reverse biased:

Accepted Answer

The barrier increases and only a small leakage current flows. The small reverse current is carried by thermally generated minority carriers.

Question 19

A junction diode is described as a unidirectional device because it:

Accepted Answer

Conducts appreciably in one direction only. This asymmetry is what makes rectification possible.

Question 20

The forward and reverse resistances of a junction diode are respectively:

Accepted Answer

Very low and very high. This large ratio, often 10^5 or more, is what makes the diode an effective one-way valve.

Semiconductor Devices — MH-CET Physics MCQs with Solutions

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