### Physics - Electronic Devices

#### Question - 1

For a germanium sample, traces of gallium are added as an impurity. The resultant sample could behave like

• A a conductor
• B an n-type semiconductor
• C a p-type semiconductor
• D an insulator

#### Question - 2

The conductivity of a semiconductor increases with increase in temperature, because

• A number density of free current carriers increases
• B relaxation time increases
• C both number density of carriers and relaxation time increase
• D number density of current carriers increases, relaxation time decreases but effect of decrease relaxation time is much less than increase in number density.

#### Question - 3

For pure Ge semiconductor, quantity of electron and hole is 1019 e/m3 . If we doped a donor impurity in it with density 1023 e/m3, then quantity of hole (e/m3) in a semiconductor is

• A 1015
• B 1019
• C 1023
• D 1027

#### Question - 4

Mobility of electrons in n-type Ge is 5000 cm2 /V-s and conductivity is 5 mho/cm. If the effect of holes is negligible, then impurity concentration will be

• A 6.25 x 1015/cm3
• B 9.25 x 1014/cm3
• C 6 x 1013/cm3
• D 9 x 1013/cm3

#### Question - 5

In sample of pure silicon 1023 e/m3  is mixed to phosphorous. If all atoms are active, then what will be the resistivity at 20oC, if mobility of electron is 1200 cm2/V-s?

• A 0.5209 ohm-cm
• B 5.209 ohm-cm
• C 52.09 ohm-cm
• D 520.9 ohm-cm

#### Question - 6

What will be conductivity of pure silicon crystal at 300 K temperature, if electron-hole pairs per cm3 are 1.072 x 1010, $\mu_e$ = 1350 cm2/V-s and  $\mu_e$ = 480 cm2/V-s ?

• A 3.14 x 10-6 mho/cm
• B 3 x 106 mho/cm
• C 10-6 mho/cm
• D 106 mho/cm

#### Question - 7

Forbidden energy gap of Ge is 0.75 eV. Maximum wavelength of incident radiation of photon for producing electron-hole pair in germanium semiconductor is

• A 4200 Å
• B 16500 Å
• C 4700 Å
• D 4000 Å

#### Question - 8

When a p-n junction diode is reverse biased

• A   electrons and holes are attracted towards each other and move towards the depletion region.
• B   electrons and hole move away from the junction depletion region.
• C height of the potential barrier decreases.
• D no change in the current takes place.

#### Question - 9

Ripple factor of half-wave rectifier is

• A 1.21
• B 1.6
• C 1.5
• D 1.4

#### Question - 10

In an n-p-n transistor circuit, the collector current is 10 mA. If 90% of the electrons emitted, reach the collector, the emitter current(IE) and base current (IB) are given by.

• A IE = 1 mA; IB=11 mA
• B IE = 11 mA; IB=1 mA
• C IE = -1 mA; IB=9 mA
• D IE = 9 mA; IB=-1 mA