Explore topic-wise InterviewSolutions in .

(a) Recombination of charge carriers

(i) The frequency of light emitted by an LED depends on the band gap of the semiconductor used in LED. 

(ii) The intensity of light emitted depends on the doping level of the semiconductor used. 

The semiconductor obtained by doping a pure semiconductor like silicon with impurity atoms to enhance its conductivity is called an extrinsic or doped semiconductor. 

(c) The change in the crystal structure

The process of adding impurities to the intrinsic semiconductor is called doping.

An extrinsic semiconductor is a semiconductor doped by a specific impurity which is able to deeply modify its electrical properties, making it suitable for electronic applications (diodes, transistors etc.) or optoelectronic applications (light emitters and detectors).

(b) NOT

 When both the inputs of a NAND gate are joined, it functions as a NOT gate.

(a) a.c into d.c

(a) intrinsic semiconductors 

(a) In intrinsic semiconductor, n_h = n_e 

(b) In P-type semiconductor, n_h >> n_e

(d) holes.

Holes are the major carriers of current in a ptype semiconductor.

Optoelectronics deals with devices which convert electrical energy into light and light into electrical energy through semiconductors.

(d) holes.

Holes are the major carriers of current in a ptype semiconductor.

(d) all of these

Ga, B and Al are all trivalent atoms, they produce p-type semiconductor.

Photodiode / Light emitting diode / photovoltaic cell or solar cell.  

Diode is called as a unidirectional device, i.e., current flows in only one direction (anode to cathode internally) when a forward voltage is applied, the diode conducts and when reverse voltage is applied, there is no conduction. A mechanical analogy is a rat chat, which allows motion in one direction only.

(i) The width of the forbidden band. 

(ii) Intrinsic charge carrier concentration. 

(a), (c), (d)

(a) R_L should be increased.

(c) input frequency should be increased.

(d) capacitors with high capacitance should be used.

(b), (d)

(b) The current through the series Resistance (R_s) changes.

(d) The resistance offered by the Zener changes.

(b), (c)

(b) The emitter current will be 10.53 mA. 

(c) The base current will be 0.53 mA. 

(a), (b), (d)

(a) there are no mobile charges
(b) equal number of holes and electrons exist, making the region neutral.

(d) immobile charged ions exist.

Correct answer is (d) oscillator.

(a) The values displayed by Shalini are :

(i) creativity.

(ii) curiosity, and

(iii) sharing knowledge

(b) LED means light emitting diode which emits the light of characteristic wavelength when conducting.

(c) We can use that semiconducting material for which forbidden energy gap (E_g) is 1.8 eV or more so that light photons emitted lies in visible light range. The compound semiconductor gallium arsenide-phosphide (GaAsP) is used for making LEDs of different colors.

(i) Zener diode.

(ii) Helpful and concerned, practical application of theoretical knowledge.

Emitter-base junction is forward biased while collector-base junction is reverse biased. 

(b) 50 Hz.

 In a half wave rectifier, fundamental frequency in the ripple = Input frequency = 50 Hz.

1. Avalanche Breakdown: 

• It occurs in junctions which are lightly and have wide depletion widths. 
• It occurs at higher reverse voltages when thermally generated electrons get enough kinetic energy to produce more electrons by collision. 
• At reverse voltage above 6 V breakdown is due to avalanche effect. Electric field produced is weak in nature. Charge carriers obtain energy from the applied potential. 

2. Zener Breakdown: 

• It occurs in junctions which are heavily doped and have narrow depletion widths. It occurs due to rupture of covalent bonds by strong electric fields set up in depletion region by the reverse voltage. 
• At reverse voltage below 6V breakdown is due to zener effect. 
• A strong electric field is produced 
• Zener current is independent of applied voltage.

Efficiency (η) is the ratio of the output dc power to the ac input power supplied to the circuit.

(i) lt is a heavily doped p-n junction.

(ii) The reverse breakdown voltages of LEDs are very low.

(iii) The semiconductor used for fabrication of visible LEDs must at least have a band gap of 1.8 eV.

The: order of band gap is about g eV to r.S eV

In forward bias, current is mainly due to major carriers while in reverse bias, current is due to minor carriers. With such fractional change in reverse current resulting from photo effects, it can be easily measured as in case forward bias current. Hence, the photo diodes will operate in reverse bias.

(b) ionic crystal semi – conductor

Correct answer is (a) Glass

(a), (c)

(a) electrons move from lower energy level to higher energy level in the conduction band. 

(c) holes in the valence band move from higher energy level to lower energy level. 

(a), (c)

(a) Electrons crossover from emitter to collector. 

(c) Electrons move from emitter to base. 

(i) n-type impurity to be added – phosphorus / antimony 

(ii) p-type impurity to be added – aluminium / boron.

(c) intrinsic semiconductor

Pure semiconductors are called intrinsic semiconductors.

(b) room temperature

During reverse bias the reverse saturation current due to minority charge carriers is small. When light is incident the fractional increase in minority charge carrier concentration is significant and is early measurable. If the photodiode is forward biased, under illumination the fractional increase in charge carriers is insignificant and is difficult to measure. Hence photodiode is usually operated in reverse bias. 

A P-N Junction diode allows current to pass only when it is forward biased. So if an alternating voltage is applied across a diode the current flows only in that part of the cycle when the diode is forward biased. This property is used to rectify alternating voltages and the circuit used for this purpose is called a rectifier.  

(a) The values displayed by Sudha are :

(i) curiosity,

(ii) creativity, and

(iii) sharing knowledge.

(b) Charger acts as a rectifier, which changes a.c input signal into unidirectional d.c. output signal.

(c) The value of current gain decreases on increasing the width of base region

It is a special purpose p-n junction diode whose reverse current strength varies with the intensity of incident light.

Reverse bias. 

The answer is (B) The base region must be very thin and lightly doped.

When emitter junction is forward biased, electron hole recombination takes place, the base region. To ensure that only a small amount (about 50%) of electron-hole recombination may take place the base is made thin and lightly doped.

(i) In half wave rectifier, the frequency of the output is same as that of the input i.e. 50 Hz.

(ii) In full-wave rectifier, the frequency of the out put is twice as that input i.e 100 Hz.

The answer is (C) Is low at high and low frequencies and constant at mid frequencies.

In a transistor, the voltage gain is high at high and low frequencies and constant at mid frequencies.

(c) computer

A computer work on binary digits 0 and 1.

(d) All the above.

1eV is the order of energy gap in a semiconductor.