Physics 3 Marks Question

Due to the diffusion of electrons and holes across the junction a region of (immobile) positive charge is created on the n-side and a region of (immobile) negative charge is created on the p-side, near the junction; this is called depletion region.
Barrier potential is formed due to loss of electrons from n-region and gain of electrons by p-region. Its polarity is such that it opposes the movement of charge carriers across the junction.

Logic Operation a) Y = A.B
b) Y = A+B
Truth Table

1.  

A	B	Y
0	0	0
1	0	0
0	1	0
1	1	1

2.  

A	B	Y
0	0	0
1	0	1
0	1	1
1	1	1

Identification:

1. AND gate.
2. OR gate.

1. R would be increased.

Resistance of S (a semi conductor) decreases on heating.

2. Photodiode diagram:

When the photodiode is illuminated with light (photons) (with energy (hv) greater than the energy gap (Eg) of the semiconductor), then electron-hole pairs are generated due to the absorption of photons. Due to junction field, electrons and holes are separated before they recombine. Electrons are collected on n-side and holes are collected on p-side giving rise to an emf. When an external load is connected, current flows.
V-I Characteristics of the diode:

Two distinguishing features:

1. In conductors, the valency band and conduction band tend to overlap (or nearly overlap) while in insulators they are seperated by a large energy gap and in semiconductors are separated by a small energy gap.
2. The conduction band, of a conductor, has a large number of electrons available for electrical conduction. However the conduction band of insulators is almost empty while that of the semi- conductor has only a (very) small number of such electrons available lectrical conduction.

To draw the logic circuit:
Truth Table:

A	B	Y
0	0	0
1	0	0
0	1	0
1	1	1

Identification: AND gate.

1. Emitter: Supplies a large number of majority charge carriers.

Base: Controls the flow of majority carriers from the emitter to the collector.
Collector: It collects the majority carriers from the base/majority of those emitted by the emitter.

2.  

Truth Table
A	B	Y
0	0	0
0	1	0
1	0	0
1	1	1

Gates, P, Q and R act as NOT, NOT and NAND gates respectively.
So

The combination acts as OR Gate.

Truth Table of OR Gate

A	B	Y
0	0	0
1	0	1
0	1	1
1	1	1

Principle
pn junction conducts when it is in forward biased and does not do so when it is reverse biased.

Formation: During the formation of p-n junction, and due to the concentration gradient across p, and n sides, holes diffuse from p-side to n-side (p → n) and electrons diffuse from n-side to p-side (n → p). This motion of charge carriers gives rise to diffusion current across the junction. When an electron diffuses from n → p, it leaves behind an immobile ionised donor (positive charge) on n-side. As the electrons continue to diffuse from n → p, a layer of positive charge (or positive space-charge region) on n-side of the junction is developed. Similarly, a layer of negative charge (or negative space-charge region) on the p-side of the junction is developed. This space-charge region, developed on either side of the junction is known as depletion region as the electrons and holes taking part in the initial movement across the junction deplete, this region of its free charges.
Width of depletion region layer decreases when the junction is forward biased and increases when it is reversed biased.

Principle: Positive feedbackAlternate Answer

Alternate Answer
Explanation: By the process of mutual induction (or inductive coupling), energy is fedback to the tank circuit, in phase. The output in an oscillator is selfsustained.

Isolated atoms have discrete energy levels in a crystal, the presence of large number of closely packed atoms, leads to inter atomic interaction and the energy levels get spread out or broadened into energy bands.
Alternate Answer

Forward bias: We apply a voltage v such that n-side is negative and p-side is positive. (Diode conducts current)
Reverse bias: We apply a voltage v such that n-side is positive and p-side is negative. (Diode is cut off)
Alternate Answer
other point relevant to forward bias/reverse bias should also be given full marks. (Explanation with /without circuit diagram)



Explanation: When the voltage at A is +ve, the diode is forward biased and it conducts. When A is negative, the diode is reverse biased and its does not conduct. Therefore, in the positive half cycle there is a current through the load resistor and we get an output voltage as shown in figure. But there is negligibly small current in the negative half cycle and hence no output. Thus, the output voltage, though still varying, is restricted to one direction only.

In forward bias, applied voltage does not support potential barrier. As a result, the depletion layer width decreases and barrier height is reduced. Due to the applied voltage, electrons from n side cross the depletion region and reach p side. Similarly holes from p side cross the junction and reach the n side. The motion of charged carriers, on either side, give rise to current. In reverse bias, applied voltage support potential barrier. As a result, barrier height is increased, depletion layer widens. This suppresses the flow of electrons from n → p and holes from p → n. Diffusion current decreases. The electric field direction of the junction is such that if electrons on p side or holes on n side in their random motion comes close to the junction, they will be swept to its majority zone. This drift of carriers give rise to the current called reverse current.

Principle:
When electrons fall from a higher to a lower energy level, containing holes, energy, in the form of light radiation, gets released.
Working:
Criterion:
The semiconductor material used in LED is chosen according to the required wavelength of emitted radiation.
Alternate Answer
Chosen to have a energy gap matching the energy of a photon of the radiation required to be emitted.
Advantages:

1. Low operation voltage.
2. Less power.
3. No warm up time.
4. Fast action.
5. Light emitted is nearly monochromatic.
6. Long life.

Intrinsic semiconductor: Pure semi-conductor having no impurities or negligible impurities.Conversion into –

1. P-type – by doping with small amount of trivalent impurities.
2. N-type- by doping with small amount of pentavalent impurity.

1. Zener diode as a Voltage Regulator.

The Zener diode makes its use as a voltage regulator due to the following property:

When a Zener diode is operated in the breakdown region, the voltage across it remains practically constant for a large change in the current.

A simple circuit of a voltage regulator using a Zener diode is shown in the figure. The Zener diode is connected across load such that it is reverse biased.



The series resistance R absorbs the output voltage fluctuations so as to maintain constant voltage across the load.

If the input dc voltage increases, the current through R and Zener diode also increases. So, voltage drop across R increases, without any change in the voltage across zener diode.

I-V Characteristics,

2. The Zener diode as fabricated by heavily doping both p and n sides of the junction so as to:

1. Push the conduction band very near to Fermi level.
2. Make depletion layer narrower.
3. Making resistance lower and allow a large current to flow with little potential difference.

A solid consists of a combination of closely spaced energy levels. These energy levels are discrete but they have very small energy gap between two consecutive levels so they are referred as band. However, the energy levels in the band are discrete.

The logic circuit is shown in fig. The logic circuit represents NOR gate.Its truth table is:

A	B	Y'	Y
0	0	0	1
1	0	1	0
0	1	1	0
1	1	1	0

Potential barrier ‘d’ = 250meV
Initial KE of hole = 300meV
We know: KE of the hole decreases when the junction is forward biased and increases when reverse blased in the given ‘Pn’ diode.
So,

1. Final KE = (300 - 250)meV = 50meV.
2. Initial KE = (300 + 250)meV = 550meV.

Holes do not exist in reality. They exist only virtually. When an electron jumps from the valence band to the conduction band, a vacancy is created at the place from where the electron had jumped. This vacancy is called a hole. So, a valence or conduction hole is a virtual concept only.

The conduction level of any element depends on the energy gap between its conduction band and valence band.
In conductors, there is no energy gap between conduction band and valence band. For insulator, the energy gap is large and for semiconductor the energy gap is moderate.
The energy gap for Sn is 0eV, for C is 5.4eV, for Si is 1.1eV and for Ge is 0.7eV related to their atomic size. Therefore Sn is a conductoc, C is an insulator, and Ge and Si are semiconductors.

1. Heavy doping makes the depletion region very thin. This makes the electric field of the junction very high, even for a small reverse bias voltage. This in turn helps the Zener diode to act as a ‘voltage regulator’.
2. When operated under reverse bias, the photodiode can detect changes in current with changes in light intensity more easily.
3. The photon energy, of visible light photons varies about 1.8eV to 3eV. Hence, for visible LED’s, the semiconductor must have a band gap of 1.8eV.