Saturday, June 27, 2009

9. Semiconductor Devices and their Applications

9. Semiconductor Devices and their Applications
1.Define semiconductors.
A material which has resistivity between conductors and insulators is
known as semiconductor. The resistivity of a semiconductor lie
approximately between 10-2 and 104 Ω m at room temperature.
Germanium and silicon are most widely used as semiconductors.
2.Define Valance band.
A band which is occupied by the valence electrons or a band having highest
energy is defined as valence band .
3.Define: Forbidden energy gap.
The separation between valence band and conduction band is
known as forbidden energy gap. The forbidden gap energy is of the order of 0.7eV for Ge and 1.1eV for Si.
4.Define intrinsic and extrinsic semiconductors.
A semiconductor which is pure and contains no impurity is
known as an intrinsic semiconductor. In an intrinsic semiconductor,
the number of free electrons and holes are equal. Common examples
of intrinsic semiconductors are pure germanium and silicon.
The process of addition of a very small amount of impurity into an intrinsic semiconductor is called doping. The impurity atoms are called dopants. The semiconductor containing impurity atoms is known as impure or doped or extrinsic semiconductor.
5.State the methods of doping a semiconductors.
There are three different methods of doping a semiconductor.
(i) The impurity atoms are added to the semiconductor in its
molten state.
(ii) The pure semiconductor is bombarded by ions of impurity
(iii) When the semiconductor crystal containing the impurity
atoms is heated, the impurity atoms diffuse into the hot crystal.
6.How N-type semiconductor formed?
When a small amount of pentavalent impurity such as arsenic is
added to a pure germanium semiconductor crystal, the resulting crystal
is called N-type semiconductor.
7.How P-type semiconductor formed?
When a small amount of trivalent impurity (such as indium,
boron or gallium) is added to a pure semiconductor crystal, the
resulting semiconductor crystal is called P-type semiconductor.
8.What is forward bias condition?
When the positive terminal of the battery is connected to P-side and negative terminal to the N-side, so that the potential difference acts in opposite direction to the barrier potential, then the PN junction diode is
said to be forward biased.
9.What is reverse bias condition?
When the positive terminal of the battery is connected to the N-side and negative terminal to the P-side, so that the applied potential difference is in the same direction as that of barrier potential, the junction is said to be reverse biased.
10.What is rectifier & rectification?
The process in which alternating voltage or alternating current is
converted into direct voltage or direct current is known as rectification.
The device used for this process is called as rectifier. The junction diode
has the property of offering low resistance and allowing current to flow
through it, in the forward biased condition. This property is used in the
process of rectification.
11.Define Avalanche breakdown
When both sides of the PN junction are lightly doped and the depletion layer becomes large, avalanche breakdown takes place. In this case, the electric field across the depletion layer is not so strong. The minority carriers accelerated by the field, collide with the semiconductor atoms in the crystal. Because of this collision with valence electrons, covalent bonds are
broken and electron hole pairs are generated. These charge carriers, so
produced acquire energy from the applied potential and in turn produce
more and more carriers. This cumulative process is called avalanche
multiplication and the breakdown is called avalanche breakdown.
12.Define: Zener breakdown : When both sides of the PN junction are
heavily doped, consequently the depletion layer is narrow. Zener
breakdown takes place in such a thin narrow junction. When a small
reverse bias is applied, a very strong electric field is produced across
the thin depletion layer. This field breaks the covalent bonds,
extremely large number of electrons and holes are produced, which
give rise to the reverse saturation current (Zener current). Zener
current is independent of applied voltage.
13.What are the different layers of transistors?
Base (B) layer : It is a very thin layer, the thickness is about
25 microns. It is the central region of the transistor.
(ii) Emitter (E) and Collector (C) layers : The two layers on the
opposite sides of B layer are emitter and collector layers.
14.What are the bias condition for a transistor to work?
For a transistor to work, the biasing to be given are as follows :
(i) The emitter-base junction is forward biased, so that majority
charge carriers are repelled from the emitter and the junction offers
very low resistance to the current.
(ii) The collector-base junction is reverse biased, so that it
attracts majority charge carriers and this junction offers a high
resistance to the current.

15.What are Transistor circuit configurations?
There are three types of circuit connections (called configurations or modes) for operating a transistor. They are (i) common base (CB) mode
(ii) common emitter (CE) mode and (iii)common collector (CC) mode.
16.Write note on:Transistor amplifier
The important function of a transistor is the amplification. An
amplifier is a circuit capable of magnifying the amplitude of weak
signals. The important parameters of an amplifier are input
impedance, output impedance, current gain and voltage gain. A good
design of an amplifier circuit must possess high input impedance, low
output impedance and high current gain.
17.What is Multistage amplifiers?
The amplification of a signal by a single amplifier may not be enough in most of the practical cases. Hence in these cases, two or more amplifiers are used in series to get sufficient amplified signal. The amplifiers are coupled in such a manner that the output of the first stage becomes the input for the next stage. Connecting the amplifiers in this manner is known as cascading the stages. When a number of amplifiers are connected in cascade, the overall voltage gain is equal to the product of voltage gain of individual stages. This is known as multistage amplifiers.
18.What is Feedback in amplifiers?State its types.
Feedback is said to exist in an amplifier circuit, when a fraction of the
output signal is returned or fed back to the input and combined with
the input signal. If the magnitude of the input signal is reduced by the
feed back, the feed back is called negative or degenerative. If the
magnitude of the input signal is increased by the feed back, such feed
back is called positive or regenerative.
19.What are the advantages of negative feedback?
The advantages of negative feedback are,
(i) Highly stabilised gain. (ii) Reduction in the noise level.
(iii) Increased bandwidth (iv) Increased input impedance and decreased output impedance. (v) Less distortion.
20.State “ Barkhausen condition” for oscillation
The gain of the amplifier with positive feedback is given by
Af =A/( 1 − A β), where A is the voltage gain without feedback, β isthe feedback ratio and Aβ is the loop gain. When Aβ = 1, thenAf= ∞. This means that output voltage is obtained, even if input voltage is zero, (i.e) it becomes an oscillator. The essential condition for the maintenance of oscillation is
Aβ = 1. This condition means that (i) the loop gain Aβ = 1 and (ii) the net phase shift round the loop is 0o or integral multiples of 2π.
These are called the Barkhausen conditions for oscillations.
21.What is Integrated circuit (IC)?
An integrated circuit (IC) consists of a single – crystal chip of
silicon, containing both active (diodes and transistors) and passive
(resistors, capacitors) elements and their interconnections.
22.What are the advantages of ICs?
(i) Extremely small in size (ii) Low power consumption
(iii) Reliability (iv) Reduced cost
(v) Very small weight (vi) Easy replacement
23.What are the types of ICs?
Digital ICs : The integrated circuits which process
the digital signals are called digital ICs.
Linear ICs : The integrated circuits which process
the analog signals are called linear ICs.
24.What are Analog signals?
The signal current or voltage is in the form of continuous, time varying voltage or current (sinusoidal). Such signals are called
continuous or analog signals.
25.What are Logic gates?
Circuits which are used to process digital signals are called logic gates. They are binary in nature. Gate is a digital circuit with one or more inputs but with only one output. The output appears only for certain combination of input logic levels. Logic gates are the basic building blocks from which most of the digital systems are built up. The numbers 0 and 1 represent the two possible states of a logic circuit. The two states can also be referred to as ‘ON and OFF’ or ‘HIGH and LOW’ or ‘TRUE and FALSE’.
26.Explain :OR gate
An OR gate has two or more inputs but only one output. It is
known as OR gate, because the output is high if any one or all of the
inputs are high. The logic symbol of a two input OR gate is shown in
Fig .The Boolean expression to represent OR gate is given by Y= A+B
27.Explain AND gate
An AND gate has two or more inputs but only one output. It is
known as AND gate because the output is high only when all the
inputs are high. The logic symbol of a two input AND gate is shown
in Fig.
The Boolean expression to represent AND gate is given by
Y = A . B ( . should be read as AND)
28.Explain NOT gate (Inverter)
The NOT gate is a gate with only one input and one output. It is
so called, because its output is complement to the input. It is also
known as inverter. Fig shows the logic symbol for NOT gate.
The Boolean expression to represent NOT operation is Y = A .
29.Explain NAND gate
This is a NOT–AND gate. It can be obtained by connecting a NOT
gate at the output of an AND gate.
The logic symbol for NAND gate is shown in Fig .
The Boolean expression to represent NAND Operation is Y = AB
30.Explain NOR gate
This is a NOT–OR gate. It can be made out of an OR gate by
connecting an inverter at its output .
The logic symbol for NOR gate
The Boolean expression to represent NOR Operation is Y = A + B
31.Write De-Morgan’s theorems
The two De Morgan’s theorems are,
First theorem
“The complement of a sum is equal to the product of the
complements.” If A and B are the inputs, then A+B=A . B
Second theorem
“The complement of a product is equal to the sum of the
complements.” If A and B are the inputs, then A . B=A+B.
32.Why NAND and NOR gates are known as Universal gates?
NAND and NOR gates are called Universal gates because they can
perform all the three basic logic functions. The construction of basic logic gates NOT, OR and AND using NAND and NOR gates is also easy.
33.State the basic Laws and theorems of Boolean algebra.
Basic laws :Commutative laws:
A + B = B + A; and AB = BA
Associative Laws
A + (B + C) = (A + B) + C and A (BC) = (AB) C
Distributive law
A (B+C) = AB + AC
New operations :
A + 0 = A; A + 1 = 1; A.0 = 0; A.1 = A
A + A = A;A + A = 1;A.A = A
A . A = 0; A = A.
34.What are Operational amplifiers (OP – AMP)?
OP-AMP is a solid state device capable of sensing and amplifying
dc and ac input signals. OP-AMP is an amplifier with two inputs
(differential inputs) and a single output. OP-AMP consists of 20
transistors, 11 resistors and one capacitor.
35.What are the characteristics of OP – AMP?
The most important characteristics of OP-AMP are : (i) very high
input impedance or even infinity which produces negligible current at
the inputs, (ii) very high gain, (iii) very low output impedance or even
zero, so as not to affect the output of the amplifier by loading.
36.State the uses of Cathode ray oscilloscope (CRO)?
(i) It is used to measure a.c and d.c voltage.
(ii) It is used to study the waveforms of a.c voltages.
(iii) It is used to find the frequency of a.c voltage.
(iv) It is used to study the beating of heart in cardiology.
37.What is a Multimeter?
Multimeter is an electronic instrument, which is used to
measure voltage, current and resistance. This is called as AVO meter
(ampere, voltage, ohm)(i)as a voltmeter
The moving coil galvanometer is converted into a voltmeter by
connecting in series a high resistance of suitable value
(ii) as an ammeter
The galvanometer is converted into an ammeter by shunting it
with suitable low resistances, one for each range
(iii)as an ohm-meter
The galvanometer is converted into an ohm-meter by connecting
a battery and a suitable resistance in series

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