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Semiconductor Fundamentals
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Semiconductor Fundamentals
Semiconductor Fundamentals
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1
Question
What is the definition of an energy band in semiconductors?
Answer
The large number of closely spaced energy levels is known as an energy band.
2
Question
What is the valence band?
Answer
The energy band formed by a series of energy levels containing valence electrons is known as the valence band. The valence band may be completely filled or partially filled, but never empty.
3
Question
What is the conduction band?
Answer
The lowest energy band just above the valence band is known as the conduction band. The conduction band is partially filled or empty but never completely filled.
4
Question
What is the forbidden energy gap?
Answer
The gap between the conduction band and the valence band is known as the forbidden energy gap.
5
Question
How do we classify materials based on the size of the energy gap?
Answer
Materials can be classified into conductors, insulators, and semiconductors based on the size of the energy gap.
6
Question
What characterizes conductors?
Answer
In conductors, the forbidden gap between the conduction band and valence band is zero, and they overlap each other. The valence band is completely filled and the conduction band is partially filled with electrons.
7
Question
What characterizes insulators?
Answer
In insulators, the energy gap is extremely large (Eg > 3eV). As a result, when an external field is applied, the electrons in the valence band find difficulty overcoming the energy gap, making the conduction band almost empty.
8
Question
What characterizes semiconductors?
Answer
In semiconductors, the energy gap between the conduction band and valence band is small (around 1-3 eV). At room temperature, some electrons in the valence band gain enough energy to reach the conduction band, resulting in a partially filled conduction band.
9
Question
What is an intrinsic semiconductor?
Answer
An intrinsic semiconductor is a semiconductor in its purest form. At room temperature, electrons from the bonds escape into the crystal lattice, creating vacancies called holes.
10
Question
What are holes in the context of semiconductors?
Answer
A vacancy of electrons in the valence band is known as a hole, which is considered a positive particle.
11
Question
What factors limit the conductivity of intrinsic semiconductors?
Answer
1. Poor conductivity due to fewer charge carriers. 2. Conductivity is temperature dependent. 3. The ratio of electron current to hole current is fixed.
12
Question
What are the energy gaps for germanium and silicon?
Answer
The energy gap of germanium is 0.72 eV and for silicon, it is 1.1 eV.
13
Question
What is doping in semiconductors?
Answer
Doping is the process of deliberately adding impurities into a semiconductor crystal to increase its conductivity.
14
Question
What is an N-type semiconductor?
Answer
An N-type semiconductor is produced by doping semiconductors like Ge and Si with pentavalent impurities (e.g., As, Sb, Bi). These impurities donate electrons to the crystal.
15
Question
What are the majority and minority charge carriers in an N-type semiconductor?
Answer
In an N-type semiconductor, electrons are the majority charge carriers and holes are the minority charge carriers.
16
Question
What is the donor energy level in N-type semiconductors?
Answer
The donor energy level is an additional energy level in the crystal that lies 0.01 eV below the conduction band of Ge crystal.
17
Question
What is a P-type semiconductor?
Answer
A P-type semiconductor is produced by doping semiconductors like Ge and Si with trivalent impurities (e.g., Al, Ga, In). These impurities create vacancies (holes) in the crystal.
18
Question
What are the majority and minority charge carriers in a P-type semiconductor?
Answer
In a P-type semiconductor, holes are the majority charge carriers and electrons are the minority charge carriers.
19
Question
What is the acceptor energy level in P-type semiconductors?
Answer
The acceptor energy level is an additional energy level in the crystal that lies 0.01 eV above the valence band of Ge crystal.
20
Question
What is the relationship between electron and hole concentration in thermal equilibrium?
Answer
The electron and hole concentration in a semiconductor at thermal equilibrium is given by n * p = n_i^2, where n_i is the intrinsic concentration.
21
Question
What is the difference between N-type and P-type semiconductors?
Answer
N-Type semiconductors are obtained by adding pentavalent impurities; majority charge carriers are electrons, and the donor energy level lies close to the conduction band. P-Type semiconductors are obtained by adding trivalent impurities; majority charge carriers are holes, and the acceptor energy level lies close to the valence band.
22
Question
What is the difference between intrinsic and extrinsic semiconductors?
Answer
Intrinsic semiconductors are pure elements with equal density of electrons and holes, while extrinsic semiconductors have added impurities resulting in unequal densities of electrons and holes.
23
Question
What is a P-N junction?
Answer
A P-N junction is formed by doping an intrinsic semiconductor with pentavalent impurities on one side and trivalent impurities on the other.
24
Question
What occurs during the formation of a P-N junction?
Answer
During P-N junction formation, holes from the P region and electrons from the N region diffuse across the junction, leading to a depletion region.
25
Question
What is forward biasing in a diode?
Answer
Forward biasing occurs when the negative terminal of the battery is connected to the P region and the positive terminal to the N region, allowing current to flow.
26
Question
What is reverse biasing in a diode?
Answer
Reverse biasing occurs when the positive terminal is connected to the N region and the negative terminal to the P region, preventing current flow.
27
Question
What is the V-I characteristic of a P-N junction diode?
Answer
The V-I characteristic graph relates current and the applied potential across the diode.
28
Question
What is the threshold voltage for germanium and silicon diodes?
Answer
The threshold voltage (or cut-in voltage) is 0.2 V for germanium diodes and 0.7 V for silicon diodes.
29
Question
What is a half-wave rectifier?
Answer
A half-wave rectifier allows current to pass only during the positive half-cycle of the AC voltage, resulting in a discontinuous output voltage.
30
Question
What are the limitations of a half-wave rectifier?
Answer
1. Only half of the cycle is rectified, resulting in discontinuous output voltage. 2. Efficiency is low (about 40.6%).