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1. Every pentavalent dopant atom which donates one electron for conduction is called a donor impurity. 

2. Each trivalent atom which can accept an electron is called an acceptor impurity.

1. These are materials doped with trivalent impurity atoms (acceptors).

2. Electrical conduction in these materials is due to majority charge carriers i.e., holes.

3. The acceptor atoms acquire electron and become negatively charged-ions.

4. Number of holes is very large compared to the number of free electrons. n_h >> n_e. Holes are majority charge carriers.

5. When energy is supplied externally, positively charged holes (majority charge carriers) and negatively charged free electrons (minority charge carriers) are available for conduction.

1. These are materials doped with pentavalent impurity (donors) atoms. 

2. Electrical conduction in these materials is due to majority charge carriers i.e., electrons. 

3. The donor atom loses electrons and becomes positively charged ions. 

4. Number of free electrons is very large compared to the number of holes, n_e >> n_h. Electrons are majority charge carriers.

5. When energy is supplied externally, negatively charged free electrons (majority charges carries) and positively charged holes (minority charges carries) are available for conduction.

The semiconductor with impurity is called a doped semiconductor or an extrinsic semiconductor.

1. Electronic properties of semiconductors can be controlled to suit our requirement.

2. They are smaller in size and light weight.

3. They can operate at smaller voltages (of the order of few mV) and require less current (of the order of pA or mA), therefore, consume lesser power.

4. Almost no heating effects occur, therefore these devices are thermally stable.

5. Faster speed of operation due to smaller size. 

6. Fabrication of ICs is possible.

i. Solar cell:

1. It converts light energy into electric energy. 

2. t is useful to produce electricity in remote areas and also for providing electricity for satellites, space probes and space stations.

ii. Photo resistor: It changes its resistance when light is incident on it.

iii. Bi-polar junction transistor:

1. These are devices with two junctions and three terminals.

2. A transistor can be a p-n-p or n-p-n transistor. 

3. Conduction takes place with holes and electrons. 

4. Many other types of transistors are designed and fabricated to suit specific requirements. 

5. They are used in almost all semiconductor devices.

iv. Photodiode: It conducts when illuminated with light.

v. LED (Light Emitting Diode): 

1. It emits light when current passes through it. 

2. House hold LED lamps use similar technology. 

3. They consume less power, are smaller in size and have a longer life and are cost effective.

vi. Solid State Laser: It is a special type of LED. It emits light of specific frequency. It is smaller in size and consumes less power.

vii. Integrated Circuits (ICs): A small device having hundreds of diodes and transistors performs the work of a large number of electronic circuits.

1. They are sensitive to electrostatic charges. 

2. Not very useful for controlling high power. 

3. They are sensitive to radiation. 

4. They are sensitive to fluctuations in temperature. 

5. They need controlled conditions for their manufacturing. 

6. Very few materials are semiconductors.

1. Solar cell:

1. It converts light energy into electric energy. 

2. It is useful to produce electricity in remote areas and also for providing electricity for satellites, space probes and space stations.

ii. Photodiode: It conducts when illuminated with light.

iii. LED (Light Emitting Diode):

1. It emits light when current passes through it. 

2. House hold LED lamps use similar technology. 

3. They consume less power, are smaller in size and have a longer life and are cost effective.

iv. Solid State Laser: It is a special type of LED. It emits light of specific frequency. It is smaller in size and consumes less power.

Thermistors are made from thermally sensitive metal oxide semiconductors. Thermistors are very sensitive to changes in temperature.

There are two types of thermistor:

i. NTC (Negative Thermal Coefficient) thermistor: Resistance of a NTC thermistor decreases with increase in its temperature. Its temperature coefficient is negative. They are commonly used as temperature sensors and also in temperature control circuits.

ii. PTC (Positive Thermal Coefficient) thermistor: Resistance of a PTC thermistor increases with increase in its temperature. They are commonly used in series with a circuit. They are generally used as a reusable fuse to limit current passing through a circuit to protect against over current conditions, as resettable fuses.