Revision: Semiconductors Physics HSC Science (General) 11th Standard Maharashtra State Board

The material with electrical conductivity between that of a conductor and an insulator, whose number of charge carriers can be controlled as per requirement, is called a semiconductor. (e.g. Silicon, Germanium)

The different energy levels with continuous energy variation are called energy bands.

The range of energies possessed by valence electrons is called valence band.

The range of energies possessed by conduction electrons is called conduction band.

The energy difference between the valence band and the conduction band is called forbidden energy gap.

The solids which have a large number of free electrons are called conductors. (e.g. Iron, Aluminium)

The solids which have very small number of free electrons are called insulators. (e.g. Glass, Wood)

A pure semiconductor in which no impurity is added intentionally.

The concentration of charge carriers in an intrinsic semiconductor, where the number of electrons equals the number of holes.

The vacancy left in the valence band when an electron leaves it behaves like a positive charge carrier in semiconductor theory.

Intrinsic semiconductors have very low conductivity at room temperature. Therefore, they are not useful for constructing electronic devices. Their electrical conductivity can be increased by adding a suitable impurity. This process is called doping.

A doped semiconductor is called an extrinsic semiconductor or impurity semiconductor.

The impurity added is called a dopant.

The semiconductor to which the dopant is added is called the host.

Since every pentavalent dopant atom donates one electron for conduction, it is called a donor impurity.

A p-type semiconductor is a semiconductor obtained by doping pure silicon or germanium with a trivalent impurity so that holes become the majority charge carriers. 

The region around the junction that is free from mobile charge carriers is called the depletion region.

The potential difference developed across the depletion layer due to immobile ions is called the barrier potential.

The two-terminal semiconductor device that allows current mainly in one direction is called a p-n junction diode.

The process in which charge carriers move from a higher concentration to a lower concentration is called diffusion.

The motion of charge carriers under the influence of an electric field is called drift.

The reverse voltage at which the current suddenly increases rapidly is called the breakdown voltage.

The small current flowing in reverse bias due to minority carriers is called the reverse saturation current.

The minimum forward voltage after which the current rises sharply is called the knee voltage.

The ratio of voltage to current at any operating point of a diode is called static resistance.

The ratio of a small change in voltage to the corresponding small change in current is called dynamic resistance.

The boundary formed when p-type and n-type semiconductor regions are joined in a single crystal is called a p-n junction.

A device that changes its resistance when light is incident on it is called a photoresistor.

A device that emits light when current passes through it is called a Light Emitting Diode (LED).

A device that emits light of specific frequency is called a solid state laser.

A small device having hundreds of diodes and transistors is called an integrated circuit.

A device that converts light energy into electric energy is called a solar cell.

A device with two junctions and three terminals is called a Bi-polar Junction Transistor.

A device that conducts electricity when illuminated with light is called a photodiode.

A temperature sensitive resistor whose resistance changes with change in its temperature is called a thermistor.

A thermistor whose resistance increases with increase in temperature and has a positive temperature coefficient is called a PTC thermistor.

A thermistor whose resistance decreases with increase in temperature and has a negative temperature coefficient is called an NTC thermistor.

If hole concentration is high, then:

This is especially important for numerical problems in board examinations and entrance tests. 

R = \[\frac {V}{I}\]

\[r_d=\frac{\Delta V}{\Delta I}\]

R_g = \[\frac {V}{I}\]

r_g = \[\frac {ΔV}{ΔI}\]

• Conductors → E_g = 0 - bands overlap, electrons flow freely.
• Semiconductors → E_g < 3 eV — small gap, conducts at room temperature.
• Insulators → E_g > 5 eV — large gap, no conduction.
• Ge = 0.72 eV, Si = 1.1 eV — both semiconductors.
• Metal conductivity decreases with temp. Semiconductor conductivity increases with temp. 

• Intrinsic semiconductors have very low conductivity at room temperature.
• Doping increases conductivity.
• A doped semiconductor is called an extrinsic semiconductor.
• The impurity added is called a dopant.
• The semiconductor receiving the impurity is called the host.
• The dopant size should be nearly the same as that of the host atom.
• Pentavalent and trivalent impurities are used as dopants.
• Extrinsic semiconductors are of two types: n-type and p-type.

• An n-type semiconductor is formed by doping silicon or germanium with a pentavalent impurity.
• Pentavalent impurities act as donor impurities.
• The fifth valence electron is weakly bound and can become free easily.
• Electrons are majority carriers and holes are minority carriers.
• For an n-type semiconductor, n_e >> n_h.
• Donor energy levels lie close to the conduction band.
• Extrinsic semiconductors are better conductors than intrinsic semiconductors.

• A p-n junction is formed by joining p-type and n-type semiconductor regions in a single crystal.
• Diffusion of carriers creates a depletion region and barrier potential.
• A p-n junction diode conducts mainly in one direction.
• In forward bias, the barrier potential decreases, and the current becomes large.
• In reverse bias, the barrier potential increases and only a small minority-carrier current flows.
• In zero bias, the diffusion and drift currents balance, so the net current is zero.
• The knee voltage is about 0.3 V for germanium and 0.7 V for silicon.
• Static resistance is given by R = V/I, and dynamic resistance is given by r_d = ΔV/ΔI.

• Size & Weight: Semiconductor devices are smaller in size and lightweight, which also enables faster speed of operation.
• Power Consumption: They operate at small voltages (few mV) and require very less current (µA or mA), hence consume lesser power and produce almost no heating effects — making them thermally stable.
• Controllability: The electronic properties of semiconductors can be controlled to suit our requirement, and fabrication of ICs is possible.
• Sensitivity: They are sensitive to electrostatic charges, radiation, and fluctuations in temperature — making them fragile in harsh environments.
• Limitations: They are not useful for controlling high power, require controlled conditions for manufacturing, and very few materials are semiconductors.

• It is a temperature sensitive resistor.
• They can measure temperature variations of a small area due to their small size.
• A small change in surrounding temperature causes a large change in resistance.