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)

The number of free electrons (nₑ) and the number of holes (nₕ) in an intrinsic semiconductor, where nₑ = nₕ = nᵢ. Here nₑ and nₕ are called the intrinsic carrier concentration.

• A pure semiconductor, such as pure silicon or pure germanium, is called an intrinsic semiconductor.
• A semiconductor free from all types of impurities is called an intrinsic semiconductor.

• The semiconductor with impurity added to it is called a doped semiconductor or extrinsic semiconductor.
• A semiconductor doped with a suitable impurity, so as to possess conductivity much higher than the pure semiconductor is called an extrinsic semiconductor.

• The semiconductor in which a silicon or germanium crystal is doped with a pentavalent impurity (donor), making electrons the majority charge carriers, is called an n-type semiconductor.
• When a few atoms of the pentavalent elements (phosphorus, arsenic, antimony and bismuth) are added to the pure germanium or silicon crystal, the resulting crystal is called an n-type semiconductor.

The pentavalent impurity atom added during doping in an n-type semiconductor is known as a donor.

The trivalent impurity atom added during doping in a p-type semiconductor is known as an acceptor.

• The semiconductor in which a silicon or germanium crystal is doped with a trivalent impurity (acceptor), making holes the majority charge carriers, is called a p-type semiconductor.
• On doping an intrinsic semiconductor with trivalent impurity like Indium (In) or Gallium (Ga), the semiconductor becomes deficient in electrons, i.e., the number of holes becomes more than the number of electrons. Such a semiconductor is called p-type.

The current flowing from n-side to p-side due to holes and electrons created in the depletion region is called drift current.

When a semiconducting material such as silicon or germanium is doped with a trivalent impurity on one side and pentavalent impurity on the other side, a p-n junction is obtained. The plane separating the two regions is called a junction.

The resistance offered by a p-n junction diode when it is in forward biased condition is called static (DC) resistance.

• When n-type and p-type semiconductor materials are fused together, the junction formed is called a p-n junction.
• The device obtained by growing a p-type semiconductor over an n-type semiconductor or vice versa is called a p-n junction.

When a high reverse voltage causes a sudden and uncontrollable increase in current, the phenomenon is called avalanche breakdown.

The resistance of a diode at a particular applied voltage is called dynamic (AC) resistance.

A p-n junction when provided with metallic connectors on each side is called a junction diode.

• The formation of a narrow region on either side of the junction which becomes free from mobile charge carriers is called depletion region.
• The small charge-free region formed near the junction where electrons combine with holes is known as the depletion region.

The difference in potential that prevents charge carriers from moving across the p-n junction is called the potential barrier.

The current flowing from p-side to n-side due to diffusion of electrons and holes because of concentration difference is called diffusion current.

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

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

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

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

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.

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

\[E=\frac{V_b}{d}\]

Where:

• \[V_b\]​ = potential barrier
• d = width of the depletion layer
• E = electric field intensity

r_a = \[\frac {ΔV}{ΔI}\]

It is the reciprocal of the slope of the I-V characteristics at that point.

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. 

• An intrinsic semiconductor is pure — free from all types of impurities.
• At 0 K, an intrinsic semiconductor behaves as an insulator with zero conductivity.
• At temperatures above 0 K, electrons gain energy and move to the conduction band, creating holes in the valence band.
• The number of free electrons always equals the number of holes in an intrinsic semiconductor.
• \[n_e, n_h,\] and \[n_i\]​ are used to denote intrinsic carrier concentrations.

• Electrical properties of semiconductors can be altered by adding small amounts of impurities.
• Doped semiconductors are known as extrinsic semiconductors.
• Two types of dopants are used for tetravalent Si or Ge:
• Pentavalent (valency 5): Arsenic (As), Antimony (Sb), Phosphorous (P)
• Trivalent (valency 3): Indium (In), Boron (B), Aluminium (Al)
• Doping increases conductivity in a controlled manner.
• Extrinsic semiconductors are used in electronic devices like transistors, diodes, and light-dependent resistors (LDRs).

• An n-type semiconductor is formed by doping with a pentavalent impurity (e.g., Phosphorus, Arsenic, Antimony, Bismuth).
• Arsenic has 5 outer electrons — 4 are used in bonding with silicon, and the 5th electron is free to move and conduct.
• The pentavalent impurity atom acts as a donor as it donates a free electron.
• Electrons are majority carriers; holes are minority carriers — ne≫nhne​≫nh.
• The donor energy level lies approximately 0.1 eV below the conduction band.
• The Fermi level shifts closer to the conduction band in n-type semiconductors.

• A p-type semiconductor is formed by doping with a trivalent impurity (e.g., Indium, Gallium, Boron).
• The trivalent impurity has 3 outer electrons, creating a hole in the crystal lattice where no electron is present.
• Due to a lack of electrons, the Fermi level shifts closer to the valence band.
• Holes are majority carriers; electrons are minority carriers — nh≫nenh​≫ne.
• The acceptor energy level lies approximately 0.01 to 0.05 eV above the valence band.

• At a p-n junction, donor impurity atoms become positively charged ions and acceptor atoms become negatively charged ions — these act like two electrodes forming a p-n junction diode.
• A strong electric field, directed from the n-type to the p-type semiconductor, exists at the junction.
• Within the depletion layer, only immobile positive and negative ions are present; material outside remains neutral.
• The potential barrier is influenced by the type of semiconductor crystal, temperature, and the level of doping.
• If the diode is ON, it has no voltage across it and acts as a short circuit; if OFF, current is zero and acts as an open circuit.
• A diode is a two-terminal device — unlike capacitors (current related to the derivative of voltage) or inductors (derivative of current related to voltage), current in a diode is not linearly related to voltage.
• In a p-n junction, there is a transfer of charge through the junction due to the concentration gradient of charge carriers with the barrier potential.

• 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.