Revision: Electronic Devices >> Semiconductor Electronics - Materials, Devices and Simple Circuits Physics Science (English Medium) Class 12 CBSE

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

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

• 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.

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.

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.

A semiconductor diode's depletion zone is the area surrounding the p-n junction where there are no mobile charge carriers, this area generates an electric field that allows the diode to conduct in one direction while blocking in another.

The barrier that the repelling forces use to stop the mobile charge carriers (at the PN junction) is known as the potential barrier.

This results from the concentration of immobile charges close to the junction after electrons and holes diffuse across the function.

(i) Rectifier: It is a device which converts alternating current into direct current.

(ii) Amplifier: An amplifier is a device which increases the energy of a weak signal by supplying energy from an external source. An amplifier increases the amplitude of a input signal.

(iii) Oscillator: An oscillator is a device which produces electrical oscillations of adjustable frequency and constant amplitude. An oscillator is basically an amplifier. A part of the output energy is fed back into the L-C circuit to produce sustained oscillations.

A semiconductor diode is basically a p-n junction with metallic contacts provided at the ends for the application of an external voltage.

An empty or partially filled band above the valence band in which electrons can move freely and conduct current.

OR

Conduction band is the wide range of energies possessed by the conduction band electrons. It is the lowest unfilled band, for insulators. But it is partially filled for conductors. Current conduction is due to the electrons in this band. 

A material having a partially filled valence band (or overlapping valence and conduction bands), allowing electrons to move easily and conduct electricity.

A material in which the valence band is completely filled and the conduction band is empty, separated by a large energy gap (a few eV), so electrons cannot move freely.

The highest occupied energy band containing valence electrons.

OR

Valence band is the wide range of energies possessed by the valence electrons. Valence band is the highest energy band, occupied by the valence electrons. It is completely filled for inert gases, but partially filled for other materials. 

A material with a small energy gap (about 1 eV) between valence and conduction bands, allowing limited conduction at room temperature.

An energy band is the wide range of energies possessed by an electron in a solid.

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

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.

• Solids are classified as metals, insulators, and semiconductors based on conductivity or band theory.
• Metals have the highest conductivity among the three classes.
• Insulators have the lowest conductivity and the highest resistivity.
• Semiconductors have conductivity intermediate between metals and insulators.
• Higher conductivity corresponds to lower resistivity, and vice versa.

• 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.

• A semiconductor diode consists of a p-n junction with metallic contacts at both ends.
• It can be made from either Silicon or Germanium, each differing in size and properties.
• Six types of diodes are: Diode, LED, Photodiode, Schottky diode, Tunnel diode, and Zener diode.
• The Anode is the p-side, and the Cathode is the n-side of the diode.
• External voltage is applied through the metallic contacts at the ends.

• A p–n junction is formed by joining p-type and n-type semiconductors.

• Electrons diffuse from n → p.

• Holes diffuse from p → n.

• Recombination occurs near the junction.

• Immobile ions are left behind.

• A depletion region is formed (no free charge carriers).

• An electric field develops across the junction.

• A barrier potential is established.

• At equilibrium:
  Diffusion current = Drift current
  Net current = 0

• Conductors have many free electrons, so electric current flows easily; insulators have almost no free electrons, so current does not flow easily.
• In conductors, resistance increases with temperature, whereas in semiconductors it decreases.
• Semiconductors have properties between conductors and insulators, and at absolute zero, they behave like insulators.

• Applied voltage reduces barrier potential.

• Depletion region width decreases.

• The majority of carriers cross the junction.

• Current increases rapidly after the threshold voltage.

• Current is in the mA range.

• Barrier potential increases.

• Depletion region widens.

• Diffusion current decreases.

• Small reverse current flows (minority carriers).

• The reverse current is almost independent of voltage (until breakdown).

• At breakdown → current increases sharply.

• An intrinsic semiconductor is a pure semiconductor (like silicon or germanium) without impurities.
• At low temperatures, it behaves like an insulator because all electrons are bound in covalent bonds.
• At room temperature, some bonds break and create electron–hole pairs.
• In an intrinsic semiconductor, the number of electrons equals the number of holes (ne = nh = ni).
• Its conductivity increases with temperature because more electron–hole pairs are produced.

• Semiconductors have a small energy gap (about 1 eV) between the valence band and conduction band.
• At absolute zero, they act like insulators because the valence band is full and the conduction band is empty.
• At room temperature, some electrons move into the conduction band, leaving holes, and both contribute to conduction; conductivity increases with temperature.