Semiconductor Electronics: Materials, Devices and Simple Circuits

The One Thing

Semiconductor electronics is based on controlling the availability and movement of charge carriers through energy bands, doping, temperature, and p-n junctions. This control enables devices such as diodes, rectifiers, voltage regulators, LEDs, photodiodes, solar cells, and transistors to perform specific electrical functions.

Reactions, Processes and Experiments

Key Terms

• Valence band: The energy band containing electrons involved in bonding. At absolute zero, it is normally filled in a semiconductor or insulator.
• Conduction band: The energy band in which electrons are sufficiently energetic to move through the material and contribute to electric current.
• Forbidden energy gap: The energy interval between the valence band and conduction band in which electrons cannot normally exist. It is denoted by Eg.
• Conductor: A material with overlapping or partially filled energy bands, allowing many free electrons and high conductivity.
• Insulator: A material with a large forbidden energy gap, so very few electrons can reach the conduction band at ordinary temperatures.
• Semiconductor: A material with a small forbidden energy gap whose conductivity lies between that of conductors and insulators.
• Intrinsic semiconductor: A chemically pure semiconductor in which the number of free electrons equals the number of holes, so ne = nh = ni.
• Extrinsic semiconductor: A semiconductor whose conductivity is increased by adding a small, controlled amount of impurity.
• Doping: The intentional addition of a suitable impurity to a pure semiconductor to increase its conductivity.
• n-type semiconductor: A semiconductor doped with a pentavalent impurity. Electrons are the majority carriers and holes are the minority carriers.
• p-type semiconductor: A semiconductor doped with a trivalent impurity. Holes are the majority carriers and electrons are the minority carriers.
• Donor impurity: A pentavalent impurity that contributes an extra electron to the crystal, producing n-type conductivity.
• Acceptor impurity: A trivalent impurity that creates a vacancy, or hole, by accepting an electron, producing p-type conductivity.
• Hole: The effective positive charge associated with the absence of a valence electron. Holes can move through a semiconductor.
• p-n junction: The boundary formed when p-type and n-type regions are joined in the same semiconductor crystal.
• Depletion region: The region near a p-n junction depleted of mobile charge carriers and containing immobile ionized donor and acceptor ions.
• Barrier potential: The potential difference across the depletion region that opposes further diffusion of majority carriers.
• Forward bias: A bias in which the p-side is connected to the positive terminal and the n-side to the negative terminal, reducing the depletion width and allowing substantial current.
• Reverse bias: A bias in which the p-side is connected to the negative terminal and the n-side to the positive terminal, increasing the depletion width and allowing only a small reverse current.
• Knee or threshold voltage: The approximate forward voltage at which diode current begins to increase rapidly; it is about 0.7 V for silicon and 0.3 V for germanium.
• Diode equation: The current-voltage relation of a diode: I = Is[exp(qV/(eta kT)) - 1], where Is is reverse saturation current, q is electronic charge, k is Boltzmann constant, T is absolute temperature, and eta is the ideality factor.
• Rectifier: A circuit that converts alternating current into unidirectional or pulsating direct current.
• Half-wave rectifier: A rectifier using one diode that conducts during only one half-cycle of an AC input.
• Full-wave rectifier: A rectifier that uses both half-cycles of an AC input, commonly through two diodes with a centre-tapped transformer or four diodes in a bridge arrangement.
• Zener diode: A specially designed diode operated in reverse breakdown to maintain an approximately constant voltage.
• LED: A light-emitting diode that produces light when forward biased because of electron-hole recombination.
• Photodiode: A light-sensitive diode usually operated in reverse bias, where incident light increases the reverse current.
• Solar cell: A p-n junction device that converts light energy directly into electrical energy through the photovoltaic effect.
• Transistor: A three-layer semiconductor device, such as an n-p-n or p-n-p transistor, used for amplification and switching.

Easily Confused

• Intrinsic and extrinsic semiconductors: An intrinsic semiconductor is chemically pure and has ne = nh = ni; an extrinsic semiconductor has increased conductivity due to controlled doping.
• n-type and p-type semiconductors: n-type material has electrons as majority carriers because of pentavalent donor impurities; p-type material has holes as majority carriers because of trivalent acceptor impurities.
• Donor and acceptor impurities: A donor contributes an extra electron; an acceptor creates a hole by accepting an electron.
• Forward and reverse bias: Forward bias connects p to positive and n to negative, reducing depletion width; reverse bias connects p to negative and n to positive, increasing depletion width.
• Knee voltage and barrier potential: Knee voltage is a practical circuit-analysis approximation for the onset of rapid forward current, whereas barrier potential is the potential difference across the depletion region.
• Zener breakdown and ordinary reverse breakdown: Zener breakdown occurs at a controlled reverse voltage and is used for regulation; uncontrolled ordinary reverse breakdown can damage a diode.
• Half-wave and full-wave rectification: A half-wave rectifier uses one AC half-cycle and has output frequency equal to the input frequency; a full-wave rectifier uses both half-cycles and has twice the input frequency.
• LED and photodiode: An LED emits light when forward biased through electron-hole recombination; a photodiode detects light through an increase in reverse current.
• Photodiode and solar cell: A photodiode is generally reverse biased to detect changes in light; a solar cell generally operates without external bias and delivers electrical power through the photovoltaic effect.

What Gets Asked

• Energy-band classification: Questions may compare conductors, insulators, and semiconductors using band structure and forbidden energy gaps. Marks are lost by omitting that conductors have overlapping or partially filled bands, while insulators have a large forbidden gap and semiconductors have a small gap.
• Carrier-concentration calculations: Questions may require ne = nh = ni, sigma = ni e(mu_e + mu_h), sigma = e(n mu_e + p mu_h), or np = ni^2. Marks are lost by confusing electron concentration with hole concentration or applying the intrinsic relation to an extrinsic semiconductor without justification.
• Doping and carrier identification: Questions may ask whether a pentavalent or trivalent impurity produces n-type or p-type material. Marks are lost by reversing donor and acceptor roles or identifying the wrong majority carrier.
• p-n junction biasing: Questions may require the polarity of forward and reverse bias, the effect on depletion width, and the direction of the internal electric field. Marks are lost by reversing the p-side and n-side connections or stating that reverse bias reduces the depletion region.
• Diode characteristics and equations: Questions may involve the diode equation I = Is[exp(qV/(eta kT)) - 1], ideality factor values, knee voltages, or forward and reverse current behaviour. Marks are lost by treating cut-in voltage as a strict physical boundary rather than a practical approximation.
• Rectifier and semiconductor-device applications: Questions may compare half-wave and full-wave rectifiers, capacitor filters, Zener diodes, LEDs, photodiodes, solar cells, and transistors. Marks are lost by confusing output frequencies, omitting the LED series resistor, or failing to distinguish light emission, light detection, photovoltaic conversion, and voltage regulation.