Semiconductor Electronics: Materials & Circuits

Semiconductor Electronics: Materials, Devices, and Simple Circuits

Introduction

• Devices allowing controlled electron flow are fundamental to electronic circuits.
• Before 1948, vacuum tubes facilitated this, including diodes, triodes, tetrodes, and pentodes.
• Vacuum tubes use heated cathodes to supply electrons, controlled by voltage between electrodes in a vacuum.
• Vacuum is essential to prevent energy loss from collisions with air molecules.
• Vacuum tubes allow electron flow in one direction only, hence the term "valves".
• Vacuum tubes are bulky, consume high power, need high voltage, have limited life, and low dependability.
• Solid-state semiconductor electronics began in the 1930s, using semiconductor junctions to control charge carrier flow.
• Semiconductors enable mobile charge manipulation through light, heat, or small voltage.
• Semiconductor devices keep charge carriers within the solid, unlike vacuum tubes needing heated cathodes in a vacuum.
• Semiconductor devices are small, consume low power, operate at low voltages, and offer long life and high reliability.
• Liquid Crystal Display (LCD) monitors are replacing Cathode Ray Tubes (CRT) that use vacuum tube principles.
• Galena crystals (lead sulfide) with metal point contacts were early radio wave detectors.
• Subsequent sections will cover semiconductor basics and devices like junction diodes and bipolar junction transistors.

Classification of Metals, Conductors, and Semiconductors

On the Basis of Conductivity

• Solids are categorized by electrical conductivity (σ) or resistivity (ρ = 1/σ).
• Metals have very low resistivity (high conductivity) with ρ ~ 10⁻² – 10⁻⁸ Ω m and σ ~ 10² – 10⁸ S m⁻¹.
• Semiconductors have intermediate resistivity and conductivity, with ρ ~ 10⁻⁵ − 10⁶ Ωm and σ ~ 10⁵ – 10⁻⁶ S m⁻¹.
• Insulators have high resistivity (low conductivity) with σ ~ 10⁻¹¹ – 10⁻¹⁹ S m⁻¹.

Semiconductor Types

• Elemental semiconductors include Si and Ge.
• Compound semiconductors include inorganic (CdS, GaAs, CdSe, InP) and organic materials.
• Organic semiconductors include anthracene, doped pthalocyanines, and organic polymers like polypyrrole.
• Current semiconductor devices primarily use Si, Ge, and inorganic compounds.
• Since 1990, some devices have utilized organic semiconductors and polymers.

Energy Bands

• Atomic model dictates electron energy by orbit.
• When atoms form a solid, close proximity or overlapping of outer electron orbits changes electron motion.
• Each electron in a crystal has a unique position and energy level, forming energy bands.
• The valence band includes valence electron energy levels.
• The conduction band is above the valence band.
• Without external energy, valence electrons stay in the valence band.
• Electrons easily move to the overlapping conduction band, characteristic of metallic conductors.
• A gap between the valence and conduction bands makes a material an insulator.
• Insulators can conduct if electrons gain enough external energy to cross the gap.
• Excitation creates conduction electrons and vacancies (holes) in the valence band, enabling conduction.
• In Si or Ge crystals, each containing N atoms, the outermost orbit is the third orbit.
• Each Si atom contains 4 electrons in the outer most orbit
• Each Ge contains 4 electrons in the outer most orbit.
• There are 4N outer electrons and 8N available states.
• Spacing between atoms causes the 8N states to split into two bands, separated by the energy gap (Eg).
• The lower band (valence) is filled by 4N valence electrons at absolute zero.
• The other band consisting of 4N energy states is called the conduction band, which is empty at absolute zero.

Energy Levels

• Lowest conduction band energy is EC, highest valence band energy is EV.
• The energy band gap (E) can vary among materials.
• Metals have partially filled or overlapping bands.
• Overlapping bands permit easy electron movement.
• Partial emptiness allows electron movement within it.
• This facilitates high conductivity.
• Large band gap (E > 3 eV) in insulators prevents conduction without thermal excitation.
• Semiconductors have a small band gap (E, < 3 eV), allowing some electrons to conduct at room temperature.
• Semiconductor resistance is lower than insulators.
• Conductivity varies among metals, insulators, and semiconductors.

Intrinsic Semiconductor

• Common examples are Ge and Si that have a diamond-like lattice structure, where each atom is surrounded by four neighbors.
• Each Si or Ge atom shares one of its four valence electrons and takes share of one electron from their neighbour. Forming a strong covalent bond.
• As temperature rises, more thermal energy can break electrons away, creating free electrons that contribute to conduction.
• This creates a vacancy in the bond leaving the atom. Effective positive charge, and this vacancy is called a "hole".
• In intrinsic semiconductors, the number of free electrons, ne equals the number of holes, nh; expressed as ne = nh = ni, where ni is the intrinsic carrier concentration."
• Semiconductors possess the property where both electrons and holes can move.
• Holes move when an electron jumps to fill a vacancy.
• The total current, I is the sum of electron current (le) and hole current (Ih) represented by the equation I = Ie + Ih.
• Simultaneously, recombination occurs where electrons recombine with holes. At equilibrium, generation equals recombination.

Extrinsic Semiconductor

• Intrinsic semiconductor is like an insulator at absolute zero.
• Thermal energy at higher temperatures stimulates electrons from valence to conduction band.
• These electrons partially occupy the conduction band.
• Improving conductivity in semiconductors involves adding impurities.
• Adding a few parts per million (ppm) of impurity increases conductivity.
• Extrinsic/impurity semiconductors involve doping with dopants, without distorting the lattice.
• Size of dopant and semiconductor should be similar and
• Dopants include pentavalent (valency 5) elements like Arsenic and trivalent (valency 3) elements like Indium.
• Pentavalent dopants: Arsenic (As), Antimony (Sb), or Phosphorus (P).
• Trivalent dopants: Indium (In), Boron (B), or Aluminum (Al).
• Choosing dopants from nearby groups ensures similar size.
• Pentavalent/trivalent dopants create distinct semiconductor types.

n-type Semiconductor

• Doping Si or Ge with a pentavalent element causes the +5 valency element to occupies crystal lattice position, bonding four electrons with silicon neighbors.
• The fifth electron is weakly bound, needs little ionization energy to move freely (~0.01 eV for germanium, 0.05 eV for silicon).
• Pentavalent dopants donate extra electrons for conduction.
• Conduction electrons from dopants depend on doping level rather than ambient temperature.
• Number of free electrons (equal number of holes) generated by Si atoms, increases slightly with temperature.
• Total conduction electrons in doped semiconductor, is the sum of contribution by donors and those generated intrinsically.
• Increased electrons accelerate hole recombination, reducing hole numbers and reducing the number of holes.
• Higher electron numbers are achieved with appropriate doping.
• Extrinsic semiconductors doped with pentavalent impurities have electrons as majority and holes as minority carriers and are known as n-type semiconductors (ne >> nh).

p-type Semiconductor

• Doping Si/Ge with trivalent impurities results in p-type semiconductors.
• Dopants like Al/B/In have one less valence electron than Si/Ge.
• This creates a hole, leading to the electron jumps from neighboring Si/Ge atoms to fill it.
• Trivalent foreign atoms effectively become negatively charged.
• One acceptor atom yields one hole. The hole is in in addition to the intrinsically generated holes
• Holes are majority and electrons are minority carriers.
• Extrinsic semiconductors doped with trivalent impurities are called p-type semiconductors.
• We have, for p-type semiconductors nn >> ne (14.4)
• Crystals stay neutral despite charge carriers due to equal and opposite charges of ionized cores in the lattice.
• Increased majority carriers boosts chances of thermally produced minority carriers being destroyed.
• Dopants indirectly reduce intrinsic minority carrier concentration, contributing large numbers of majority current carriers.

Semiconductor Energy Band Structure

• Doping influences energy band structure in semiconductors.
• Additional energy states exist because of donor (ED) and acceptor (E") impurities.
• n-type Si semiconductor has a donor energy level (E")) slightly below the bottom, (EC) of the conduction band.
• Donor electrons move into the band with minimal energy.
• Conduction band gains electrons mostly from donor impurities, as donor atoms get ionized but few silicon atoms do.
• Acceptor energy level is slightly above valence band top (EV) in p-type semiconductors, allowing electrons to jump.
• This has with little energy where acceptors get ionized negatively.
• Valence band hole concentration results from impurity, dominating thermal equilibrium.

p-n Junction Basics

• The p-n junction is a building block of semi-conductor devices
• Knowledge of junction is important for how they work.
• Goal is to comprehend junction formation/behavior under external voltage.
• External voltage influencing p-n junction known as "bias".

p-n Junction Formation

• Begin with thin p-type silicon (p-Si) semiconductor wafer.
• Part of p-Si wafer converted to n-Si with small amount of pentavalent impurity.
• Two important processes occur in p-n junction formation: diffusion/drift.
• Diffusion: electrons more concentrated in n-type, holes in p-type.
• Holes diffuse across, creating diffusion current.
• Electrons leave ionized donors, holes leave ionized acceptors.
• Layer of positive charge develops on n-side, negative charge on p-side.
• Charge regions together form depletion region, reducing free charges, with a thickness of one-tenth of a micrometre.
• Electric field from positive to negative charge develops.
• Electrons move to n-side and holes to p-side to resist electric field.
• Movement of charge carriers called "drift" creates opposed current, opposite to diffusion and starts diffusion current.