Semiconductor Devices

Introduction

• Devices for controlled electron flow are fundamental to electronic circuits
• Before the 1948 transistor, vacuum tubes were used:
  • Vacuum diode: anode (plate) and cathode
  • Triode: cathode, plate, and grid
  • Tetrode and pentode: 4 and 5 electrodes respectively
• Vacuum tubes use a heated cathode to supply electrons
• Voltage is varied between electrodes for control
• Vacuum is needed to prevent energy loss from collisions with air molecules
• Electron flow is unidirectional (cathode to anode)
• Vacuum tubes were large, power-hungry, high voltage, unreliable, and had short lifespans
• The concept of solid-state semiconductor electronics dates back to the 1930s
• Semiconductors and their junctions allow control of the number/direction of charge carriers
• Light, heat, or small voltage can alter mobile charges in semiconductors

Semiconductor Devices

• Charge carriers flow within the solid material itself
• Earlier vacuum tubes required heated cathodes and evacuated spaces
• Semiconductor devices:
  • Small in size
  • Low power consumption
  • Operate at low voltages
  • Long life and high reliability
• Liquid Crystal Displays (LCD) are replacing Cathode Ray Tubes (CRT) which are used in TVs
• Crystal of galena (lead sulphide, PbS) with metal contact was an early radio wave detector
• Junction diodes (2-electrode device) and bipolar junction transistors (3-electrode device) are types of semiconductor devices

Classification Of Materials

• Solids are classified based on electrical conductivity (σ) or resistivity (ρ = 1/σ)
• Metals: very low resistivity (high conductivity)
• ρ ≈ 10^-2 to 10^-8 Ω m
• σ ≈ 10^2 to 10^8 S m^-1
• Semiconductors: intermediate resistivity/conductivity:
• ρ ≈ 10^-5 to 10^6 Ω m
• σ ≈ 10^5 to 10^-6 S m^-1
• Insulators: high resistivity (low conductivity):
• ρ ≈ 10^11 to 10^19 Ω m
• σ ≈ 10^-11 to 10^-19 S m^-1
• Resistivity values indicate the approximate magnitude
• Relative resistivity isn't the sole factor to distinguish them.
• Semiconductors can be:
  • Elemental: Si and Ge
  • Compound:
  • Inorganic: CdS, GaAs, CdSe, InP, etc.
  • Organic: anthracene, doped pthalocyanines, etc.
  • Organic polymers: polypyrrole, polyaniline, polythiophene, etc.

Semiconductor Types

• Most semiconductor devices use Si or Ge and inorganic compounds
• Organic semiconductors and polymers signal the rise of polymer/molecular electronics
• This chapter focuses on inorganic semiconductors (Si, Ge)
• Elemental semiconductor concepts apply to compound semiconductors

Energy Bands

• In isolated atoms (Bohr model), electron energy depends on its orbit
• In solids, atoms are close, altering electron motion
• Each electron has a unique position and surrounding charge pattern
• Each electron has a different energy level
• Continuous energy variation forms energy bands
• Valence band: energy band of valence electrons
• Conduction band: energy band above the valence band
• Without external energy, valence electrons stay in the valence band
• If the conduction band’s lowest level is below the valence band’s highest level, electrons move to the conduction band
• Normally, the conduction band is empty unless it overlaps with the valence band
• Overlap occurs in metallic conductors where electrons move freely
• A gap between the conduction and valence bands creates an insulator
• Valence band electrons remain bound, and no free electrons are available in the conduction band
• Insulators: some electrons gain enough energy to cross the gap
• Electrons in the conduction band and vacancies in the valence band (holes) allow conduction
• In Si/Ge crystals with N atoms:
• Si’s outermost orbit is the third (n=3)
• Ge’s outermost orbit is the fourth (n=4)
• There are 4 valence electrons
• Total outer electrons= 4N
• Maximum electron number is 8N (2s + 6p)
• 8N discrete energy levels form bands based on atomic distance
• Atomic spacing in Si/Ge crystals splits 8N states into 2 bands separated by gap (Eg)
• Valence band: completely occupied by 4N valence electrons at absolute zero
• Conduction band: 4N energy states, completely empty at absolute zero

Energy Level Diagrams and Conductivity

• EC represents the lowest energy level in the conduction band
• EV represents the highest energy level in the valence band
• There are a lot of closely spaced energy levels between EC and EV
• Energy band gap (Eg) is the space between the top of the valence band and the bottom of the conduction band
• The band gap (Eg) can be large, small, or non-existent depending on the material
• Metals, semiconductors, and insulators have differing band gaps.
• Metals happen when the conduction band is partly filled and the balanced band is partially empty, or when the conduction bands overlap Electrons move easily, creating conductivity
• Insulators contain a large band gap (Eg > 3 eV).
• Conduction is impossible because electrons cannot jump into the conduction band
• Semiconductors contain a small band gap (Eg < 3 eV)
• At room temperature, a few electrons acquire the ability to cross the energy gap and join the conduction band
• The resistance of semiconductors is not high

Intrinsic Semiconductors

• Ge and Si structures are diamond-like
• Each atom is surrounded by four neighbours
• Si and Ge have 4 valence electrons
• In crystals, every atom shares one valence electron with neighbors
• Shared pairs create covalent/valence bonds
• Shared electrons move between atoms, binding them strongly
• Heating increases thermal energy, freeing electrons for conduction
• Thermal energy ionizes atoms and creates vacancies
• Free electrons leave behind positive charge vacancies
• The effective positive vacancies with electronic charge are called a holes
• A hole behaves as an apparent free particle with effective positive charge.
• In intrinsic semiconductors, free electrons equal the number of holes: ne = nh = ni
• ni = intrinsic carrier concentration
• Semiconductors possess the unique property that holes also move apart from the electrons

Hole Movement

• If a hole exists at site 1, it can move
• An electron from the covalent bond at site 2 jumps to the vacant site 1 (hole)
• After the electron jumps, the hole is at site 2, so the hole has moved from site 1 to site 2
• Free electron travel happens separately as conduction electron and create the electron current
• Electrons motion is used to describe to motion of bound electrons whenever a bond is empty in a crystal
• Holes shift towards negative potential when electric field action is present and creates current
• Net current (I) = electron (Ie) + hole (Ih) currents
• Besides electron/hole creation, recombination happens when electrons recombine the holes
• Balance occurs when generation and recombination have equal rates
• Recombination is caused by electrons having a collision with a hole.

An introduction to semiconductor devices, contrasting them with earlier vacuum tube technology. Explains how semiconductors control charge carriers, offering advantages over vacuum tubes in size, power consumption, and reliability.