Electronic Devices: Valves and Semiconductors

• Electronics devices are the fundamental components of circuits that enable electron flow.
• Two basic types of electronic devices exist: vacuum valves, and solid-state semiconductors.

Valve in Electronics

• Valves are also called vacuum tubes.
• Valves consist of varying numbers of electrodes/plates based on their type: diode (two), triode (three), tetrode (four), and pentode (five).
• Thermionic electrons are supplied in a vacuum tube by heating a cathode or filament.
• Voltage variations between electrodes control electron flow within the vacuum.
• A vacuum is essential to prevent energy loss from electron collisions with air or gas molecules.
• Valves only allow electron flow in one direction (cathode to anode).

Solid State Semiconductor Electronics

• Solid-state semiconductors and their junctions allow control over the quantity and direction of charge carriers.
• The number of mobile charges in a semiconductor can be altered by light, heat, or small voltage.
• Charge carrier supply and flow occur within the solid material itself.
• Solid state electronics devices include the junction diode (two electrodes) and the transistor (three electrodes).
• Integrated circuits contain numerous electrodes.

Vacuum Tubes and Semiconductors

• Vacuum tubes are larger than semiconductors and remain stationary.
• Semiconductors are smaller and portable.
• Vacuum tubes need more electrical power (9V to 100V) than semiconductors (1.5V to 9.0V).
• Vacuum tubes are more prone to damage due to their glass construction, resulting in a shorter lifespan.
• Semiconductors are more durable with a longer lifespan due to being solid-state.
• Vacuum tubes require voltage to the filament and different electrodes for controlling electron flow.
• Semiconductors use ambient light and heat to obtain electrons and can change charge particles with small voltage.
• Cathode-ray tubes were previously used as monitors.
• Liquid crystal displays are utilized in modern televisions and computers.

Solid Substance Classification

• Solid substances are classified based on electrical conductivity (σ) and resistivity (ρ).
• Metals have low resistivity (10⁻² to 10⁻⁸ Ωm) and high conductivity (10² to 10⁸ Sm⁻¹).
• Semiconductors have intermediate resistivity (10⁻⁵ to 10⁶ Ωm) and conductivity (10⁵ to 10⁻⁶ Sm⁻¹).
• Insulators have high resistivity (10¹¹ to 10¹⁹ Ωm) and low conductivity (10⁻¹¹ to 10⁻¹⁹ Sm⁻¹).
• Resistivity values are not the only criteria for distinguishing the materials.

Semiconductors

• Semiconductors can be original elements or compounds.
• Elemental semiconductors include silicon(Si) and germanium(Ge).
• Compound semiconductors include:
  • Inorganic: CdS, GaAs, CdSe, InP.
  • Organic: anthracene, doped pthaloc-yanines.
  • Organic polymers: polypyrrole, polyaniline, polythiophene etc.
• Most semiconductor devices utilize Si, Ge, and compound inorganic semiconductors.
• Since 1990, a few semiconductor devices use organic semiconductors/polymers.

Electron Motion

• An electron's motion differs in solids versus isolated atoms.
• In Bohr's model, electrons orbit the nucleus with energy dependent on the orbit.
• Atoms in solids are close, causing outer electron orbits to interact and merge.

Energy Bands

• Crystalline solids feature systematic atom arrangements.
• Neighboring atoms interact when closely arranged, changing electron energies.
• Inner shell electron energy levels are stable, but outer (valence) electrons change as they are shared.
• Electrons in crystals have closely spaced energy levels, forming an energy band, instead of the discrete.
• Valence electrons in the outermost orbit form the valence band.
• The conduction band lies above the valence band.
• Valence bands usually have electrons, while conduction bands do not.
• A band gap (Eg) separates the conduction and valence bands.

Conduction

• When the conduction band's minimum energy is lower than the valence band's maximum, electrons easily move to the conduction band (metals/conductors).
• A gap between bands prevents free electron availability in the conduction band (insulators).
• Semiconductors have relatively small gaps; some valence electrons gain external energy, cross the gap, and allow conduction.

Silicon and Germanium

• Silicon's atomic number is 14, with electron configuration 1s² 2s² 2p⁶ 3s² 3p².
• Germanium has atomic number 32, with electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p².
• Both Si and Ge are tetravalent, with 4 outermost electrons.
• In crystalline structures, Si and Ge atoms share valence electrons with four neighbors, forming covalent bonds including two electrons.

Semiconductor Holes

• At absolute zero, valence electrons in semiconductors bind in covalent bonds, making them insulators.
• Thermal oscillations at room temperature break covalent bonds, freeing electrons for conduction.
• Thermal energy ionizes atoms creating vacancies (holes) in the crystal lattice.
• Holes act as if they have an effective, positive charge.
• Both free electrons and holes act as electric charges.
• In intrinsic semiconductors, free electron numbers (ne) equal hole numbers (nh), giving intrinsic carrier concentration.

Electric Current

• Semiconductors have holes that move in addition to electrons. Holes move from high to low electric potential under external electric field influence.
• Semiconductor current is the sum of electron (Ie) and hole (Ih) currents.

Intrinsic Semiconductor

• At absolute zero (0 K), intrinsic semiconductors act as insulators as all electrons are bound.
• At temperatures above 0 K, thermally excited electrons partially occupy the conduction band.

Recombination

• Creation of an electron-hole pair when an electron moves to the conduction band is unstable.
• Electrons and holes interact, with electrons eventually occupying holes again; this is the recombination process which occurs simultaneously.
• Thermal equilibrium exists when electron-hole pair formation and recombination rates are consistent.

Energy Release

• C, Si, and Ge share a lattice structure, but C is an insulator due to greater energy. Metals and Conductors.
• Atoms require varied energy to release electrons from their nucleus (least for Ge, most for C).
• Resistivity depends on the energy difference between valence and conduction bands (5.4 eV for C, 1.1 eV for Si, 0.7 eV for Ge).
• The number of free electrons for conduction is significant in Ge and Si but negligible in C.
• Therefore Ge and Si are semiconductors, and C is an insulator.