Electronic Devices: Valves and Semiconductors

• Electronic devices manipulate electron flow in circuits, with two basic types: vacuum valves and solid-state semiconductors.

Valve Description

• Valves, also known as vacuum tubes, consist of multiple electrodes/plates.
• Electrode count dictates valve name: diode (2 electrodes), triode (3 electrodes), tetrode (4 electrodes), pentode (5 electrodes).
• Thermionic emission, achieved by heating a cathode or filament, supplies electrons inside vacuum tubes.
• Valves need a vacuum to prevent moving electrons losing energy through collisions with air.
• Valves allow electron flow only from cathode to anode, providing directionality.

Solid-State Semiconductor Electronics

• These semiconductors and their junctions control the number/direction of charge carriers.
• External stimuli such as light, heat or apply voltage change the number of them mobile charges.
• Charge carrier supply and flow occur within the solid material itself.
• Solid-state semiconductor devices include junction diodes (2 electrodes) and transistors (3 electrodes) and integrated circuits (numerous electrodes)

Vacuum Tube vs. Semiconductor

• Vacuum tubes are larger/stationary, while semiconductors are smaller/portable.
• Vacuum tubes need more electrical power (9-100V), while semiconductors are more efficient (1.5-9V).
• Vacuum tubes are fragile (glass), semiconductors are durable (solid-state), and have longer lifespans.
• Vacuum tubes require voltage for electron emission, semiconductors can use light and heat, for electron generation or small voltage for charge change
• Cathode-ray tubes are used in old TVs/computers, liquid crystal displays (LCDs) are used in modern TVs/computers.

Classifying Solids by Electrical Properties

• Classification is based on electrical conductivity (σ) or resistivity values.
• Metals exhibit low resistivity (10^-2 to 10^-8 Ωm) and high conductivity (10^2 to 10^8 Sm^-1).
• Semiconductors possess intermediate resistivity (10^-5 to 10^6 Ωm) and conductivity (10^5 to 10^-6 Sm^-1).
• Insulators have high resistivity (10^11 to 10^19 Ωm) and low conductivity (10^-11 to 10^-19 Sm^-1).

Semiconductor Types

• Semiconductors can be elemental or compound materials.
• Elemental exmaples include Si and Ge
• Compound semiconductors: inorganic (CdS, GaAs, CdSe, InP), organic (anthracene, doped pthaloc-yanines) and organic polymers (polypyrrole, polyaniline, polythiophene).
• Most semiconductor devices use Si, Ge, or inorganic compounds, but electronics have emerged after 1990 based on organic semiconductors and semiconducting polymers.

Electron Motion Comparison

• The Bohr model dictates electron motion in isolated atoms depends on orbital position
• When atoms form a solid, outer electron orbits merge due to close proximity.
• The way electrons move in solids differs greatly, making the study of semiconductors quite novel

Energy Bands in Crystals

• Crystalline solids feature systematic atom/molecule arrangements.
• Atom interaction in crystals alters electron energy levels in the shells, the outer one in particular (valence electrons), as they exist b/w multiple atoms
• Electrons in crystals occupy closely spaced energy levels, forming energy bands instead of discrete energy levels of isolated ones
• The valence band contains valence electron energy levels,while the conduction exists above the other
• Valence bands typically contain electrons, conduction bands typically do not
• A band gap represents the energy difference between the two bands (Eg).
• Metals have conduction bands whose minimum energy levels sits below maxium level of the one below it, or have both overlapping
• Insulators have a significant gap between them

Energy Band Diagrams

• Conduction and valence band spacing affects material conductivity. Conductivity occurs with external energy input in insulators.
• A relatively small gap that external energies can surpass lead to flow in both bands, only possible in semiconductors

Energy Levels of Si/Ge

• Silicon (Si) has atomic number 14 (1s² 2s² 2p⁶ 3s² 3p²), containing four valence electrons in its incomplete M shell.
• Germanium (Ge) has atomic number 32 (1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p²), also with four valence electrons but in its N shell.
• Both Si and Ge are tetravalent, forming crystals where each atom shares one valence electron with four neighbors. The maximum electron count is 8 (2s + 6p).
• In Si/Ge lattices, 8N states split into two energy bands separated by a band gap (Eg)
• 4N valence electrons fill the lower band (valence band) at absolute zero, while the upper band (conduction band) remains empty

Band Diagrams for Conductivity

• The minimum conduction band energy is denoted as Ec, while the maximum valence band energy is Ev
• Region between each contains no states, representing a forbidden gap
• Metals can occur two ways
• partially filled conduction, partially empty valence band,
• when there's overlap the movement is easy
• Insulators have a large gap (Eg > 3 eV), no conduction electrons,

Semiconductors (Case III)

• Semiconductors possess a smaller energy gap (Eg < 3 eV).
• Valence electrons gain room temperature enoughto cross the gap, hence the conductivity.
• Limited electron movement makes them semiconductors.
• Eg for Si is 1.1 eV, and 0.7 eV for Ge.

Crystalline Structure

• Silicon and Germanium are elementary semiconductors.
• Si has 14 electrons (1s², 2s² 2p⁶, 3s² 3p²), while Ge has 32 electrons (1s², 2s² 2p⁶, 3s² 3p⁶ 3d¹⁰, 4s² 4p²).
• Si and Ge are tetravalent, with each Si/Ge atom sharing its four valence electrons with its neighbors
• Shared pairs form convalent/valence bonds containing two electrons each

Hole Concepts

• Semiconductors at absolute zero act as insulators due to bound valence electrons due to covalent bonds
• At room temperature, the valence electrons become free to move across the covalent bonds, hence the conductivity
• Vacancies created are known as (hole) in the bond

Hole Movement

• The thermal generation creates holes and conduction electrons. With charge -q comes vacancies with a counterpart.
• Vacancy with effective pos. charge = hole.
• Holes attract electrons despite lacking charge
• Intrinsic semiconductivity means free electrons = holes. They are intrinsically linked
• For intrinsic materials, ne = nh = ni, the latter referring to intrinsic carreir concentration

Electron and Hole Current

• Semiconductors enable electron movement along with hole movement.
• With holes, they can jump from the site with hole on it, to another close to that same hole.
• Free electron movement = electron current (Ie).
• Hole movement = hole current (Ih).
• The net semiconductor = additon of the currents

Band Changes

• Intrinsic semiconductors are insulator-esque at absolute zero as all electrons exist in the valence band and none in the conduction
• At > 0K, thermally excited electrons partially occupy the conduction band
• Electrons entering bands leaves holes
• Four electrons in the conduction band = corresponding holes.

Recombination Coefficient

• The relationship between recombination and intrinsic semiconductors.
• Creating heat through the process is not stable
• The electrons and holes are not separate the same temperature
• Creation and recombination happens simultaneously
• Equality b/w the two exists during thermal equilibrium positions
• Recombination rate is proportional to electron-hole density
• Recombination rate = Rnenn.

Carbon, Silicon & Germanium

• Atoms need less ionization energy than some orbiting nucleus.
• They each have 4 electrons
• Least energy to greates is as follows, Ge, then Si then C as well as reisitivity.
• Therefore semiconductors due to higher electron count, and Carbon is insulator