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To examine electronic materials, including insulators, dielectrics, conductors, semiconductors, and superconductors, as well as their conductivity properties.

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1. Ohm’s Law and Electrical Conductivity

2. Band Structures of Solids

3. Conductivity of Metals and Alloys

4. Conductivity in Other Materials

5. Semiconductors

6. Applications of Semiconductors

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Classification of technologically useful electronic materials

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Current density: The current flowing through per unit cross-sectional area.

Electric field: The voltage gradient or volts per unit length.

Drift velocity: The average rate at which charge carriers move through a material under an electric or magnetic field.

Mobility: The ease with which a charge carrier moves through a material.

Dielectric constant: The ratio of a material's permittivity to the permittivity of a vacuum, indicating its relative ability to polarize and store charge, also known as relative permittivity.

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(a) Charge carriers like electrons are deflected by atoms or defects, resulting in an irregular path with an average drift velocity

(b) valence electrons in metals move easily

(c) electrons in semiconductors and insulators require breaking covalent bonds to move

(d) in ionically bonded materials, entire ions must diffuse to carry charge.

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Valence band: The energy levels occupied by electrons in their lowest energy states.

Conduction band: The energy levels electrons can occupy to enable conductivity.

Holes: Unfilled energy levels in the valence band that facilitate current as electrons move to fill them.

Energy gap (Band gap): The energy difference between the top of the valence band and the bottom of the conduction band that a charge carrier must overcome to transfer a charge.

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Band Structure in Solids (Part 2) | Engineering Chemistry | S Chand Academy

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The energy levels broaden into bands as the number of electrons grouped together increases.

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The simplified band structure for sodium. The energy levels broaden into bands. The 3s band, which is only half filled with electrons, is responsible for conduction in sodium

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(a) At absolute zero, all of the electrons in the outer energy level have the lowest possible energy.

(b) When the temperature is increased, some electrons are excited into unfilled levels. Note that the Fermi energy is unchanged

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The band structure of carbon in the diamond form. The 2s and 2p levels combine to form two hybrid bands separated by an energy gap, Eg.

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Schematic of band structures for (a) metals,

(b) semiconductors, and

(c) dielectrics or insulators. (Temperature is 0 K.)

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- Mean free path: The average distance electrons travel without scattering by other atoms.

- Temperature Effect: Increased temperature causes metal atoms to vibrate, affecting electron movement.

- Effect of Atomic Level Defects: Crystal structure imperfections scatter electrons, reducing mobility and conductivity.

- Matthiessen’s rule: The resistivity of a metal is the sum of temperature-dependent resistivity (ρT) and a temperature-independent term due to atomic defects and impurities (ρd).

- Effect of Processing and Strengthening: Not explicitly defined, likely refers to how material processing and strengthening methods impact conductivity and other properties.

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Movement of an electron through (a) a perfect crystal, (b) a crystal heated to a high temperature, and (c) a crystal containing atomic level defects. Scattering of the electrons reduces the mobility and conductivity.

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The effect of temperature on the electrical resistivity of a metal with a perfect crystal structure. The slope of the curve is the temperature resistivity coefficient.

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The electrical resistivity of a metal is composed of a constant defect contribution ρd and a variable temperature contribution ρT.

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(a) the effect of solid-solution strengthening and cold working on the electrical conductivity of copper

(b) the effect of addition of selected elements on the electrical conductivity of copper

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- Conduction in Ionic Materials: Conducted by the movement of entire ions due to a large energy gap, making most ionic materials insulators.

- Conduction in Polymers: Low electrical conductivity due to valence electrons being involved in covalent bonding and a large energy gap in their band structure.

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Effect of carbon fibers on the electrical resistivity of nylon

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Effect of carbon fibers on the electrical resistivity of nylon

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- Intrinsic semiconductor: A semiconductor whose properties are determined by its inherent material composition, without dopants or impurities.

- Extrinsic semiconductor: A semiconductor whose properties are modified by adding dopants to control the number and type of charge carriers.

- Doping: The intentional addition of controlled amounts of other elements to increase the number of charge carriers in a semiconductor.

- Thermistor: A semiconductor device highly sensitive to temperature changes, used for precise temperature measurements.

- Radiative recombination: The process where holes and electrons recombine, emitting light, occurring in direct bandgap materials.

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When a voltage is applied to a semiconductor, the electrons move through the conduction band, which the electron holes move through the valence band in the opposite direction.

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The distribution of electrons and holes in the valence and conduction bands (a) at absolute zero and (b) at an elevated temperature

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The electrical conductivity versus temperature for intrinsic semiconductors compared with metals. Note the break in y-axis scale.

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When a dopant atom with a valence greater than four is added to silicon, an extra electron is introduced and a donor energy state is created. Now electrons are more easily excited into the conduction band.

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In an n-type semiconductor, at low temperatures, donor atoms are not ionized, but as temperature rises, ionization completes and carrier concentration increases based on doping levels. Conductivity remains stable until very high temperatures, where thermally generated carriers dominate, causing the semiconductor to exhibit intrinsic behavior.

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When a dopant atom with a valence of less than four is substituted into the silicon structure, a hole is created in the structure and an acceptor energy level is created just above the valence band. Little energy is required toexcite the holes into motion.

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- Diodes, transistors, lasers, and LEDs: Devices made using semiconductors, with silicon being crucial for very large scale integrated (VLSI) circuits.

- Forward bias: Connecting a p-n junction so the p-side is positive, reducing the energy barrier and allowing significant current flow.

- Reverse bias: Connecting a p-n junction so the p-side is negative, resulting in minimal current flow.

- Avalanche breakdown: A reverse-bias voltage causing a large current in a lightly doped p-n junction.

- Transistor: A semiconductor device used to amplify electrical signals.

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Applications of Semiconductors - Semiconductor - Engineering Physics 1