Materials Ch. 18 - Electrical Properties

Piezoelectricity

Application of stress induces voltage, and application of voltage induces dimensional change. An electric field is generated when mechanical stresses are applied.

Temperature & Metal Resistivity

At higher temperature, there are more vigorous vibrations, and thus more scattering. Thus, resistivity increases with temperature.

Conductor Energy Band Structure

Empty energy states are adjacent to filled states, so it is easy to excite electrons. Thermal energy (kB*T) excites electrons into higher energy states. Can either have a partially filled band structure, or an empty band overlapping a filled band.

As atoms approach each other

each distinct atomic state splits into a series of closely spaced electro states

Charge Carriers in Insulators and Semiconductors

1. Free electrons (negatively charged, exist in the conduction band). 2. Holes (positively charged, exist as vacant electron states in the valance band).

Scattering & Resistivity

Scattering by lattice vibrations and defects (vacancies, impurities, dislocation, grain boundaries) makes electrons take a less direct path, and thus increases resistivity. *Charge carriers in metals = electrons

Factors Increasing Metal Resistivity

Scattering, temperature, impurity, and cold work

Material classification

Based on conductivity. Conductors (metals) >> Semiconductors >> Insulators (ceramics and polymers)

% Cold Work & Metal Resistivity

Cold work mechanism: more dislocations lead to a higher dislocation density, and therefore more scattering centers. More scattering leads to higher resistivity, so resistivity increases with cold work as well.

Semiconductors: Conductivity and Temperature

Conductivity increases with T (opposite of metals!). EXPLAIN WHY

Intrinsic Semiconductors

Contain no impurities (without 'doping'). The number of electrons in the conduction band is always equal to the number in the valence band. Include elemental semiconductors (Si and Ge) or compound semiconductors (group III - V or group II - VI compounds). Electrical properties are inherent in the pure material.

Magnitude of Conductivity Dependence

Dependent on the number of electrons available to participate in the conduction process.

Electrical Conductivity

Ease with which a material is capable or transmitting an electric current. Its reciprocal is resistivity.

Extrinsic Semiconductors

Electrical behavior is dictated by impurities.

Semiconductor Energy Band Structure

Have a narrow band gap (< 2eV), so more, but not all, electrons are excited across the gap.

Insulator Energy Band Structure

Have a wide band gap (> 2eV), so few electrons are excited across this gap

% Impurity & Metal Resistivity

Impurities act as a scattering center, thus increasing scattering and therefore resistivity. More impurities lead to a higher resistivity.

Ferroelectricity

Materials that exhibit spontaneous polarization with out applied electric field. Contains permanent electric dipoles below ferroelectric Curie temperature (Tc is around 120 C). Above Tc, there is no more ferroelectricity.

P-N Rectifying Junction

No applied potential - no current flows. Forward Bias - carriers flow through p-type and n-type regions. holes and electrons recombine at junction. current does flow. Reverse Bias - carriers flow away from the junction. the region is depleted of charge carriers so no current flows.

Fermi Level

energy corresponding to the highest filled energy state at 0K

N-Type Extrinsic Semiconductors

n>>p, majority charge carrier is the electron. addition of group V impurities (donors - introduce excess electrons)

P-Type Extrinsic Semiconductors

p>>n, majority charge carrier is the hole. addition of group III impurities (acceptors - introduce excess holes)