ENGR 205 Chapter 18

Ohm’s Law

The applied voltage is equal to the product of the current and resistance; equivalently, the current density is equal to the product of the conductivity and electric field intensity. V = IR

Using Ohm's law V = IR, if an applied voltage stays constant and current increases, then resistance

decreases

Electrical _____ measures a material's resistance to current flow.

resistivity

If a material's circular cross-sectional area's radius doubles, then the electrical resistivity changes by a factor of _____.

4

resistivity (p)

The reciprocal of electrical conductivity; a measure of a material's resistance to the passage of electric current.

Electrical conductivity measures how easily a material conducts electrical _____.

current

The expression _____ is equivalent to Ohm's law given as V = IR

J = (1/p)*E

The voltage is measured at a material's opposite ends. If the material length increases, then the current density _____.

stays the same

Metals

High conductivity

Insulators

Low conductivity

conductivity, electrical

The proportionality constant between current density and applied electric field; also, a measure of the ease with which a material is capable of conducting an electric current.

metal

The electropositive elements and the alloys based on these elements. The electron band structure of metals is characterized by a partially filled electron band.

insulator (electrical)

A nonmetallic material that has a filled valence band at 0 K and a relatively wide energy band gap. Consequently, the room-temperature electrical conductivity is very low.

As the separation distance between atoms decreases, an atom's energy levels _____.

split

For a material with N=8 atoms, the _____ band will have _____ different electron states.

p,24

In semiconductors, the _____ has a lower energy level than the _____.

valance band, conduction band

electron energy band

A series of electron energy states that are very closely spaced with respect to energy.

Fermi energy (E_f)

For a metal, the energy corresponding to the highest filled electron state at 0 K.

valence band

For solid materials, the electron energy band that contains the valence electrons.

conduction band

For electrical insulators and semiconductors, the lowest-lying electron energy band that is empty of electrons at 0 K. Conduction electrons are those that have been excited to states within this band.

energy band gap

For semiconductors and insulators, the energies that lie between the valence and conduction bands; for intrinsic materials, electrons are forbidden to have energies within this range

A free electron's minimum energy must be greater than the _____.

highest filled valance band energy

When an electron is promoted to a free electron, a hole is typically left behind in the electron's place for _____ materials.

insulator and semiconductor

For an insulating material at a specified temperature, as the band gap width increases, the conductivity _____.

decreases

Increasing temperature decreases _____ conductivity.

a pure metal’s

An insulator has relatively low conductivity because the material typically has _____ bonds.

ionic or strongly covalent

free electron

An electron that has been excited into an energy state above the Fermi energy (or into the conduction band for semiconductors and insulators) and may participate in the electrical conduction process.

hole (electron)

For semiconductors and insulators, a vacant electron state in the valence band that behaves as a positive charge carrier in an electric field.

Drift velocity

V_d = μ_eE

Free electrons accelerate in the _____ direction as an applied electric field.

opposite

Electron mobility,μ_e , is greatest for a ______ material structure.

perfectly ordered crystal lattice

If a metallic material is cooled through its melting temperature at an extremely rapid rate, it forms a noncrystalline solid (i.e., a metallic glass). Will the electrical conductivity of the noncrystalline metal be greater or less than its crystalline counterpart? Why?

metallic glasses have lower electrical conductivity than crystalline metals

mobility

The proportionality constant between the carrier drift velocity and applied electric field; also, a measure of the ease of charge carrier motion.

Which of ZnS and CdSe has the larger band gap energy E_g? Cite reason(s) for your choice

ZnS, higher difference in electronegativity

In a semiconductor, an electron in the valence band that is excited to the conduction band leaves a _____ charged hole behind in the valence band.

positively

In semiconductors, the hole mobility μ_h is _____ the electron mobility μ_e.

less than

For an intrinsic semiconductor, the number of free electrons is _____ the number of holes.

equal to

For an intrinsic semiconductor, conductivity in terms of intrinsic carrier concentration

σ = n_i[e](μ_e + μ_h)

intrinsic semiconductor

A semiconductor material for which the electrical behavior is characteristic of the pure material—that is, in which electrical conductivity depends only on temperature and the band gap energy

extrinsic semiconductor

A semiconducting material for which the electrical behavior is determined by impurities.

Intrinsic semiconductors have _____, and extrinsic semiconductors have _____.

virtually no impurities, impurities

For an n-type semiconductor, each impurity creates an excess _____ charge

negative

Impurity atoms in _____ semiconductors introduce an energy level within the band gap.

both n and p type

Holes in the valence band are generated by impurities in _____ semiconductors.

p type

donor state (level)

For a semiconductor or insulator, an energy level lying within and near the top of the energy band gap and from which electrons may be excited into the conduction band. It is normally introduced by an impurity atom.

acceptor state (level)

For a semiconductor or insulator, an energy level lying within yet near the bottom of the energy band gap that may accept electrons from the valence band, leaving behind holes. The level is normally introduced by an impurity atom.

doping

The intentional alloying of semiconducting materials with controlled concentrations of donor or acceptor impurities.

A p-n rectifying junction has _____.

a single crystal

_____ are the dominant charge carriers for the n-side.

electrons

A forward bias will cause the semiconductor's charge carriers to _____.

move towards the junction

A photon _____ when a hole and electron recombine.

can be released

Many light emitting diodes (LEDs) are designed as p-n junctions. When switched on, LEDs are _____.

forward biased

As the magnitude of input voltage across a p-n junction increases, the _____ bias output current _____ in magnitude.

forward, increases

As the magnitude of input voltage across a p-n junction increases, the _____ bias output current _____ in magnitude.

forward, positive, p

When an alternating current is applied to a p-n rectifying junction, the output I_f current component is _____ the I_r component.

much larger than, I_f dominates I_r

A p-n rectifying junction transforms an alternating input current into _____.

a direct current in the forward bias direction

To fully charge a large battery from a wall outlet, using one diode in the circuit is _____.

somewhat efficient

diode

Electronic device that allows current flow in only one direction.

rectifying junction

A semiconductor that is p-type on one side, and n-type on the other side.

forward bias

In a semiconductor, the electrons and holes move towards each other due to an applied voltage

reverse bias

In a semiconductor, the electrons and holes move away from other due to an applied voltage.

A transistor's function is to _____.

amplify electrical signals or serve as a switching device

In a junction transistor, the base region _____ with minority charge carriers.

reacts partially

A MOSFET's gate current is typically _____ a junction transistor's current.

much smaller than

diode

An electronic device that rectifies an electrical current—that is, allows current flow in one direction only.

rectifying junction

A semiconductor p-n junction conductive for a current flow in one direction and highly resistive for the opposite direction

forward bias

The conducting bias for a p-n junction rectifier such that electron flow is to the n side of the junction.

reverse bias

The insulating bias for a p-n junction rectifier; electrons flow into the p side of the junction

junction transistor

A semiconducting device composed of appropriately biased n-p-n or p-n-p junctions, used to amplify an electrical signal

MOSFET

Metal-oxide-semiconductor field-effect transistor, an integrated circuit element.

integrated circuit

Millions of electronic circuit elements (transistors, diodes, resistors, capacitors, etc.) incorporated on a very small silicon chip.

Capacitance

C = Q/V

Parallel plate capacitor

C = ε₀(A/l)

Capacitance of parallel plate capacitor, with dielectric material

C = ε(A/l)

Dielectric constant

ε_r = ε/ε₀

A capacitor stores _____ charged particles on opposite plates.

oppositely

As the separation distance between plates increases, the capacitance _____.

decreases

dielectric

Any material that is electrically insulating.

electric dipole

A pair of equal and opposite electrical charges separated by a small distance.

capacitance (C)

The charge-storing ability of a capacitor, defined as the magnitude of charge stored on either plate divided by the applied voltage.

permittivity (ε)

The proportionality constant between the dielectric displacement D and the electric field E. The value of the permittivity ε₀ for a vacuum is 8.85e-12 F/m

dielectric constant (ε_r)

The ratio of the permittivity of a medium to that of a vacuum. Often called the relative dielectric constant or relative permittivity.