semiconductor 1

How does the band structure of a semiconductor differ from an insulator?

Both have a filled valence band and empty conduction band at 0 K but semiconductors have a much smaller band gap (Eg < 3.0 eV) allowing thermal excitation at room temperature

What is an intrinsic semiconductor?

A pure semiconductor crystal with no significant impurities where charge carriers (n = p) are generated solely by thermal excitation

Define the band gap energy (Eg).

The energy difference between the top of the valence band (Ev) and the bottom of the conduction band (Ec)

What are the two types of charge carriers in semiconductors?

Electrons in the conduction band and holes in the valence band

What is a hole?

A missing electron in a filled valence band that behaves as a positively charged particle

What is the distinction between direct and indirect band gap semiconductors?

In direct gap materials the conduction band minimum and valence band maximum occur at the same k whereas in indirect gap they occur at different k requiring a phonon for transition

Why are direct band gap materials essential for LEDs and lasers?

Radiative recombination is efficient because momentum is conserved by photon emission making light emission effective

What is an extrinsic semiconductor?

A semiconductor doped with impurity atoms to modify its electrical properties

What is an n-type semiconductor?

A semiconductor doped with donor atoms that provide extra electrons making electrons the majority carrier

What is a p-type semiconductor?

A semiconductor doped with acceptor atoms that create holes making holes the majority carrier

Where is the Fermi level located in an n-type semiconductor?

Shifted toward the conduction band closer to Ec

Where is the Fermi level located in a p-type semiconductor?

Shifted toward the valence band closer to Ev

What is the law of mass action?

At equilibrium n times p equals ni squared

How does temperature affect carrier concentration in the intrinsic region?

Thermal generation dominates and the material behaves intrinsically with n approximately equal to p approximately equal to ni

What is the freeze-out region?

Low temperature region where dopants are not fully ionized leading to low carrier concentration

What is the extrinsic region?

Intermediate temperature range where dopants are fully ionized and carrier concentration is nearly constant

How does mobility vary with doping concentration?

Mobility decreases as doping increases due to impurity scattering

What is the conductivity formula for semiconductors?

Sigma equals e times (n mue plus p muh)

Which has higher mobility in silicon electrons or holes?

Electrons have higher mobility than holes

What is effective mass?

A parameter that accounts for lattice effects allowing electrons to be treated with modified mass in equations of motion

How does temperature affect mobility in lightly doped semiconductors?

Mobility decreases with increasing temperature due to phonon scattering

What is a donor impurity?

A group V atom that donates an extra electron to the conduction band

What is an acceptor impurity?

A group III atom that creates a hole by accepting an electron

What is the intrinsic carrier concentration formula?

ni equals square root of Nc Nv times exp minus Eg over 2kT

Why is silicon widely used as a semiconductor?

It has a stable oxide adequate band gap is abundant and inexpensive to process

What happens to the Fermi level in an intrinsic semiconductor as temperature increases?

It remains near the middle of the band gap

What is the Hall effect used for?

Determining carrier type carrier concentration and mobility

What is the band gap of silicon at room temperature?

About 1.12 eV

What is the unit of mobility?

Square metres per volt second or centimetres squared per volt second

How does effective mass affect mobility?

Lower effective mass leads to higher mobility

What is scattering by ionized impurities?

Deflection of charge carriers by charged dopant ions reducing mobility especially at low temperature or high doping