Electromagnetic Properties Exam 2

Location of Fermi energy in semiconductors

Halfway between conduction and valence bands, E_g/2

Intrinsic semiconductor conductivity

sigma = N_e*e*mu_e + N_h*e*mu_h

increasing temperature = _______ conductivity

increasing

Why are direct bandgap semiconductors used in optoelectronic devices?

direct = vertical/radiative recombination, indirect = nonradiative recombination

Radiative recombination reaction

electron + hole = photon (light)

Nonradiative recombination reaction

electron + hole = phonon (heat)

n-doped semiconductors

semiconductors doped with donors, extra electrons weakly bound to dopant atoms, extra negative charge

p-doped semiconductors

semiconductors doped with acceptors, available electron states, extra positive charge

What are the regions in this graph? (left to right, include region as T→0)

intrinsic region, extrinsic region, dopant ionization, freeze-out range

Freeze-out range

close to T=0, electrons/holes are bound to the donor/acceptor atoms

Dopant ionization

extra electrons/holes are available for conduction, dissociate from dopants

Extrinsic region

donors/acceptors are completely ionized, carrier concentration is relatively constant

Intrinsic region

thermally excited carriers are dominant

Why is the binding/ionization energy in semiconductors lower than that of the hydrogen Bohr model?

semiconductors have a small effective mass and large permittivity, so the binding energy is much lower

p-n junction

the interface between p-type and n-type doped semiconductors

depletion region

the region around the interface of a p-n junction where an electric field is present, free carriers diffuse and recombine, creating a depletion of free carriers

contact potential (p-n)

the potential at the p-n interface due to the equilibrium of the Fermi energy level and the associated electric field in the depletion region

Schottky barrier

a potential barrier seen by electrons tunneling from the metal to the semiconductor, resulting in Schottky type contact with nonlinear I-V characteristics

drift current

current induced by the drift of thermally-generated (minority) carriers in response to an electric field

diffusion current

current induced by the diffusion of electrons and holes toward the interface in a p-n junction due to a concentration gradient

forward/reverse bias

when a p-n junction is connected to a battery, the applied potential works against/with the built-in voltage, suppresses/improves drift, and improves/suppresses carrier diffusion

reverse breakdown

a phenomenon that occurs at high reverse bias, resulting in a dramatic current increase and high loss of energy from atom ionization or Si-Si bond rupture

rectifying contact

a contact in which a Schottky junction is formed from the relative magnitudes of the metal/semiconductor work functions, resulting in nonlinear I-V characteristics (like a diode)

Ohmic contact

a contact in which an Ohmic junction is formed from the relative magnitudes of the metal/semiconductor, resulting in linear 1-V characteristics (like a standard resistor)

work function

the energy required to remove an electron from the Fermi energy level to the vacuum level

electron affinity

the energy required to free electrons at the bottom of the conduction band to the vacuum level

pinning

defect states at the semiconductor interface introduce available energy levels, so the Fermi level at the semiconductor surface does not change with the addition/removal of electrons

how solar cells work

p-n junctions with narrow/heavily-doped n-sides, light waves are absorbed in the depletion and p regions, a charge difference is created in the diode and generates a current

how LEDs work

p-n junctions with a direct bandgap, radiative recombination emits a photon

rectification

limiting the direction of current by controlling free carrier diffusion with a potential barrier

how transistors work

3 differently-doped regions work as 2 diodes, the provided voltage controls the output current by affecting how many holes diffuse through the transistor without recombination

how MOSFETs work

metal-oxide semiconductor field-effect transistor, applied voltage through gate repels holes and creates a depletion region, high voltage creates an n-channel for electron flow

Types of reverse breakdown

Avalanche (impact ionization), Zener tunneling (electron tunneling from the valence to conduction band)

Contact potential (metal/semiconductor)

the potential barrier seen by electrons tunneling from the semiconductor to the metal, resulting in Schottky type contact with nonlinear I-V characteristics