Junctions 10 Surface ~ simplest one; junction between vacuum/surface Metal/Metal Metal/Semiconductor Schottky contact ~ blocking barrier Ohmic contact ~ low resistance Semiconductor/Semiconductor Homojunction ~ junctions are composed of the same material Heterojunction ~ each side is composed of different material Iso-type ~ types are same to both sides <n-types or p-types> Anisotype <p-type and n-type, vise versa> M/I/M, MIS, SIS, MOS

Surfaces Chemisorption of oxygen on n-type Semiconductor surface Termination of the periodic potential  localized states at the surface Chemisorption of oxygen on n-type Semiconductor surface Ec EF Ev Surface state O2- chemisorption bending of band gives E + separation of charge - d - negatively charged surface state positively charged depletion region As more O2- are formed, the energy bands bend at the surface of the semiconductor because of the local field that is built up.

Metal/Metal B A B A E contact potential Transfer of electron until EF are the same for both of metals A potential difference known as the contact potential is set up between two metals. EF has to be the same in thermal equilibrium for whole system Although an internal field exists, no potential can be measured in an external circuit connecting the two metals together.

Metal/Semiconductor: Schottky Barrier ≡ electron affinity Eb - - ++ Wd Blocking contact Blocking contact if for n-type and for p-type A flow of electrons from the semiconductor to the metal in order to equalize the Fermi energies in the two materials. An internal field is developed in the semiconductor. : diffusion potential, built-in potential : Energy barrier

Metal/Semiconductor: Schottky Barrier The width of the depletion region Ionized donor density = ND+

Metal/Semiconductor: Schottky Barrier In the depletion region Wd, potential change If we include the effects of an applied voltage

Metal/Semiconductor: Schottky Barrier depletion region conductor dielectric nearly conductor slope → One method to measure

Metal/Semiconductor: Schottky Barrier P-type semiconductors - - Eb Wd Schottky barrier of hole If semiconductor is p-type, e- transferred from M → S h+ transferred from S → M In the depletion region (negatively charged depletion region),

Metal/Semiconductor: Schottky Barrier J V JV characteristics for Schottky barrier (n-type semiconductor) Thermionic emission +

Metal/Semiconductor: Schottky Barrier Metal + forward Metal – reverse J Φapp J0 Basic equation for current in junction ≡ J0 , not depend on the Φapp

Metal/Semiconductor: Schottky Barrier Rectifying Φapp logJ -J0 T

Junction vocabulary Depletion layer Depletion layer width Accumulation layer ~ carrier having added to the materials Band bending Surface states Contact potential ~ difference in work function Electron affinity ~ Junction capacitance Rectifying I-V for junction forward bias reverse bias

Junction formation Fermi level alignment throughout whole system in thermal equilibrium Schottky barrier n-type semiconductor transfer e- from semiconductor to metal p-type semiconductor transfer e- from metal to semiconductor Diffusion potential Δ(work function)

Junction formation For n-type, Eb J J0 Φapp Metal – reverse ++ Metal + forward Metal – reverse J Φapp J0 for thermionic emission

Ohmic contact n-type: p-type: The accumulation layer in the semiconductor serves as a ready reservoir of electrons for conduction in the material available at the contact, and thus application of an electric field measures only the conductivity of the semiconductor. J V R n-type: p-type: (opposite of Schottky barrier)

Ohmic contact Accumulation region (reservoir of e-) e- n-semiconductor metal Transfer of e- from metal to semiconductor Electrons flow without any barrier contact does not make any change in J. Drift current = diffusion current

Ohmic contact: electric potential

Ohmic contact - + n-type Virtual cathode Extra electrons (source of carriers) Ohmic contacts when electric field is applied. For sufficiently high applied voltage, however, injection of electrons from the contact may dominate carriers in the material, charge neutrality in the material is violated, and one enters into a new regime of space charge limited current with . Critical voltage of SCLC Transit time through the material  dielectric relaxation time

p-n junction - homojunctions p-type n-type h+ e- Field free p-type Field free n-type Wp Wn W = Wp+ Wn Similar to two Schottky barriers

p-n junction - homojunctions When we apply Φapp, Φapp = 0 Φapp = Φapp Minority np y y Majority carrier Majority Minority x x pn Majority carriers ~ e- in n-type, h+ in p-type Minority carriers ~ h+ in n-type, e- in p-type

p-n junction - homojunctions Hole density in n-type material Diffusion length for hole Lifetime of hole Diffusion controlled flow

p-n junction - homojunctions Φapp J0 Same form as the J vs. Φapp dependence of a Schottky barrier, but the pre-exponential reverse saturation current J0 has a different definition.

Applications of p-n junctions Rectifier Because of asymmetry in J-V curve Amplifier Applying small signal to get large signal source collector larger modulator (small) p-n-p type transistor

Application of p-n junctions p-n-p type transistor n-type base is very narrow, only a fraction of a hole diffusion length; the collector current is nearly equal to the emitter current V2 > V1 IV2 > IV1  Small V1 results in large emitter collector current FET (High resistance) IV1 IV2 large current collector source Small current

Application of p-n junctions Photodetector: operate under reverse bias Increase of current by light - + light electron/hole pair

Application of p-n junctions Phototransistor Photoexcitation is used to produce an electron-hole pair with a diffusion length of the p-n2 junction. The electron is collected but the hole remains in the base until it diffuses out or until recombination occurs. Many electrons transit per photo-excited hole flow before the hole is lost out of the base across the junction forward reverse light

Application of p-n junctions Solar cell: no applied bias - + light electron/hole pair Open circuit voltage O Short circuit current The band gap of about 1.4 eV proves to be optimal for solar energy conversion application.

Application of p-n junctions Light emitter: Forward biased p-n junction Direct band gap materials are favored for this application. GaAsP (R), InGaN (B), GaN (B) By coating a blue LED with a phosphor material, a portion of the blue light can be converted to green, yellow and red light to get white light. Laser possible - + light electron/hole pair + - LED, Laser

Application of p-n junctions Tunnel diode A p-n junction formed between a degenerate p-type material and a degenerate n-type material. The maximum current flows when the occupied states in the n-type material are energetically exactly equal to the unoccupied states in the p- type material.

Semiconductor-semiconductor junctions: Heterojunctions A positive value of or a negative value of implies a spike impeding the transport of electrons or holes, respectively. An isotope heterojunction between two n-type materials with the same electron density, but showing a discontinuity at the interface because of a difference in electron affinities.

Semiconductor-semiconductor junctions: Heterojunctions Energy band diagrams for p-n heterojunctions. The materials in (a) and (b) have the same band gaps, but in (a) the p-type material has a smaller electron affinity than the n-type material, whereas in (b) the situation is reversed.

Quantum wells and superlattices Avalanche photodiode Avalanche photodiode using superlattices with electron ionization but no hole ionization to minimize noise. Noise is caused by feedback between slight fluctuations in the gain due to multiplication by one carrier as these are then amplified by changes in the multiplication due to the other carrier.

Quantum wells and superlattices Tunneling If the allowed levels in the quantum well formed by the low band gap material are limited to discrete values, however, resonant tunneling can occur only for specific electron energies. negative resistance regions In superlattices with relatively thick barriers, sequential tunneling can occur in which electrons tunnel from the ground state in one well to an excited state in the next, where they then relax to the ground state and continue the process.

Quantum wells and superlattices Effective mass filter and formation of a miniband A superlattice can act as an effective mass filter, making it possible to localize holes in a superlattice, for example, while permitting tunneling transport of electrons. Photoexcited holes remain localized in the quantum wells since their tunneling probability is small because of their larger effective mass. The transport of electrons by tunneling through the thin barriers can be described as the formation of a miniband.