ECE 663Metal-Semiconductor InterfacesMetal-Semiconductor contactSchottky Barrier/DiodeOhmic ContactsMESFET

ECE 663

ECE 663Device Building BlocksSchottky (MS)p-n junctionHBTMOS

ECE 663

ECE 663Energy band diagram of an isolated metal adjacent to an isolated n-type semiconductorq(fs-c) = EC EF = kTln(NC/ND) for n-type = EG kTln(Nv/NA) for p-type

ECE 663

ECE 663Energy band diagram of a metal-n semiconductor contact in thermal equilibrium.qfBn = qfms + kTln(NC/ND)

ECE 663

ECE 663

Measured barrier height fms for metal-Si and metal-GaAs contactsTheory still evolving (see review article by Tung)

ECE 663

ECE 663Energy band diagrams of metal n-type and p-type semiconductors under thermal equilibrium

ECE 663

ECE 663Energy band diagrams of metal n-type and p-type semiconductors under forward bias

ECE 663

Energy band diagrams of metal n-type and p-type semiconductors under reverse biasECE 663

ECE 663

ECE 663Charge distributionelectric-field distribution(Vbi-V) = - E(x)dx = qNDW2/Kse00WVbi = fms (Doping does not matter!)fBn = fms + kTln(NC/ND)

ECE 663

ECE 663Depletionq

ECE 663

ECE 663CapacitancePer unit area:Rearranging:Or:

ECE 663

ECE 6631/C2 versus applied voltage for W-Si and W-GaAs diodes

ECE 663

ECE 6631/C2vs VIf straight line constant doping profile slope = doping concentrationIf not straight line, can be used to find profileIntercept = Vbi can be used to find Bn

ECE 663

ECE 663Current transport by the thermionic emission processThermal equilibriumforward biasreverse biasJ = Jsm(V) Jms(V) Jms(V) = Jms(0) = Jsm(0)

ECE 663

Barrier from metal side is pinned

Els from metal must jump over barrier

Current is limited by speed of jumping electrons (that the ones jumping from the right cancel at equilibrium)

Unipolar majority carrier device, since valence band is entirely inside metal band

Note the difference with p-n junctions!! Barrier is not pinned

Els with zero kinetic energy can slide down negative barrier to initiate current

Current is limited by how fast minority carriers can be removed (diffusion rate)

Both el and hole currents important (charges X-over and become min. carriers) In both cases, were modulating the population of backflowing electrons, hence the Shockley form, but V > 0V < 0V > 0V < 0

ECE 663Lets roll up our sleeves and do the algebra !!Jsm = 2qf(Ek-EF)vxvx > vmin,vy,vz= 2q Ek-EF = (Ek-EC) + (EC -EF) EC - EF = q(fBn-Vbi)Ek - EC = m(vx2 + vy2 + vz2 )/2m*vmin2/2 = q(Vbi V)kx,y,z = m*vx,y,z/V > 0Vbi - V

ECE 663

ECE 663This meansx e-q(fBn-Vbi)/kT

= qm*k2T2/2p23e-q(fBn-V)kT = A*T2e-q(fBn-V)kT A* = 4pm*qk2/h3 = 120 A/cm2/K2

ECE 663

J = A*T2e-qfBN/kT(eqV/kT-1)

ECE 663

In regular pn junctions, charge needs to move throughdrift-diffusion, and get whisked away by RG processes

MS junctions are majority carrier devices, and RG is notas critical. Charges that go over a barrier already have high velocity, and these continue with those velocities togive the current

ECE 663

Forward current density vs applied voltage of W-Si and W-GaAs diodes

ECE 663

ECE 663Thermionic Emission over the barrier low doping

ECE 663

ECE 663Tunneling through the barrier high dopingSchottky barrier becomes Ohmic !!

ECE 663

ECE 663

ECE 663

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