Metal-Semiconductor Junctions: Ohmic Contact and Schottky Junction

Home , Depletion region, Ohmic contact, Schottky barrier

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provide n overview of hottky rrier dio desD ohmi onttsD nd interonnet dely issuesF BAND PROFILE OF A METAL AND SEMICONDUCTOR JUNCTION

Metal Work functions of some metals

Element Work function, φ (volt) n-type m semiconductor Ag, silver 4.26 Contact Al, aluminum 4.28 Au, gold 5.1 (a) Cr, chromium 4.5 Mo, molybdenum 4.6 Ni, nickel 5.15 Pd, palladium 5.12 Pt, platinum 5.65 Ti, titanium 4.33 W, tungsten 4.55

Electron affinity of some semiconductors

Element Electron affinity, χ (volt)

Ge, germanium 4.13 Si, silicon 4.01 GaAs, gallium arsenide 4.07 AlAs, aluminum arsenide 3.5 φ φ Band profiles of disconnected m > s n-type metal and semiconductor Vacuum energy ≈ ≈ φ eχ e s s φ e m Ec EFs

EFm

Ev n-semiconductor (b) Metal

Vacuum Formation of a Schottky Energy χ junction e s

+ + eφ Ð eφ = eV eφ eφ + m s bi m b ++ Ð E Ð c EFm EFs

METAL-SEMICONDUCTOR JUNCTION AT EQUILIBRIUM

p-type E Metal and semiconductor band vac ≈ ≈ φ χ profiles e s e s eφ m Ecs

EFm EFs Evs Metal Semiconductor

(a)

Ec Formation of a Schottky junction for p-type materials p-semiconductor + + Metal + EFs AAAAAAAAAAeφ AAAAA Ev b φ φ AAAAAe s Ð e m = eVbi

(b)

Semiconductor surfaces have a large number of defect states (from broken bands, impurities, etc.) Defect levels in the bandgap at the metal-semiconductor interface φ φ e b = Eg Ð e ο Ec eφο EFm EFs

SCHOTTKY METAL n Si p Si n GaAs

Aluminum, Al 0.7 0.8

Titanium, Ti 0.5 0.61

Tungsten, W 0.67

Gold, Au 0.79 0.25 0.9

Silver, Ag 0.88

Platinum, Pt 0.86

PtSi 0.85 0.2

NiSi2 0.7 0.45

Schottky barrier heights are determined by the semiconductor and have a rather weak dependence on the metal.

¥ Metal-to-semiconductor barrier is unchanged by external bias ¥ Semiconductor-to-metal barrier is increased (reverse bias) or decreased (forward bias) by an external bias.

Ð + V Ð V+ Electron flow

e(Vbi Ð V) E φ φ χ c φ χ e b = e( m Ð ) e( m Ð ) E eV Fs E E Fm Fm e(Vbi + V)

Ev Ec Forward bias EFs (a)

Ev Reverse bias (b) I Current dominated by electron flow from the semiconductor to the metal

Current dominated by electron V flow from the metal to the semiconductor (c) Diode with area A: eV I = Is exp Ð1 ( kB T ) 2 φ m*ekB Ðe b Is = A T2 exp ()2π2h3 ( kB T ) φ Ðe b = A R* T2 exp ( kB T ) m* Richardson constant: R* = 120 Ð2 Ð2 mo Acm K

The Schottky diode is a majority carrier device. Unlike a p-n diode, in forward bias no minority carrier injection occurs. Thus there is no diffusion capacitance and the device response can be very fast.

Equivalent circuit of a diode in series with a resistor and inductor

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R L Cd s s

Depletion capacitance: ε 1/2 eNd Cd = A 2(VbiÐV)

Diode resistance:

dV eI Rd = = dI kBT

p-n DIODE SCHOTTKY DIODE

Reverse current due to minority Reverse current due to majority carriers diffusing to the depletion carriers that overcome the barrier layer strong temperature less temperature dependence dependence

Forward current due to minority Forward current due to majority carrier injection from n- and p-sides injection from the semiconductor

Forward bias needed to make the The cut-in voltage is quite small device conducting (the cut-in voltage) is large

Switching speed controlled by Switching speed controlled by recombination (elimination) of thermalization of "hot" injected minority injected carriers electrons across the barrier ~ few picoseconds

Ideality factor in I-V characteristics Essentially no recombination in ~ 1.2-2.0 due to recombination in depletion region ideality factor depletion region ~ 1.0

OHMIC CONTACT Current is linear in an ohmic contact resistance is very small

Schottky

barrier

URRENT C

VOLTAGE

OHMIC CONTACT

Heavy doping in the OHMIC CONTACT semiconductor causes a very thin depletion width Electrons tunnel and electrons can tunnel through narrow across this barrier leading depletion region to ohmic behavior e e Ec EF

n+ ETAL

M REGION n-type SEMICONDUCTOR Ev

As device dimensions shrink interconnect cross-sections also must shrink. This increases the interconnect resistance and the associated time delay for signal propagation.

TRANSISTOR GATE LENGTHS: 40 1999: 0.2 µm µ 35 2003: 0.1 m Interconnect delay for 30 Al-based interconnects

25 (ps)

20 Transistor delay DELAY

IME T 10 Interconnect delay for Cu-based interconnects 5

0.1 0.2 0.3 0.4 0.5 0.6 0.7

GATE LENGTH (µm) Based on Semiconductor Industry Associates Roadmap.