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Metal-Semiconductor Junctions: Ohmic Contact and Schottky Junction

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ghpter U wievE iwsgyxh gy t xgsyx wetlEsemiondutor juntions re ritil omp onent of miro eletronisF he following gures 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 – eφ = eV eφ eφ + m s bi m b ++ – E – c EFm EFs Ev (c) W METAL-SEMICONDUCTOR JUNCTION AT EQUILIBRIUM © Prof. Jasprit Singh www.eecs.umich.edu/~singh SCHOTTKY JUNCTION ON A P-TYPE SEMICONDUCTOR 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 – e m = eVbi W (b) © Prof. Jasprit Singh www.eecs.umich.edu/~singh SCHOTTKY JUNCTION IN REAL SYSTEMS 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 – e ο Ec eφο EFm EFs Ev 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. © Prof. Jasprit Singh www.eecs.umich.edu/~singh CURRENT FLOW IN A SCHOTTKY DIODE • 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. – + V – V+ Electron flow e(Vbi – V) E φ φ χ c φ χ e b = e( m – ) e( m – ) 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 –1 ( kB T ) 2 φ m*ekB –e b Is = A T2 exp ()2π2h3 ( kB T ) φ –e b = A R* T2 exp ( kB T ) m* Richardson constant: R* = 120 –2 –2 mo Acm K © Prof. Jasprit Singh www.eecs.umich.edu/~singh SMALL SIGNAL MODEL OF A SCHOTTKY DIODE 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 Cgeom R L Cd s s Rd Depletion capacitance: ε 1/2 eNd Cd = A 2(Vbi–V) Diode resistance: dV eI Rd = = dI kBT © Prof. Jasprit Singh www.eecs.umich.edu/~singh A COMPARISON BETWEEN THE PROPERTIES OF A P-N AND A SCHOTTKY DIODE 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 © Prof. Jasprit Singh www.eecs.umich.edu/~singh METAL-SEMICONDUCTOR JUNCTIONS: OHMIC CONTACT AND SCHOTTKY JUNCTION 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 © Prof. Jasprit Singh www.eecs.umich.edu/~singh INTERCONNECT DELAY: GOING FROM AL TO CU 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 15 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. © Prof. Jasprit Singh www.eecs.umich.edu/~singh.
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