Slides-March17 - Metal-Semiconductor Contacts lCo • M-S contacts exist in every electronic or optoelectronic device • Barrier may be formed

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Unformatted text preview: Metal-Semiconductor Contacts lCo • M-S contacts exist in every electronic or optoelectronic device • Barrier may be formed when metal makes contact with a semiconductor – Responsible for controlling the current conduction as well as the capacitance behavior capacitance behavior – In some cases, we need Ohmic contacts (no contact resistance) – In some other cases, we need rectifying contacts (directional current conduction) Additional reading: S. M. Sze and K. K. Ng, Physics of Semiconductor Devices (3rd ed.), Chap. 3 S. M. Sze, Semiconductor Devices: Physics and Technology (2nd ed.), §7.1 #1 Energy Energy Level Diagram of a M-S Contact MBarrier height for electron injection from M to S qBn qm q Barrier height for hole injection from M to S (n-type) qVbi qm qS qBp q Eg qm qBn qBp Eg Built-in potential (total band bending) qVbi qm qS #2 Metal Metal Work Function #3 Charge and E-Field Distribution Char EMetal/n-type semiconductor contact Depletion width 2 s WD qND E x qND s kT Vbi q WD x Em EC x qBn qND x s q 2N D x2 WD x s 2 #4 M-S Contact Under Bias Depletion width qBp 2 s WD qND kT Vbi V q (a) Thermal equilibrium Capacitance CD (b) Forward bias s WD 2Vbi V kT / q 2 q s ND CD 1 (c) Reverse bias #5 Thermionic Thermionic Emission Theory qV Jn JTE exp 1 kT Effective Richardson constant qBn JTE A T exp kT 4qm * k 2 A* h3 *2 #6 Ohmic Ohmic Contact A M-S contact that has a negligible junction resistance relative to the total resistance of the semiconductor device 1 dJ RC dV V 0 Specific contact resistance From thermionic emission current expression: qBn qBn RC * exp exp A Tq kT kT k Ohmic contact requires negligible barrier height Alternatively, we may use degenerate doped semiconductor to form contact with metal: Very thin depletion layer width and strong built-in electric field at the contact ⇒ high tunneling probability ⇒ small contact resistance #7 Contact Contact Resistance Resistance for M-Si Contacts B ↓, RC ↓ ND ↑, RC ↓ #8 MOS FieldMOS Field-Effect Transistors Additional reading: S. M. Sze and K. K. Ng, Physics of Semiconductor Devices (3rd ed.), Chap. 6 Sze and Ng Physics of Semiconductor Devices (3 Chap S. M. Sze, Semiconductor Devices: Physics and Technology (2nd ed.), Chap. 6 #9 Energy Energy Band Diagram of an Ideal MOS Capacitor at Zero Bias Bi Assumptions: • No interface traps • No charge in oxide charge in oxide • No dc current through oxide • m = s #10 MOS MOS Capacitor under Bias Accumulation Depletion Inversion #11 Band Band Diagram At Semiconductor Surface Ei x Ei Potential x q q x n p x n p0 exp kT q x pp x pp0 exp kT S < 0 Accumulation of holes of holes S = 0 Flat-band condition Depletion of holes 0 < S < B S = B Fermi level at mid-gap B < S < 2B Weak inversion, np(0) > pp(0) S > 2B Strong inversion, np(0) > pp0 or NA #12 MOSFET MOSFET Output Characteristics (1) Low drain voltage – linear region VG > VT (threshold voltage): voltage): strong inversion in semiconductor under the gate electrode ⇒ formation of an n-channel in p-type substrate At low drain voltage (VD), ID is linear with VD, i.e. conductivity of the n-channel is not significantly perturbed by the drain-source voltages. W VD ID nCox VG VT VD 2 L #13 MOSFET MOSFET Output Characteristics (2) Onset of saturation: VD = VD,sat An increase in VD will raise the potential in the semiconductor esp. in regions close to the drain electrode; The thickness of strong inversion layer near drain will be reduced to zero when VD thi reaches VD,sat (called “pinch-off”). #14 MOSFET MOSFET Output Characteristics (3) Saturation region: VD > VD,sat Point-off point moves closer to the source electrode, or the channel length is reduced; Drain current remains constant (at point P, the voltage VD,sat remains the same, and current is limited by charge transport from point P to drain). ID,sat W nCox VG VT 2 2L #15 Nonlinear Nonlinear region Idealized Drain Characteristics Characteristics of of a MOSFET Linear region Saturation region Linear region ( VD VG VT ): ID W V nCox VG VT D VD L 2 Saturation region ( VD VD,sat ): ID,sat W nCox VG VT 2 2L #16 ...
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This note was uploaded on 11/16/2011 for the course EMA 3413 taught by Professor Xue during the Spring '11 term at University of Florida.

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