Chapter 4.1 Metal-semiconductor (MS) junctions

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Presentation transcript:

Chapter 4.1 Metal-semiconductor (MS) junctions Many of the properties of pn junctions can be realized by forming an appropriate metal-semiconductor rectifying contact (Schottky contact) Simple to fabricate Switching speed is much higher than that of p-n junction diodes Metal-Semiconductor junctions are also used as ohmic-contact to carry current into and out of the semiconductor device

Diode Layout

Ideal MS contacts Vbi=m-s

MS contacts Vacuum level, E0 - corresponds to energy of free electrons. The difference between vacuum level and Fermi-level is called workfunction,  of materials. Workfunction, M is an invariant property of metal. It is the minimum energy required to free up electrons from metal. (3.66 eV for Mg, 5.15eV for Ni etc.) The semiconductor workfunction, s, depends on the doping. where  = (E0 – EC)|SURFACE is a a fundamental property of the semiconductor. (Example:  = 4.0 eV, 4.03 eV and 4.07 eV for Ge, Si and GaAs respectively)

Energy band diagrams for ideal MS contacts (a) and (c) An instant after contact formation (b) and (d) under equilibrium conditions M > S M < S

MS (n-type) contact with M > S Soon after the contact formation, electrons will begin to flow from S to M near junction. Creates surface depletion layer, and hence a built-in electric field (similar to p+-n junction). Under equilibrium, net flow of carriers will be zero, and Fermi-level will be constant. A barrier B forms for electron flow from M to S. B = M –  ... ideal MS (n-type) contact. B is called “barrier height”. Electrons in semiconductor will encounter an energy barrier equal to M – S while flowing from S to M.

MS (n-type) contact with M > S Response to applied bias for n-type semiconductor Note: An applied positive voltage lowers the band since energy bands are drawn with respect to electron energy.

MS (n-type) contact with  M < S No barrier for electron flow from S to M. So, even a small V > 0 results in large current. As drawn, small barrier exists for electron flow from M to S, but vanishes when V< 0 is applied to the metal. The MS(n-type) contact when M < S behaves like an ohmic contact. VA I

Table 4.1 Electrical nature of ideal MS contacts

MOSFET作业有一n沟道MOSFET，衬底掺杂浓度NA=51015cm-3，栅金属为Al，作为栅绝缘体的SiO2膜厚度为100nm，并具有QOX=11010cm-2。试计算该MOS晶体管阈电压是多少伏？该MOS晶体管是耗尽型还是增强型？（已知：0=8.85410-14F/cm， Si=11.8， SiO2=3.9，MS=-0.8V）