454Fall07HW2_Solution

454Fall07HW2_Solution - ECEN 454 Digital Integrated Circuit...

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Fall 2007 Homework 2 Solutions 1. (10 points) In the textbook and the lecture notes, three cases of external bias for MOS structure with P-type substrate are described: accumulation, depletion and inversion. Please describe these three cases for MOS with N-type substrate. Please draw the energy band diagrams and provide corresponding justifications. Answer : Shown above is the diagram for Accumulation . Since V > 0, the electrons from the n type substrate are attracted towards the silicon / oxide interface. The electric field points away from the gate which leads to the shown bending of the bands. Since the applied positive voltage elevates the electrostatic potential of the metal relative to the semiconductor, the electron energies decrease in the metal relative to the semiconductor. As a result, the Fermi Level for the metal E FM lies below the equilibrium position by qV G , where V G is the applied voltage. E
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454Fall07HW2_Solution - ECEN 454 Digital Integrated Circuit...

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