MIT6_012s09_lec07

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MIT OpenCourseWare http://ocw.mit.edu 6.012 Microelectronic Devices and Circuits Spring 2009 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms .
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Lecture 7 PN Junction and MOS Electrostatics(IV) Metal-Oxide-Semiconductor Structure (contd.) Outline 1. Overview of MOS electrostatics under bias 2. Depletion regime 3. Flatband 4. Accumulation regime 5. Threshold 6. Inversion regime Reading Assignment: Howe and Sodini, Chapter 3, Sections 3.8-3.9 6.012 Spring 2009 Lecture 7 1
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1. Overview of MOS electrostatics under bias Built-in potential across MOS structure increases from φ B to φ B + V GB Oxide forbids current flow J=0 everywhere in semiconductor Need drift = -diffusion in SCR Must maintain boundary condition at Si/SiO 2 interface E ox / E s 3 Application of bias: How can this be accommodated simultaneously? quasi-equilibrium situation with potential build-up across MOS equal to φ B + V GB 6.012 Spring 2009 Lecture 7 2
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Important consequence of quasi-equilibrium: Boltzmann relations apply in semiconductor [they were derived starting from J n = J p =0] n ( x ) = n i e q φ ( x ) kT q φ ( x ) kT p ( x ) = n i e and 2 np = n i at every x 6.012 Spring 2009 Lecture 7 3
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2. Depletion regime For V GB >0, metal attracts electrons and repels holes
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