MIT6_012S09_lec07

MIT6_012S09_lec07 - Lecture 7 PN Junction and MOS...

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Unformatted text preview: 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 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 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 2. Depletion regime For V GB >0, metal attracts electrons and repels holes ⇒ Depletion region widens For V GB <0, metal repels electrons and attracts holes ⇒ Depletion region shrinks 6.012 Spring 2009 Lecture 7 4 In...
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This note was uploaded on 11/07/2011 for the course COMPUTERSC 6.012 taught by Professor Charlesg.sodini during the Spring '09 term at MIT.

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MIT6_012S09_lec07 - Lecture 7 PN Junction and MOS...

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