Lec_23 - EE143 F2010 Lecture 23 Metal-Oxide-Semiconductor...

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Professor N Cheung, U.C. Berkeley Lecture 23 EE143 F2010 1 Negligible electron concentration underneath Gate region; Source-Drain is electrically open High electron concentration underneath Gate region; Source-Drain is electrically connected V G < V threshold V G > V threshold Metal -Oxide-Semiconductor Transistor [ n-channel]
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Professor N Cheung, U.C. Berkeley Lecture 23 EE143 F2010 2 MOSFET I-V Analysis n + n + V S V G W V B=0 V D I D L Q n N-MOSFET In general, inversion charge Qn ( [V G -V T ]) decreases from Source toward Drain because channel potential V C increases. V T increases
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Professor N Cheung, U.C. Berkeley Lecture 23 EE143 F2010 3 Let V T defined to be threshold voltage at Source   2 V V V C ) average ( V V C ) ( Q 2 V V ~ ) ( V DS T G OX T G n DS T T [ This is an approximation ] I D = W t (-q n v drift ) = W Q n v drift Inversion layer thickness Inversion layer concentration Approximate Analysis Note: I D is constant for all positions along channel
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Professor N Cheung, U.C. Berkeley Lecture 23 EE143 F2010 4 L V E v With DS n n drift T G OX D V 2 V V V C L W I V DS I D Linear with V DS Quadratic with V DS
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Professor N Cheung, U.C. Berkeley Lecture 23 EE143 F2010 5 V D saturation n + n + V S=0 V D Q n =0 at the drain Lateral E-field   Electrons moves saturation velocity V Dsat is defined to be the value of V D with Q n =0 at drain. From Q n = C ox (V G -V T -V D ), we get V Dsat =V G -V T
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Professor N Cheung, U.C. Berkeley Lecture 23 EE143 F2010 6
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Professor N Cheung, U.C. Berkeley Lecture 23 EE143 F2010 7 V D I D
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Professor N Cheung, U.C. Berkeley Lecture 23 EE143 F2010 8 DS T G OX n D V V V V C L W I 2 MOSFET I-V Characteristics Summary For V D < V Dsat   2 2 T G n Dsat D V V C L W I I For V D > V Dsat Note: V Dsat = V G - V T
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Professor N Cheung, U.C. Berkeley Lecture 23 EE143 F2010 9 E x SiO2 inversion layer Mobility of inversion charge carriers *Carrier will experience additional scattering at the Si/SiO2 interface *Channel mobility is lower than bulk mobility * (effective) is extracted from MOSFET I-V characteristics * Typically ~0.5 of (bulk)
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Professor N Cheung, U.C. Berkeley Lecture 23 EE143 F2010 I D vs. V DS Characteristics The MOSFET I D - V DS curve consists of two regions:
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Lec_23 - EE143 F2010 Lecture 23 Metal-Oxide-Semiconductor...

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