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Lecture 16

# On eect is less than that of short l ee105 fall 2013

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Unformatted text preview: . EE105, Fall 2013 Lecture 16 – Slide 19 Prof. Ming C. Wu Diﬀerent Regions of Opera?on EE105, Fall 2013 Lecture 16 – Slide 20 Prof. Ming C. Wu 10 How to Determine ‘Region of Opera?on’ §༊  When the poten?al diﬀerence between gate and drain is greater than VTH, the MOSFET is in triode region. §༊  When the poten?al diﬀerence between gate and drain becomes equal to or less than VTH, the MOSFET enters satura?on region. EE105, Fall 2013 Lecture 16 – Slide 21 Prof. Ming C. Wu Triode or Satura?on? §༊  When the region of opera?on is not known, a region is assumed (with an intelligent guess). Then, the ﬁnal answer is checked against the assump?on. EE105, Fall 2013 Lecture 16 – Slide 22 Prof. Ming C. Wu 11 Channel- Length Modula?on 1 W 2 I D = µ nCox (VGS − VTH ) (1 + λVDS ) 2 L §༊  The original observa?on that the current is constant in the satura?on region is not quite correct. The end point of the channel actually moves toward the source as VD increases, increasing ID. Therefore, the current in the satura?on region is a weak func?on of the drain voltage. EE105, Fall 2013 Lecture 16 – Slide 23 Prof. Ming C. Wu λ and L §༊  Unlike the Early voltage in BJT, the channel- length modula?on factor can be controlled by the circuit designer. §༊  For long L, the channel- length modula?on eﬀect is less than that of short L. EE105, Fall 2013 Lecture 16 – Slide 24 Prof. Ming C. Wu 12 Transconductance g m = µnCox W (VGS − VTH ) L gm = 2µnCox W ID L gm = 2I D VGS − VTH §༊  Transconductance is a measure of how strong the drain current changes when the gate voltage changes. §༊  It has three diﬀerent expressions. Lecture 16 – Slide 25 EE105, Fall 2013 Prof. Ming C. Wu Velocity Satura?on I D = vsat ⋅ Q = vsat ⋅ WCox (VGS − VTH ) gm = ∂I D = vsatWCox ∂VGS §༊  Since the channel is very short, it does not take a very large drain voltage to velocity saturate the charge par?cles. §༊  In velocity satura?on, the drain current becomes a linear func?on of gate voltage, and gm becomes a func?on of W. EE105, Fall 2013 Lecture 16 – Slide 26 Prof. Ming C. Wu 13 Body Eﬀect VTH = VTH 0 + γ ( 2φF + VSB − 2φF ) §༊  As the source poten?al departs from the bulk poten?al, the threshold voltage changes. EE105, Fall 2013 Lecture 16 – Slide 27 Prof. Ming C. Wu Large- Signal Models §༊  Based on the value of VDS, MOSFET can be repres...
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