lecture_15_first_posting

lecture_15_first_posting - Lecture 15 MOS Transistor...

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MOS Transistor Sub-threshold operation (EKV model) Small signal model Reading Assignment Sedra and Smith Chap 4 Howe Chap 4 Handout on MOS (see blackboard) Lecture 15
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Subthreshold operation and EKV model Up to this point we have viewed the threshold point as a sharp change in operating conditions ie. the transistor is either inverted or it is not In reality the transistor continuously changes it’s state of conduction In recognition of this we divide the operating regions into regions of strong and weak inversion
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( 29 ( 29 x C x C C x Q notes posted see terms g rearrangin and ating Differenti C Q Q Q Q t then E E ce e ch mobile is Q Q Q E and t E where V V know we as s s dep ox m ox m dep m dep ox ox ox Si ox ox si m si m dep Si ox ox ox ox s FB G = + = + - = + - = = + = - = + = - ψ ψ ε ψ ε ε ε ψ ψ ψ ) ( sin arg
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( 29 ( 29 ( 29 m I Q T m I m Q T m L m t m m s m L m t s m m t s m m t s m m t m diff s m x m drift R D F s dQ U C Q S dQ U C Q S I dx x Q U x Q Q Q L W I dx x Q U x Q L W I g Integratin x Q U x Q W x Q U W x Q W x I relation Einstein g u x Q U W WQ x D x I Idiff x Q W E WQ x I Idrift x Idiff x Idrift x I drift to addition in conduction diff consider now we new O O O O O O O O O O O O O O O O O O + - - + - = + - = + - = + - = + - = = = - = = + = 0 0 0 0 ) ( sin ) ( ) ( ) ( ) ( μ μ ψ μ ψ μ ψ μ μ ψ μ μ ψ μ μ
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dep ox m I Q T m I m Q T m C C C L W S dQ U C Q S dQ U C Q S I R D F s + = = + - - + - = , 0 0 e e e e e e e e e e e e e e e e e e μ μ Current is a function of two components forward and reverse The mobile charge at the source depends on the gate-bulk voltage and source-bulk voltage. The mobile charge at the drain depends on the drain-bulk voltage and the gate-bulk voltage When I F >>I R the channel current is independent of the drain voltage and the device is in saturation Expression is general and exact
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