Lect.6.Intrinsic.Cap

Dutton b murmann ee114214a 14 t r a n s is t o r in t

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Unformatted text preview: ive regions – T r a n s is to r " o ff" • Sub-threshold operation R. Dutton, B. Murmann EE114/214A 14 T r a n s is t o r in T r io d e R e g io n Electrons respond to gate via the S and D; this effectively “short out” the effects of CCB • G a te te r m in a l a n d c o n d u c tiv e c h a n n e l fo r m a p a r a lle l p la te capacitor across gate oxide CGC= WLεox/tox= WLCox – We can approximately model this using lumped capacitors of size ½ CGC each from gate-source and gate-drain • Changing either voltage will change the channel charge • T h e d e p l e t i o n c a p a c i t a n c e CC B a d d s e x t r a c a p a c i t a n c e f r o m d r a in a n d s o u r c e to s u b s tr a te – U s u a lly n e g lig ib le ( s e e a b o v e c o m m e n t) R. Dutton, B. Murmann EE114/214A 15 T r a n s is t o r in S a t u r a t io n R e g io n • A s s u m in g a lo n g c h a n n e l m o d e l, if w e c h a n g e th e s o u r c e v o lta g e – T h e v o lta g e d iffe r e n c e b e tw e e n th e g a te a n d c h a n n e l a t th e drain end remains at Vt, but the voltage at the source end changes – T h is m e a n s th a t th e " b o tto m p la te " o f th e c a p a c ito r d o e s n o t c h a n g e u n ifo r m ly • Detailed analysis shows that in this case Cgs=2/3WLCox – F o r y o u r w e e k e n d a m u s e m e n t y o u c a n d e r iv e th is ; w r ite Q(V(y)) and integrate 0->L (review Lecture 2, Slide 7) • In th e lo n g c h a n...
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