15+MOSFET - Review: MOS-C EC M O S Ef EV Consider special...

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Review: MOS-C M O S E f E C E V Consider special case: φ M = φ S Ideal MOS Structure
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Review: Inversion ρ ()( ) [] surface E bulk E q i i S = 1 φ () F i F E bulk E q = 1 MO Onset of Inversion: S q φ F q φ s F S 2 = qN W S εφ 2 = Depletion region width: qN W F T ε 4 = Maximum depletion width:
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Review: Gate Voltage Relation Include voltage drop across oxide Apply electrostatic boundary conditions OS q φ F q φ S S ox ox S S G x V E ε φ+ = M S S S qN φ 2 = E E x d S ox ox E x ox ox E x ox ox S ox S S S G S S ox ox S G C Q C N q V N q x V + = + = + = 2 2 ox V S G V x
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Review: C-V Characteristics MOS: 2 capacitors in series Gate SiO 2 Si C ox C S 1 1 1 + = + = S ox S ox S ox C C C C C C C + + = ox s T ox ox ox ox s ox ox ox x W C C x W C C C ε 1 1 Acc. Depl. Inv. ( ω Æ 0) Inv. ( ω Æ )
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EECS 320 MOSFET Operation
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MOSFET Operation: Qualitative Control current flow between source/drain by gate potential |Q ch | (logscale) Accumulation Flat Band Depletion Inversion 0 φ S
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Band Diagrams
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Band Diagrams Under Bias y
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MOSFET, OFF State 0<V GS <V T (depletion) V GS <0 (accumulation) Device is off for V GS < V T Threshold voltage, V T , is the V G at the onset of inversion. ) 2 ( 2 2 F A S ox ox F T N q x V φ ε + = N-MOSFET: (p-bulk) ) 2 ( 2 2 F D S ox ox F T N q x V = P-MOSFET: (n-bulk)
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MOSFET, Linear Region V GS > V T V DS =0 V DS >0, small Low V DS Channel resistance determined by inversion charge, controlled by V GS
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MOSFET, Linear Region V GS > V T V DS >0 Depletion width and inversion charge varies along channel at larger V DS
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MOSFET, Pinch-Off V GS > V T V DS = V sat Channel pinches off at larger V DS (V DS sat ) Why can we still get a current after pinch-off?
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Qualitative I-V Characteristics Gate I D V DS V GS Source Drain n + n + Depletion Inversion layer p-type I D V DS V GS linear saturation V DS sat Gate Source Drain n + n + Depletion Inversion layer Pinch off p-type
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I-V Characteristics – Square Law Theory L DS DS T GS ox n D V V V V C L Z I = 2 µ Calculate I D Determining Channel Charge (Drift current only) SD Z G Gate Source Drain N A n + n + y x () = DS V T GS ox n D dV V V V C L Z I 0 ) ( ) ( y v y ZQ I n D = dy y dV y y v n n ) ( ) ( ) ( = = E ) ( y dV Q Z dy I n n D = ) ( y V V V C Q T GS ox n = Includes variation of inversion layer charge with V DS
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I-V Characteristics – Saturation Linear Saturation DS
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15+MOSFET - Review: MOS-C EC M O S Ef EV Consider special...

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