EE 261 Krish Chakrabarty 1 MOS Transistor Theory • So far, we have viewed a MOS transistor as an ideal switch (digital operation) – Reality: less than ideal EE 261 Krish Chakrabarty 2 Introduction • So far, we have treated transistors as ideal switches • An ON transistor passes a finite amount of current – Depends on terminal voltages – Derive current-voltage (I-V) relationships • Transistor gate, source, drain all have capacitance – I = C (∆V/∆t) -> ∆t = (C/I) ∆V – Capacitance and current determine speed • Also explore what a “degraded level” really means
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MOS Transistor Theorypeople.ee.duke.edu/~krish/teaching/Lectures/MOS-theory... · 2004-08-29 · 2 EE 261 Krish Chakrabarty 3 MOS Transistor Theory • Study conducting channel between
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1
EE 261 Krish Chakrabarty 1
MOS Transistor Theory
• So far, we have viewed a MOS transistor as an ideal switch (digital operation)– Reality: less than ideal
EE 261 Krish Chakrabarty 2
Introduction• So far, we have treated transistors as ideal
switches• An ON transistor passes a finite amount of current
– Depends on terminal voltages– Derive current-voltage (I-V) relationships
• Transistor gate, source, drain all have capacitance– I = C (∆V/∆t) -> ∆t = (C/I) ∆V– Capacitance and current determine speed
• Also explore what a “degraded level” really means
2
EE 261 Krish Chakrabarty 3
MOS Transistor Theory
• Study conducting channel between source and drain• Modulated by voltage applied to the gate (voltage-controlled device)• nMOS transistor: majority carriers are electrons (greater mobility), p-substrate doped (positively doped)• pMOS transistor: majority carriers are holes (less mobility), n-substrate (negatively doped)
EE 261 Krish Chakrabarty 4
Terminal Voltages• Mode of operation depends on Vg, Vd, Vs
– Vgs = Vg – Vs
– Vgd = Vg – Vd
– Vds = Vd – Vs = Vgs - Vgd
• Source and drain are symmetric diffusion terminals– By convention, source is terminal at lower voltage– Hence Vds ≥ 0
• nMOS body is grounded. First assume source is 0 too.• Three regions of operation
– Cutoff– Linear– Saturation
Vg
Vs Vd
VgdVgs
Vds+-
+
-
+
-
3
EE 261 Krish Chakrabarty 5
Gate Biasing
p-substrate
n+ n+
Source Gate Drain
Channel
+ -
E
SiO2
VSS (Gnd)
• Vgs=0: no current flows fromsource to drain (insulated by two reverse biased pn junctions
• Vgs>0: electric field created across substrate
• Electrons accumulate under gate: region changes from p-typeto n-type
• Conduction path between source and drain
EE 261 Krish Chakrabarty 6
nMOS Device Behavior
Vgs << Vt
Polysilicon gatep-substrate
Accumulationmode
• Enhancement-mode transistor: Conducts when gate biasVgs > Vt
• Depletion-mode transistor: Conducts when gate bias is zero
Vgs = Vt
Depletion mode
Depletion region
Oxide insulator
Vgs > Vt
Inversion mode
Depletion region
InversionRegion(n-type)
4
EE 261 Krish Chakrabarty 7
nMOS Cutoff
• No channel• Ids = 0
+-
Vgs = 0
n+ n+
+-
Vgd
p-type body
b
g
s d
EE 261 Krish Chakrabarty 8
nMOS Linear• Channel forms• Current flows from d to s
– e- from s to d
• Ids increases with Vds
• Similar to linear resistor
+-
Vgs > Vt
n+ n+
+-
Vgd = Vgs
+-
Vgs > Vt
n+ n+
+-
Vgs > Vgd > Vt
Vds = 0
0 < Vds < Vgs-Vt
p-type body
p-type body
b
g
s d
b
g
s d Ids
5
EE 261 Krish Chakrabarty 9
nMOS Saturation
• Channel pinches off• Ids independent of Vds
• We say current saturates• Similar to current source
+-
Vgs > Vt
n+ n+
+-
Vgd < Vt
Vds > Vgs-Vt
p-type bodyb
g
s d Ids
EE 261 Krish Chakrabarty 10
I-V Characteristics
• In linear region, Ids depends on– How much charge is in the channel?– How fast is the charge moving?
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EE 261 Krish Chakrabarty 11
Channel Charge
• MOS structure looks like parallel plate capacitor while operating in inversion– Gate – oxide – channel
• Qchannel =
n+ n+
p-type body
+
Vgd
gate
+ +source
-
Vgs
-drain
Vds
channel-
Vg
Vs Vd
Cg
n+ n+
p-type body
W
L
tox
SiO2 gate oxide(good insulator, εox = 3.9)
polysilicongate
EE 261 Krish Chakrabarty 12
Channel Charge
• MOS structure looks like parallel plate capacitor while operating in inversion– Gate – oxide – channel
• Qchannel = CV• C =
n+ n+
p-type body
+
Vgd
gate
+ +source
-
Vgs
-drain
Vds
channel-
Vg
Vs Vd
Cg
n+ n+
p-type body
W
L
tox
SiO2 gate oxide(good insulator, εox = 3.9)
polysilicongate
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EE 261 Krish Chakrabarty 13
Channel Charge• MOS structure looks like parallel plate capacitor
while operating in inversion– Gate – oxide – channel
• Qchannel = CV• C = Cg = εoxWL/tox = CoxWL• V =
n+ n+
p-type body
+
Vgd
gate
+ +source
-
Vgs
-drain
Vds
channel-
Vg
Vs Vd
Cg
n+ n+
p-type body
W
L
tox
SiO2 gate oxide(good insulator, εox = 3.9)
polysilicongate
Cox = εox / tox
EE 261 Krish Chakrabarty 14
Channel Charge• MOS structure looks like parallel plate capacitor while