Introduction toMetal-Oxide-

SemiconductorField Effect Transistors

(MOSFETs)

Chapter 7, Anderson and Anderson

MOSFET• History• Structure• Future• Review• Threshold Voltage• I-V Characteristics• Modifications to I-V:

– Depletion layer correction (Sup. 3)– Mobility, Vsat– Short Channel Effects– Channel Length Modulation– Channel Quantum Effects

• MOSFET Scaling and Current Topics (Literature + Sup. 3)• Subthreshold Behavior• Damage and Temperature (Sup. 3)• Spice (Sup. 3)• HFET, MESFET, JFET, DRAM, CCD (Some in Sup. 3)

MOSFET History (Very Short!)• First Patents:

– 1935• Variable Capacitor Proposed:

– 1959• Silicon MOS:

– 1960• Clean PMOS, NMOS:

– Late 1960s, big growth!• CCDs:

– 1970s, Bell Labs• Switch to CMOS:

– 1980s

Structure: n-channel MOSFET(NMOS)

pn+n+

metal

LW

sourceS

gate: metal or heavily doped poly-Si G drain

Dbody

B

oxide

IG=0

ID=ISIS

x

y

(bulk or substrate)

MOSFET Future (One Part of)

• International Technology Roadmap for Semiconductors, 2008 update.

• Look at size, manufacturing technique.

From Intel

Structure: n-channel MOSFET(NMOS)

pn+n+

metal

LW

sourceS

gate: metal or heavily doped poly-Si G drain

Dbody

B

oxide

IG=0

ID=ISIS

x

y

(bulk or substrate)

MOSFET ScalingECE G201

Fin (30nm)

Gate

BOX

prevents “top” gate

Circuit Symbol (NMOS)enhancement-type: no channel at zero gate voltage

G

D

S

B (IB=0, should be reverse biased)

ID= IS

IS

IG= 0

G-GateD-DrainS-SourceB-Substrate or Body

Structure: n-channel MOSFET(NMOS)

pn+n+

metal

LW

sourceS

gate: metal or heavily doped poly-Si G drain

Dbody

B

oxide

IG=0

ID=ISIS

x

y

(bulk or substrate)

Energy bands

(“flat band” condition; not equilibrium) (equilibrium)

Flatbands! For this choice of materials, VGS<0 n+pn+ structure ID ~ 0

pn+n+

n++

LW

sourceS

gateG drain

Dbody

B

oxide

+-

VD=Vs

Flatbands < VGS < VT (Includes VGS=0 here). n+-depletion-n+ structure ID ~ 0

pn+n+

n++

LW

sourceS

gateG drain

Dbody

B

oxide

+-

+++

VD=Vs

VGS > VT n+-n-n+ structure inversion

pn+n+

n++

LW

sourceS

gateG drain

Dbody

B

oxide

+-

+++++++++

- - - - -

VD=Vs

VGS>VT

Channel Charge (Qch)

Qch

Depletion regioncharge (QB) is dueto uncovered acceptor ions

pn+n+

n++

L W

Ec(y) with VDS=0

(x)

Increasing VGS decreases EB

EB

y0 L

EF ~ EC

Triode Region A voltage-controlled resistor @small VDS

G

pn+n+

metal

S DB

oxide

+-

++++++

- - - -

VGS1>Vt

pn+n+

metal

S DB

oxide

+-

++++++

+++

- - - - - -

VGS2>VGS1

pn+n+

metal

S DB

oxide

+-

++++++

+++

- - - - - - - - -

VGS3>VGS2+++

ID

VDS

0.1 v

increasingVGS

Increasing VGS puts more charge in the channel, allowing more drain current to flow

cut-off

Saturation Regionoccurs at large VDS

pn+n+

metal

sourceS

gateG drain

Dbody

B

oxide

+-

+++++++++

VDS large

As the drain voltage increases, the difference in voltage between the drain and the gate becomes smaller. At some point, the difference is too small to maintain the channel near the drain pinch-off

Saturation Regionoccurs at large VDS

pn+n+

metal

sourceS

gateG drain

Dbody

B

oxide

+-

+++++++++

VDS large

The saturation region is when the MOSFET experiences pinch-off.

Pinch-off occurs when VG - VD is less than VT.

Saturation Regionoccurs at large VDS

pn+n+

metal

sourceS

gateG drain

Dbody

B

oxide

+-

+++++++++

VD>>Vs

VGS - VDS < VT or VGD <

VDS > VGS - VT

VT

Saturation Regiononce pinch-off occurs, there is no further increase in

drain currentID

VDS

0.1 v

increasingVGS

triode

saturation

VDS>VGS-VT

VDS<VGS-VT

Band diagram of triode and saturation

Simplified MOSFET I-V EquationsCut-off: VGS< VT

ID = IS = 0Triode: VGS>VT and VDS < VGS-VT

ID = kn’(W/L)[(VGS-VT)VDS - 1/2VDS

2]Saturation: VGS>VT and VDS > VGS-VT

ID = 1/2kn’(W/L)(VGS-VT)2

where kn’= (electron mobility)x(gate capacitance)

= n(ox/tox) …electron velocity = nE

and VT depends on the doping concentration and gate material used (…more details later)

Energy bands

(“flat band” condition; not equilibrium) (equilibrium)

VGS>VT

Channel Charge (Qch)

Qch

Depletion regioncharge (QB) is dueto uncovered acceptor ions

Threshold Voltage Definition

VGS = VT when the carrier concentration in the channel is equal to the carrier concentration in the bulk silicon.

Mathematically, this occurs when s=2f ,

where s is called the surface potential