nmos n' pmos

8/8/2019 nmos n' pmos

1/20

Lecture 20-1

MOSFET transistor I-V characteristics

iD K 2 vGS Vt( )vDS vDS2

[ ]=

iD

vGS

vDS--

++

iD

K vGS

Vt

( )2

[ ] 1 vDS

+( )=

KW

2L------K

n=

Kn

Cox

n

=iD

K 2 vGS

Vt

( )vDS

[ ]=Linear region:

vDS

sat

vGS

Vt

=

vDS

vGS

Vt

Triode region:

vDS

vGS

Vt

0

VS1B=0

VBVS2B=0

VS1B=0

VS2B>0


4/20

Lecture 20-4

Body Effect

Positive VSB for NMOSFETs tends to increase QB, hence decrease QI, for a

fixed VGS

VB

VGS > Vt

+

n+n+ QB0

VS>0

QI

VDS > 0


5/20

Lecture 20-5

Body Effect

Modeled as a change in the threshold voltage as a function of VSB

The source is, by definition for NMOSFET, at a lower positive potential thanthe drain, which is why we use it as our reference voltage

Vt

Vt0

2f

VSB

+ 2f

( )+=

SPICE will calculate this variation in threshold voltage, or you can over-rideits calculation by directly specifying gamma


6/20

Lecture 20-6

Temperature Variations

The threshold voltage varies with temperature due to carrier generation in thesubstrate --- tends to decrease with increasing temperature

~2mV for every 1C increase

K also changes with temperature due to change in mobility

Tends to dominate temperature variation for large iD

Will iD increase or decrease with temperature?

Vt Vt0 2f VSB+ 2f( )+=

T1

T2 > T1

I1

2---

nC

ox

W

L----- v

GSV

t( )

2


7/20

Lecture 20-7

Where is drain, where is source?

S

D

GB

D

GB

S

n-channel transistor p-channel transistor


8/20

Lecture 20-8

PMOSFETs

All of the voltages are negative Carrier mobility is about half of what it is for n channels

p+

n

S DG

B

p+

The bulk is now connected to the most positive potential in the circuit

Strong inversion occurs when the channel becomes as p-type as it was n-type

The inversion layer is a positive charge that is sourced by the larger potential

and drained at the smallest potential The threshold voltage is negative for an enhancement PMOSFET

Note that the flatband voltage (which is negative) effects now tend toincrease the PFET threshold while they decreased the NFET threshold


9/20

Lecture 20-9

PMOS

The equations are the same, but all of the voltages are negative Triode region:

iD K 2 vGS Vt( ) vDS vDS2[ ]=

vGS

Vt

vDS

vGS

Vt

K 12---nCoxWL

-----= AV

2-------

iD is also negative --- positive charge flows into the drain

Saturation expression is the same as it is for NFETs:

iD

satK v

GSV

t( )

2[ ] 1 v

DS+( )=

+V dd


10/20

Lecture 20-10

PMOS

Characteristic appears to be the same, except that all of the voltages arenegative

-5 -4 -3 -2 -1 0

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

10

W=1 micronL=1 micronsV t0= -1 volt

Kp=2e-5 (A/v2)

phi =-0.6

ND=1e15

IDS

(A)

VDS

VGS=-2.5V

VGS=-2.0V

VGS=-1.5V

VGS=-1.0V

VGS=-3.0V


11/20

Lecture 20-11

PMOS

But it is generally displayed as:


Kp=2e-5 (A/v 2)phi =-0.6

ND=1e15

-IDS(A)

-VDS

VGS=-2.5V

VGS=-2.0V

VGS=-1.5V

VGS=-1.0V

VGS=-3.0V

0 1 2 3 4 5

-10

0

10

20

30

40

50

60

70

80

90

100


12/20

Lecture 20-12

Depletion Mode NMOSFET

Depletion mode FETs have a channel implanted such that there is conductionwith VGS=0

The operation is the same as the enhancement mode FET, but the thresholdvoltage is shifted

Vt is negative for depletion NMOS, and positive for depletion PMOS

VGS

n+n+

VS VDS

n+

p


13/20

Lecture 20-13


Negative gate voltage is required to turn the channel off


Kp=2e-5 (A/v2

)

IDS(mA)

VDS

VGS

=1.0V

VGS=0.0V

VGS

=-1.0V

VGS=-2.0V

VGS=2.0V

0 1 2 3 4 5

0.0

0.2

0.4


14/20

Lecture 20-14


The iDS vs. vGS characteristic is still quadratic in saturation

W=1 micronL=1 micronsV t0= -2 voltKp=2e-5 (A/v

2)

IDS(m

A)

VGS

-4 -3 -2 -1 0 1 2 3 4 5

0

1

2


15/20

Lecture 20-15

Examples

Find the largest value that RD can have before the transistor fails to operate insaturation

5V

-5V

5k

RD

Vt

2V=

Kn

20A V2

=

L 10m=

W 400m=

0=


16/20

Lecture 20-16

Examples

Find the drain currents and voltages for both transistors

10V

10k 15k

Vt

2V=

Kn

20A V2

=

L 10m=

W 100m=

0=

10V

M2 M1


17/20

Lecture 20-17

Examples

What is the effective resistance of the transistor in the triode region?

10V

24.8k Vt 1V=

K 0.5m A V2=


18/20

Lecture 20-18

Examples

Select the Rs so that the gate voltage is 4V, the drain voltage is 4V and thecurrent is 1mA.

10V

RD

Vt

2V=

K 1m A V

2

= 0=

RS

10V

RG1

RG2


19/20

Lecture 20-19

Examples

Select the Rs so that the transistor is in saturation with a drain current of1.0mA and a drain voltage of 5V

Vt 1 V=K 0.5m A V

2=

0=

RD

10V

RG1

RG2


20/20

Lecture 20-20

Examples

Solve for the drain current and voltage

20V

32k

Vt

2 V=

K 1m A V2

=

0=

4k10M

Documents

nmos n' pmos