PMOS NMOS Equations and Examples

Equations and Terms for nMOS and pMOS devices Term

nMOS

pMOS

D

S

Voltages and Currents +

_

VDS

_

+

VGS _

VSB

VGS

ID G

+

B

G

_

+

B

VSB

ID

+

+

VDS

S

VGS, VDS, VSB, VT Cut-off

Positive When VGS < VT ID=0

Negative When VGS > VT ID=0

Linear

When VGS ≥ VT

When VGS ≤ VT

VDS < VGS - VT Saturation

ID = kn [(VGS – VT)VDS – VDS2/2] When VGS ≥ VT

ID = kp [(VGS – VT)VDS – VDS2/2] When VGS ≤ VT

ID = (kn/2) [(VGS – VT) 2] (1 + VDS) kn = k’n W/L

ID = (kp/2) [(VGS – VT) 2] (1 + VDS) Kp = k’p W/L

Negative Negative

Positive Postitive

Positive

Negative

VDS ≥ VGS - VT

k Fermi potential F Depletion region charge densities QB0 and QB Substrate bias coefficient 

VDS > VGS - VT

k’n = nCox = nox/tox

VDS ≤ VGS - VT

k’p = pCox = pox/tox

Example: What is the value of the capacitor after the pulse. Assume:  VDD = 5V  VTN=1V  VTP=-1V

Case 1:

5V

Vout=5V

Vin

0V t=0

Case 2: VDD

5V

ID

0V t=0

Vin

Vout=0V

Case 3:

VDD

5V 0V t=0

Vin

Vout = 0V

Case 4:

5V

Vout= 5V

0V t=0

Vin