Transistor nMOS

1
Transistor nMOS

• Qchannel CV
• C Cg eoxWL/tox CoxWL
• V Vgc Vt (Vgs Vds/2) Vt
• v mE m (mobility)
• E Vds/L
• Time for carrier
• to cross channel
• t L / v

Cox eox / tox
2
nMOS Linear I-V

• Now we know
• How much charge Qchannel is in the channel
• How much time t each carrier takes to cross

3
nMOS Operation
Cutoff Linear Saturated
Vgs lt Vt Vgs gt Vt Vds lt VgsVt Vgs gt Vt Vds gt Vgs Vt
Ids ? 0 Ids ?(VgsVtVds/2)Vds Ids ?(VgsVt)2
4
Esempio

• 180 nm process
• W/L 4/2 l (360nm/180nm)
• tox40Å
• m 180 cm2/(V?s)
• Vt 0.4V
• ? m Cox W/L 180 (3.98.85 10-14
  F/cm)/(4010-8)
• 155 mA/V2
• Idsmax _at_ (Vgs1V) 155 mA/V2 (0.6 V)2 55 mA
• Ids(Vgs2V, Vds1V) 155 mA/V2 (1.61) 248 mA

5
I-V Characteristics
6
CMOS Inverter
substrate contact (p)
n-well contact (n)
n-well
polysilicon
diffusion contacts
n diffusions
polysilicon contacts
p diffusions
7
DC Transfer Curve

• For a given Vin
• Plot Idsn, Idsp vs. Vout
• Vout must be where currents are equal in
• Transcribe points onto Vin vs. Vout plot

8
Operating Regions
Region nMOS pMOS
A Cutoff Linear
B Saturation Linear
C Saturation Saturation
D Linear Saturation
E Linear Cutoff
9
Beta Ratio

• If bp / bn ? 1, switching point will move from
  VDD/2

10
Inverter a Carico Resistivo
Vout
I
11
Inverter a Carico Attivo
Vout
I