this is about mosfet velosity saturation.

1. PRESENTATION
ON
VELOCITY SATURATION
ARVIND DAUTANIYA
B.TECH (ECE)
PRATAP UNIVERSITY

2. VELOCITY SATURATION
 When A strong enough electric field is applied:
 the carrier velocity in the semiconductor reaches a
maximum value saturation velocity
 the carriers lose energy through increased levels of
interaction with the lattice, collisions and by emitting phonons

3. VELOCITY SATURATION
We assumed carrier velocity is proportional to E-field
 v = µElat = µVds/L
At high fields, this ceases to be true
 Carriers scatter off atoms
 Velocity reaches vsat
 Electrons: 6-10 x 106 cm/s
 Holes: 4-8 x 106 cm/s
 Better model
μElat
v=
⇒ vsat = μEsat
Elat
1+
Esat
νsat
ν
νsat / 2
slope = µ
0
0
Esat
2Esat
Elat
3Esat

4. VELOCITY SATURATION

5. VELOCITY SATURATION

6. VELOCITY SATURATION

7. VELOCITY SATURATION
At high E lat , carrier velocity rolls off
 Carriers scatter off atoms in silicon lattice
 Velocity reaches vsat
 Electrons: 107 cm/s
 Holes: 8 x 106 cm/s
 Better model

8. VEL SAT I-V EFFECTS
Ideal transistor ON 2current increases with V DD 2
2
W ( Vgs − Vt )
β
I ds = µCox
= ( Vgs − Vt )
L
2
2
Velocity-saturated ON current increases with V DD
I ds = CoxW ( Vgs − Vt ) vmax
Real transistors are partially velocity saturated
 Approximate with α-power law model
 Ids ∝ VDDα
1 < α < 2 determined empirically ( ≈ 1.3 for 65 nm)