281
Compact MOSFET Models for RF Applications
L is the channel length
the parameter β models the external gate resistance
Now, R gch includes the static channel resistance (R st ) that accounts for the DC
channel resistance and the excess-diffusion channel resistance (R ed ) due to
the change of channel charge distribution by the AC excitation of the gate
voltage. Both R st and R ed together determine the time constant of the NQS
effect. R st is modeled by integrating the resistance along the channel under
the QS assumption [58]
R
dR
dV
I
V
I
V V
V
V
I
V V
V
st
d
ds
ds
gs
th
ds
dsat
ds
gs
th
=
=
=
<
−
(
) ≥
<
−
(
) ≤
∫
∫
;
;
0
0
d ds







(7.54)
where:
V dsat  ≅ (V gs  − V th ) is the saturation drain voltage
Both I dsat and V dsat can be calculated from the target compact model. And, R ed
can be derived from the diffusion current (Equation 4.122) as
R
L
W C v
ed
eff
eff
o x kT
= η
µ
(7.55)
where:
η is a technology-dependent constant
C ox is the gate capacitance as shown in Figure 7.8
S
D
R gch
C ox
ρ sh,geltd
FIGURE 7.8
Distributed nature of gate electrode resistance, channel resistance network, and gate capacitance for modeling the efective gate resistance for RF analysis of MOSFET devices. (Data from
X. Jin et al., IEDM Technical Digest, 961–964, 1998.)