ELEC 302 FET High Frequency Models 1
HIGH-FREQUENCY MODELS OF THE FET
• For high-frequency analysis, the relationships must be modified to include the following two
effects:
1. The JFET structure acts as a parallel plate capacitor when viewed from the gate and
source terminals, with the gate and channel forming the two plates.
2. The majority carriers require a finite transition time to cross the source to gate
channel.
• The frequency-dependent components are: Cgs - gate-to-source capacitance, Cgd - gate-to-
drain capacitance (sometimes called the overlap capacitance), and Cds - drain-to-source
capacitance. Since Cgs >> Cds , Cds can usually be ignored.
• The drain-source capacitance, Cds , is small and therefore does not appreciably affect the
high-frequency response of the FET. The two remaining capacitances can be modeled as
voltage dependent capacitors with values determined by the following expressions:
m
o
GS
gso
gs V
C
C
÷
÷
ø
ö
ç
ç
è
æ+
=
y
1
and
m
o
GD
gdo
gd V
C
C
÷
÷
ø
ö
ç
ç
è
æ+
=
y
1
, where
Cgso, Cgdo are the zero-bias gate-source and gate-drain junction capacitances,
respectively, in Farads;
VGS, VDS are the quiescent gate-source and drain-source voltages, respectively;
m is the gate p-n grading coefficient (SPICE default = 0.5);
y
o is the gate junction (barrier) potential, typically 0.6 V (SPICE default = 1V).
• The frequency dependent elements for the MOSFET can be obtained in the same manner as
the JFET.
• The capacitance formed by the oxide layer at the gate is defined as
CWL
t
CWL
ox
ox
ox
ox
= = ¢
Î
,
where
Cox is the oxide capacitance formed by the gate and channel
¢
Cox
is the oxide capacitance per unit area
Î
ox is the permittivity of the oxide layer (silicon oxide -SiO2: 3.9
Î
o)
tox is the thickness of the oxide layer (separation between the gate and channel)
W, L are the width and the length of the channel under the gate, respectively.
• The permittivity of vacuum is,
Î
o = 8.851´ 10-12 F/m. The oxide capacitance per unit area
can be calculated from physical parameters:
÷
÷
ø
ö
ç
ç
è
æ
=
¢2
2
1
PO
DSS
ox V
I
C
µ
, for depletion MOSFETs (10.7-4a)
( )
KCox 2
1
µ
=
¢
, for enhancement MOSFETs, (10.7-4b)
where
µ
is the charge mobility (typically 600 cm2/V-s for n-channel, 200 cm2/V-s for
p-channel.
