227
MOSFET Capacitance Models
6.2.1 Intrinsic Charges and Capacitances
In a typical steady-state operation, the current flow through a MOSFET device
is due to the transport of the mobile carriers (e.g., electrons in n- channel
MOSFETs or nMOSFETs and holes in p-channel MOSFETs or pMOSFETs)
from the source to drain under the applied drain voltage. This current is
referred to as the transport current in transient analysis. In a dynamic operation, additional currents flow through the device due to the stored charges
at the device terminals and are called the charging currents as shown in
Figure 6.2.
Figure 6.2a shows the transient or dynamic currents i g , i s , i d , and i b flowing
through the gate (g), source (s), drain (d), and bulk (b) terminals, respectively,
of a MOSFET device. In Figure 6.2a, Q G , Q S , Q D , and Q B are the total gate,
source, drain, and bulk charges, respectively, corresponding to four terminals of the MOSFET. These terminal charges are functions of the gate, source,
drain, and bulk terminal voltages V g , V s , V d , and V b , respectively. Thus, in
general
Q f V V V V
j G S D B
j
g
s
d
b
= (
)
=
, , ,
,
, , ,
where
(6.1)
From Kirchhoff’s current law (KCL) for the total current, we have
i i i i
g
s
d
b
+ + + = 0
(6.2)
and from the law of conservation of charge, we have
Q Q Q Q
G
S
D
B
+ +
+
=0
(6.3)
In order to calculate various charges of a MOSFET device, we assume quasistatic operation of the device [1]. In quasistatic operation, the terminal voltages
are assumed to vary sufficiently slowly so that the distribution in the stored
Source
Q S
Q B
Q G
Q D
V d
V ds V b
V g V gs
−V gd
+
+
−
V s
Drain
Drain
Body
Source
Body
Gate
(a)
(b)
Gate
i b
i s
i g
i d
FIGURE 6.2
Schematic of a MOSFET as a circuit element: (a) transient currents i g , i s , i d , and i b flowing through
the gate, source, drain, and body terminals, respectively and (b) terminal DC voltages, V g , V d , V s
and V b at the gate, drain, source, and body terminals, respectively, where V ds = V gs − V gd .
MOSFET Capacitance Models
6.2.1 Intrinsic Charges and Capacitances
In a typical steady-state operation, the current flow through a MOSFET device
is due to the transport of the mobile carriers (e.g., electrons in n- channel
MOSFETs or nMOSFETs and holes in p-channel MOSFETs or pMOSFETs)
from the source to drain under the applied drain voltage. This current is
referred to as the transport current in transient analysis. In a dynamic operation, additional currents flow through the device due to the stored charges
at the device terminals and are called the charging currents as shown in
Figure 6.2.
Figure 6.2a shows the transient or dynamic currents i g , i s , i d , and i b flowing
through the gate (g), source (s), drain (d), and bulk (b) terminals, respectively,
of a MOSFET device. In Figure 6.2a, Q G , Q S , Q D , and Q B are the total gate,
source, drain, and bulk charges, respectively, corresponding to four terminals of the MOSFET. These terminal charges are functions of the gate, source,
drain, and bulk terminal voltages V g , V s , V d , and V b , respectively. Thus, in
general
Q f V V V V
j G S D B
j
g
s
d
b
= (
)
=
, , ,
,
, , ,
where
(6.1)
From Kirchhoff’s current law (KCL) for the total current, we have
i i i i
g
s
d
b
+ + + = 0
(6.2)
and from the law of conservation of charge, we have
Q Q Q Q
G
S
D
B
+ +
+
=0
(6.3)
In order to calculate various charges of a MOSFET device, we assume quasistatic operation of the device [1]. In quasistatic operation, the terminal voltages
are assumed to vary sufficiently slowly so that the distribution in the stored
Source
Q S
Q B
Q G
Q D
V d
V ds V b
V g V gs
−V gd
+
+
−
V s
Drain
Drain
Body
Source
Body
Gate
(a)
(b)
Gate
i b
i s
i g
i d
FIGURE 6.2
Schematic of a MOSFET as a circuit element: (a) transient currents i g , i s , i d , and i b flowing through
the gate, source, drain, and body terminals, respectively and (b) terminal DC voltages, V g , V d , V s
and V b at the gate, drain, source, and body terminals, respectively, where V ds = V gs − V gd .