77
Review of Basic Device Physics
to C j . However, under reverse bias, C j decreases much more slowly with
increasing V d (=–V r ), as compared to C dif . Therefore, C j is the dominant capacitance for reverse bias and small for forward bias (V d < f bi /2), while diffusion
capacitance C dif is dominant for forward bias (V d > f bi /2).
2.3.7.3 Small Signal Conductance
In the model discussed in Section 2.3.7.2, referred to as the large-signal model,
we did not place any restriction on the allowed voltage variation. However,
in some circuit situations, voltage variations are sufficiently small so that the
resulting small current variations can be expressed using linear relationships.
This is the so called small signal behavior of a pn-junction. An example of
linear relations are the capacitances C j and C dif in Equations 2.141 and 2.143,
respectively, as they represent an overall nonlinear charge storage effect in
terms of linear circuit elements (capacitors), although we did not label them
as such.
For small variations about the operating point, which is set by the DC condition, the nonlinear junction current can be linearized so that the incremental diode current is proportional to the incremental applied bias. This linear
relationship is used to calculate the small signal conductance g d
g
dI
dV
d
d
d
=
(2.144)
Using (2.119) for I d , we have
g
I
v
V
v
v
I I
d
s
kT
d
kT
kT
d
s
=
=
+
(
)
exp
1
(2.145)
Thus, Equation 2.145 clearly shows that g d is proportional to the slope of the
DC characteristics at the operating point. When the diode is forward biased,
I d is much larger than I s and therefore, g d is proportional to I d . However, when
the diode is reverse biased, I d = –I s and therefore, from Equation 2.145, g d
becomes zero. But in real diodes, g d ≠ 0 in the reverse bias condition due to
the fact that the generation current I gen (Equation 2.126) is dominant conduction mechanism.
2.3.8 Diode Equivalent Circuit for Circuit CAD
The small signal equivalent circuit of a pn-junction is shown in Figure 2.29.
In Figure 2.29, r s represents the series resistance due to ohmic drop across the
neutral n- and p-regions; C j is junction capacitance; C d is the diffusion capacitance due to the minority carrier diffusion through the neutral regions; and
g d is the small signal conductance of the pn-junctions.
Review of Basic Device Physics
to C j . However, under reverse bias, C j decreases much more slowly with
increasing V d (=–V r ), as compared to C dif . Therefore, C j is the dominant capacitance for reverse bias and small for forward bias (V d < f bi /2), while diffusion
capacitance C dif is dominant for forward bias (V d > f bi /2).
2.3.7.3 Small Signal Conductance
In the model discussed in Section 2.3.7.2, referred to as the large-signal model,
we did not place any restriction on the allowed voltage variation. However,
in some circuit situations, voltage variations are sufficiently small so that the
resulting small current variations can be expressed using linear relationships.
This is the so called small signal behavior of a pn-junction. An example of
linear relations are the capacitances C j and C dif in Equations 2.141 and 2.143,
respectively, as they represent an overall nonlinear charge storage effect in
terms of linear circuit elements (capacitors), although we did not label them
as such.
For small variations about the operating point, which is set by the DC condition, the nonlinear junction current can be linearized so that the incremental diode current is proportional to the incremental applied bias. This linear
relationship is used to calculate the small signal conductance g d
g
dI
dV
d
d
d
=
(2.144)
Using (2.119) for I d , we have
g
I
v
V
v
v
I I
d
s
kT
d
kT
kT
d
s
=
=
+
(
)
exp
1
(2.145)
Thus, Equation 2.145 clearly shows that g d is proportional to the slope of the
DC characteristics at the operating point. When the diode is forward biased,
I d is much larger than I s and therefore, g d is proportional to I d . However, when
the diode is reverse biased, I d = –I s and therefore, from Equation 2.145, g d
becomes zero. But in real diodes, g d ≠ 0 in the reverse bias condition due to
the fact that the generation current I gen (Equation 2.126) is dominant conduction mechanism.
2.3.8 Diode Equivalent Circuit for Circuit CAD
The small signal equivalent circuit of a pn-junction is shown in Figure 2.29.
In Figure 2.29, r s represents the series resistance due to ohmic drop across the
neutral n- and p-regions; C j is junction capacitance; C d is the diffusion capacitance due to the minority carrier diffusion through the neutral regions; and
g d is the small signal conductance of the pn-junctions.