387
Bipolar Junction Transistor Compact Models
Let us look at the impact of parasitic resistors shown in Figures 11.13 and
11.14 on the characteristics of a typical BJT device.
• Effect of ohmic bulk collector resistor, r c : The collector series resistance r c
decreases the slope of I C versus V CE characteristics in the saturation
region of BJT operation and improves the accuracy of modeling DC
device characteristics as shown in Figure 11.15. Figure 11.15 shows
(I C , I B ) as a function of V CE of an npn-BJT. As shown in Figure 11.15, r c
increases the transition voltage V CE,sat of BJTs. The typical value of r c
of modern IC BJTs is about 200 ohm.
E′
C′
B′
C
E
B
I B
r b I B
I C
r e I E
I E
r c I C
FIGURE 11.14
The parasitic series resistances on a typical npn-BJT device causing ohmic voltage drop at the
respective neutral regions of the device.
I C
V CE
I B1
I B1 < I B2 < I B3
I B2
I B3
EM1
Effect of r c
FIGURE 11.15
The effect of collector series resistance on the BJT device characteristics is to increase the saturation to linear region transition voltage V CE,sat of BJTs.
Bipolar Junction Transistor Compact Models
Let us look at the impact of parasitic resistors shown in Figures 11.13 and
11.14 on the characteristics of a typical BJT device.
• Effect of ohmic bulk collector resistor, r c : The collector series resistance r c
decreases the slope of I C versus V CE characteristics in the saturation
region of BJT operation and improves the accuracy of modeling DC
device characteristics as shown in Figure 11.15. Figure 11.15 shows
(I C , I B ) as a function of V CE of an npn-BJT. As shown in Figure 11.15, r c
increases the transition voltage V CE,sat of BJTs. The typical value of r c
of modern IC BJTs is about 200 ohm.
E′
C′
B′
C
E
B
I B
r b I B
I C
r e I E
I E
r c I C
FIGURE 11.14
The parasitic series resistances on a typical npn-BJT device causing ohmic voltage drop at the
respective neutral regions of the device.
I C
V CE
I B1
I B1 < I B2 < I B3
I B2
I B3
EM1
Effect of r c
FIGURE 11.15
The effect of collector series resistance on the BJT device characteristics is to increase the saturation to linear region transition voltage V CE,sat of BJTs.