Using the BJT Quasi-Saturation Model

Use the BJT quasi-saturation model (LEVEL=2), an extension of the Gummel-Poon model (LEVEL 1 model), to model bipolar junction transistors that exhibit quasi-saturation or base push-out effects. When a device with lightly doped collector regions operates at high injection levels, the internal base-collector junction is forward biased, while the external base-collector junction is reversed biased; DC current gain and the unity gain frequency fT falls sharply. Such an operation regime is referred to as quasi-saturation, and its effects have been included in this model.

Vertical npn Bipolar Transistor (SUBS=+1) show the additional elements of the LEVEL 2 model. The current source Iepi and charge storage elements Ci and Cx model the quasi-saturation effects. The parasitic substrate bipolar transistor is also included in the vertical transistor by the diode D and current source Ibs.

Figure 16-11: Vertical npn Bipolar Transistor (SUBS=+1)

Figure 16-12: Lateral npn Bipolar Transistor (SUBS=-1)

Using Epitaxial Current Source Iepi

The epitaxial current value, Iepi, is determined by the equation:

where:

In special cases when the model parameter GAMMA is set to zero, ki and kx become one and,

Using Epitaxial Charge Storage Elements Ci and Cx

The epitaxial charges are determined by:

and:

The corresponding capacitances are calculated as:

and:

In the special case where GAMMA=0 the Ci and Cx become zero.

Example

*quasisat.sp comparison of bjt LEVEL1 and LEVEL2

*model

.options nomod relv=.001 reli=.001 absv=.1u absi=1p

.options post

q11 10 11 0 mod1

q12 10 12 0 mod2

q21 10 21 0 mod1

q22 10 22 0 mod2

q31 10 31 0 mod1

q32 10 32 0 mod2

vcc 10 0 .7

i11 0 11 15u

i12 0 12 15u

i21 0 21 30u

i22 0 22 30u

i31 0 31 50u

i32 0 32 50u

.dc vcc 0 3 .1

.print dc vce=par('v(10)') i(q11) i(q12) i(q21)
+ i(q22) i(q31) i(q32)

*.graph dc i(q11) i(q12) i(q21) i(q22)

*.graph dc i(q11) i(q12)

.MODEL MOD1 NPn IS=4.0E-16 BF=75 VAF=75

+ LEVEL=1 rc=500 SUBS=+1

.MODEL MOD2 NPn IS=4.0E-16 BF=75 VAF=75

+ LEVEL=2 rc=500 vo=1 qco=1e-10

+ gamma=1e-9 SUBS=+1

.end

Figure 16-13: Comparing BJT LEVEL 1 and LEVEL 2 Models