Applet Tutorial
The Emitter-junction forward bias is fixed at 0.3V, and the Collector-junction reverse bias is fixed at 6.0V.
I. Dependence of injection current on the doping level.
1.Make a note of the excess minority carrier concentration at E-junction, the three terminal currents
(Je, Jb, Jc) and the current components, Jep, Jen, and Jcp (or Jcn).
2.Change the Emitter doping level to another value, say 1E18. Does the injected minority carrier
density change ? Why or why not ?
If it does not affect injected minority density in the Base (and thus the minority diffusion
current in the Base), why do they keep the Emitter doping level much higher than the Base
doping level, typically x100 ?
Which current components change their values ? Use the current expressions in the Math.Analysis
part and explain.
Which terminal currents change their values ? Why ?
3.Change the Base-doping level to another value, say 1E16. Does it change the minority density of
Base at E-junction ? Why or why not ?
If it does, why can't you lower the Base doping still further ?
Which of the current components change their values? Which terminal currents change and why ?
II. Dependence of Collector Current on the Diffusion Length to Base-Width ratio.
Set PNP, Ne=1E19, Nb=1E17, Nc=1E15, lifetime=0.01us, Dp=15 cm2/Vs.
While decreasing the base width, observe how it affects the injected minority density profile in the
base, observe how it affects the slope of the minority carrier density at the E and C junctions.
While increasing the base width, do the same as above.
Why is the minority density at the C-junction always zero (or near zero) ?
When Wb << Lp, Compare the slopes dp/dx at E and C junctions. Are they close, or very different
? What function does p(x) approximate ? Compare Jep and Jcp.
For an effective BJT, is this condition (Wb < Lp) important ?
When Wb >> Lp, Compare dp/dx at E and C junctions. What function
does p(x) become ? Compare Jep and Jcp.
What does Jcp << Jep mean physically ? (Hint: consider the recombination loss in the Base).
III. Others
Diffusion length is controlled by the lifetime and diffusion coefficient D by L = sqrt(lifetime * D).
Change the lifetime and D and observe the Lp value.
Applet Worksheet
1.For Si with ni = 1E10 cm-3, and doped with acceptor impurities to
Na = 1E17 cm-3, what is the minority electron concentration at
equilibrium ? see the answer.
2.If a forward bias V=0.71 Volts is applied to a Si PN junction where
the p-type doping is 1E17 cm-3, find the (injected) minority electron
concentration at the junction edge. see the answer.
3.Change the input signal voltage until a Base-Emitter bias Vbe =
0.72 Volt is applied. Find the Base transport factor, the Emitter
injection efficiency, and the transistor Beta. see the answer.