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 ?

Set PNP, Ne=1E19, Nb=1E17, Nc=1E15, lifetime=0.1us, Dp=50 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).

• 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.