6.7. PROBLEMS 297
B-C JUNCTION
Figure 6.26: Figure for problem 6.33.
methods to arrive at the answer. For the impulse function, make sure the impulse charge is
initiated at the base-emitter junction by the application of an impulse voltage. Think about
how the charge density is distributed based on the fact that the base has a varying width.
Problem 6.34An n-p-n AlGaAs-GaAs HBT is grown with the emitter-base junction
graded from Al 0. 2 Ga 0. 8 As to GaAs over 0.19μm. Assume that the emitter is doped
5 × 1016 cm−^3 , and that the base is doped at 10^18 cm−^3. Assume the conduction band
offset between Al 0. 2 Ga 0. 8 As and GaAs to be 0.16 eV, and the bandgap of Al 0. 2 Ga 0. 8 As
and GaAs to be 1.67 eV and 1.4 eV respectively.
(a) Draw the equilibrium band diagram of the emitter-base junction, indicating the band
bending due to depletion charges and quasi-electric field. Calculate the depletion width of
the junction.
(b) Calculateβfor this device.
(c) Now, if a forward bias of 1 V is applied to this junction, what is the new depletion
width? Calculate the conduction band profile and draw the band diagram for the device.
(d) Calculate theβwhen the emitter-base junction is forward biased.
Problem 6.35Consider a GaAs npn BJT with the structure shown in figure 6.27. In this
problem, we consider the effect of a non-zero lateral base resistance,RB. The effective
emitter base potential,VBE(x), drops along the lateral directionx(shown in the figure)
