352 CHAPTER 7. TEMPORAL RESPONSE OF DIODES AND BIPOLAR TRANSISTORS
RE=1μm
RC=3μm
(RC - R )
n
h
p
RE
RB
RC
B
Figure 7.26: Figure for problem 7.15.
delay for each of these structures in terms of the base width, WB, diffusion constant Dn,
and carrier velocity vs. Do not assume Shockley boundary conditions. Use charge control
analysis to derive these delays.
Problem 7.18To maintain a high breakdown voltage in the collector of an InP-based
HBT, it is preferable to use an InP collector. In problem 7.14, you calculated the additional
base delay introduced by a charge accumulation layer at the interface for a current density
of 10 kA/cm^2.
- To eliminate the above delay, I linearly grade the bandgap from InGaAs to InP across
the base. Derive an expression for the new base delay. Based on physical arguments,
sketch the expected minority charge profile. Compare this with a device whose base
is not graded. Physically explain your result. Donotassume Shockley boundary
conditions to calculate the delay. Instead, assume a saturated velocityvs.
