SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

6.5. BJT DESIGN LIMITATIONS: NEED FOR BAND TAILORING 267

NEEDS CHALLENGES

DEMANDS AND PROBLEMS FOR A

BIPOLAR JUNCTION TRANSISTOR

• High gain


• High emitter efficiency


• High speed

Heavy emitter doping

• Low base doping


• Narrow base width

Bandgap shrinkage causing

base injection

High base resistance

SOLUTION: HETEROJUNCTION BIPOLAR TRANSISTORS

Figure 6.11: Figure showing the conflicting requirements for high-performance BJTs. Het-
erostructure devices offer reconciliation of all these requirements.

Example 6.1Consider annpnGaAs BJT that has a doping of

Nde=5× 1017 cm−^3 ,Nab=10^17 cm−^3. Compare the emitter efficiency of this device

with that of a similarly doped HBT where the emitter is Al 0. 3 Ga 0. 7 As and the base is

GaAs. The following parameters characterize the devices at 300 K:

Electron diffusion constant in the base, Db = 100 cm^2 s−^1

Hole diffusion constant in the emitter, De =15cm^2 s−^1

Base width, Wb =0. 5 μm

Bandgap discontinuity, ΔEg =0.36 eV

Minority carrier length for holes, Le =1. 5 μm