SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

254 CHAPTER 6. BIPOLAR JUNCTION TRANSISTORS

Emitter

contact

Base

contact

SiO 2

AA'

p

n

n

IC

InEB= Emitter current injected into the base = IEn

InBC = Electron current injected across reverse-biased base collector junction

IBEp= Base current injected into the emitter = IEp

IBER = Recombination current in the base region

IBCp= Hole current injected across reverse-biased base collector junction

InC= Electron current coming from the emitter (= I~C)

IEBn Electron flow

n n

IBEp

p

IBER

IBCp

IBCn

InC

IB

IE

Hole flow

II I

III

IV

IBE V

R

BE + +

A

A'

CB

VI

Collector

contact

Figure 6.6: A schematic of an Si BJT showing the three-dimensional nature of the structure and
the current flow. Along the sectionAA′, the current flow can be assumed one-dimensional. The
various current components in a BJT are discussed in the text.

diffusion length while the base width is small compared to the electron diffusion length. We will
use the different axes and origins shown in figure 6.7. The distances are labeledxe,xb,andxc
as shown and are measured from the edges of the depletion region. The base width isWb,but
the width of the “neutral” base region isWbnas shown. We assume thatWbandWbnare equal.