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Bipolar Transistor
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CHAPTER OBJECTIVES
This chapter introduces the bipolar junction transistor (BJT) operation and then presents
the theory of the bipolar transistor I-V characteristics, current gain, and output
conductance. High-level injection and heavy doping induced band narrowing are
introduced. SiGe transistor, transit time, and cutoff frequency are explained. Several
bipolar transistor models are introduced, i.e., Ebers–Moll model, small-signal model, and
charge control model. Each model has its own areas of applications.
he bipolar junction transistor or BJT was invented in 1948 at Bell Telephone
Laboratories, New Jersey, USA. It was the first mass produced transistor,
ahead of the MOS field-effect transistor (MOSFET) by a decade. After the
introduction of metal-oxide-semiconductor (MOS) ICs around 1968, the highdensity
and low-power advantages of the MOS technology steadily eroded the
BJT’s early dominance. BJTs are still preferred in some high-frequency and analog
applications because of their high speed, low noise, and high output power
advantages such as in some cell phone amplifier circuits. When they are used, a
small number of BJTs are integrated into a high-density complementary MOS
(CMOS) chip. Integration of BJT and CMOS is known as the BiCMOS
technology.
The term bipolar refers to the fact that both electrons and holes are involved
in the operation of a BJT. In fact, minority carrier diffusion plays the leading role
just as in the PN junction diode. The word junction refers to the fact that PN junctions
are critical to the operation of the BJT. BJTs are also simply known as bipolar
transistors.
INTRODUCTION TO THE BJT
A BJT is made of a heavily doped emitter (see Fig. 8–1a), a P-type base, and an N-type
collector. This device is an NPN BJT. (A PNP BJT would have a P+ emitter, N-type
base, and P-type collector.) NPN transistors exhibit higher transconductance and FIGURE 8–1 (a) Schematic NPN BJT and normal voltage polarities; (b) electron injection
from emitter into base produces and determines IC ; and © IC is basically determined by
VBE and is insensitive to VCB.
BASE CURRENT
Whenever the base–emitter junction is forward biased, some holes are injected from
the P-type base into the N+ emitter. These holes are provided by the base current, IB.1
IB is an undesirable but inevitable side effect of producing IC by forward biasing the BE
junction. The analysis of IB, the base to emitter injection current, is a perfect parallel of
the IC analysis. Figure 8–6b illustrates the mirror equivalence. At an ideal ohmic
contact such as the contact of the emitter, the equilibrium condition holds and p' = 0
similar to Eq. (8.2.4).
Narrow Band-Gap Base and Heterojunction BJT
To further elevate βF, we can raise niB by using a base material that has a smaller
band gap than the emitter material. Si1-ηGeη is an excellent base material candidate
for an Si emitter. With η = 0.2, EgB is reduced by 0.1 eV. In an SiGe BJT, the base is
made of high-quality P-type epitaxial SiGe. In practice, η is graded such that η = 0
at the emitter end of the base and 0.2 at the drain end to create a built-in field that
improves the speed of the BJT (see Section 8.7.2).
Because the emitter and base junction is made of two different
semiconductors, the device is known as a heterojunction bipolar transistor or
HBT. HBTs made of InP emitter (Eg = 1.35 eV) and InGaAs base (Eg = 0.68 eV)
and GaAlAs emitter with GaAs base are other examples of well-studied HBTs.
The ternary semiconductors are used to achieve lattice constant matching at the
heterojunction (see Section 4.13.1).