Chapter 3
Bipolar Junction Transistor
(BJT)
➢ The first bipolar transistor was realized in 1947 by Brattain, Bardeen and
Shockley.
➢ The three of them received the Nobel prize in 1956 for their invention.
➢ In a bipolar transistor the current is due to transport of both electrons and
holes
➢ There are two types of bipolar transistors: the NPN transistor, in which a Ptype region is sandwiched between two N-type regions, and the PNP
transistor, where N-type silicon is confined between two P-type regions.
➢ If no bias is applied to the device terminals
both junctions are at thermal equilibrium and there is no current flow.
Energy
band
diagram
nEo=nnEo=NdE
nnCo=NdC
Distribution
of Carrier
concentration
ppBo=NaB
ni
npBo
pnCo
pnEo
Long Base BJT
➢ If the emitter-base junction is forward biased current flows through the
emitter-base junction.
➢ Holes are injected from the base into the emitter where they recombine with
majority carriers (electrons). Similarly, electrons are injected from the emitter
into the base where they recombine with the local majority carriers (holes).
➢ If the collector-base junction is reverse-biased, only a small reverse current
(the collector-base junction saturation current) flows between base and
collector.
➢ If the width of the neutral base, is large enough (larger than the electron
diffusion length ),
all the electrons injected by the emitter into the
base recombine in the base
➢ No interaction between both junctions and therefore no current flowing
between emitter and collector.
Short Base BJT
➢ Consider now a device with a short base. The term "short base" implies
that the neutral base width is smaller than the electron diffusion length
➢ Because the length of the neutral base is smaller than the diffusion length for
electrons in the base, a number of electrons injected from the emitter into the
base can diffuse to the collector-base junction depletion region.
➢ These electrons are accelerated by the electric field of the depletion region
and transported into the collector
Forward active mode
Carrier distribution
nnEo=NdE
nnCo=NdC
ppBo=NaB
ni
npB
pnE
pnC
➢ In modern bipolar transistors a large portion (99% or more) of the electrons
injected by the emitter into the base reach the collector.
➢ The magnitude of current flowing in the collector does not depend on
magnitude of the collector voltage
➢ The collector current is fixed by the bias applied to the emitter-base diode.
This effect, in which the current in a junction is controlled by the bias applied to
another junction, is called "transistor effect"
➢ An NPN bipolar transistor with a forward-biased emitter-base junction and
a reverse-biased collector-base junction is said to operate in the forward
active mode
➢ It is possible to bias the device differently than in the forward active mode:
✓ If both junctions are forward biased the transistor is said to be in
saturation mode. In that case electrons are injected from the emitter
through the base into the collector and from the collector through the base
into the emitter.
✓ If both junctions are reverse biased there is no current flow at all and the
device is in the cut-off mode.
✓ If the emitter junction is reverse biased and the collector junction is
forward biased the transistor operates in the reverse active mode.
The carrier- current components in
NPN BJT in Forward active mode
❖ In the forward active mode,the current flowing through the emitter junction
is given by the sum of the hole current injected from the base and the
electron current injected from the emitter.
➢ The collector current is due to the diffusion through the base of
electrons injected by the emitter into the base.
=
is the current due to the recombination of electrons in the base
➢ Since high gain values are desirable, a higher doping concentration is
used in the emitter than in the base
➢ Since the transistor is designed in such a way that
➢ Defining a parameter called the "common-base gain", noted
parameter is the "common-emitter gain" which describes the relationship
between collector and base currents when the emitter is grounded.
Super
transistors have current gains higher than 1,000 or even 10,000
Ebers-Moll model
➢ In 1954 J.J. Ebers and J.L. Moll developed a model for the bipolar
transistor which is still used in modern circuit simulators
➢To simplify the study of the device we will assume that the impurity
concentrations in the emitter, base and collector are constant.
The matrix of the model is:
Where,
Note:
PoE= PEo
noB= nBo
PoC= pCo
➢ ICO or ICBO is the saturation current flowing in the collector when the
collector junction is reverse biased and the emitter is left open.
➢ Similarly, IEO or IEBO is defined as the saturation current flowing in the
emitter when the emitter junction is reverse biased and the collector is left
open.
Emitter efficiency
➢ In an "ideal" bipolar transistor the base current should be much smaller
than the emitter and collector currents.
➢ The emitter efficiency, γF is defined as the ratio of the electron current
injected from the emitter to the total current in the emitter-base junction
Transport factor in the base
➢The success rate at which the electrons injected into the base reach the
collector is measured by a parameter called "transport factor in the base" and
noted
➢It is defined as the current of electrons reaching the collector after crossing
the base divided by the current of electrons injected by the emitter into the base:
❖ Large gain transistors can be achieved by varying some
processing parameters during device fabrication, such as:
➢ A reduction of base width which yields devices with higher transport
factor in the base, and hence higher gain. The base width of bipolar
transistors has been reduced from tens of micrometers in 1954 to
0.1µm or less today.
➢ A higher doping concentration in the emitter than in the base to
increase the emitter efficiency.
➢ Polysilicon can be used as the emitter material. In that case the
interface between the silicon base and the polysilicon decreases the hole
current injected by the base, which increases the emitter efficiency.
➢ Alternative base materials, such as silicon-germanium alloys, can be
used to decrease the hole current injected by the base, which increases
the emitter efficiency.
Remember
Regions of operation
The Ebers-Moll Model can be used to describe the different possible
regimes of operation of the bipolar transistor, which depend on the bias
applied to the different device terminals.
Early effect
➢ We have so far considered that the collector current is independent of the
collector-base voltage when the device operates in the forward active mode,
this is not completely true.
➢ Actually, the collector current increases with the collector-base bias. This
effect was explained by J. Early in 1952 and is due to the modulation of the
neutral base width, by the applied collector-base reverse bias
➢ Any variation of
induces a variation of the width of the depletion region
in the base, at the collector-base junction. That variation induces a variation of
the neutral base width, and therefore, a variation of the current gain.
Small-signal model
➢ If the amplitude of the small ac signal (a music signal, for example) is small
compared to kT/q, it is possible to linearize the transistor equations around
the dc operating point.
➢ This greatly simplifies the equations. The model obtained from this
simplification is called a "small signal" model.
In the forward active mode we obtain:
The parameter
is called the transconductance of the
transistor. Note that the transconductance is proportional to the collector
current.
π- model
Low fr.
High fr.
T- model
Small-signal model Parameters
Unity Gain Frequency, fT