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Chapter 3: Bipolar Junction Transistors (BJTs)
Adersh [email protected]
I. INTRODUCTION
1) The electrons and holes contribute to the current-conduction
process in BJTs.2) Basic principle involved in BJT is that the
voltage between two terminals controls the current flowing in the
third terminal.
In this way, the three ternimal device can be used to realize a
controlled source which is the basis of amplifier design.
The control signal can be used to control the current in the
third terminal from zero to a large value, thus allowing the
device to act as a switch that is the basis for the logic
inverter.
3) BJT is preferred in very high frequency applications such as
radio-circuits (RF).
4) Bipolar transistors can be combined with MOSFETs to create
innovative circuits that take advantage of high input
impedance and low power operation of MOSFETs and the very high
frequency operation and high current driving
capability of bipolar transistors. The resulting technology is
known as BiCMOS or BiMOS.
5) By the end of this chapter, we should be able to perform
first-order analysis of transistor circuits and to design
single
stage transistor amplifiers and simple logic circuits.
I I . DEVICE S TRUCTURE ANDP HYSICAL O PERATION
A. Simplified Structure and Modes of Operation1) Active Mode:
The emitter-base-junction is forward biased and collector-base
junction is reverse biased. This is also called
forward active mode. It is used when transistor is used to
operate as an amplifier.
2) Cut-off Mode: The emitter-base-junction is reverse biased and
collector-base junction is also reverse biased. Switching
applications utilize both the cut-off and saturation modes.
3) Saturation Mode: The emitter-base-junction is forward biased
and collector-base junction is also forward biased.
4) Reverse Active Mode: The emitter-base-junction is forward
biased and collector-base junction is reverse biased. This
mode has very limited application. We study this from concept
point of view.
B. Operation of the npn Transistor in the Active Mode
Two external voltage sources are used to establish the required
bias conditions for active-mode operation.
1) The voltage VBE causes the p-type base to be higher in
potential than the n-type emitter, thus forward biasing
theemitter-base-junction. The collector base voltage VBC causes the
n-type collector to be at a high potential than the
p-type base, thus reverse-biasing the collector-base-junction.2)
The current that flow aross the emitter-base junction will
constitute the emitter currentiE. The direction ofiE is out of
the emitter lead, which is in the direction of hole current and
opposite to the direction of electron current. The current
iEis sum of these two current components.3) Since, base is small
and lightly doped than emitter which is larger and highly doped,
the emitter current will be dominated
by the electron components. The electron concentration in the
base at the edge of base-emitter depletion region.
np(0) =np0eVBE/VT
wherenp0 is the thermal equilibrium value of minority carrier
concentration in the base region, vBEis the forward
biasbase-emitter voltage, and VT is the thermal voltage, which is
approximately 26mV are room temperature.
4) The electron diffusion current in the base is given by the
following expression.
In= AEqDndnp(x)
dx =AEqDn
np(0)
W
where AE is the cross sectional area of the base-emitter
junction, Dn is electron diffusion coefficient, and W is
theeffective width of the base region. Oberve that the negative
slope of the minority-carrier concentration results in the
current flow in negative x direction.5) The slope of the
concentration profile at the emitter-base-junction is slightly
higher than that at the collector-base-junction,
with the difference accounting for the small number of electrons
lost in the base region through recombination.
6) Most of the electrons diffused in the base are swept across
the base-collector depletion region and collected at the
collector terminal, because collector is more positive than the
base. Thus iC=In which will yield a negative value ofiCindicating
it flows in the negative x direction.
iC=ISeVBE/VT
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whereISis saturation current and is given by
IS=AEqDnnp0W
substitutingnp0 = n2
i/NA, we get
Is=AEqDnnp0n
2
i
NAW
7) An important observation here is that the magnitude ofiC is
independent ofvCB . That is, as long as the collector is
positive with repect to the base, the electrons that reach the
collector side of the base region will swept into the
collector and register as collector current. If collector is not
positive with respect to base, vCB would also affect thecollector
current.
8) The saturation current ISis directly proportional to the
emitter-base-junction area and inversely proportional the
basewidth. It is also a strong function of temperature. With
every5C rise in temperature, it doubles. It is directly
proportionalto the junction area, it will also be referred to as
the scale current. If two transistors that are identical except
that one
has an emitter-base-junction, say, twice that of the other. The
ratio of their saturation currents is same as ratio of their
emitter-base-junction area. Thus, for the same vBEthe larger
device will have a collector current twice that in the
smallerdevie. This concept is frequently employed in
integrated-circuit design.
9) The base current is imployed to two components. The first
component iB1 is due to the holes injected from the baseregion into
the emitter region. This current component is proportional to
eVBE/VT .
iB1 =AEqDpn
2
i
NDLpeVBE/VT
The second component of the base current iB2 is due to the holes
that have to be supplied by the external circuitin ordrer to
replinish the holes lost from the base through the recombination
process. The minority carrier life time is
denoted by b and minority carrier charge that recombines in base
is denoted by Qn.
iB2 =Qn
b
Qn is computed by the area of carrier concentration in the
base,
Qn= AEq1
2np(0)W
10) The final base current is
iB = iC/= IS
eVBE/VT
where is called common-emitter current gainand is given by
= 1
DpDn
NAND
WLp
+ 12
W2
Dnb
11) For modern npn transistors, is in the range 50 to 200, but
it can be as high as 1000 for special devices.12) From the above
expression, it is clear that is highly influenced by the two
factors: the width of the base region, W,
and the relative dopings of the base and emitter regions,
(NA/ND). To obtain a high (which is highly desirable sinceit
represents a gain parameter), the base should be thin (W small and
lightly doped and emitter heavily doped (making
NA/ND small). We assume that it is constant for a transistor.13)
The emitter currentiE is equal to the sum of the collector current
iCand the base current iB; that is,
iE=iC+ iB =iC+
iC
=
+ 1
iC=
+ 1
ISeVBE/VT
=
+ 1, =
1
iC= iE
iE= (IS/)eVBE/VT
where is called common base current gain.
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