Bipolar Junction Transistor

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Prepared by: Er. Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS
Chapter 2: Bipolar Junction Transistor (BJT)
2.1 Introduction
It is a three layer semiconductor device. Emitter is highly doped and supplies the charges. Collector is moderately
doped and collects the charges from emitter. Whereas, Base is lightly doped and controls the charges flowing from
emitter to collector. There are two p-n junctions: emitter-base (EB) junction and collector-base (CB) junction.
For the amplification purpose EB junction is forward biased and CB junction is reverse biased. Here, VBB forward
biases the EB junction and VCC reverse biases the CB junction.

Prepared by: Er. Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS
The electrons from emitter are injected into the base. Similarly, holes from the base are injected into the emitter. The
number of holes in base is very low due to light doping as compared to the number of electrons in heavily doped
emitter. The base holes recombine with some of the electrons thereby leaving large number of injected electrons in
the base as minority carriers. VBB supplies electrons to emitter continuously so as to set up emitter current. Some of
the electrons in base are drawn by VBB and sets the base current. Due to reverse biased BC junction, the minority
electrons in base are attracted to the positive collector and set the collector current.
    …. (i)
Where, IB very small (leakage current)
We define the term:   
   
   
 known as current gain parameters.
Then dividing equation (i) by IB, we will get: 

        …. (ii)
If we divide equation (i) by IC, we will get: 
   

    
   
     
 …. (iii)
Combining equations (ii) and (iii):       
    

       
  
2.2 Transistor Configuration & Input-Output Characteristics
As the Bipolar Transistor is a three terminal device, there are basically three possible ways to connect it within an
electronic circuit with one terminal being common to both the input and output. Each method of connection responding
differently to its input signal within a circuit as the static characteristics of the transistor vary with each circuit
arrangement.
# Common Emitter Configuration
In CEC, the emitter of the transistor is made common to both other terminals. The input is applied between base and
emitter while the output is taken between collector and emitter. This configuration is most widely used.
Figure: Common Emitter Circuit
Input Characteristics Curve
It shows the relationship between the input current IB and input voltage VBE for a constant output voltage VCE. The
input curve for VCE = 0 resembles the forward V-I characteristics curve of the diode because when VCE = 0, the CB
junction has no effect in the performance of the transistor. When VCE increases, the CB junction becomes reverse
biased and the positive collector starts to attract the electrons in the base which have diffused from the emitter. Due

Prepared by: Er. Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS
to this reason, the base current IB for the same VBE when VCE not equal to zero will be less than the base current for
VCE equal to zero.
Output Characteristics Curve
This shows the relationship between the output current IC and the output voltage VCE for a constant base current IB.
The output curves are divided into three regions: (a) Saturation Region, (b) Active Region and (c) Breakdown Region.
When VBE = 0.7V and VCC = 0V, both the BE junction and the CB junction are forward biased. Here, VCE = 0V and
hence IC = 0. When both junctions are forward biased, the transistor is in the saturation region of its operation. As VCC
is increased. VCE increases gradually and hence increases IC. When VCE exceeds 0.7V, the CB junction becomes reverse
biased and the transistor goes into the active or linear region of its operation. Once the CB junction is reverse biased,
IC levels off and remains essentially constant for a given value of IB as VCE continues to increase. When VCE reaches
a sufficiently high voltage, the reverse biased CB junction goes into breakdown and the collector current increases
rapidly.
# Common Base Configuration
In CBC, the base of the transistor is made common to both emitter and collector terminal. The input is applied between
the emitter and base while the output is taken between the collector and base.
Figure: Common Base Configuration
The collector current consists of two parts: (a) the current produced by normal transistor action, i.e. component
depending upon emitter current (αIE) which is produced by majority carriers and (b) the leakage current due to the
movement of minority carriers across CB junction on account of it being reverse biased. This leakage current flows
in the same direction as that of IC and is abbreviated as ICBO (Collector Base current with Emitter open).
Then total collector current, IC = αIE + ICBO. (ICBO is usually very small and may be neglected in transistor calculations)
Then, IC = αIE + ICBO. = IC = α(IB + IC)+ ICBO = αIB + αIC +ICBO
Or, IC(1-α) = αIB + ICBO

Prepared by: Er. Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS
Or, IC = IB[α/(1-α)] + ICBO [1/(1-α)] = β.IB + (β+1).ICBO = β.IB + γ.ICBO
Input Characteristics Curve
The graph of IE versus VBE resembles that of a forward biased diode. However, the exact shape of this curve depends
on the reverse biasing output voltage, VCB. The greater the value of VCB, the more readily minority carriers in the base
are swept through the CB junction. The figure alongside represents the different input DC characteristics curves of
CB transistor for different values of VCB.
Output Characteristics Curve
From the figure, we note that each curve starts at IC = 0 and rises rapidly for a small positive increase in VCB. A very
small portion of emitter current is able to enter the collector region until the reverse biasing voltage VCB is allowed to
reach 0 value large enough to propel all carriers across the junction. When VCB is negative, the junction is actually
forward biased. Thus the portion of the plot where VCB is negative is called the saturation region of the transistor.
Once VCB reaches a value large enough to ensure that a large portion of carriers enter the collector (close to 0), we see
that the curves more or less level off. In other words, for a fixed emitter current, the collector current remains
essentially constant for further increase in VCB. Note that IC ~ IE in active region.
# Common Collector Configuration
In CCC, the collector terminal is made common to both emitter and base terminals. The input is applied between base
and collector while the output is taken between emitter and collector.
Figure: Common Collector Circuit

Prepared by: Er. Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS
Input Characteristics Curve
IB versus VCB for VCE = constant (as we have VCB = VCE – VBE, since, VBE ~ 0.7V, VCB = VCE – 0.7). Therefore, in
order to keep the CB junction reversed biased (VCB > 0), it is necessary that VCB be larger than VCE – 0.7 V.
From the figure alongside, we can see that each curve is drawn for a different fixed value of VCE and that each shows
the base current going to 0 very quickly as VCB increases slightly. This behavior can be explained by remembering
that VBE must remain in the neighborhood of 0.5V to 0.7V in order for any appreciable base current to flow. But, VBE
= VCE – VCB. Therefore, if the value of VCB is allowed to a point where it is near the value of VCE, the value of VBE
approaches 0, and no base current will flow. Let, VCE = 2V, when VCB = 1.3V, then VBE = 2-1.3 = 0.7V and we
therefore expect a substantial base current. If VCB is now allowed to increase to 2V, then VBE = 2-2 = 0V, and the BE
junction is no longer forward biased and hence IB = 0 when VCE = VCB = 2V.
Output Characteristics Curve
Plot of IE versus VCE for IB = constant
These characteristics curves are very much similar to those of CE configuration. Only the difference is that IE = (β+1)IB
or IE = IC + IB has been used as output current here instead of IC.
# Properties of Transistor Circuit Configuration
Properties
CBC
CEC
CCC
Voltage Gain: Vout/Vin
Highest
< that of CBC
Least (< 1)
Current Gain: Iin/Iout
Least (< 1)
< that of CCC
Highest
Power Gain: Pout/Pin
Not very high
Highest
Not very high
Input Resistance (Rin)
Least (10-100 ohms)
> that of CBC
Highest (10 – 100kΩ)
Output Resistance (Rout)
Highest (1KΩ to MΩ)
< that of CBC
Least (10-100Ω)
Polarity of Output Signal
No change
Inverts the Polarity
No change
Harmonic Distortion
Few %
Maximum (5 – 20%)
Least (< 1%)
#Note:
- Due to the very low input resistance and very high output resistance, CBC is not often used in the preamplifier stage.
- Due to the low harmonic distortion the CBC is very often used in the power amplifier stage.
- Due to the highest power gain, the CEC is very often used in reapplication and power amplification stage.
- Due to small power gain, CCC is seldom used.

Prepared by: Er. Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS
2.3 DC Load Line in Transistor
The figure below are for the DC biased CE configuration with the help of DC Load Line. We can easily analyze the
performance of an amplifier circuit. Applying KVL to the above circuit
VCC = ICRC + VCE.
When, VCE = 0V, then VCC = IcRC => IC = VCC/RC and when, IC = 0V, then VCC = VCE
So, joining the two points (Vcc, 0) and (0, VCC/RC) one in VCE axis and the other in IC axis gives the DC Load Line
as shown. This load line depends on the value of RC which is the DC load of the circuit.
# Operating Point
The point intersected by the load line and output characteristic curve is known as the operation point. This point gives
the value of DC collector current IC and DC collector to emitter voltage VCE for a particular value of DC input base
current, IB. This point is also known as Quiescent Point (stable point) or simply Q-point. Since it is a point on output
characteristic curve when the transistor is in a silent condition (i.e. the absence of ac input signal).
2.4 Analysis of Amplifier Using DC Load Line
The figure alongside shows an
amplifier circuit in common
emitter (CE) configuration. The
EB junction is said to be forward
biased while battery VCC is used
to reverse bias the CB junction.
The resistors R1 and R2 are
connected to limit the input or
base current. The resistor RL
acts as a load resistance. The
amplified ac output voltage
appears across load resistor RL.
The input ac signal to be
amplified is Vin which is
applied to the base through
coupling capacitor C1.
Figure : CE Amplifier

Prepared by: Er. Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS
The input coupling capacitor block DC, hence DC current flows only through RB and is prevented from reaching the
input signal source. The coupling capacitor C2 also blocks the DC from reaching the output terminals only amplified
output ac signal voltage Vout appears. A transistor may be used to amplify ac signals after fixing its operating point
suitably. The Q-point must lie in the middle portion on the output characteristics of a transistor. This helps the
transistor to amplify ac signals faithfully i.e. without producing distortion in wave shape of input signal.
In quiescent condition, the base current is fixed at a constant dc value. When we apply an ac signal at the input of the
amplifier circuit, the base voltage varies as per the signal voltage VS. Due to this, the base current will also vary. Now,
as the base current varies, the instantaneous operating point of the transistor moves along the DC Load Line. This
means that the instantaneous values of the collector current and voltage also vary according to the input signal. This
variation in collector voltage is several times larger than the variation of the input signal. Also, this collector voltage
variation reaches the output terminals through coupling capacitor. Thus, the output signal is several time larger than
the input signal.
Example:
Let biased condition be IB = 50A, IC = 4.8mA and VCE = 5.8 V.
Now, let us apply a small ac signal voltage of 10mV at the input. By the application of this input ac voltage, the base
current varies between 20A and 80A as shown in the wave diagram below.
Figure: CE Amplifier Example

Prepared by: Er. Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS
As the instantaneous operating point moves along the DC Load Line between point M and N, both the collector current
and collector voltage would vary. The collector current varies between the points 2mA and 7.7mA while the voltage
VCE varies between the points 2V and 9.3V as shown. The input peak-to-peak voltage is just 20mV in our example.
This has resulted 7.34V peak-to-peak output voltage VCE. Hence, there is voltage amplification. The input peak-to-
peak current is just 60A. This has resulted 5.7mA peak-to-peak output current (collector current). Hence, there is
current amplification.
Then
(a) Voltage gain (Av) = VCE/VBE ,
(b) Current Gain (Ai) = IC/IB, and
(c) Power Gain (Ap) = Av X Ai.
2.5 Transistor as a Switch
Transistor switches can be used to switch a low voltage DC device (e.g. LED’s) ON or OFF. The areas of operation
for a transistor switch are known as the Saturation Region and the Cut-off Region. This means then that we can
ignore the operating Q-point biasing and voltage divider circuitry required for amplification, and use the transistor as
a switch by driving it back and forth between its “fully-OFF” (cut-off) and “fully-ON” (saturation) regions.
# Cut-off Characteristics
# Saturation Characteristics

Prepared by: Er. Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS
2.5 BJT Switching Time
When a transistor is used as a switch, it is usually made to operate alternately in the cut-off and saturation mode. Let
us consider a transistor circuit as shown in figure. It is driven by the pulse waveform. This waveform makes transitions
between the voltage levels V2 and V1. At V2, the transistor is at cut-off and at V1, the transistor is in saturation. The
collector current does not immediately respond to the input signal. Instead, there is a delay and the time that elapses
during this delay, together with the time required for the current to rise to 10% of its maximum (saturation) value is
called the Delay Time (td).
Figure: BJT Switching Time
The current waveform has a non-zero Rise Time ‘tr’ here, which is the time required for the current to rise from 10 to
90% of ICS. The total turn ON time ‘tON’ is the sum of the Delay Time and Rise Time.
i.e. tON = td + tr
When the input signal returns to its initial state at t = T, the current again fails to respond immediately. The interval
which elapses between the transition of the input waveform and the time when IC has dropped to 90% of ICS is called
the Storage Time ‘ts’. The storage interval is followed by the Fall Time ‘tf’, which is the time required for IC to all
from 90% to 10% of ICS.
i.e. tOFF = ts + tf
# The Delay Time
Three factors contribute to the delay time: first, when the driving signal is applied to the transistor input, a non-zero
time is required to charge up the emitter junction transition capacitance so that the transistor may be brought from cut
off to the active region. Second, even when the transistor has been brought to the point where minority carriers have
begun to cross the emitter junction, into the base, a time interval is required before these carriers can cross the base
region to the collector junction and be recorded as collector current. Finally, a time is required for the collector current
to rise to 10% of its maximum.
# The Storage Time
The failure of the transistor to respond to the trailing edge of the driving pulse for the time interval ‘ts’ results from
the fact that a transistor in saturation has saturation charge of excess minority carriers stored in the base. The transistor
cannot respond until this saturation excess charge has been removed.

Prepared by: Er. Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS
# Maximum Voltage Rating
Even though the rated dissipation of a transistor is not exceeded, there is a an upper limit to maximum allowable
collector junction voltage. As we know that CB is reverse biased and if VCB is very high, then the operation which is
not desirable. There are two process restricting the operation of transistor.
(a) Reach Through/Punch Through: As CB junction is reverse biased, the width of the depletion region increases
as reverse bias voltage increases. The depletion region is the area of uncovered charges on both sides of the junction
at the position occupied by impurity atoms. As reverse bias voltage is increased, the depletion region penetrates
deeper and deeper in the CB junction. As neutrality of the charge should be maintained, number of uncovered
charges on each side are equal. As base is lightly doped, the depletion region penetrates deeper in base than in
collector. This causes the depletion region to cross the base region and enter to the emitter region even at moderate
values of voltage, preventing the transistor to operate in normal region of operation. This phenomenon occurs at
fixed VCB and is independent of circuit configuration.
(b) Avalanche Breakdown: Avalanche breakdown is due to the maximum reverse bias potential applied across the
base and collector junction.
2.6 The Ebers-Moll Bipolar Junction Transistor Model
The bipolar junction transistor can be considered essentially as two p-n junctions placed back-to-back, with the base
p-type region being common to both diodes. This can be viewed as two diodes having a common third terminal as
shown in figure below.
Figure: Bipolar Transistor Shown as Two Back-to-Back p-n Junctions
However, the two diodes are not in isolation, but are interdependent. This means that the total current flowing in each
diode is influenced by the conditions prevailing in the other. In isolation, the two junctions would be characterized by
the normal Diode Equation with a suitable notation used to differentiate between the two junctions as can be seen in
Fig. 2.2. When the two junctions are combined, however, to form a transistor, the base region is shared internally by
both diodes even though there is an external connection to it.
(a) B – E Junction in Isolation (b) B – C Junction in Isolation
In the forward active mode, αF of the emitter current reaches the collector. This means that αF of the diode current
passing through the base-emitter junction contributes to the current flowing through the base-collector junction.
Typically, αF has a value of between 0.98 and 0.99. This is shown as the forward component of current as it applies to
the normal forward active mode of operation of the device.

Prepared by: Er. Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS
Note this current is shown as a conventional current in figure below. It is equally possible to reverse the biases on the
junctions to operate the transistor in the “reverse active mode”. In this case, αR times the collector current will
contribute to the emitter current. For the doping ratios normally used the transistor will be much less efficient in the
reverse mode and αR would typically be in the range 0.1 to 0.5.
Combined B-E and B-C Junctions
Figure: The n-p-n Transistor Considered as Combined p-n Junctions
# Ebers-Moll Equations
The Ebers-Moll transistor model is an attempt to create an electrical model of the device as two diodes whose currents
are determined by the normal diode law but with additional transfer ratios to quantify the interdependency of the
junctions as shown below.
Figure: The Ebers-Moll Model of an n-p-n Bipolar Junction Transistor
Two dependent current sources are used to indicate the interaction of the junctions. The interdependency is quantified
by the forward and reverse transfer ratios, αF and αR. The diode currents are given as:
  
 
  
 
Applying Kirchhoff’s laws to the model gives the terminal currents known as Ebers-Moll Equations for BJT.
     
   
 
     
   
 , and
           
⸫    
     
   

Prepared by: Er. Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS
# Modes of Operation
The Ebers-Moll BJT Model is a good large-signal, steady-state model of the transistor and allows the state of
conduction of the device to be easily determined for different modes of operation of the device. The different modes
of operation are determined by the manner in which the junctions are biased. The charge profiles for each mode are
shown in Fig. 2.4.
(a) Cut-Off Mode
 B-E unbiased, VBE = 0V  
 
 B-C reverse-biased, VBC negative  
 , then
   (leakage current in nA range)
   (leakage current in nA range)
    
(b) Forward Active Mode
 B-E forward-biased, VBE positive  
 
 B-C reverse biased, VBC negative  
 , then from the model,
   
 (relatively large)
   
  (relatively large)
   
   (small)
(c) Reverse Active Mode
 B-E reverse-biased, VBE negative  
 
 B-C forward-biased, VBC positive  
 , essentially the transistor conducts in the opposite direction.
This mode mainly used for current steering in switching circuits. e.g. TTL. From the model,
   
 (moderately high)
   
 (moderate)
   
   (as high as 0.5|Ic|)
(d) The Saturation Mode
 B-E forward biased, VBE positive  
 
 B-C forward-biased, VBC positive  
 , then
   
  

   
  
, and
   
    
  
In this case, with both junctions forward biased, VBE ≈ 0.8V, VBC ≈ 0.7V and VCE = VBE - VBC = 0.1V. Hence, there
is a 0.1V drop across the transistor from collector to emitter which is quite low while a substantial current flows
through the device. In this mode it can be considered as having a very high conductivity and acts as a closed switch
with a finite resistance or conductivity.

Prepared by: Er. Shree Krishna Khadka
Lecturer: AITM, KIST, NCIT, MAMTS
Figure: Charge Profiles for Modes of Operation of n-p-n BJT
Figure: Transistor Equivalent Circuit during Cut-off and Saturation Mode