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Bipolar Junction Transistor is a three layer semiconductor device, which has a wide range of application in the semiconductor industries. Three layers Emitter, Base and Collector of a BJT transistor are formed by sandwiching alternative P and N layers. Emitter is a heavily doped region of the BJT transistor which provides majority carriers into the base region. Base region is a thin, lightly doped region and is sandwiched between emitter and collector. Majority carriers from the emitter pass through the base region and its flow can be externally controlled. Collector region is moderately doped. Majority carriers from emitter are finally collected at the collector region of the BJT transistor.
A BJT transistor can be constructed in two ways. In one method, N layer is sandwiched between two P layers called PNP transistor and in the other one, the P layer is sandwiched between two N layers called NPN transistor.
Structure and Symbol of Bipolar Junction Transistor - NPN Transistor, PNP Transistor
Structure and circuit symbol of PNP transistor and NPN transistor are shown in the above figures. In NPN transistor, electrons are the majority carriers and in PNP transistor, holes are the majority carriers. Mobility of electrons is higher than that of holes and therefore, NPN transistor are more preferred in electronic circuits. All other properties of NPN transistor and PNP transistor are same.
Bipolar Junction Transistor can be defined as a three terminal current controlled device. Working of a BJT transistor is analogous to a water tap. Quantity of water flowing through the pipe can be controlled by moving the knob of the tap. Similarly, majority carrier flow from emitter to collector can be controlled by base current. The controlled power can be higher than that of the input controlling power. Therefore, a BJT transistor can be used as an amplifier.
Working of BJT Transistor - NPN Transistor
The figure shows the most common configuration of NPN transistor. Here, Emitter-Base junction is forward biased and Collector-Base junction is reverse biased. That is, for a NPN transistor, Base is more positive with respect to Emitter and Collector is more positive with respect to Base. When the circuit is turned on, higher potential at the base region of the NPN transistor pulls the electrons from the emitter of NPN transistor towards it. The electron flow that reaches the base region of the NPN transistor is again pulled by the more positive voltage of the collector region of the NPN transistor. Only very little current flows out though the base terminal of NPN transistor , since it is very thin and lightly doped, so the downward movement of electrons is restricted by its higher resistance (for downward movement, large length and small area is required).
When BJT transistor is not biased, that is, there is no voltage drop across its junctions and thus, no current flows through it. If Emitter-Base junction is forward biased and Collector-Base junction is reverse biased, the voltage across the device causes electrons from emitter to flow to collector. In this, electrons pass through P type lightly doped base region and some of the electrons recombine with holes. Therefore, collector current is less than that of emitter current. Emitter current, Base current and Collector current can be related by.
Emitter current = Base current + Collector current
Mainly three parameters are used to define BJT transistor performance. Current Amplification factor, Base Transport Factor, Emitter Injection Efficiency parameters shows the performance of NPN transistor and PNP transistor.
Current amplification factor in a BJT transistor is defined as the ratio of output current to its input current. In a common base configuration, current amplification factor is the ratio of collector current to the emitter current.
α = Ic / Ie
It is defined as the factor of base current required to transfer emitter current to collector of the BJT transistor. Base transport factor is the ratio of collector current to base current of a BJT transistor. That is, it is the ratio of output current to input current in common emitter configuration.
β = Ic / Ib
Emitter injection efficiency in a BJT transistor defines the efficiency of majority carrier injection from emitter. It is the ratio of current due to emitter majority carriers to the total emitter current. It defines the injection capability of an emitter. Heavily doped region will have high injection factor.
BJT Transistor can be modeled as two PN junctions connected back to back. Depending on the application, each junction can be forward or reverse biased independently. Thus there are four different biasing methods.
In forward active mode of BJT transistor Emitter-Base junction is forward biased and collector base junction is reverse biased. When transistor operates in this mode, collector current increases linearly with the increase in base current. Therefore when BJT transistor is used as an amplifier it is biased to operate in active mode.
Reverse active mode is also called inverse active because its biasing conditions are just opposite to that of forward active mode. That is emitter-Base junction is reverse biased and Collector-Base junction is forward biased.
BJT Transistor is symmetrical and so if bias conditions are inversed the emitter and collector of a transistor gets interchanged and current flows in the opposite direction. Because of the difference in the doping concentration and size of the collector and emitter region, current gain of the transistor is two to three times less than that in the forward active mode.
In saturation mode both Emitter-Base and Collector-Base junctions are forward biased. Maximum current flows through the transistor because in both junctions depletion width is very narrow. BJT Transistor behaves like a closed switch in this mode.
Both Emitter-Base and Collector-Base junctions are reverse biased in cut-off mode. In this mode transistor is inactive that is no current flows from emitter to collector. Transistor behaves like open switch in cut-off mode.
The three transistor configuration modes are analogous to the movement of a car. Forward active mode is when the car is moving at an average speed and its speed can by controlled by accelerator liver. Similarly current flow in the transistor in forward active mode is controlled by the base current.
Cut off mode is when the engine to the car is turned off and even if accelerator is pressed to its maximum nothing happens. Similarly for a transistor in cut off mode collector current is close to zero, increase in base current has no effect on it.
Saturation in BJT transistor is analogous to a car moving down a steep hill such that it has already achieved its maximum speed. No further increase in the speed is possible for a car in this state. For a transistor in saturation region maximum current flows through the device. Increase in base current has no effect on the collector current.
Modes of Operation of Bipolar Junction Transistor (BJT Transistor)
A BJT transistor is configured to operate in saturation and cut off mode for applications when its used as a switch. BJT Transistor in cut-off mode behaves like an open switch and transistor in saturation mode behaves like a closed switch.
For applications like amplifiers, NPN transistor and PNP transistor is biased to operate in active mode. BJT Transistor amplifies the magnitude of the signal given at the base terminal without affecting any other parameters.
In electronic circuits depending upon the applications, NPN transistor and PNP transistor can be configured as common base, common collector or common emitter. The term 'common' means that the terminal is common to both input and output. Best method to identify the configuration of NPN transistor and PNP transistor in a complicated electronic circuit is by checking terminals to which input and output is connected. Then we can conclude that third terminal is the common terminal.
It is the most frequently used BJT transistor configuration. In this, input voltage is given across Base-Emitter junction and output is taken across Collector-Emitter junction. Voltage gain, current gain and power gain of a common emitter configuration is high when compared to other transistor configurations. As input is given across forward biased junction, input resistance of a common emitter amplifier is low. Output resistance is high as the output is taken across reverse biased junction. Another important property of common emitter configuration is that, its output is phase shifted by 180 degree.
Common Emitter Configuration of NPN Transistor
In common base configuration of a BJT transistor, input is given to emitter terminal and output is taken across collector terminal. It is used in applications, where, low input impedance and high output impedance are required. Unlike common emitter, in common base configuration, input and output are in same phase. Following figure shows a PNP transistor connected in common base configuration.
Common Base Configuration of PNP Transistor
Common collector configuration of a BJT transistor is also called voltage follower or emitter follower. In this, collector is common to both input and output. As in the figure shown below, input is connected across reverse biased base-collector junction and output is taken across forward biased emitter-collector junction. Therefore, input resistance is high and output resistance is low and common collector configuration is used in applications for impedance matching. It gives high current gain but voltage gain of common collector configuration is less than unity.
Common Collector Configuration of NPN Transistor
JIS is an acronym for Japanese Industrial Standard, used in Japan. The format used is
Examples: The transistors that use JIS coding format are:
JEDEC is an acronym for Joint Electron Device Engineering Council. This system takes the format given below:
A – Represents Low gain, B – Represents Medium gain, C – Represents High Gain.
Examples: 2N3906 is a PNP transistor which comes under the JEDEC System.
Examples: BC107, BD139, AD140 etc.
By following the simple tips and tricks given below, you can easily identify your transistor.
Usually, the Pro- Electron coding system is used commonly than the JEDEC system. But both are used by many device manufacturers to code the transistors. The coding system is really helpful and helps in easy identification.
Note: Other than the JIS, JEDEC and Pro- Electron Coding system some of the electronic component manufactures bring out their own type for the commercial purposes. Some examples for this are ZTX302, TIS43 etc. Here the characters represent the particular manufacturer. Some of them are given in the table below:
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