1. Presentation On BIPOLAR JUNCTION TRANSISTOR
Department of ECE / EEE

2. Introduction
A bipolar junction transistor (BJT) is a three terminal semiconductor device in which the operation depends on the interaction of both majority and minority carriers and hence the name Bipolar.
The BJT is analogous to a vacuum triode and is comparatively smaller in size.
It is used in amplifier and oscillator circuits, and as a switch in digital circuit.
It has wide applications in computers, satellites and other modern communication system.
Department of ECE / EEE

3. Construction
The BJT consists of a silicon (or germanium) crystal in which a thin layer of N-type silicon is sandwiched between two layers of P-type silicon.
This transistor is referred to as PNP. Alternatively, in a NPN transistor, a layer of P-type material is sandwiched between two layers of N-type material.
Department of ECE / EEE

4. NPN and PNP N P B E C P N B E C
NPN
PNP
Department of ECE / EEE

5. Base is lightly doped and very thin.
The three portions of the transistor are Emitter, Base and Collector, shown as E, B, and C, respectively.
The arrow on the emitter specifies the direction of current flow when the EB junction is forward biased.
Emitter is heavily doped so that it can inject a large number of charge carriers into the base.
Base is lightly doped and very thin.
It passes most of the injected charge carriers from the emitter into the collector.
Collector is moderately doped.
Department of ECE / EEE

6. Symbol of NPN transistor Symbol of PNP transistorE C B E C
Symbol of NPN transistor
Symbol of PNP transistor
Department of ECE / EEE

7. Transistor biasing
The following figure shows, usually the emitter-base junction is forward biased and collector-base junction is reverse biased.
Due to the forward bias on the emitter-base junction, an emitter current flows through the base into the collector.
Through the collector-base junction is reverse biased, almost the entire emitter current flows through the collector circuit.
Department of ECE / EEE

8. NPN Transistor biasing
Department of ECE / EEE

9. PNP Transistor biasing
Department of ECE / EEE

10. Active-mode NPN transistors in circuits
Department of ECE / EEE

11. The diagram opposite is a schematic representation of an NPN transistor connected to two voltage sources. To make the transistor conduct appreciable current (on the order of 1 mA) from C to E, VBE must be above a minimum value sometimes referred to as the cut-in voltage.
The cut-in voltage is usually about 600 mV for silicon BJTs at room temperature but can be different depending on the type of transistor and its biasing.
This applied voltage causes the lower P-N junction to 'turn-on' allowing a flow of electrons from the emitter into the base. In active mode, the electric field existing between base and collector (caused by VCE) will cause the majority of these electrons to cross the upper P-N junction into the collector to form the collector current IC.
Department of ECE / EEE

12. The remainder of the electrons recombine with holes, the majority carriers in the base, making a current through the base connection to form the base current, IB.
As shown in the diagram, the emitter current, IE, is the total transistor current, which is the sum of the other terminal currents (i.e., IE = IB + IC ).
The arrows representing current point in the direction of conventional current – the flow of electrons is in the opposite direction of the arrows because electrons carry negative electric charge.
In active mode, the ratio of the collector current to the base current is called the DC current gain.
Department of ECE / EEE

13. This gain is usually 100 or more, but robust circuit designs do not depend on the exact value (for example - op-amp).
The value of this gain for DC signals is referred to as hFE, and the value of this gain for AC signals is referred to as hfe.
However, when there is no particular frequency range of interest, the symbol β is used
Department of ECE / EEE

14. Active-mode PNP transistors in circuits
Department of ECE / EEE

15. The diagram opposite is a schematic representation of a PNP transistor connected to two voltage sources. To make the transistor conduct appreciable current (on the order of 1 mA) from E to C, VEB must be above a minimum value sometimes referred to as the cut-in voltage.
The cut-in voltage is usually about 600 mV for silicon BJTs at room temperature but can be different depending on the type of transistor and its biasing.
This applied voltage causes the upper P-N junction to 'turn-on' allowing a flow of holes from the emitter into the base.
In active mode, the electric field existing between the emitter and the collector (caused by VCE) causes the majority of these holes to cross the lower P-N junction into the collector to form the collector current IC.
Department of ECE / EEE

16. The remainder of the holes recombine with electrons, the majority carriers in the base, making a current through the base connection to form the base current, IB.
As shown in the diagram, the emitter current, IE, is the total transistor current, which is the sum of the other terminal currents (i.e., IE = IB + IC).
The arrows representing current point in the direction of conventional current – the flow of holes is in the same direction of the arrows because holes carry positive electric charge. In active mode, the ratio of the collector current to the base current is called the DC current gain.
Department of ECE / EEE

17. This gain is usually 100 or more, but robust circuit designs do not depend on the exact value.
The value of this gain for DC signals is referred to as hFE, and the value of this gain for AC signals is referred to as hfe.
However, when there is no particular frequency range of interest, the symbol β is used
Department of ECE / EEE

18. Types of configuration
When a transistor is to be connected in a circuit, one terminal is used as an input terminal, the other terminal is used as an output terminal and the third terminal is common to the input and output.
Depending upon the input, output and common terminal, a transistor can be connected in three configurations.
Common base configuration
Common emitter configuration
Common collector configuration
Department of ECE / EEE

19. CB Configuration
This is also called grounded base configuration. In this configuration, emitter is the input terminal, collector is the output terminal and base is the common terminal.
Department of ECE / EEE

20. NPN common-base circuit
Department of ECE / EEE

21. CE Configuration
This is also called grounded emitter configuration. In this configuration, base is the input terminal, collector is the output terminal and emitter is the common terminal.
Department of ECE / EEE

22. NPN common-emitter circuit
Department of ECE / EEE

23. CC Configuration
This is also called grounded collector configuration. In this configuration, base is the input terminal, emitter is the output terminal and collector is the common terminal.
Department of ECE / EEE

24. NPN common-collector circuit
Department of ECE / EEE

25. Comparison of CB CE and CC configuration
S .No
Characteristics CB CE CC 1
Input impedance
Low
Medium
High 2
Output impedance
medium
low 3
Current gain 4
Voltage gain
Unity 5
Power gain 6
Phase reversal No
Yes 7
application
AF amplifiers
Voltage & power amplifiers
Impedance matching
Department of ECE / EEE

26. Transistor as a switch
When a transistor is used as a switch, it is usually required to be brought alternatively in the saturation and cut-off conditions.
When in saturation condition, it should carry heavy current, so the voltage drop across the transistor is as near to zero as possible. It may be considered as “closed switch”.
When in cut-off condition, it should carry almost no current so that it may be considered to be an “open switch”.
Department of ECE / EEE

27. Applications
The BJT remains a device that excels in some applications, such as discrete circuit design, due to the very wide selection of BJT types available, and because of its high transconductance and output resistance compared to MOSFETs.
The BJT is also the choice for demanding analog circuits, especially for very-high-frequency applications, such as radio-frequency circuits for wireless systems. Bipolar transistors can be combined with MOSFETs in an integrated circuit by using a BiCMOS process of wafer fabrication to create circuits that take advantage of the application strengths of both types of transistor.
Department of ECE / EEE

28. Temperature sensors
Because of the known temperature and current dependence of the forward-biased base–emitter junction voltage, the BJT can be used to measure temperature by subtracting two voltages at two different bias currents in a known ratio
Department of ECE / EEE

29. Logarithmic converters
Because base–emitter voltage varies as the log of the base–emitter and collector–emitter currents, a BJT can also be used to compute logarithms and anti-logarithms.
A diode can also perform these nonlinear functions, but the transistor provides more circuit flexibility.
Department of ECE / EEE

30. Vulnerabilities
Exposure of the transistor to ionizing radiation causes radiation damage. Radiation causes a buildup of 'defects' in the base region that act as recombination centers.
The resulting reduction in minority carrier lifetime causes gradual loss of gain of the transistor.
Power BJTs are subject to a failure mode called secondary breakdown, in which excessive current and normal imperfections in the silicon die cause portions of the silicon inside the device become disproportionately hotter than the others.
Department of ECE / EEE

31. The doped silicon has a negative temperature coefficient, meaning that it conducts more current at higher temperatures.
Thus, the hottest part of the die conducts the most current, causing its conductivity to increase, which then causes it to become progressively hotter again, until the device fails internally.
The thermal runaway process associated with secondary breakdown, once triggered, occurs almost instantly and may catastrophically damage the transistor package.
Department of ECE / EEE

32. The End
M.S.P.V.L Polytechnic College, Department of ECE, Pavoorchatram.
Department of ECE / EEE