Bipolar Junction Transistors: Working and Operation Modes

1. Bipolar Junction Transistors Dr.S Sujatha Lekshmy Assistant Professor Dept. of Physics NSS College Pandalam

2. Bipolar Junction Transistors • The transistor is a three-layer semiconductor device consisting of either two n- and one p- type layers of material or two p- and one n- type layers of material. i) There are two junction, so transistor can be considered as two diode connected back to back. • two types of BJT- i) pnp transistor ii) npn transistor

3. ii) There are three terminals. • Emitter: supplies charge carriers (electrons or holes) • Always forward biased w.r.t. Base • Collector: collects charges • Reverse biased • Remove charges from the junction with the base • Base: middle section between emitter and collector • Base –emitter junction is forward biased allowing low resistance for emitter circuit • Base-collector region reverse biased allowing high resistance in the collector circuit

5. Some important factors to be remembered- • The transistor has three region named emitter, base and collector. • The Base is much thinner than other region. • Emitter is heavily doped so it can inject large amount of carriers into the base. • Base is lightly doped so it can pass most of the carrier to the collector. • Collector is moderately doped

6. Some important factors to be remembered- • The junction between emitter and base is called emitter-base junction(emitter diode) and junction between base and collector is called collector-base junction(collector diode). • The emitter diode is always forward biased and collector diode is reverse biased. • The resistance of emitter diode is very small(forward) and resistance of collector diode is high(reverse).

7. Transistor Operation 1) Working of npn transistor: Forward biased EB junction causes electrons in the n-type emitter to flow towards the base. This constitutes emitter current (IE )  As this electrons flow toward p-type base, they try to recombine with holes. As base is lightly doped only few electrons recombine with holes within the base (<5%). These recombined electrons constitute small base current(IB). The remainder electrons (>95%)crosses base and constitute collector current (IC) So emitter current is the sum of collector and base current IE =IC+IB

9. . Transistor Operation 2) Working of pnp transistor:  Forward bias is applied to emitter- base junction and reverse bias is applied to collector- base junction. The forward bias in the emitter-base junction causes holes to move toward base. This constitute emitter current, IE  As this holes flow toward n-type base, they try to recombine with electrons. As base is lightly doped only few holes recombine with electrons within the base. These recombined holes constitute small base current. The remainder holes crosses base and constitute collector current. So IE =IC+IB

10. Transistor Operating Modes • Active Mode  Base- Emitter junction is forward and Base- Collector junction is reverse biased. • Saturation Mode  Base- Emitter junction is forward and Base- Collector junction is forward biased. • Cut-off Mode  Both junctions are reverse biased.

12. Transistor Amplifier • A transistor acts as an amplifier by raising the strength of a weak signal • . The DC bias voltage applied to the emitter base junction, makes it remain in forward biased condition. •  The emitter current caused by the input signal contributes the collector current, flows through the RL, results in a large volt. drop across it. Thus a small input volt. results in a large output volt.

13. Transistor Connection • Transistor can be connected in a circuit in following three ways- 1) Common Base 2) Common Emitter 3) Common Collector

14. BE junction JE is F- biased by the supply volt. VBE & CB junction JC is reverse biased by the supply voltage VCB. • Due to the F- bias volt. VBE, the free electrons (majority carriers) in the emitter region experience a repulsive force from the negative terminal of the battery &  holes in the base region experience a repulsive force from the positive terminal of the battery.  • free electrons start flowing from emitter to base similarly holes start flowing from base to emitter. Thus free electrons which are flowing from emitter to base and holes which are flowing from base to emitter conducts electric current. 

15. only a small percentage of free electrons from emitter region will combine with the holes in the base region • remaining large number of free electrons cross the base region and enters into the collector region, will experience an attractive force from the positive terminal of the battery. • Therefore, the free electrons in the collector region will flow towards the positive terminal of the battery. Thus, electric current is produced in the collector region.

16. electric current produced at the collector region is primarily due to the free electrons from the emitter region & electric current produced at the base region is also primarily due to the free electrons from emitter region. • Therefore, the emitter current is greater than the base current and collector current. • The emitter current is the sum of base current and collector current. • IE = IB + IC

17. The output collector current is less than the input emitter current, so the current gain of this amplifier is actually less than 1. • In other words, the common base amplifier attenuates the electric current rather than amplifying it. • BE junction JE at input side acts as a forward biased diode. So the CB amplifier has a low input impedance (low opposition to incoming current). CB junction JC at output side acts like a reverse biased diode. • So the common base amplifier has high output impedance. • Therefore, the common base amplifier provides a low input impedance and high output impedance.

18. Transistors with low input impedance and high output impedance provide a high voltage gain.  • Even though the voltage gain is high, the current gain is very low and the overall power gain of the common base amplifier is low as compared to the other transistor amplifier configurations. • The common base transistor amplifiers are primarily used in the applications where low input impedance is required.

19. Current Amplification factor (α) The ratio of output current to input current is known as a current amplification factor. ΔIC is the change in the collector & ΔIE is changed in emitter current at constant VCB

20. Collector Current • The base current is because of the recombination of the electrons and holes in the base region. The whole emitter current will not flow through the current. • The collector current increase slightly because of the leakage current flows due to the minority charge carrier. The total collector current consists; • The large percentage of emitter current that reaches the collector terminal, i.e., αIE. • The leakage current Ileakage. The minority charge carrier is because of the flow of minority charge carrier across the collector-base junction as the junction is heavily reversed. Its value is much smaller than αIE.

21. This leakage current is represented by as ICBO, i.e., collector-base current with emitter circuit is open. Characteristics of Common Base (CB) Configuration

22. Input Characteristic

23. Out put characteristics the variation of collector current, IC with VCB when the emitter current, IE is held constant Current amplification factor

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