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EST 130, Bipolar Junction Transistors
- 1. 1 EST 130, Bipolar Junction Transistors MEC
- 2. 2 Contents • Introduction. • BJT (Transistor) Types. • Operating Regions. • Early Effect. • Transistor Configurations. • Transistor Characteristics. • Comparison. • Transistor Testing.
- 4. 4 Transistor Invention • First transistor called a point-contact transistor on December 23, 1947. • Walter H. Brattain and John Bardeen demonstrated the amplifying action of first transistor at Bell Telephone Laboratories. • Was the predecessor to the junction transistor invented by Shockley in 1948. • Shared Nobel Prize in Physics in 1956.
- 5. 5
- 6. 6 Bipolar Junction Transistor • Three terminal device – Emitter, Base and Collector. • Two p-n junctions – Emitter-Base junction and Collector-Base junction. • PNP and NPN transistors. • Current controlled – base current controls the device. • Can be used in amplifiers or as switches. • Four regions of operation. • Three Configurations – Common Emitter, Common Base and Common Collector.
- 7. 7 Bipolar Junction Transistor • Three doped regions. • Base narrow region sandwiched between the larger collector and emitter regions. • Emitter region heavily doped. • Emitter injects current carriers into the base. • Base region very thin and lightly doped. Electrons are the majority carriers in a n type material. Holes are the majority carriers in a p type material.
- 8. 8 Bipolar Junction Transistor • Most of the current carriers injected into the base from the emitter pass on to the collector, do not flow out of the base lead. • Collector region moderately doped, largest of all three regions. • Collector region attracts carriers injected into the base region. • Collector region the largest region, must dissipate more heat than emitter or base regions.
- 9. 9 Doping in Transistor Structures Emitter Heavily Doped Base Lightly Doped Collector Moderately Doped e b c Thin Largest Two types of carriers – holes and electrons, hence bipolar.
- 10. 10 Depletion Regions (Unbiased) • Diffusion of electrons from both n regions into the p -type base causes a barrier potential for both p-n junctions. • e-b depletion region narrower than the c-b depletion region due to difference in doping levels. Barrier Potential = 0.7 V
- 11. 11 Depletion Regions (Unbiased) • Heavy doping in the emitter region. • Penetration into the n material minimal due to the availability of many free electrons. • Fewer free electrons due to moderate doping level in the collector region. • Depletion layer penetrate deeper into the collector region.
- 12. 12 Bipolar Junction Transistors Can two back to back connected pn junction diodes function as a npn transistor? Base width higher? Recombination in the base?
- 13. 13 Bipolar Junction Transistors • Two back-to-back pn diodes can't function as a single BJT. • Transistor operation not achieved with standalone pn diodes which conduct a negligible currents under reverse bias. • Excess electrons from the p side of the forward biased diode can not be swept to the p side of the reverse biased diode through the metal wire in a "BJT like diode configuration".
- 14. 14 Bipolar Junction Transistors • Excess electrons from the p side of the forward biased diode are swept to the power supply (stronger pull) providing a voltage bias to the common terminal of the diodes. • For transistor functionality, semiconductor only thin lightly doped Base region required. • With a metal introduced in the path (two back-to-back diodes), no BJT functionality possible.
- 15. 15 Transistor Operating Regions Operating Region Emitter – Base Junction Collector – Base Junction Applications Active Region Forward Biased Reverse Biased Amplifiers Saturation Region Forward Biased Forward Biased Switches (on/off) Cut off Region Reverse Biased Reverse Biased Inverse Active Region Reverse Biased Forward Biased -
- 16. 16 Transistor Currents • For a transistor amplifier, emitter-base junction forward-biased, and collector- base junction reverse-biased. • Common connection for voltage sources at the base lead. • Electrons in the n -type emitter repelled into the base by the negative terminal of the emitter supply voltage.
- 17. 17 Transistor Currents • Base is very thin and lightly doped. • Only a few electrons combine with holes in the base. • Small current flowing out of the base lead called recombination current. • Free electrons injected into the base must fall into a hole before they can flow out the base lead.
- 18. 18 Transistor Currents • Most of the emitter-injected electrons pass through the base region to the collector region. • A small voltage creates a strong electric field in the collector-base junction to attract almost all free electrons injected into the base. • Positive collector-base voltage attracts free electrons in the p - type base to the collector side before they recombine with holes in the base.
- 19. 19 Early Effect/Base Width Modulation • As reverse bias across the collector-base junction increases, depletion region penetrates more into the base. • Thin base lightly doped, penetration into the base region larger than into the collector, effective base width decreases/ modulated. • Punch through – large reverse bias, depletion region penetrates the entire base, transistor action lost, large current flow damage.
- 21. 21 Common Base Configuration • Collector current nearly identical to the emitter current. IC ≈ IE RC, RE for Current Limiting. RC RE RC RE Common Base Configuration Base is Common
- 22. 22 Common Base Current Gain • dc alpha (αdc) describes how closely the emitter and collector currents are in a common base circuit. • In most cases, the dc alpha 0.99 or greater. • The thinner and more lightly doped the base, the closer the value of alpha to one.
- 23. 23 Circuit for Common Base Characteristics Input Side Output Side
- 24. 24 Common Base Characteristics Input Characteristics Output Characteristics
- 25. 25 Common Emitter Configuration Emitter common to input and output. @ 25oC Phase Inversion.
- 26. 26 Common Emitter Current Gain • dc current gain of a transistor in common- emitter connection called dc beta. • Again, • Or, DC Equivalent of a Transistor
- 27. 27 Circuit for Common Emitter Characteristics
- 29. 29 Common Emitter Output Characteristics Active Region Saturation Region Cut Off Region CE ac C V r I C ac B I I
- 30. 30 Common Emitter Amplifier RC Coupled Amplifier Voltage Divider Bias Coupling Capacitor For Current Limiting
- 32. 32 CB and CE Parameters
- 33. 33 Common Collector Configuration Collector common to input and output Emitter Follower. No phase inversion for the output. For Impedance Matching. Voltage Gain ≈ 1. Used in cascade amplifiers.
- 34. 34 Common Collector Current Gain • Common Collector Current Gain: • Again and E dc B I I 1 dc dc 1 1 dc dc
- 35. 35 Circuit for Common Collector Characteristics
- 38. 38 Brief Comparison Characteristic CB CE CC Input Resistance Low Medium High Output Resistance Very High High Low Voltage Gain High Medium Low Current Gain Low Medium High
- 39. 39 Darlington Pair Current Gain Improves β1 β2 βd = β1.β2
- 41. 41
- 42. 42 Transistor Testing • Identify whether pnp or npn. • Identify transistor leads. • Use of Ohmmeter/Multimeter. • Consider the transistor as two diodes. • Emitter-base and collector-base junctions forward biased – low resistance. • Emitter-base and collector-base junctions reverse biased – high resistance. • Emitter to collector – high resistance.
- 44. 44 Thank You