Introduction to bjt npn &pnp

BJT-CE,CB&CC

1. STUDENTS

2. R.M.K COLLEGE OF ENGINEERING AND TECHNOLOGY DEPARTMENT OF ECE EC8252-ELECTRONIC DEVICES SECOND SEMESTER-I YEAR- (2020-2024 BATCH) Mrs.P.Sivalakshmi AP/ECE SESSION:11 DATE: 3.05.2021

3. 2017- Regulation syllabus

4. Lecture Overview on BJT What is a Transistor? History Types Characteristics Applications

5. What is a Transistor? Semiconductors: ability to change from conductor to insulator Can either allow current or prohibit current to flow Useful as a switch, but also as an amplifier Essential part of many technological advances

6. A Brief History Guglielmo Marconi invents radio in 1895 Problem: For long distance travel, signal must be amplified Lee De Forest improves on Fleming’s original vacuum tube to amplify signals Made use of third electrode Too bulky for most applications

7. The Transistor is Born Bell Labs (1947): Bardeen, Brattain, and Shockley Originally made of germanium Current transistors made of doped silicon

8. How Transistors Work Doping: adding small amounts of other elements to create additional protons or electrons P-Type: dopants lack a fourth valence electron (Boron, Aluminum) N-Type: dopants have an additional (5 th ) valence electron (Phosphorus, Arsenic) Importance: Current only flows from P to N

9. Diodes and Bias Diode: simple P-N junction. Forward Bias: allows current to flow from P to N. Reverse Bias: no current allowed to flow from N to P. Breakdown Voltage: sufficient N to P voltage of a Zener Diode will allow for current to flow in this direction.

10. The Invention Important Features ( compared to Vacuum tubes ) : - three terminal solid-state device - requires less power - smaller and lightweight - lower operating voltage - has rugged construction - more efficient - no heater requirement The First Transistor : On Dec 23, 1947 , three scientists led by Dr. William Shockley at the Bell Telephone Laboratories demonstrated the amplifying action of the first transistor . ( Courtesy Bell Telephone Laboratories .) Co-inventors: Dr. William Shockley (seated); Dr. John Bardeen (left); Dr. Walter H. Brattain. Honored with Nobel Prize in Physics in 1956

11. Bipolar Junction transistor Holes and electrons determine device characteristics Three terminal device Control of two terminal currents Amplification and switching through 3 rd contact The term transistor was coined by John R. Pierce as a contraction of the term trans resistance.

12. The Structure Bipolar : both electrons and holes are involved in current flow. Junction : has two p-n junctions. Transistor : Transfer + Resistor. It can be either n-p-n type or p-n-p type. Has three regions with three terminals labeled as i. Emitter (E) ii. Base (B) and iii. Collector (C) The Bipolar Junction Transistor (BJT)

13. The Structure: npn & pnp Base is made much narrow. Emitter is heavily doped ( p + , n + ). Base is lightly doped ( p - , n - ). Collector is lightly doped ( p, n ).

14. The Structure: npn & pnp Transistors can be constructed as two diodes that are connected together.

15. Circuit Symbol The arrow indicates the direction of current flow. The current flows from collector to emitter in an n-p-n transistor. The arrow is drawn on the emitter. The arrow always points towards the n-type. So the emitter is n-type and the transistor is n-p-n type. Layout and Circuit Symbol: n-p-n Transistor

16. Circuit Symbol The arrow indicates the direction of current flow. The current flows from emitter to collector in an p-n-p transistor. The arrow points towards the n-type. So the base is n-type and transistor is p-n-p type. Layout and Circuit Symbol: p-n-p Transistor

17. Modes of Operation Based on the bias voltages applied at the two p-n junctions, transistors can operate in three modes : 1. Cut-off (both EB and CB junctions are reversed biased) 2. Saturation (both EB and CB junctions are forward biased) 3. Active mode (EBJ is forward biased and CBJ is reversed biased) Cut-off and Saturation modes are used in switching operation. Active mode is used in amplification purposes.

18. Modes of Operation npn bipolar junction transistor pnp bipolar junction transistor

19. Modes of Operation Both the junctions are reversed biased. No current can flow through either of the junctions. So the circuit is open. Cut-off V BC + + - - V BE Ideal model of BJT in cut-off.

20. Modes of Operation Both the junctions are forward biased. So the equivalent circuit can be represented by short-circuit between the base, emitter and collector. Saturation: Ideal Model V BC + + - - V BE Ideal model of BJT in saturation.

21. Active Mode Operation EBJ: Forward Biased CBJ: Reverse Biased Forward bias of EBJ injects electrons from emitter into base ( Emitter current ). Most electrons shoot through the base into the collector ( Collector current ). Some emitted electrons recombine with holes in p-type base ( Base Current )

22. + + + + + + + + + + - - - - - - - - - - N P N E B C electron Hole Active Mode Operation

23. + + + + + + - - - - - - N P N E C B V BE V CB E-Field Electron diffusion Hole diffusion Active Mode Operation

24. + + + + + + - - - - - - N P N E-Field E C B V BE V CB Electron hole recombination Active Mode Operation

25. Collector current The equation above shows that the BJT is indeed a voltage-dependent current source; thus it can be used as an amplifier . Electrons that diffuse across the base to the CBJ junction are swept across the CBJ depletion to the collector because of the higher potential applied to the collector

26. Active Mode Operation Biasing for Active Mode Carriers injected from forward bias junction (from the emitter labeled E) travel through the intermediate layer (BASE, labeled B) and swept into the COLLECTOR, labeled C by the reverse biased voltage. EBJ: Forward Biased CBJ: Reverse Biased

27. Conceptual Biasing Circuits npn Transistor

28. Conceptual Biasing Circuits pnp Transistor

29. DC Output Characteristics I C vs V CE Characteristics of an npn Transistor Active Breakdown Saturation 

30. 31 Active Breakdown Saturation   Cutoff DC Output Characteristics I C vs V CE Characteristics of an npn Transistor

31. Bipolar Junction Transistors: Basics + - + - I E I B I C I E = I B + I C ………(KCL) V EC = V EB + V BC ……… (KVL)

32. Current Gain : α is the fraction of electrons that diffuse across the narrow Base region 1- α is the fraction of electrons that recombine with holes in the Base region to create base current The current Gain is expressed in terms of the β (beta) of the transistor (often called h fe by manufacturers ). β (beta) is Temperature and Voltage dependent . It can vary a lot among transistors (common values for signal BJT: (20 - 200). BJT characteristics

33. Active Mode: Terminal Currents Current Relationships and Amplification As a is close to unity, b is very large, typically around 100. b represents the current amplification factor from base to collector. The base current is amplified by a factor of b in the collector circuit in the Active mode. b is called the Forward Current Gain , often written as b F .

34. Operation Region I B or V CE Char. BC and BE Junctions Mode Cutoff I B = Very small Reverse & Reverse Open Switch Saturation V CE = Small Forward & Forward Closed Switch Active Linear V CE = Moderate Reverse & Forward Linear Amplifier Break-down V CE = Large Beyond Limits Overload Operation region summary

35. BJT configurations GAIN CONFIG

36. How transistor works: https ://www.youtube.com/watch?v=7ukDKVHnac4