SSGMCE, Shegaon Electronic Devices And Circuits

1. SSGMCE, ShegaonElectronic Devices And Circuits Lecture 11 Transistor Prof. A. N. Dolas, Dept. of E&TC

2. Transistors Three-terminal, solid-state electronic device used for amplification and switching. The transistor is an arrangement of semiconductor materials that share common physical boundaries. Materials most commonly used are silicon, gallium-arsenide, and germanium, into which impurities have been introduced by a process called doping. In n-type semiconductors the impurities or dopants result in an excess of electrons, or negative charges; in p-type semiconductors the dopants lead to a deficiency of electrons and therefore an excess of positive charge carriers or "holes. The PNP and NPN Junction Transistor The Field-Effect Transistor The Metal-Oxide Semiconductor Field-Effect Transistor (MOSFET) Prof. A. N. Dolas, Dept. of E&TC

3. 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 (5th) valence electron (Phosphorus, Arsenic) • Importance: Current only flows from P to N

4. npn bipolar junction transistor pnp bipolar junction transistor Bipolar Junction Transistor (BJT) • 3 adjacent regions of doped Si (each connected to a lead): • Base. (thin layer,less doped). • Collector. • Emitter. • 2 types of BJT: • npn. • pnp. • Most common: npn (focus on it). Developed by Shockley (1949)

5. BJT npn Transistor • 1 thin layer of p-type, sandwiched between 2 layers of n-type. • N-type of emitter: more heavily doped than collector. • With VC>VB>VE: • Base-Emitter junction forward biased, Base-Collectorreverse biased. • Electrons diffuse from Emitter to Base (from n to p). • There’s a depletion layer on the Base-Collector junction no flow of e- allowed. • BUT the Base is thin and Emitter region is n+ (heavily doped) electrons have enough momentum to cross the Base into the Collector. • The small base current IB controls a large current IC

6. BJT characteristics • 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 hfe by manufacturers). • β (beta) is Temperature and Voltage dependent. • It can vary a lot among transistors (common values for signal BJT: 20 - 200).

7. npn Common Emitter circuit • Emitter is grounded. • Base-Emitter starts to conduct with VBE=0.6V,IC flows and it’s IC=b*IB. • Increasing IB, VBE slowly increases to 0.7V but IC rises exponentially. • As IC rises ,voltage drop across RC increases and VCE drops toward ground. (transistor in saturation, no more linear relation between IC and IB)

8. Common Emitter characteristics Collector current controlled by the collector circuit. (Switch behavior) In full saturation VCE=0.2V. Collector current proportional to Base current The avalanche multiplication of current through collector junction occurs: to be avoided No current flows

9. Operation region summary

10. BJT as Switch • Vin(Low ) < 0.7 V • BE junction not forward biased • Cutoff region • No current flows • Vout = VCE = Vcc • Vout = High • Vin(High) • BE junction forward biased (VBE=0.7V) • Saturation region • VCE small (~0.2 V for saturated BJT) • Vout = small • IB = (Vin-VB)/RB • Vout = Low

11. BJT as Switch 2 • Basis of digital logic circuits • Input to transistor gate can be analog or digital • Building blocks for TTL – Transistor Transistor Logic • Guidelines for designing a transistor switch: • VC>VB>VE • VBE= 0.7 V • IC independent from IB (in saturation). • Min. IB estimated from by (IBmin» IC/b). • Input resistance such that IB > 5-10 times IBminbecause b varies among components, with temperature and voltage and RB may change when current flows. • Calculate the max IC and IB not to overcome device specifications.

12. BJT as Amplifier • Common emitter mode • Linear Active Region • Significant current Gain • Example: • Let Gain, b = 100 • Assume to be in active region -> VBE=0.7V • Find if it’s in active region

13. BJT as Amplifier VCB>0 so the BJT is in active region

14. 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 What is a Transistor?

15. Point-contact-transistor

16. The First Transistor The bipolar junction transistor was the first solid-state amplifier element and started the solid-state electronics revolution. Bardeen, Brattain and Shockley at the Bell Laboratories invented it in 1948 as part of a post-war effort to replace vacuum tubes with solid-state devices (Nobel Prize in 1956). Prof. A. N. Dolas, Dept. of E&TC

17. Bipolar Junction Transistors A Bipolar Transistor essentially consists of a pair of PN Junction Diodes that are joined back-to-back. This forms a sort of a sandwich where one kind of semiconductor is placed in between two others. There are therefore two kinds of Bipolar sandwich, the NPN and PNP varieties. The three layers of the sandwich are conventionally called the Collector, Base, and Emitter. Prof. A. N. Dolas, Dept. of E&TC

18. Bipolar Junction Transistors The direction of the emitter arrow defines the type transistor. Biasing and power supply polarity are positive for NPN and negative for PNP transistors. The transistor is primarily used as an current amplifier. When a small current signal is applied to the base terminal, it is amplified in the collector circuit. Prof. A. N. Dolas, Dept. of E&TC

19. Figure 1 shows the energy levels in an NPN transistor when we aren't externally applying any voltages. In each of the N-type layers conduction can take place by the free movement of electrons in the conduction band. In the P-type (filling) layer conduction can take place by the movement of the free holes in the valence band. However, in the absence of any externally applied electric field, we find that depletion zones form at both PN-Junctions, so no charge wants to move from one layer to another. Prof. A. N. Dolas, Dept. of E&TC

20. When we apply a moderate voltage between the Collector and Base and the polarity of the applied voltage is chosen to increase the force pulling the N-type electrons and P-type holes apart. (i.e. we make the Collector positive with respect to the Base.) This widens the depletion zone between the Collector and base and so no current will flow. In effect we have reverse-biased the Base-Collector diode junction. Prof. A. N. Dolas, Dept. of E&TC

21. When we apply a relatively small Emitter-Base voltage whose polarity is designed to forward-bias the Emitter-Base junction. This 'pushes' electrons from the Emitter into the Base region and sets up a current flow across the Emitter-Base boundary. Once the electrons have managed to get into the Base region they can respond to the attractive force from the positively-biased Collector region. As a result the electrons which get into the Base move swiftly towards the Collector and cross into the Collector region. Hence we see a Emitter-Collector current whose magnitude is set by the chosen Emitter-Base voltage we have applied. Prof. A. N. Dolas, Dept. of E&TC

22. Characteristic curves from a working Bipolar Transistor Each curve shows how the collector current, IC, varies with the Collector-Emitter voltage, VCE, for a specific fixed value of the Base current, IB. Prof. A. N. Dolas, Dept. of E&TC

23. Outline Types of Transistors • Bipolar Junction Transistor (BJT) Fundamentals Representation Common emitter mode (active) Operation region Applications • Field Effect Transistors (FET) Fundamentals MOSFET Operating regimes MOSFET Fundamentals JFET Operating regimes JFET application areas • Power Transistors

24. Fundamentals BJT Collector Base Emitter

25. Vc Vb Fundamentals npn BJT Common emitter mode (active) Collector Reverse bias Base Forward Bias Emitter Hole E-

26. Representation BJT N-type emitter: more heavily doped than collector

27. Common emitter mode BJT • Emitter grounded. • VBE<0.6V: transistor inactive • VBE>=0.6V :Base-Emitter conduct • IB ↑, VBE ↑ (slow) 0.7V , IC↑ exponentially.(IB =βIC) • As IC↑,voltage drop across RC increases and VCE↓ 0 V. (saturation) IB≠βIC • Q: Operating point Q

28. Operation region BJT

29. Switch Applications BJT • logic circuits • TTL • lab

30. Amplifier Applications BJT • Assume to be in active region -> VBE=0.7V • Find if it’s in active region by solving the equations

31. Other Types of Transistors

32. Discussion!! Prof. A. N. Dolas, Dept. of E&TC

33. ME 6405 Student Lecture Transistors Angela Sodemann Jin Yang Louis Nucci Friday, December 20, 2019

34. Lecture outline • Brief History of Transistors • Introduction to Transistors • Bipolar Junction Transistors (BJT) • Field Effect Transistors (FET) • Power Transistors

35. Transistor History Invention of transistors is a milestone of modern microelectronics industry • Invention: • 1947, at Bell Laboratories, by John Bardeen, Walter Brattain, and William Schockly (a three-point transistor, made with Germanium) • 1956, They received Nobel Prize in Physics "for their researches on semiconductors and their discovery of the transistor effect" • First application: replacing vacuum tubes (big & inefficient) • Today: • Advanced microprocessor uses as many as 1.7 billion transistors (MOSFETs) First model of Transistor

36. Packaging Format of Transistor • Two main categories: • Through-Hole • Surface-Mount • Packaging Materials: • Glass, metal, ceramics or plastic Through-hole transistor Surface Mount Transistor • Large transistor array uses latest Ball grid array (BGA) packaging • Power transistors have large packages clamped to heat sinks for enhanced cooling

37. What is Transistor ? • Definition: Transistor is three-terminal, solid-state semiconductor device • Function: Control electric current or voltage between two of the terminals by applying an electric current or voltage to third terminal. Transistor is an active component. • Application: • Switch in digital circuits Two operating positions: on and off. This switching capability allows binary functionality and permits to process information in a microprocessor • Amplifier in analog circuits

38. Transistor Chemistry • Silicon • Semiconductor that is able to be doped with other elements to adjust its electrical response • Arsenic, phosphorous • N-type dopants which add an electron to the silicon • Boron, aluminum • P-type dopants which have an extra “hole”

39. PN Junction • It is also called Junction Diode • Allows current to flow from P to N only • Electronsfrom n regiondiffuse to occupy holes in pregion • Thin depletion region forms near junction, resulting in contact potential (For silicon, on order of 0.6-0.7 V) • Two types of behavior: Forward and Reverse biased

40. PN Junction – Forward Biased • http://www.tpub.com/neets/book7/25k.htm

41. PN Junction – Reverse Biased • http://www.tpub.com/neets/book7/25k.htm

42. Types of Transistor Two Main Categories: • Bipolar Junction Transistor - BJT • Field Effect Transistor - FET • JFET - Junction FET • MOSFET - Metal Oxide Semiconductor FET

43. BJT

44. npn bipolar junction transistor Bipolar Junction Transistor • 3 adjacent doped regions (each layer connected to a lead) • Base (B) • Collector (C) • Emitter (E) • 2 types of BJT: • npn • pnp • Most common type: npn pnp bipolar junction transistor

45. npn BJT • 1 thin layer of p-type silicon, sandwiched between 2 layers of n-type silicon • Emitter is more heavily doped than collector • With VC>VB>VE: • Base-Emitter junction forward biased, Base-Collector reverse biased. • Electrons diffuse from Emitter to Base (from n to p) • Depletion layer on the Base-Collector junction no flow of electron allowed • BUT Base is thin and Emitter region is heavily doped  electrons have enough momentum to cross Base into Collector • Small base current IB controls large current IC, functioning as a current amplifier

46. β (beta) is amplification factor for transistor (often called hFE by manufacturers) β is temperature and voltage dependent, no precise relationship can be assumed when designing transistor circuit β varies a lot among transistors (for typical BJT: on order of 100) IC is controlled by IB (Current Control) BJT Characteristics

47. Common Emitter Transistor Circuit • Emitter is grounded and input voltage is applied to Base • Base-Emitter starts to conduct when VBE is about0.6V, IC flows with IC= β*IB • As IB further increases, VBE slowly increases to 0.7V, IC rises exponentially • As IC rises, voltage drop across RC increases and VCE drops toward ground (transistor in saturation, no more linear relation between IC and IB)

48. Common Emitter Characteristics Collector current IC proportional to Base current IB Collector current controlled by the collector circuit (Switch behavior) In full saturation VCE=0.2V No current flows

49. Biased Point Q Load-line curve Operation Point of BJT in Active Region • Every IB has corresponding I-V curve. • Applying Kirchoff laws to base, emitter and collector circuits

50. Operation Region Summary