ME 6405 Student Lecture

1. ME 6405 Student Lecture Transistors Angela Sodemann Jin Yang Louis Nucci Wednesday, April 2, 2014 Georgia Institute of Technology

2. Lecture outline • Brief History of Transistors • Introduction to Transistors • Bipolar Junction Transistors (BJT) • Field Effect Transistors (FET) • Power Transistors Georgia Institute of Technology

3. 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 Georgia Institute of Technology

4. 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 Georgia Institute of Technology

5. 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 Georgia Institute of Technology

6. 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” Georgia Institute of Technology

7. 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 Georgia Institute of Technology

8. PN Junction – Forward Biased http://www.tpub.com/neets/book7/25k.htm Georgia Institute of Technology

9. PN Junction – Reverse Biased http://www.tpub.com/neets/book7/25k.htm Georgia Institute of Technology

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

11. BJT Georgia Institute of Technology

12. 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 Georgia Institute of Technology

13. 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 Georgia Institute of Technology

14. 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 Georgia Institute of Technology

15. 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) Georgia Institute of Technology

16. 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 Georgia Institute of Technology

17. 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 Georgia Institute of Technology

18. Operation Region Summary Georgia Institute of Technology

19. BJT as Switch When Vin < 0.7V • BE junction not forward biased • Cutoff state of transistor • IC=IE=0 • Vout = VCE=VC Vout= High When Vin > 0.7 • BE junction forward biased (VBE=0.7V) • IB = (Vin-VB)/RB • Saturation region • VCE small (~0.2 V for saturated BJT) Vout= Low Georgia Institute of Technology

20. Practical Example – LED Switch Transistor model is known: 2N3904 npn Assuming LED requires 20-40 mA to provide a bright display and has 2 voltage drop when forwarded biased; • When digital output is 0V, transistor is off • When digital output is 5V, the transistor is in saturation, with base current Collector current (LED current) is limited by collector resistor LED is lighted Georgia Institute of Technology

21. BJT as Amplifier • Assume Gain β= 100 • Assume BJT in active region VBE=0.7V Georgia Institute of Technology

22. Practical Example – Speaker Amplifier Georgia Institute of Technology

23. FET Georgia Institute of Technology

24. Field Effect Transistor • Described as a transconductance amplifier, meaning output current is controlled by an input voltage • Contrarily, BJT is a current amplifier, a large output current is controlled by a much smaller base current • In structure, FET is similar to BJT: • Three terminals • Different terminal names Georgia Institute of Technology

25. Types of Field Effect Transistor • Types of Field Effect Transistors • MOSFET (metal-oxide-semiconductor field-effect transistors) • Enhancement mode • Depletion mode • JFET (Junction Field-effect transistors) • MESFET (Metal Semiconductor Field-effect transistor) • HFET (Heterostructure Field-effect transistor) • MODFET (Modulation Doped Field-effect transistor) • Mostly used one is n-channel enhancement mode MOSFET, also called NMOS Conducting Region Nonconducting Region Nonconducting Region Enhanced n-MOSFET Depleted MOSFET JFET Georgia Institute of Technology

26. Enhancement mode n-channel Enhancement Mode MOSFET • N-channel => Source and Drain are n type • Enhancement mode => Increase VGS to make the travel from D to S easier for the electrons Georgia Institute of Technology

27. Modes of MOSFET • Enhancement mode • Sub-threshold – Vg < Vth • Transistor is off • Linear Region – Vg > Vth and Vds < Vgs-Vth • Transistor is on • Saturation – Vg > Vth and Vds > Vgs-Vth • Transistor is on, with a portion of the channel being off • Depletion mode • Similar to enhancement, always on, use negative voltage • Power mode • Better behavior in saturation Georgia Institute of Technology

28. For VDS > VPinchoff , the base current is a function of VGS Active region Saturation region Pinchoff Point NMOS Characteristics Georgia Institute of Technology

29. VGS <Vth IDS=0 NMOS Behavior VGS > Vth : 0 < VDS < VPinch off Depletion mode (or active region), gate holes are repelled.  variable resistor (controled by VGS) VDS > VPinch off Inversion mode (or saturation region), IDS constant. VDS > VBreakdown IDS increases quickly Should be avoided Georgia Institute of Technology

30. Symbols of FET Georgia Institute of Technology

31. JFET Can be used with VG < 0 N-type N- doping is done by adding carrier electrons, phosphorus, arsenic, and antimony Georgia Institute of Technology

32. JFET Can be used with VG=0 P-Type P- doping is done by adding boron to silicon to create holes Georgia Institute of Technology

33. JFET Behavior Georgia Institute of Technology

34. Application of FET • Switch • Voltage Controlled Resistor • Small Signal Amplifier Georgia Institute of Technology

35. Differences Between BJT and FET Georgia Institute of Technology

36. Power Transistor • In General • Power transistor is one that has a power dissipation of 1W or more • Conduct a large maximum collector current and have maximum collector power dissipation • Interface from low-output current devices such as IC and computer ports to other devices requiring large currents • Generally shielded by or have a structure with heat sinks in order to dissipate more heat • BJT Power Transistor • FET Power Transistor Georgia Institute of Technology

37. Application of BJT Power Transistor Used in LCD Inverter From Toshiba Semiconductor Company http://www.semicon.toshiba.co.jp Georgia Institute of Technology

38. Application of BJT Power Transistor Used in Battery Charger From Toshiba Semiconductor Company http://www.semicon.toshiba.co.jp Georgia Institute of Technology

39. Application of MOSFET Power Transistor Used in Battery Protection Circuit www.Lovoltech.com Georgia Institute of Technology

40. References • David G. Alciatore and Michael B. Histand, “Introduction to Mechatronics and Measurement Systems”, Second Edition, Mc Graw Hill, 2002 • http://en.wikipedia.org • http://www.semicon.toshiba.co.jp • http://www.brookdale.cc.nj.us/fac/engtech/aandersen/engi242/powerpoint • Old student lecture slides Georgia Institute of Technology