Introduction to Transistors

1. Introduction to Transistors Presented: October 23, 2001 Chris Green Carl Hanna Ancil Marshall Kwame Ofori

2. Overview Introduction & History Semiconductors Operation of Transistors Transistor Types Applications Examples Questions Conclusion

3. Background Invented at Bell Laboratories in 1947. John Bardeen, Walter Brattain, and William Schockly received Nobel Prize in Physics in 1956 for Inventing Transistors. First application: telephone signal amplification Replaced cumbersome and inefficient vacuum tubes Transistors can now be found on a single silicon wafer in most common electronic devices

4. Background Model of First Transistor

5. What are Transistors? Versatile three lead semiconductor devices whose applications include electronic switching and modulation (amplification) Transistors are miniature electronic switches. Configuration of circuit determines whether the transistor will serve a switch and amplifier Building blocks of the microprocessor, which is the brain of the computer. Have two operating positions- on and off. Binary functionality of transistors enables the processing of information in a computer.

6. Semiconductors Silicon Basic building material of most integrated circuits Has four valence electrons, which allow it to form four covalent bonds. Silicon crystal is an insulator-- no free electrons.

7. Semiconductors Resistance to current flow in the silicon crystal is reduced by adding small amounts of foreign impurities, which is referred to as doping. Doping transforms a silicon crystal from a good insulator into a viable conductor; hence, the name semiconductor.

8. Semiconductors Two Dopant Types N-type (Negative) �Free flowing electrons are added to the silicon crystal structure. Examples include Group V elements including Phosphorous, Arsenic, and Antimony. P-type(Positive)- Lack electrons and serve as potential slots for migrating electrons. Examples include Group III elements such as Boron, Aluminum, and Gallium

9. Comparison of Energy Bands Semiconductor resembles an insulator, but with a smaller energy band. Small energy band makes it a marginal conductor

10. Simple Semiconductors: Diodes Diode is the simplest semiconductor. Allows current to flow in one direction only.

11. Diode Sign Conventions Power dissipated by a load = (+) quantity Current flows from (+) ? (-) Forward Biased Supplied Current flows with natural (hole) diffusion current Reversed Biased Supplied Current fights against natural diffusion (hole) current and diode orientation

13. Reverse-Bias Example Charges cannot diffuse unless supplied current flows towards �n�

14. Diodes States Forward biased (on)- Current flows Real: Need about 0.7 V to initiate electron-hole combination process. Reversed biased (off)- Diode blocks current Ideal- Current flow = 0 Real : Iflow= 10-6 Amps

15. Bipolar Junction Transistors (BJT) Three Layers in a BJT Collector Base (very thin) has fewer doping atoms Emitter Two Types of BJT�s PNP (figure on left) operates with outgoing base current NPN (figure on right) operates with incoming base current

16. BJT Schematic Representation

17. BJT Operation Characteristics IC vs. VCE graph allows us to determine operating region. Works for any IB or VCE VBE tops out around ~0.7V

18. BJT Operation Regions

21. Active Linear NPN BJT

22. Possible Uses for BJT�s Can act as Signal Current Switch (Cutoff Mode) Can act as Current Amplifier (Active Region) Where: Beta = intrinsic amp property (20 - 200)

23. FIELD-EFFECT TRANSISTORS In 1925, the fundamental principle of FET transistors was establish by Lilienfield. In 1955, the first successful FET was made. Types of Transistors MOSFET (metal-oxide-semiconductor field-effect transistors) JEFT (Junction Field-effect transistors)

24. MOSFET Four types: n-channel enhancement mode Most common since it is cheapest to manufacture p-channel enhancement mode n-channel depletion mode p-channel depletion mode

25. MOSFET

26. MOSFET

27. n-channel Enhancement Mode Cutoff region VGS < VT.

28. n-channel Enhancement Mode Ohmic region VDS < 0.25 (VGS-VT), VGS>VT Voltage controlled resistor.

29. n-channel Enhancement Mode Saturation region VDS = VGS-VT, VGS > VT Constant-current source.

30. Breakdown region VDS > VB n-channel Enhancement Mode

31. Comparison p-type charge carrier. Direction of drain current is opposite. VDS and VGS are negative. n-channel, p-channel behave the same way.

32. Depletion MOSFET Addition of an n-type region between the oxide layer and p-type substrate. Thus, depletion MOSFETs are normally on. VT, threshold voltage, is negative. Unlike enhancement MOSFET, depletion MOSFET : Allows positive and negative gate voltages. Can be in the saturation region for VGS= 0

33. JFET JFET n-channel p-channel

34. JFET

35. JFET Cutoff region VGS < -VP, -VP is the threshold voltage. VDS = 0

36. JEFT Ohmic region VDS < 0.25(VGS + VP), VGS > -VP. Resistance controlled by VGS

37. JFET Saturation region VDS = VGS +VP, VGS > -VP. Constant- current source.

38. JFET Breakdown regions. VDS > VB.

39. JFET

40. Use the I-V characteristic curves of BJT and MOSFET Use the regions of operation of these transistors BJT Cutoff Region Active Linear Region Saturation Region MOSFET Cutoff Region Ohmic or Triode Region Saturation (Active Region) Transistors as Amplifiers and Switches

41. I-V Characteristic Curves

42. I-V Characteristic Curves

43. Transistors as Amplifiers

44. Transistors as Amplifiers

45. Transistors as Switches Basis of digital logic circuits Used in microprocessors Input to transistor gate can be analog or digital Common names are TTL � Transistor Transitor Logic CMOS � Complementary Metal Oxide Semiconductor

46. Transistors as Switches � BJT Inverter

47. Transistors as Switches � BJT Inverter

48. Transistors as Switches- MOSFET

49. Transistors as Switches- MOSFET Inverter

50. Transistors as Switches- CMOS Inverter

51. References