Intro to Mechatronics 18 February 2005 Student Lecture: Transistors

1. Intro to Mechatronics18 February 2005Student Lecture: Transistors Andrew Cannon Shubham Saxena

2. Outline • What is a Transistor? • Transistor Properties • Characteristics and Applications of • Bipolar Junction Transistor (BJT) • Field Effect Transistors (FET) • Power Transistors

3. What is a Transistor? • Electrically Actuated Switch • Two operating positions: on and off • Binary functionality – Permits processing of information • Three-terminal semiconductor device • Control current or voltage between two of the terminals by applying a current or voltage to the third terminal • Amplifiers or Switches • Configuration of circuit determines whether the transistor will work as switch or amplifier

4. A Brief History • Invented in 1947 at Bell Labs • John Bardeen, Walter Brattain, and William Schockly • Nobel Prize in Physics in 1956 • Initial Application • Replaced Vacuum Tubes: Big and Inefficient • Today • - Millions of transistors are built on a single silicon chip

5. What Are The Building Blocks? • Silicon • Basic building material of most integrated circuits • Four valence electrons: Possibility for 4 covalent bonds • Silicon crystal itself is an insulator: no free electrons

6. Building Blocks • Electric conductivity in the Silicon crystal is increased by doping • Doping: Adding small amounts of neighbor elements

7. Building Blocks Two Dopant Types • N-type (Negative) • Group V - Dominant mobile charge carrier: negative electrons Phosphorous, Arsenic, and Antimony • P-type (Positive) • Group III - Dominant mobile charge carrier: positive holes Boron, Aluminum, and Gallium N-type P-type

8. P-N Junction (Junction Diode) • Allows current to flow from P to N only • Density Gradient • - Electrons diffuse to the p region • - Holes diffuse to the n region • Recombination • - Region near the junction is depleted of mobile charges • Two types of behavior: Forward and Reverse Biasing

9. Forward Biasing • External Voltage lowers the potential barrier at the junction • P-N junction drives holes (from the p-type material) and electrons (from the n-type material) to the junction • A current of electrons to the left and a current of holes to the right: total current is the sum of these two currents

10. Reverse Biasing • Reverse voltage increases the potential barrier at the junction • There will be a transient current as both electrons and holes are • pulled away from the junction • When the potential formed by the widened depletion region equals the applied voltage, the current will cease except for the small thermal current. It’s called reverse saturation • current and is due to hole-electrons pairs generated by • thermal energy

11. V threshold Diode Characteristics • Forward biased (on)- Current flows • Conduction begins around 0.7 V (Vd ) • Reversed biased (off)- Diode blocks current • Ideal: Current flow = 0 • Real : Iflow= 10-6 Amps (reverse saturation current)

12. Types of Transistors • Bipolar Junction Transistor (BJT) • Field Effect Transistors (FET) • Power Transistors

13. 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

14. Fundamentals BJT Collector Base Emitter

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

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

17. 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

18. Operation region BJT

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

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

21. Field Effect Transistors (FET) FET: three types • Metal oxide semiconductor FET (MOSFET) • Enhancement mode • Depletion mode • Junction FET (JFET)

22. Id Fundamentals MOSFET Gate, Vg ++++++ Drain, Vd Source, Vs P-substrate n n Reverse bias N-channel enhancement MOSFET

23. Operating regimes MOSFET Cut-off regime: VGS < VT , VGD < VT with VDS > 0. Linear or Triode regime:VGS > VT, VGD > VT , with VDS> 0. Saturation regime:VGS > VT, VGD < VT (VDS > 0). • In the linear regime: • – VGS ") ID ": more electrons in the channel • – VDS ") ID ": stronger field pulling electrons out • of the source • • Channel debiasing: inversion layer ”thins down” from • source to drain)current saturation as VDS approaches: • VDSsat = VGS − VT

24. Saturation region Active region Pinch-off region Operating regimes MOSFET NMOS PMOS Gate: G Source: S Drain: D

25. Depletion Mode Devices FET • Physically implanted channel: An n-channel depletion type MOSFET has an n-type silicon region connecting the n+ source and drain regions at the top of the p-type substrate. • The channel depth and its conductivity can be controlled by Vgs in exactly the same manner as in the enhancement-type device. • Negative value of Vgs is the threshold voltage

26. Field Effect Transistors (FET) • FETs are useful because there is essentially no input • current • – Thus the output current can be controlled with nearly no • input power • – In this sense, FETs are more nearly ideal transistors than • bipolar junctions are • • Integrated circuits (“chips”) are made by forming many • FET’s on layers of silicon • • Main limitation of FETs is maximum current they can • handle • – For high-current applications the bipolar junction is a better • choice

27. Fundamentals JFET Depletion region grows as the reverse bias across the PN junction is increased

28. Operating regimes JFET

29. Application areas MOSFET • Switches: High-current voltage-controlled and Analog switches • Drive Motor: DC and stepper motor • Current sources • Chips and Microprocessors • CMOS: Complementary fabrication JFET • differential amplifier

30. Power Transistors • Designed to conduct large currents and dissipate more heat. Usually physically larger than a regular transistor • Applications where low current devices are interfaced with high current devices • Lower gain than signal transistors • RF amplifiers, motors, solenoid control, lighting control. • MOSFET base (flyback) diode

31. References • “Introduction to Mechatronics and Measurement Systems” by D.G. Alciatore, McGraw-Hill • “Microelectronics” by J. Millman, McGraw-Hill • http://www.phys.ualberta.ca/~gingrich/phys395/notes/phys395.html • http://ocw.mit.edu/NR/rdonlyres/Electrical-Engineering-and-Computer-Science/6-012Microelectronic-Devices-and-CircuitsSpring2003/C1EC60A4-4196-4EE6-AAC3-2775F2200596/0/lecture9.pdf • http://people.deas.harvard.edu/~jones/es154/lectures/lecture_4/jfet/jfet.html • Previous Mechatronics course lectures • www.howstuffworks.com