Intro to Mechatronics 18 February 2005 Student Lecture: Transistors Andrew Cannon Shubham Saxena

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

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

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

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

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

Building Blocks Two Dopant Types N-type (Negative) P-type (Positive) 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

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

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

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

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) V threshold

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

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

Fundamentals BJT Collector Base Emitter

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

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

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

Operation region BJT Operation Region IB or VCE BC and BE Junctions Mode Cutoff IB = Very small Reverse & Reverse Open Switch Saturation VCE = Small Forward & Forward Closed Switch Active Linear VCE = Moderate Reverse & Forward Linear Amplifier Break-down VCE = Large Beyond Limits Overload

Switch Applications BJT Vin(Low ) < 0.7 V Vin(High) BE forward bias BE not biased Saturation Cutoff logic circuits TTL lab

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

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

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

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

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

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

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

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

Operating regimes JFET

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

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

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