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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.

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Intro to mechatronics 18 february 2005 student lecture transistors

Intro to Mechatronics18 February 2005Student Lecture: Transistors

Andrew Cannon

Shubham Saxena


  • What is a Transistor?

  • Transistor Properties

  • Characteristics and Applications of

    • Bipolar Junction Transistor (BJT)

    • Field Effect Transistors (FET)

    • Power Transistors

What is a transistor
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
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
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
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)

    • 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



P n junction junction diode
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
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 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

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)

Types of transistors
Types of Transistors

  • Bipolar Junction Transistor (BJT)

  • Field Effect Transistors (FET)

  • Power Transistors

Outline types of transistors
Outline Types of Transistors

  • Bipolar Junction Transistor (BJT)



    Common emitter mode (active)

    Operation region


  • Field Effect Transistors (FET)

    Fundamentals MOSFET

    Operating regimes MOSFET

    Fundamentals JFET

    Operating regimes JFET

    application areas

  • Power Transistors

Fundamentals bjt
Fundamentals BJT




Fundamentals npn bjt



Fundamentals npn BJT

Common emitter mode (active)


Reverse bias


Forward Bias




Representation bjt
Representation BJT

N-type emitter: more heavily doped than collector

Common emitter mode bjt
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


Switch applications bjt
Switch Applications BJT

  • logic circuits

  • TTL

  • lab

Amplifier applications bjt
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
Field Effect Transistors (FET)

FET: three types

  • Metal oxide semiconductor FET (MOSFET)

    • Enhancement mode

    • Depletion mode

  • Junction FET (JFET)

  • Fundamentals mosfet


    Fundamentals MOSFET

    Gate, Vg


    Drain, Vd

    Source, Vs




    Reverse bias

    N-channel enhancement MOSFET

    Operating regimes 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 mosfet1

    Saturation region

    Active region

    Pinch-off region

    Operating regimes MOSFET



    Gate: G

    Source: S

    Drain: D

    Depletion mode devices fet
    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 fet1
    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
    Fundamentals JFET

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

    Application areas
    Application areas


    • Switches: High-current voltage-controlled and Analog switches

    • Drive Motor: DC and stepper motor

    • Current sources

    • Chips and Microprocessors

    • CMOS: Complementary fabrication


    • differential amplifier

    Power transistors
    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


    • “Introduction to Mechatronics and Measurement Systems” by D.G. Alciatore, McGraw-Hill

    • “Microelectronics” by J. Millman, McGraw-Hill




    • Previous Mechatronics course lectures