Intro to Mechatronics18 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

Two Dopant Types1. N-type (Negative)

• Group V- Dominant mobile charge carrier: negative electrons

Phosphorous, Arsenic, and Antimony

2. P-type (Positive)• Group III

- Dominant mobile charge carrier: positive holesBoron, Aluminum, and Gallium

Building Blocks

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 bythermal energy

• 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)

Diode Characteristics

V threshold

Types of Transistors

• Bipolar Junction Transistor (BJT)

• Field Effect Transistors (FET)

• Power Transistors

Outline Types of Transistors• Bipolar Junction Transistor (BJT)FundamentalsRepresentationCommon emitter mode (active)Operation regionApplications

• Field Effect Transistors (FET) Fundamentals MOSFETOperating regimes MOSFET Fundamentals JFETOperating regimes JFET application areas

• Power Transistors

Fundamentals BJT

Emitter

Base

Collector

Fundamentals npn BJT

Emitter

Base

Collector

Common emitter mode (active)

Forward Bias

Reverse bias

Vc

Vb

HoleE-

Representation BJT

N-type emitter: more heavily doped than collector

EC II

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

C

CE

C

CCC R

V

R

VI

Operation region BJT

Operation Operation RegionRegion

IB or VCE BC and BE BC and BE JunctionsJunctions

ModeMode

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

• logic circuits• TTL• lab

Vin(Low ) < 0.7 V

Vin(High)

BE forward bias

BE not biased

Saturation Cutoff

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)

P-substrate

Fundamentals MOSFET

n nSource, Vs

Gate, Vg

Drain, Vd

N-channel enhancement MOSFET

++++++

Reverse bias

Id

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

Saturation region

Active region

Pinch-off region

Operating regimes MOSFET

Gate: G Source: S Drain: D

NMOS

PMOS

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/ph

ys395.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