1. Intro to Mechatronics 18 February 2005 Student 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) 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

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. 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

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. Common emitter mode (active)
Fundamentals npn BJT
Common emitter mode (active) Vc Vb
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 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

19. Switch Applications BJT
Vin(Low ) < 0.7 V
Vin(High)
BE forward bias
BE not biased
Saturation
Cutoff
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. N-channel enhancement MOSFET
Fundamentals MOSFET
Gate, Vg
++++++
Drain, Vd
Source, Vs n
P-substrate n
Reverse bias Id
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. Operating regimes MOSFET
NMOS
Active region
Saturation region
Pinch-off region
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
Previous Mechatronics course lectures