1. Transistors Student Lecture by: Giangiacomo Groppi Joel Cassell
Pierre Berthelot
September 28th 2004

2. Lecture outline Historical introduction Semiconductor devices overview
Bipolar Junction Transistor (BJT)
Field Effect Transistors (FET)
Power Transistors

3. Transistor History Invention: 1947,at Bell Laboratories.
John Bardeen, Walter Brattain, and William Schockly developed the first model of transistor (a Three Points transistor, made with Germanium)
They received Nobel Prize in Physics in 1956 "for their researches on semiconductors and their discovery of the transistor effect"
First application: replacing vacuum tubes (big & inefficient).
Today: millions of Transistors are built on a single silicon wafer in most common electronic devices
First model of Transistor

4. What is a transistor ?
The Transistor is a three-terminal, semiconductor device.
It’s possible to control electric current or voltage between two of the terminals (by applying an electric current or voltage to the third terminal). The transistor is an active component.
With the Transistor we can make amplification devices or electric switch. Configuration of circuit determines whether the transistor will work as switch or amplifier
As a miniature electronic switch, it has two operating positions: on and off. This switching capability allows binary functionality and permits to process information in a microprocessor.

5. Semiconductors Most used semiconductor: Silicon
Basic building material of most integrated circuits
Has four valence electrons, in its lattice there are 4 covalent bonds.
Silicon crystal itself is an insulator: no free electrons
Intrinsic concentration (ni) of charge carriers: function of Temperature (at room temp. 300K ni = 1010 /cm3)

6. Semiconductors 2
Electric conductibility in the Silicon crystal is increased by rising the temperature (not useful for our scope) and by doping.
Doping consists in adding small amounts of neighbor elements.

7. Semiconductors 3: Doping
Two Dopant Types
N-type (Negative)
Donor impurities (from Group V) added to the Si crystal lattice.
Dominant mobile charge carrier: negative electrons.
Group V elements such as Phosphorous, Arsenic, and Antimony.
P-type (Positive)
Acceptor impurities (from Group III) added to the Si crystal lattice.
Dominant mobile charge carrier: positive holes.
Group III elements such as Boron, Aluminum, and Gallium.
P-type
N-type

8. The simplest example: p-n junction
It’s also called Junction Diode
Allows current to flow from P to N only.
Because of the density gradient, electrons diffuse to the p region, holes to the n region.
Because of the recombination, the region near the junction is depleted of mobile charges.
Two types of behavior: Forward and Reverse biased.

9. Forward bias Forward biasing:
The external Voltage lowers the potential barrier at the junction.
The 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: the total current is the sum of these two currents.

10. Reverse bias Reverse biasing:
Reverse voltage increases the potential barrier at the junction.
There will be a transient current to flow 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
It needs about 0.7 V to start conduction (Vd )
Reversed biased (off)- Diode blocks current
Ideal: Current flow = 0
Real : Iflow= 10-6 Amps (reverse saturation current)
V threshold

12. BJT

13. Bipolar Junction Transistor (BJT)
npn bipolar junction transistor
pnp bipolar junction transistor
3 adjacent regions of doped Si (each connected to a lead):
Base. (thin layer,less doped).
Collector.
Emitter.
2 types of BJT:
npn.
pnp.
Most common: npn (focus on it).
Developed by Shockley (1949)

14. BJT npn Transistor
1 thin layer of p-type, sandwiched between 2 layers of n-type.
N-type of emitter: more heavily doped than collector.
With VC>VB>VE:
Base-Emitter junction forward biased, Base-Collector reverse biased.
Electrons diffuse from Emitter to Base (from n to p).
There’s a depletion layer on the Base-Collector junction no flow of e- allowed.
BUT the Base is thin and Emitter region is n+ (heavily doped)  electrons have enough momentum to cross the Base into the Collector.
The small base current IB controls a large current IC

15. BJT characteristics Current Gain:
α is the fraction of electrons that diffuse across the narrow Base region
1- α is the fraction of electrons that recombine with holes in the Base region to create base current
The current Gain is expressed in terms of the β (beta) of the transistor (often called hfe by manufacturers).
β (beta) is Temperature and Voltage dependent.
It can vary a lot among transistors (common values for signal BJT: ).

16. npn Common Emitter circuit
Emitter is grounded.
Base-Emitter starts to conduct with VBE=0.6V,IC flows and it’s IC=b*IB.
Increasing IB, VBE slowly increases to 0.7V but IC rises exponentially.
As IC rises ,voltage drop across RC increases and VCE drops toward ground. (transistor in saturation, no more linear relation between IC and IB)

17. Common Emitter characteristics
Collector current controlled by the collector circuit. (Switch behavior)
In full saturation VCE=0.2V.
Collector current proportional to Base current
The avalanche multiplication of current through collector junction occurs: to be avoided
No current flows

18. BJT as Switch Vin(Low ) < 0.7 V BE junction not forward biased
Cutoff region
No current flows
Vout = VCE = Vcc
Vout = High
Vin(High)
BE junction forward biased (VBE=0.7V)
Saturation region
VCE small (~0.2 V for saturated BJT)
Vout = small
IB = (Vin-VB)/RB
Vout = Low

19. BJT as Switch 2 Basis of digital logic circuits
Input to transistor gate can be analog or digital
Building blocks for TTL – Transistor Transistor Logic
Guidelines for designing a transistor switch:
VC>VB>VE
VBE= 0.7 V
IC independent from IB (in saturation).
Min. IB estimated from by (IBmin» IC/b).
Input resistance such that IB > 5-10 times IBmin because b varies among components, with temperature and voltage and RB may change when current flows.
Calculate the max IC and IB not to overcome device specifications.

20. Operation point of BJT Every IB has a corresponding I-V curve.
Selecting IB and VCE, we can find the operating point, or Q point.
Applying Kirchoff laws around the base-emitter and collector circuits, we have :
IB = (VBB-VBE)/RB
VCE = Vcc – IC*RC

21. Operation point of BJT 2
Load-line curve Q

22. BJT as amplifier Common emitter mode Linear Active Region
Significant current Gain
Example:
Let Gain, b = 100
Assume to be in active region -> VBE=0.7V
Find if it’s in active region

23. BJT as amplifier 2
VCB>0 so the BJT is in active region

24. Operation region summary
IB or VCE Char.
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

25. FET

26. Field Effect Transistors
1955 : the first Field effect transistor works
Increasingly important in mechatronics.
BJT Terminal
FET Terminal
Base
Gate
Collector
Drain
Emitter
Source
Similar to the BJT:
Three terminals,
Control the output current

27. Field Effect Transistors
Three Types of Field Effect Transistors
MOSFET (metal-oxide-semiconductor field-effect transistors)
Enhancement mode
Depletion mode
JFET (Junction Field-effect transistors)
Each in p-channel or n-channel
The more used one is the n-channel enhancement mode MOSFET, also called NMOS

28. MOSFET (enhancement mode n-channel)
Depletion mode
Symbols (base connected to the source or not)
The arrow head indicates the direction of the pn substrate-channel junction
N-channel => Source and Drain are n type
Enhancement mode =>
Increase VGS to make the travel from D to S easier for the electrons

29. NMOS Behavior VGS < Vth VGS > Vth : 0 < VDS < VPinch off
IDS=0
VGS > Vth :
0 < VDS < VPinch off
Depletion mode (or active region), gate holes are repelled.
 variable resistor (controled by VGS)
VDS > VPinch off
Inversion mode (or saturation region), IDS constant.
VDS > VBreakdown
IDS increases quickly
Should be avoided

30. NMOS Characteristic Active region Saturation region
For VDS > VPinchoff , the base current is a function of VGS
Pinchoff Point

31. NMOS Vs PMOS
Symbols:

32. NMOS Vs PMOS VGS > Vth Vth < 0 VGS < Vth :
IDS=0
VGS < Vth :
0 < VDS < VPinch off
Depletion mode (or active region), gate holes are repelled.
 variable resistor (controled by VGS)
VDS > VPinch off
Inversion mode (or saturation region), IDS constant.
Analogous to the pnp BJT
VDS > VBreakdown
IDS increases quickly
Should be avoided

33. NMOS uses High-current voltage-controlled switches Analog switches
Drive DC and stepper motor
Current sources
Chips and Microprocessors
CMOS: Complementary fabrication

34. NMOS Example
For Vpinchoff < VDS < 0
And VGS > VTH

35. JFET overview
The circuit symbols:
JFET design:

36. JFET Behavior
Can be used with VG=0

37. JFET Behavior
Can be used with VG < 0

38. JFET Behavior VGS > Vth VGS < Vth : 0 < VDS < VPinch off
IDS=0
VGS < Vth :
0 < VDS < VPinch off
Depletion mode (or active region), gate holes are repelled.
 variable resistor (controled by VGS)
VDS > VPinch off
Inversion mode (or saturation region), IDS constant.
Analogous to the pnp BJT
VDS > VBreakdown
IDS increases quickly
Should be avoided

39. JFET uses
Small Signal Amplifier
Voltage Controlled Resistor
Switch

40. FET Summary General: Signal Amplifiers Switches JFET:
For Small signals
Low noise signals
Behind a high impedence system
Inside a good Op-Ampl.
MOSFET:
Quick
Voltage Controlled Resistors
RDS can be really low : 10 mOhms

41. Power Transistors In General BJT MOSFET
Fabrication is different in order to:
Dissipate more heat
Avoid breakdown
So Lower gain than signal transistors
BJT
essentially the same as a signal level BJT
Power BJT cannot be driven directly by HC11
MOSFET
base (flyback) diode
Large current requirements

42. References
“Introduction to Mechatronics and Measurement Systems” by D.G. Alciatore, McGraw-Hill
“Microelectronics” by J. Millman, McGraw-Hill
Several Images from Internet: some websites are:
SDM-2/