1. Georgia Institute of Technology
ME 6405 Student Lecture
Transistors
Angela Sodemann Jin Yang Louis Nucci
Wednesday, November 14, 2018
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Lecture outline
Brief History of Transistors
Introduction to Transistors
Bipolar Junction Transistors (BJT)
Field Effect Transistors (FET)
Power Transistors
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Transistor History
Invention of transistors is a milestone of modern microelectronics industry
Invention:
1947, at Bell Laboratories, by John Bardeen, Walter Brattain, and William Schockly (a three-point transistor, made with Germanium)
1956, They received Nobel Prize in Physics "for their researches on semiconductors and their discovery of the transistor effect"
First application: replacing vacuum tubes (big & inefficient)
Today:
Advanced microprocessor uses as many as 1.7 billion transistors (MOSFETs)
First model of Transistor
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4. Packaging Format of Transistor
Two main categories:
Through-Hole
Surface-Mount
Packaging Materials:
Glass, metal, ceramics or plastic
Through-hole transistor
Surface Mount Transistor
Large transistor array uses latest Ball grid array (BGA) packaging
Power transistors have large packages clamped to heat sinks for enhanced cooling
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What is Transistor ?
Definition:
Transistor is three-terminal, solid-state semiconductor device
Function:
Control electric current or voltage between two of the terminals by applying an electric current or voltage to third terminal. Transistor is an active component.
Application:
Switch in digital circuits
Two operating positions: on and off. This switching capability allows binary functionality and permits to process information in a microprocessor
Amplifier in analog circuits
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Transistor Chemistry
Silicon
Semiconductor that is able to be doped with other elements to adjust its electrical response
Arsenic, phosphorous
N-type dopants which add an electron to the silicon
Boron, aluminum
P-type dopants which have an extra “hole”
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PN Junction
It is also called Junction Diode
Allows current to flow from P to N only
Electrons from n region diffuse to occupy holes in p region
Thin depletion region forms near junction, resulting in contact potential (For silicon, on order of V)
Two types of behavior: Forward and Reverse biased
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8. PN Junction – Forward Biased
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9. PN Junction – Reverse Biased
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Types of Transistor
Two Main Categories:
Bipolar Junction Transistor - BJT
Field Effect Transistor - FET
JFET - Junction FET
MOSFET - Metal Oxide Semiconductor FET
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BJT
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12. Bipolar Junction Transistor
npn bipolar junction transistor
3 adjacent doped regions (each layer connected to a lead)
Base (B)
Collector (C)
Emitter (E)
2 types of BJT:
npn
pnp
Most common type: npn
pnp bipolar junction transistor
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npn BJT
1 thin layer of p-type silicon, sandwiched between 2 layers of n-type silicon
Emitter is 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)
Depletion layer on the Base-Collector junction no flow of electron allowed
BUT Base is thin and Emitter region is heavily doped  electrons have enough momentum to cross Base into Collector
Small base current IB controls large current IC, functioning as a current amplifier
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BJT Characteristics
β (beta) is amplification factor for transistor (often called hFE by manufacturers)
β is temperature and voltage dependent, no precise relationship can be assumed when designing transistor circuit
β varies a lot among transistors (for typical BJT: on order of 100)
IC is controlled by IB (Current Control)
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15. Common Emitter Transistor Circuit
Emitter is grounded and input voltage is applied to Base
Base-Emitter starts to conduct when VBE is about 0.6V, IC flows with IC= β*IB
As IB further increases, VBE slowly increases to 0.7V, 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)
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16. Common Emitter Characteristics
Collector current IC proportional to Base current IB
Collector current controlled by the collector circuit (Switch behavior)
In full saturation VCE=0.2V
No current flows
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17. Operation Point of BJT in Active Region
Every IB has corresponding I-V curve.
Applying Kirchoff laws to base, emitter and collector circuits
Load-line curve
Biased Point Q
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18. Operation Region Summary
IB or VCE Char.
Mode
Cutoff
IB = Very small
Open Switch
Saturation
VCE = Small
Closed Switch
Active
VCE = Moderate
Amplifier
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BJT as Switch
When Vin < 0.7V
BE junction not forward biased
Cutoff state of transistor
IC=IE=0
Vout = VCE=VC
Vout= High
When Vin > 0.7
BE junction forward biased
(VBE=0.7V)
IB = (Vin-VB)/RB
Saturation region
VCE small (~0.2 V for saturated BJT)
Vout= Low
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20. Practical Example – LED Switch
Transistor model is known: 2N3904 npn
Assuming LED requires mA to provide a bright display and has 2 voltage drop when forwarded biased;
When digital output is 0V, transistor is off
When digital output is 5V, the transistor is
in saturation, with base current
Collector current (LED current) is limited
by collector resistor
LED is lighted
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BJT as Amplifier
Assume Gain β = 100
Assume BJT in active region VBE=0.7V
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22. Practical Example – Speaker Amplifier
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FET
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24. Field Effect Transistor
Described as a transconductance amplifier, meaning output current is controlled by an input voltage
Contrarily, BJT is a current amplifier, a large output current is controlled by a much smaller base current
In structure, FET is similar to BJT:
Three terminals
Different terminal names
BJT Terminal
FET Terminal
Base
Gate
Collector
Drain
Emitter
Source
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25. Types of Field Effect Transistor
Types of Field Effect Transistors
MOSFET (metal-oxide-semiconductor field-effect transistors)
Enhancement mode
Depletion mode
JFET (Junction Field-effect transistors)
MESFET (Metal Semiconductor Field-effect transistor)
HFET (Heterostructure Field-effect transistor)
MODFET (Modulation Doped Field-effect transistor)
Mostly used one is n-channel enhancement mode MOSFET, also called NMOS
Conducting Region
Nonconducting Region
Nonconducting Region
Enhanced n-MOSFET
Depleted MOSFET
JFET
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26. n-channel Enhancement Mode MOSFET
N-channel => Source and Drain are n type
Enhancement mode =>
Increase VGS to make the travel from D to S easier for the electrons
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Modes of MOSFET
Enhancement mode
Sub-threshold – Vg < Vth
Transistor is off
Linear Region – Vg > Vth and Vds < Vgs-Vth
Transistor is on
Saturation – Vg > Vth and Vds > Vgs-Vth
Transistor is on, with a portion of the channel being off
Depletion mode
Similar to enhancement, always on, use negative voltage
Power mode
Better behavior in saturation
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NMOS Characteristics
Active region
Saturation region
For VDS > VPinchoff , the base current is a function of VGS
Pinchoff Point
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NMOS Behavior
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.
VDS > VBreakdown
IDS increases quickly
Should be avoided
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Symbols of FET
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JFET
Can be used with VG < 0
N-type
N- doping is
done by adding carrier electrons,
phosphorus, arsenic, and antimony
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JFET
Can be used with VG=0
P-Type
P- doping is
done by adding boron to
silicon to create holes
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JFET Behavior
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Application of FET
Switch
Voltage Controlled Resistor
Small Signal Amplifier
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35. Differences Between BJT and FET
Input current controls output current
Input voltage controls output current
Base, collector, emitter
Gate, drain, source
Cheaper
More expensive
Can be used as a variable resistor
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Power Transistor
In General
Power transistor is one that has a power dissipation of 1W or more
Conduct a large maximum collector current and have maximum collector power dissipation
Interface from low-output current devices such as IC and computer ports to other devices requiring large currents
Generally shielded by or have a structure with heat sinks in order to dissipate more heat
BJT Power Transistor
FET Power Transistor
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37. Application of BJT Power Transistor
Used in LCD Inverter
From Toshiba Semiconductor Company
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38. Application of BJT Power Transistor
Used in Battery Charger
From Toshiba Semiconductor Company
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39. Application of MOSFET Power Transistor
Used in Battery Protection Circuit
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References
David G. Alciatore and Michael B. Histand, “Introduction to Mechatronics and Measurement Systems”, Second Edition, Mc Graw Hill, 2002
Old student lecture slides
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