Transistors S08 VF - Georgia Institute of...
Transcript of Transistors S08 VF - Georgia Institute of...
ME 4447 / 6405Student Lecture
Transistors
Abiodun OtolorinMichael Abraham
Waqas Majeed
Lecture Overview• Transistor?• History• Underlying Science• Properties• Types of transistors
– Bipolar Junction Transistors (BJT)– Field Effect Transistors (FET)– Power Transistors
• Applications
Transistor ?• Transistor = transconductance + varistor• 3 terminal electronics device.• Solid state Semiconductor material.• Part and parcel of almost every circuit (digital / analog)
Transistor ?
• Two main categories:– Through-Hole– Surface-Mount
• Packaging Materials:– Plastic, Glass, metal or ceramics
* Power transistors packages clamped to heat sinks.
HistoryVacuum Tube (Predecessors)Edison effect in the Light Bulb (1879-83).John Fleming implements it in a diode.Lee DeForest extends it using third electrode. This triode
was used as both an amplifier and a switch (1906).
Diode Triode
First Transistor
William Shockley, John Bardeen, and Walter Brattain @ Bell Labs in 1947
Replaced vacuum tubesSmaller, robust, durable, no warm upsSolid state1956 Nobel Prize
Current Transistorssemiconductor
• The world's first single chip microprocessor2,300 MOS (metal oxide semiconductor) transistors Equivalent 18,000 vacuum tubes contained in 3,000 ft3
Nov. 1971the Intel 4004
• Today’s microprocessor has 2B transistors.• SD cards has 50B transistors.• Brought revolution like IC, Chips.
Current Transistor
Underlying Science
• FREE ELECTRONS are required for current flow.• Conductors have abundant of them.• Insulators & semiconductors have scarcity.• TEMPERATURE increase produces free electrons
but not possible.• SMALLER BAND GAPS of semiconductors than
insulators help yield free electrons.• Hence, DOPING is done.
Underlying Science• 8 electrons for stable valence shell.
GIII > P-dopant has 3 valence shell electrons (e.g. B, Ga).GIV > Semiconductor has 4 valence shell electrons (e.g. Si, Ge).GV > N-dopant has 5 valence shell electrons (e.g. P, As)
Underlying Science
• Covalent bonding/pair sharing.GIII + GIV => -1 electron => Hole > P-typeGV + GIV => +1 electron => Electron > N-type
DopantsSemiconductor
Properties
• Forward Bias (on)– Current flows from P
to N.– Vd = 0.7 V to start
conduction.
• Reverse Bias (off)– No Current flows
(ideally).– Excessive voltage or
heat can cause degradation to diode.
Properties
V threshold
Properties
PNP Transistor
NPN Transistor
Properties
No current flows
Collector current controlled by the collector circuit(Switching)
In full saturation VCE=0.2V
(Variable resistor)
Collector current IC proportional to Base current IB (Amplification + Regulation)
Properties
No current flows
Collector current controlled by the collector circuit(Switching)
In full saturation VCE=0.2V
(Variable resistor)
Collector current IC proportional to Base current IB (Amplification + Regulation)
Bipolar Junction Transistor (BJT)
• 3 layers of Semiconductor• 3 Terminal (Base,
Collector & Emitter)– Each terminal may act as
Input, Output or Common
• 3 possible configurations– Common Emitter
• Both Current and Voltage gain
– Common Base • Voltage gain but no current
gain
– Common Collector• Current gain but no voltage
gain
NPN Common Emitter
BJT Types• NPN
– Base is energized to allow current flow
– High potential at collector– Low potential at emitter– Allows current flow when the
base is given a high potential
• PNP– Base is connected to a lower
potential to allow current flow– High potential at emitter– Low potential at collector– Allows current flow when base
is connected to a low potential
BJT Characteristics• Cut-off Region: VBE < VTH, IB=0
– off switch• Active Linear Region:
– VBE=VTH, IB≠0, IC=βIB– current amplifier– Current gain (β)
• 100 for most transistors– Voltage Gain = β(IC/IB)
• Saturation Region: – VBE=VTH, IB>IC,max/ β– on switch
Operation region overview
Operation Operation RegionRegion
IB or VCE Char.
BC and BEBC and BEJunctionsJunctions
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
Types of BJT Circuits
• Major BJT Circuits– Transistor Switch– Common-Emitter Amplifier– Emitter Follower– Current Source
NPN Transistor Switch•Vin(Low ) < 0.7 V
•BE junction is notforward 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
•Linear Active Region•Significant current Gain
Example:•Let Gain, β = 100
•Assume to be in active region -> VBE=0.7V
•Is device in active region?
NPN Common Emitter Amplifier
NPN Common Emitter Amplifier
V
VRIRIVVmAII
mARR
VVI
IIIIVV
BEEECCCCCB
BC
EB
BEBBB
BCBE
BE
93.37.0)0107.0*101)(2()07.1)(3(10
**07.10107.0*100*
0107.0402
7.05101*
)1(7.0
==−−−=
=−−−====
=−
=+−
=
+=+==
β
β
VCB>0 so the BJT is in active region
Power Across BJT
• PBJT = VCE * ICE
• Should be below the rated transistor power• Should be kept in mind when considering
heat dissipation• Reducing power increases efficiency
Darlington Transistors
• Two BJT devices combined– BE junction in series
• Allow for much greater gain in a circuit
• β = β1 * β2
• VBE=VBE1 + VBE2
Field Effect Transistors (FET)• Analogous to BJTs• FETs switch by voltage rather
than by current• FETs have 4 terminals (except
for J-FET which have 3)
BJT FETCollector Drain
Base Gate
Emitter Source
N/A Body
B
S
G
D
FET: The Basic Idea• Current flow is controlled via the “field effect”• Electrons gather when a field is formed• Current flows when an electron bridge is created
Semi-conductor
Plate
Types of FETs
•MOSFET (Metal-Oxide-Semiconductor)•JFET (Junction Gate)•MESFET (Metal-Semiconductor)•MODFET (Modulated-Doping)•HFET (Hetero-structure)•HEMT (High Electron Mobility Transistor)
(Most common are MOSFET and JFET)
Boba Fett
The Two Most Common TypesMOSFET
2 varieties: enhancement mode (shown)
depletion mode
JFET
P
N sourcedrain
gate
P
sourcedrain
gate
NN
Both MOSFETs and JFETs can be n-channel orp-channel depending upon the doping of the drain and source
Junction Gate FET (JFET)•Source and Drain are connected to n-doped material (or p-doped material for p-channel)
•Gate is connected to p-doped material
•When a negative biased voltage is applied to the Gate, the size of the depletion layer increases and impedes current flow from the source to the drain.
•JFETs can thus behave as voltage-controlled variable resistors
•JFETs work by “depletion”
p-doped n-doped depletion layer
MOSFET
Depletion Mode Enhancement Mode
n-Channel Enhanced MOSFET
•n-Channel because source and drain are connected to n-doped regions•With no voltage applied, the MOSFET behaves as shown (no channel is formed between the source and drain)•Because a channel is being created where none existed, this is an “enhancement mode” MOSFET
•When a voltage is applied, the resulting field causes electrons from the p-doped region to collect near the gate and a channel is created that connects the source and the drain.•The size and shape of the channel vary depending upon the amount of voltage applied.
n-Channel Enhanced MOSFET
Circuit Symbols
• In practice the body and source leads are almost always connected
• Most packages have these leads already connected
B
S
G
D
B
S
G
D
S
G
D
MOSFET
JFET
Performance CharacteristicsCurrent flow
B
S
G
D
Performance RegionsCurrent flow
B
S
G
D
Region Criteria Effect on Current
Cut-off VGS < Vth IDS=0
Linear VGS > Vth
AndVDS <VGS-Vth
Transistor acts like a variable resistor, controlled by Vgs
Saturation VGS > Vth
AndVDS >VGS-Vth
Essentially constant current
JFET vs MOSFETCurrent flow
B
S
G
D
MOSFET JFETHigh switching speed
Will operate at VG<0
Can have very low RDS
Better suited for low signal amplification
Susceptible to ESD
More commonly used as a power transistor
Power Transistors
• Additional material for current handling and heat dissipation
• Can handle high current and voltage
• Functionally the same as normal transistors
Applications
• Switching• Amplification (Voltage / Current)• Variable Resistor (VDR)• Voltage Regulation
Switching
Switching
DC motor• Power to motor is proportional to duty cycle
• MOSFET transistor is ideal for this use
Analog signal (e.g. sensor, audio, etc.).
Op-amps does same but suitable for voltage amplification and this is for current/power amplification.
Amplification
• Transistors can be used in series to produce a very high current gain
Amplification
• FET operating in Linear or OhmicMode
• Lesser usage
Variable Resistor
• Transistors can be used in regulating voltage for high power devices.
• Inefficient• Power supplies
Voltage Regulation
Application Examples• Digital logic circuits• Microprocessors, microcontrollers, chips (TTL)• Photo-transistors• Replaces normal switches, mechanical relays.• ADC• Opamp• Encoders• Multiplexers• Power supplies
References• http://www.owlnet.rice.edu/~elec201/Book/images/img95.gif• http://nobelprize.org/educational_games/physics/transistor/
function/p-type.html• http://www.electronics-for-
beginners.com/pictures/closed_diode.PNG• http://people.deas.harvard.edu/~jones/es154/lectures/lecture_
3/dtob.gif• http://en.wikipedia.org/wiki/Image:IvsV_mosfet.png• http://en.wikipedia.org/wiki/Transistor• http://www.physlink.com/Education/AskExperts/ae430.cfm• http://www.kpsec.freeuk.com/trancirc.htm
References (contd.)
• Sabri Cetinkunt; MechatronicsJohn Wiley and sons; 2007
• Mechatronics by D. Bradley• Mechatronics System Design by Shetty• Mechatronics : Principles and Application
by Onwubolu• Applied Mechatronis by A. Smaili et al
Questions?