Intro to Mechatronics 18 February 2005 Student Lecture: Transistors Andrew Cannon Shubham Saxena.
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Transcript of Intro to Mechatronics 18 February 2005 Student Lecture: Transistors Andrew Cannon Shubham Saxena.
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