Diodes, Triodes, Thermistors, Opto-isolators, & Phototransistors
ME 6405 – Spring 2005Danny NguyenWei TanQiulin Xie
Presentation Outline
Diodes – Danny Triacs & Thermistors – Qiulin Opto-isolators & Phototransistors – Wei
Diodes: Overview
Meet the Diode Junction Diodes Analysis and Applications Zener Diodes and Applications
What is a Diode?
Simplest semiconductor device Allows current to flow in one direction but
not the other Symbols:
ID
+ VD − Anode Cathode
Schematic Internal View
p n
Junction Diodes
Start out with Silicon or Germanium (Group IV elements)
P-type - doping with Group III elementsBoron, Aluminum, GalliumAdds positive ‘holes’ to the region
N-type - Group V dopingPhosphorous, ArsenicAdd electrons to the region
p n+
+
+
+
+
+
−
−
−
−
−
−
+
+
−
−
Junction Diodes
Due to thermal energy, some electrons diffuse into the p-type region, creating a depletion region
No current flows through the diode at this point
p n+
+
+
−
−
−
+
+
−
−
Depletion Region
Junction Diodes
Forward BiasDepletion region decreasesCurrent flow when voltage is high enough
(0.6-0.7 Volts)Current sustained by majority carriers
p n+
+
+
−
−
−
+
+
−
−
+
+
+
−
−
−
VDID
Junction Diodes
Reverse BiasDepletion region increasesSmall leakage current by minority carriersReverse saturation current (I0)
On the order of 10-9 to 10-15 AVD
p n+
+
+
−
−
−
+
+
−
−
Analysis of Diodes
Mathematical Model
Ideal ModelOn: Off:
Constant Voltage Drop ModelOn:Off:
I D = I 0[exp(qVDkT
) ¡ 1]
VD = 0;I D > 0 I D = 0;VD · 0
VD = Von ; I D > 0
Von = 0:6» 0:7VI D = 0;VD · Von
VD
ID
Ideal CVD
VonDiode Eq.
Analysis and Applications
Half-wave rectifier
CVD Analysis:On: Replace diode with Von voltage source
Off: Replace diode with open circuit
ID
+ VD −
1kΩ VoVi = 5VAC
Von = 0.7V
ID1kΩ VoVi = 5VAC
Von = 0.7V
Von
Analysis and Applications
Half-wave rectifier
CVD Analysis:On:Off:
Vi > 0:7V;I D > 0! Vo = Vi ¡ 0:7V
1kΩVi = 5VAC Vo
Analysis and Applications
Half-wave rectifier
CVD Analysis:On:Off:
Vi > 0:7V;I D > 0! Vo = Vi ¡ 0:7VVi · 0:7V; I D = 0! Vo = 0V
Analysis and Applications
Full-wave bridge rectifier
Peak Detector
ViVo
Vi Vo
Zener Diodes
Operated by reverse bias instead of forward bias
All diodes have a breakdown region – point where the diode can not handle anymore negative voltage
Voltage remains nearly constant in the breakdown region (Vz: Zener Voltage) under widely varying current for Zeners
Zener Diodes: I-V Graph
Slope = 1/Rz
ID
+ VD −
IZ
− VZ + VZ RZ
IZ
SchematicReverse Breakdown
Model
Zener Diodes: Applications
Ability to maintain a constant voltage allows it to act as a voltage regulator
Vi
R
Iz Vo = Vz RL
Vz = 6:2V;R = 1k ;RL = 10k ;Vi = 7» 11V
Zener Diodes: Specifications
VZ (Zener Voltage): Common range is between 3.3V and 75V
Tolerance: Commonly 5 to 10% Power Handling: ¼, ½, 1, 5, 10, 50 W
Contents
Shockley Diode Silicon-Controlled Rectifier (SCR) Triac Thermistor
Shockley Diode
Shockley diode after its inventor, William Shockley
four-layer diode, also known as a PNPN
on if applying sufficient voltage between anode and cathode
Off if reducing to a much lower voltage
Silicon-Controlled Rectifier (SCR) Shockley diode becomes SCR if gate addition to
PNPN it behaves exactly as a Shockley diode If an SCR's
gate is left disconnected. gate terminal may be used as an alternative means
to latch the SCR SCRs are unidirectional (one-way) current devices,
making them useful for controlling DC only
Triode AC Switch (Triac) A triac can be regarded as a "bidirectional (AC) SCR” because it conducts in
both directions.
• 5 layer device• Region between MT1 and MT2 are parallel switches (PNPN and NPNP)
• Allows for positive or negative gate triggering
Triggering Quadrant
Triac Characteristic Curve
Triac Characteristic Curve VDRM refers to the maximum peak forward voltage which may be continuously
applied to the main terminals and the highest voltage that can be blocked IDRM is the leakage current of the Triac when VDRM is applied to MT1 and MT2 ,
which is several orders of magnitude smaller than the “on” rating VRRM: Peak Repetitive Reverse Voltage
Maximum peak reverse voltage that may be continuously applied to the main terminals
IGT Gate trigger current VGT Gate trigger voltage Latching Current: the value of on-state current required to maintain conduction
at the instant when the gate current is removed Holding current :Value of on-state current required to maintain conduction once
the device has fully turned on and the gate current has been removed. The on-state current is equal to or lower in value than the latching current
Triac Advantages and Applications Advantages
Controllable trigger Four quadrant device Triacs provide the lowest cost
and simplest route to reliable, interference-free switching and power control.
Application Light dimmer control Motor speed control (a phase-
control circuit is used to vary the power to brush motors.)
Reason Trigger pulse can control any
percentage of half cycle
Thermistor
Thermistor - Temperature sensitive resistor Their change in electrical resistance is very large and
precise when subjected to a change in temperature. Thermistors exhibit larger parameter change with
temperature than thermocouples and Resistance Temperature Detectors (RTD’s). Thermistor - sensitive Thermocouple - versatile RTD – stable
Generally composed of semiconductor materials. Very fragile and are susceptible to permanent
decalibration.
Thermistor Probe
One of many available probe assemblies
Thermistor Characteristics
Most thermistors have a negative temperature coefficient (NTC); that is, their resistance decreases with increasing temperature.
Positive temperature coefficient (PTC) thermistors also exist with directly proportional R vs. T.
Extremely non-linear devices (high sensitivity) Common temperature ranges are –100 °F (~-75
°C) to +300 °F (~150 °C) Some can reach up to 600 °F
Thermistor R-T Curve An individual thermistor curve can be very
closely approximated by using the Steinhart-Hart equation:
A B ln R( ) C ln R( )31
T=
T = Degrees Kelvin
R = Resistance ofthe thermistor
A,B,C = Curve-fitting constants• Typical Graph
Thermistor (sensible)RTD (stable)
Thermocouple (versatile)
T
V o
r R
Thermistor Applications•Resistor is set to a desired temperature (bridge unbalance occurs)
•Unbalance is fed into an amplifier, which actuates a relay to provide a source of heat or cold.
•When the thermistor senses the desired temperature, the bridge is balanced, opening the relay and turning off the heat or cold.
Temperature Control
high gain amplifier
relay
thermistor
variable resistor for setting desired temperature
Phototransistor
Introduction Package and Scheme Operation Advantages Example and applications
Phototransistor Introduction
A transistor which is sensitive to the input light intensity
Operation similar to traditional transistors; Have collector, emitter, and base
Phototransistor base is a light-sensitive collector-base junction
Dark Current: Small collector can emit leakage current when transistor is switched off.
Phototransistor Packages
Phototransistor Scheme
Photocurrent: The electrons are amplified by the transistor and appear as a current in the collector/emitter circuit.
The base is internally left open and is at the focus of a plastic lens.
Phototransistor Operation
The phototransistor must be properly biased
A light sensitive collector base p-n junction controls current flow between the emitter and collector
As light intensity increases, resistance decreases, creating more emitter-base current
The small base current controls the larger emitter-collector current
Collector current depends on the light intensity and the DC current gain of the phototransistor
Why Use Phototransistors?
More sensitive than photodiodes of comparably sized area
Available with gains form 100 to over 1500 Moderately fast response times Available in a wide range of packages Usable with almost any visible or near infrared
light source such as IREDs, lasers, sunlight, and etc
Same general electrical characteristics as familiar signal transistors
Obstacle
Application Example: Avoiding Obstacles
Automated Cart LED
Baffle
Phototransistor
Phototransistor Applications
Computer/Business EquipmentWrite protect control – floppy driverMargin controls – printers
IndustrialLED light source – light pensSecurity systems
ConsumerCoin countersLottery card readers
Optoisolator
Introduction Scheme and Package Optocoupler Interrupter Example Advantages and applications
Optoisolator Introduction
A device that uses a short optical transmission path to accomplish electrical isolation between elements of a circuit.
Note 1: The optical path may be air or a dielectric waveguide;
Note 2: The transmitting and receiving elements may be contained within a single compact module.
Optoisolator Scheme
The light emitted form the LED is detected by a photodetector which sits across from the LED inside the chip, and output a current.
Since the input signal is passed from the LED to the photodetector, and cannot be passed form the photodetector to the LED, the input device is optically isolated from the circuit connected to the output side.
Optoisolator Package
An IRED is typically a controllable light source and a phototransistor employs as the detector element. The input and output sides have separate grounds Optoisolators sensitive to input voltages.
Optocoupler Interrupter Example
Integrated emitter and detector pair Setup Similar to Lab L3 Used to calculate speed or distance
Optoisolator Advantages & Applications
AdvantagesOutput signals have no effect on inputHigh reliability and high efficiencyNoise isolationSmall size
ApplicationsOptical switchSignal transmission devicesUsed to control motors, solenoids, etc.
Questions?
References
“Introduction to Mechatronics and Measurement Systems, 2nd Ed.” by D.G. Alciatore and M.B. Histand
http://www.semiconductors.philips.com http://www.omega.com “Microelectronic Circuit Design, 1st Ed.” by
Richard C. Jaeger Fall 2000 Slides