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