Diodes, Triodes,
Thermistors, Opto-isolators,
& Phototransistors
ME 6405 – Spring 2005
Danny Nguyen
Wei Tan
Qiulin 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:

Schematic
+ VD −
Internal View
Anode
Cathode
p
ID
n
Junction Diodes
Start out with Silicon or Germanium
(Group IV elements)
 P-type - doping with Group III elements

 Boron, Aluminum,
Gallium
 Adds positive ‘holes’ to the region

N-type - Group V doping
 Phosphorous, Arsenic
 Add
electrons to the region
+
+
+
p
+
+
+
−
+
−
+
−
−
n
−
−
−
−
Junction Diodes

Due to thermal energy, some electrons
diffuse into the p-type region, creating a
depletion region
+
+
+
+
−
p
n
+
−
−
−
−
Depletion Region

No current flows through the diode at this
point
Junction Diodes

Forward Bias
 Depletion
region decreases
 Current flow when voltage is high enough
(0.6-0.7 Volts)
 Current sustained by majority carriers
VD
ID
+
+
+
+
p
+
+ −
+ −
+ −
−
n
−
−
−
−
Junction Diodes

Reverse Bias
 Depletion
region increases
 Small leakage current by minority carriers
 Reverse saturation current (I0)

On the order of 10-9 to 10-15 A
VD
+ +
+ p
+ +
− −
n −
− −
Analysis of Diodes

Mathematical Model
qVD ¡
I
=
I
[exp(
) 1]
 D
0
kT
Ideal Model
·0
V
=
0;
I
>
0
I
=
0;
V
D
D
D
 On:
Off: D
ID
 Constant Voltage Drop Model
VD = Von ; ID > 0
 On:
·V
Ideal
CVD
I
=
0;
V
D
D
on
 Off:
» 0:7V
VD
V
=
0:6
on

V

Diode Eq.
on
Analysis and Applications

Half-wave rectifier
+ VD −
Vi = 5VAC
ID
1kΩ
Vo
Von = 0.7V

CVD Analysis:
 On:
Replace diode with Von voltage source
 Off: Replace diode with open circuit
Analysis and Applications

Half-wave rectifier
Von
Vi = 5VAC
ID
1kΩ
Vo
Von = 0.7V

CVD Analysis:
! V = V ¡ 0:7V
V
>
0:7V;
I
>
0
D
o
i
 On: i
 Off:
Analysis and Applications

Half-wave rectifier
Vi = 5VAC

1kΩ
Vo
CVD Analysis:
! V = V ¡ 0:7V
V
>
0:7V;
I
>
0
D
o
i
 On: i
· 0:7V; I = 0 ! V = 0V
V
i
D
o
 Off:
Analysis and Applications

Full-wave bridge rectifier
Vi
Vo

Peak Detector
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
Schematic
− VZ +
+ VD −
IZ
ID
Reverse Breakdown
Model
VZ
RZ
IZ
Zener Diodes: Applications

Ability to maintain a constant voltage
allows it to act as a voltage regulator
R
Vi
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 onstate 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 phasecontrol 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 SteinhartHart equation:
T = Degrees Kelvin
1
T
= A
B ln( R)
C ln( R)
3
R = Resistance of
the thermistor
A,B,C = Curve-fitting
constants
V or R
• Typical Graph
Thermistor (sensible)
RTD (stable)
T
Thermocouple
(versatile)
Thermistor Applications
Temperature Control
variable resistor
for setting
desired
temperature
relay
thermistor
high gain
amplifier
•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.
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 emittercollector 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

Application Example: Avoiding
Obstacles
Automated
Cart
LED
Baffle
Phototransistor
Obstacle
Phototransistor Applications

Computer/Business Equipment
protect control – floppy driver
 Margin controls – printers
 Write

Industrial
light source – light pens
 Security systems
 LED

Consumer
 Coin
counters
 Lottery 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

Advantages
 Output
signals have no effect on input
 High reliability and high efficiency
 Noise isolation
 Small size

Applications
 Optical
switch
 Signal transmission devices
 Used 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