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Zener Diodes/Thermistors Gustavo
Four layer devices
Class of semiconductor components
Wide range of devices, SCR (silicon controlled rectifier), SCS (silicon controlled switch), Diacs, Triacs, and Shockley diodes
Used in high power switching applications
i.e. hundreds of amps / thousands of watts
The Triac is a three terminal AC semiconductor switch
Turned on with a low energy signal to the Gate
MT1 and MT2 are the current carrying terminals
G is the gate terminal, used for triggering
5 layer device
Region between MT1 and MT2 are parallel switches (PNPN and NPNP)
Allows for positive or negative gate triggering
Triac Characteristic Curve
1st quadrant - MT2 is (+) with respect to MT1
VDRM is the break-over voltage of the Triac
and the highest voltage that can be blocked
IRDM is the leakage current of the Triac when VDRM is applied to MT1 and MT2
IRDM is several orders of magnitude smaller than the “on” rating
Real World Triacs
Come in various shapes and sizes
Essentially all the same operationally
Different mounting schemes
Simple Triac Switch
Small control current/voltage
Eliminates Mechanical wear in a Relay
Brief review of semiconductors
Applications of Junction Diodes
Review of Semiconductors
The two semiconductors of greatest importance are Silicon (Si)and Germanium (Ge)
Both elements have four valence electrons
The conduction band is defined as the lowest unfilled energy band
The valence band is an energy region wherethe states are filled or partially filled by valence electrons
Electrons in the valence band can be moved to the conduction band with the applicationof energy, usually thermal energy
A material can be classified as: 1. Insulator – has valence and conduction bands well separated 2. Semiconductor – has valence band close to conduction band (the energy gap is about 1eV). 3. Conductor – has the conduction and valence bands overlapping
Pure semiconductors (Si, Ge) are poor conductors
Semiconductors are valuable for two unusual properties:
1. Conductivity increases exponentially with temperature (ex: Thermistor)
2. Conductivity can be increased and precisely controlled by adding small impurities in a process called doping.
n-type doping – adds impurities from column V of the periodic tableto a semiconductor material. Negative free charge carriers (electrons)become available.
p-type doping – adds impurities from column III of the periodic table to a semiconductor material. Positive free charge carriers (holes) become available.
A diode is created when a p-type semiconductor is joined with and n-type semiconductor by the addition of thermal energy.
When both materials are joined, the thermal energy causes positivecarriers in the p-type material to diffuse into the n-type region and negative carriers in the n-type material to diffuse into the p-type region.
This creates the depletion region within the diode.
The depletion region contains an internal electric field caused by theseparation of charge. This is called the potential barrier and it acts tooppose the diffusion of majority carriers across the junction.
Current flow in the diode
A diode is forward biased if the positive terminal of the batteryis connected to the p-type material. The majority carriers are forcedtowards the junction and the depletion region decreases.
If the voltage is high enough the depletion region can be entirelyeliminated.
Current is sustained by the majority carriers.
A diode is reverse biased if the positive terminal of the batteryis connected to the n-type material. The majority carriers are forcedaway from the junction and the depletion region increases.
The majority carriers are unable to create a current
There is a small reverse current or leakage current sustained by the minority carriers
If reverse bias is sufficiently increased, a sudden increase in reverse current is observed. This is known as the Zener or Avalancheeffect
Zener diodes operate in the breakdown region.
Zener diodes have a specified voltage drop when they are used in reverse bias.
Every pn junction (i.e. diode) will break down in reverse bias if enough voltage is applied.
Zener diodes are operated in reverse bias for normal voltage regulation.
Able to maintain a nearly constant voltage under conditions of widely varying current.
Types of Breakdowns
Zener breakdown - the electric field near the junction becomes large enough to excite valence electrons directly into the conduction band.
Avalanche breakdown –minority carriers are accelerated in the electric field near the junction to sufficient energies that they can excite valence electrons through collisions.
Note: The predominance of one breakdown over the other depends on the room temperature.
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 RTD’s.
Thermistor - sensitive
Thermocouple - versatile
RTD – stable
Generally composed of semiconductor materials.
Very fragile and are susceptible to permanent decalibration.
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 oF (~-75 oC) to +300 oF (~150 oC)
Some can reach up to 600 oF
Operation similar to traditional transistors
Have a collector, emitter, and base
Phototransistor base is a light-sensitive collector-base junction
Small collector to emitter leakage current when transistor is switched off, called collector dark current
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.
They must be properly biased
They are sensitive to temperature changes
They must be protected against moisture
Hermetic packages are more tolerant of severe environments than plastic ones
Plastic packages are less expensive than hermetic packages
Operation similar to relays
Used to control high voltage devices
Excellent noise isolation because switching circuits are electrically isolated
Coupling of two systems with transmission of photons eliminates the need for a common ground
Ideal for for applications requiring
High isolation surge voltage
Signal cannot travel in opposite direction
Used to control motors, solenoids, etc.