Diodes, Triacs, Thermistors, Opto-isolators, Phototransistors

Raj GhoshPrateek Shah

Diodes

• Diodes are semiconductor devices consisting of two terminals

– Junction Diodes

– Zener Diodes

• A diode is created when a p-type semiconductor is joined with and

n-type semiconductor by the addition of thermal energy.

• On joining, the positive carriers diffuse into the n-type region and vice versa, to create a depletion region within the diode.

Diodes

• An internal electric field forms in the depletion layer which acts as a

barrier to diffusion of the majority carriers across it.

• Size of the depletion layer determines current flow across the diode.

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Depletion Region

Majority carriers Majority carriers

Diodes

• Forward biased configuration when positive side of battery is connected to

p-type semiconductor

• The majority carriers are forced towards the junction and the depletion

region decreases

• Current is sustained by the majority carriers.

Vo

Vo-V

Potential BarrierForward Biased

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depletion regionshrinks

V

Diodes

• Reverse biased configuration when positive side of battery is connected to n-

type semiconductor

• The majority carriers are forced away from the junction which increases the

depletion region

• A small reverse current or leakage current is sustained by the minority carriers

• At sufficiently high voltages, a sudden increase in reverse current is observed

(Zener effect).

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Reverse Biased

depletion regionincreases

irV

Vo

Vo+V

Potential Barrier

Diodes

V

I

conductionregion

non-conductionregion

Ideal Curve

The following graphs show the current flow through a diode as a function of

voltage across its terminals in forward biased configuration.

Junction Diode Applications

• Radio Demodulation

• Power conversion

• Over-voltage Protection

• Logic Gates

• Current Steering

Zener Diodes

• A Zener diode is a type of diode that permits

current to flow in the forward direction like a

normal diode, but also in the reverse direction if

the voltage is larger than the breakdown voltage

known as "Zener voltage“.

• They are usually used in the reverse biased

configuration, and operate near their breakdown

voltage.

• Zener diodes are specifically designed so that

the breakdown voltage is low.

Zener Diode Applications

• Zener diodes are used as “Voltage Regulators” and placed in parallel

across a load to be regulated.

TRIACs

• TRIAC (TRIode for Alternating Current) is a three

terminal switch which can conduct current in either

direction.

• It is triggered by a positive or negative voltage applied to

the Gate (G).

• Once triggered, it continues to conduct current, till the

current falls below a threshold value.

• A1 and A2 are the current carrying terminals.

TRIAC operations

• It is a 5 layer device equivalent to two silicon-controlled rectifiers (thyristors)

• Region between the main terminals are parallel switches (PNPN and NPNP)

TRIAC structure TRIAC Characteristic Curve

TRIAC Characteristic Curve

• 1st quadrant - A2 is (+) with respect to A1

• 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 A1 and A2

• IRDM is several orders of magnitude smaller than the “on” rating

TRIAC Applications

High Power TRIACS

• Switch for AC circuits, allowing the control of very large power flows with

milliampere - scale control currents

• Eliminate Mechanical wear in a Relay

Low Power TRIACS

• light dimmers

• speed controls for electric fans and other electric motors

• modern computerized control circuits

Opto-Isolator

• Opto-coupler

• Photocoupler

• PhotoMOS

• Used to transmit current

between terminals without

an electrical connection

Functioning

• Electric signal converted to optical signal and back

• Usually an LED along with a photo-diode is used to achieve this

Configuration Modes

• Photovoltaic mode– Diode acts as current source– Output current and voltage depend on load

impedance, light intensity

• Photoconductive mode– Diode connected to supply voltage– Magnitude of current directly proportional to

light intensity

Applications

• Cut down ground loops– Current between terminals supposed to be at

same potential but actually are not

• Block voltage spikes

• Isolate low current control circuitry from power supplies, high-current circuitry

Opto-isolators in digital logic

Phototransistors

• Also photodiodes

• Convert light into current or voltage

Functioning

• Similar to regular transistor with light acting as base/gate

• PN junction creates electron-hole pair when photon strikes

PIN structure

• Sometimes used instead of PN junction

• Intrinsic layer between P and N layers• Increases area where electron-hole pair can be

generated

Modes of Operation

• Photovoltaic– Photocurrent restricted– Voltage builds up – device forward-biased

• Photoconductive– Diode reverse biased– Decreases junction capacitance– Faster but noisier

Applications

• Measuring light intensity– CD players– Smoke detectors

• PIN diodes (fast)– Optical communication– Light regulation

Thermistor

• “Thermal Resistor”• Resistance changes with temperature

Types

• Positive Temperature Coefficient (PTC)– Resistance rises with temperature– Also known as “posistors”

• Negative Temperature Coefficient (NTC)– Resistance decreases as temperature rises

Functioning

• NTC Thermistors– Semiconductor cast chip– Increasing temperature increases conductivity

• PTC Thermistors– Mostly switching – resistance rises at

particular temperature– Ferroelectric material - Curie temperature

Applications

• Current-limiting devices for circuit protection –resistive heating (PTC)

• Heating in temperature-controlled ovens (PTC)• Current limiting devices in power supply circuits

(NTC)• Monitor temperatures (NTC)

Questions