Semiconductor Physics Mine Aa4

8/11/2019 Semiconductor Physics Mine Aa4

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SEMICONDUCTOR PHYSICS

Breakdown etc.


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Breakdown in P-N junction diode

In Electronics, the term breakdown stands for release ofelectron-hole pairs in excess.

1. Avlanche Breakdown (uncontrolled)2. Zener Breakdown (controlled)

The critical value of the voltage, at which the breakdown of a P-N junction diode occurs is called the breakdown voltage. The

breakdown voltage depends on the width of the depletion region,which, in turn, depends on the doping level.

There are two mechanisms by which breakdown can occur at a

reverse biased P-N junction:


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Avalanche breakdown


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Avalanche breakdown

If the reverse bias is made very high, the thermally generatedelectrons and holes get sufficient K.E from applied voltage to breakthe covalent bonds near the junction and a large no. of electron-hole pairs are released. These new carriers, in turn, produceadditional carrier again by breaking bonds. Thus reverse currentthen increase abruptly and may damage the junction by theexcessive heat generated.

The avalanche breakdown occurs in lightly doped junctions, whichproduce wide depletion region.

The avalanche breakdown voltage increases as the temp. of the junction increases due to the increased probability of collisions ofelectron and holes with crystal atoms.


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Zener breakdown (controlled)

Zener Breakdown occurs at low voltage in heavily doped reversebiased p-n junction.

Strong electric field directly (without impact of electron) pull out theelectrons from the covalent bond.

Zener breakdown voltage decreases as the temp. of the junctionincreases. Since an increase in temp. increase the energy of valenceelectron. So escape from covalent bond become easier for theseelectrons. Thus a smaller reverse voltage Is sufficient to pull thevalence electron out of the covalent bonds.


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Zener breakdown (controlled)Zener Breakdown occurs at low voltage (


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Working of Zener Diode

At a certain reverse bias voltage (Zener voltage), in heavily doped p-ndiode, the bottom of conduction band in n-region becomes lower

than the top of valence band in p-region.

Electron now tunnel directly across the potential barrier from thevalence band in p-region into the conduction band in n-region .

Hence a large reverse current flows.


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IntroductionThe basic function of zener diode is to maintain a specific voltage across itsterminals within given limits of line or load change. Typically it is used forproviding a stable reference voltage for use in power supplies and otherequipment.

This particular zener circuit will work to maintain 10 V across the load.


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Zener Diode ApplicationsRegulation

In this simple illustration of zener regulation circuit, the zener diode will adjust itsimpedance based on varying input voltages and loads (R L) to be able to maintain itsdesignated zener voltage. Zener current will increase or decrease directly withvoltage input changes. The zener current will increase or decrease inversely withvarying loads. Again, the zener has a finite range of operation.


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Zener LimitingZener diodes can used for limiting just as normal diodes. Recall in previouschapter studies about limiters. The difference to consider for a zener limiter isits zener breakdown characteristics.


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The diode

Diode structure and symbol The diode is a single pn junction device with

conductive contacts and wire leads The p region is called anode and the n region

is called cathode The arrow points in the direction of

conventional current (opposite to electronflow)


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Forward and reverse bias of a diode


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The ideal diode model

The ideal model ofa diode is a simpleswitch. The barrier

potential, theforward dynamicresistance and thereverse current areall neglected


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The practical diode model

R

V V I F BIAS

F


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The IV characteristics of the practical diodemodel


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The complex diode model


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The IV characteristics of the complex diodemodel


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Photodiodes Photodiodes are semiconductor light sensors that generates

current or voltage when PN junction is illuminated by light. Photodiode working as photo-detector is essentially a reverse

biased PN junction diode which is designed to respond to photonabsorption.

When photodiode is kept under dark condition and sufficientreverse voltage is applied then almost constant current,independent of magnitude of reverse bias is obtained. Which iscalled dark current.


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Photodiode fundamentals Based on PN or PIN junction diode

photon absorption in the depletionregion induces current flow Depletion layer must be exposed

optically to source light and thickenough to interact with the light

Spectral sensitivity

Material Band gap(eV)

Spectral sensitivity

silicon (Si) 1.12 250 to 1100 nm

indium arsenide (InGaAs) ~0.35 1000 to 2200 nm

Germanium (Ge) .67 900 to 1600 nm

I

P

N

+

-

h

RLIL

electron

hole


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Semiconductor Physics Mine Aa4

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