Lecture 4 (Diodes)

8/12/2019 Lecture 4 (Diodes)

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Diode and Transistor

Dr. Abdul Majid Sandhu

Dr. Abdul Majid Sandhu, Department of

Physics, University of Gujrat.


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Doping (N- and P-type semiconductors)Extrinsic semiconductors



Elemental semiconductors consists of group-IV of periodic table.Doping with group-V elements make them N-type.

Doping with group-III elements make them P-type.


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N-type material

P

SiSi

Si P

SiSiSi

SiSi

P

SiSi

Si P

SiSiSi

SiSi

P Si P SiSi

Si

Si

Si

Si

Si

Phosphorus, a pentavalent atom,

is donating an electron to the

Crystal so it is called donor

impurity. There will be as manysuch electrons as the number of

P dopant atoms. These electrons

are free to move and available for

conduction.


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P-type material

Al

SiSi

Si Al

SiSiSi

SiSi

Al

SiSi

Si Al

SiSiSi

SiSi

Al Si Al SiSi

Si

Si

Si

Si

Si

Aluminum, a trivalent atom,

produces hole and ready to accept

and electron, so it is called an

acceptor impurity. There will beas many such holes as the number

of Al dopant atoms. These holes

are free to move and available for

conduction.


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PN-Junction

Si

SiSi

Si Al

SiSi

Si

Al Si AlSi

SiSi SiSi

Al Si AlSi

P

SiSi

Si P

SiSi

Si

P

SiSi

Si P

SiSi

Si

P Si PSi

Si

Si

Si

Si

Si

A junction is formed when on side of semiconductor is doped by n-type and other side by p-

type impurity.


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Diodes (The PN Junction)


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Diodes (The PN Junction)

The free electrons in

the n region are

randomly drifting in

all directions. At the

instant of thepn

junction formation, the

free electrons near the

junction in the n

region begin to diffuse

across the junctioninto thep region

where they combine

with holes near the

junction. The same is

true for thep-typematerial.


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Diodes (The Depletion

Region)When thepnjunctionis formed, the n regionloses free electrons as

they diffuse across the

junction. This creates

a layer of positive

charges (ions) near the

junction. As the

electrons move across

the junction, thepregion loses holes as

the electrons and holes

combine. This creates

a layer of negativecharges (ions) near the


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PN-Junction

Si

SiSi

Si Al

SiSi

Si

Al Si AlSi

SiSi SiSi

Al Si AlSi

P

SiSi

Si P

SiSi

Si

P

SiSi

Si P

SiSi

Si

P Si PSi

Si

Si

Si

Si

Si

Electrons and holes present near the junction recombine. Electrons diffuse to the left leaving

behind pentavalent atom as positive ion in N-type and trivalent atom as a negative ion in P-type. Hole disappears and electrons becomes a valance electron. A pair of ions form a dipole.


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Electron hole diffusion

Al

SiSi

Si

SiSi

Si

Al Si Si

SiSi SiSi

Al Si Si

SiSi

Si P

SiSi

Si

SiSi

Si P

SiSi

Si

Si PSi

Si

Si

Si

Si

Si


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Depletion Region and Barrier potential



Electrons diffuse to the left forming dipoles, positive ions are formed in N-type side and

negative ions in P-type side. As dipoles build up, region near the junction is emptied of

carriers. This region is charged region (having +ve andve ions) but have no charge carrier

(electrons and holes), and is called depletion region. Since dipole has an electric field

between +ve andve ions, when fresh electrons try to enter the depletion region, electric field

try to push them back. Strength of field increase with each crossing electron until equilibrium

is reached and stops further diffusion of electrons. This is barrier potential whose value is0.7V for silicon.


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Diodes (Forward Bias )

To bias apnjunction, apply an external dc voltage across it. Forward bias

is the condition that allows current through apnjunction. The picture

shows a dc voltage source connected by conductive material (contacts and

wire) across apnjunction in the direction to produce forward bias.This external bias voltage is designated as VBIAS. Notice that the negative

side of VBIASis connected to the n region of thepnjunction and the

positive side is connected to thep region. This is one requirement for

forward bias. A second requirement is that the bias voltage, VBIAS, must be

greater than the barrier potential (0.7V in silicon and 0.3 in germanium).


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Diodes (Forward Bias )

Because like charges repel, the negative side of the bias-voltagesource "pushes" the free electrons, which are the majority carriers

in the n region, toward thepnjunction. This flow of free electrons

is called electron current. The negative side of the source also

provides a continuous flow of electrons through the external

connection (conductor) and into the n region as shown.


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Forward biased pn-junction Depletion region and potential barrier reduced

bi d di d


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Reverse biased diode

Depletion region becomes wider,

barrier potential higher


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Biasing of diode



Biasing is to apply voltage for current flow in a diode. It is of two types

Forward biasing

Reverse biasing


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Forward biasing

In forward biasing, P-side is connected to +ve and N-side tove terminal of battery. Battery pushes

electrons and holes towards the junction. If battery voltage is less than barrier potential (0.7V for

Silicon), free electrons do not have enough energy to get through depletion region. Otherwise, free

electrons have enough energy to pass through depletion region and combine with holes which

produces a continuous current. Therefore current easily flows when diode is forward biased andwidth of depletion region becomes smaller.

Diode act as a complete circuit and conducts during such forward biasing.


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In reverse biasing, P-side is connected toveand N-side to +ve terminal of battery. Negativeterminal of battery attracts holes and positive terminal of battery attracts electrons away from the

junction. Therefore depletion region widens and opposes the flow of current across junction. The

wider the depletion layer, greater the junction potential. Depletion region stop growing when

junction potential equals the applied reverse voltage. Therefore electrons and holes stop moving

away from junction then.

Diode act as an open circuit and does not conduct during such reverse biasing.

Reverse biasing


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Reverse minority saturation current



In reverse biasing a small current exists which is due to flow of minority charge carriers. Such

minority charge carriers are electrons in P-side and holes in N-side produce due to bonds broken by

thermal energy. Most of these minority carriers recombine with majority carriers but those present

inside depletion region may cross the junction produces small current calledreverse current. This

is also called minority saturation current, since it does not depend upon battery but temperature.Besides this thermally produced minority carrier current, a small current flows on the surface of

crystal (due to surface impurities) called surface leakage current.


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Diode breakdown

(Process of avalanche)



Diodes have maximum voltage ratings. There is a

limit to how much reverse voltage a diode can

withstand before it destroyed. If reverse voltage is

continuously increases, there will be breakdown

voltage. For most of diodes it is nearly 50 V. Afterincreases applied voltage more than breakdown

voltagea large number of minority charge carriers

appear and diode conducts heavily. These large

number of carriers are produced by avalanche

effect.

When reverse voltage is increased it forces

minority carriers more quickly which collide withatoms of crystals and knock valence electrons

producing free electrons. The process is geometric

producing 1,2,4,8. Electrons.

Exceeding breakdown voltage does not

necessary mean the destroy of diode.


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Forward (closed) and Reveres (open)biasing of diode.




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Half wave rectifier



Rectification is process of conversion of AC to DC.

Input A.C Output D.C


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Full wave rectifier


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Full wave rectifier (Bridge rectifier)


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Lecture 4 (Diodes)

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