(07) PN-Junction.ppt

7/28/2019 (07) PN-Junction.ppt

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PN -Junction


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PN-Junction 22 July 2013

PN -junction is formed by growing a singlecrystal of Si or Ge, which is half P -typeand half N -type.

Note that a PN -junction diode cannot bemade by simply pushing a piece of P -typeagainst another of N -type.

PN -junction Diode


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Symbol


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Formation of Depletion Region


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7

Because of diffusion of electrons from N -typeinto P- type, and of hole from P- type into N -type, depletion region or space-chargeregion is formed .This region has extremely high resistance.Electric field is called barrier.

Also called barrier potential or contactpotential or diffusion potential .

V 0 = k T ln (N A ND /n i2) VoltV 0 = 0.7 V for Si, 0.3 V for Ge


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The barrier discourages the diffusion of majority carriers across the junction.

But the same barrier helps the minoritycarriers to drift across the junction.The minority carriers are constantly

generated due to thermal energy.Does it mean that there should beconstant current due to the minoritycarriers crossing the junction ?If yes, how can there be current withoutany external connection ?


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Ans.: The net current across the junctionhas to be zero. The majority carrier havinghigh energy still diffuse across the junction.The two currents counterbalance eachother.

0dif dr I I


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PN -Junction with Forward Bias


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Applied voltage opposes the contactpotential.

Net barrier potential V B is reduced.Diffusion current increases.Drift current slightly decreases.The forward basing current is

dr dif f I I I


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PN -Junction with Reverse Bias


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PN-Junction13

2 July 2013

V B increase.

I dif due to majority carriers reduces toalmost zero.I dr slightly increases.The net current I r remains constanttill breakdown , hence calledsaturation current I s .


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PN-Junction14

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Summary of Biasing Conditions


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Reverse Breakdown

At some high reverse voltage, calledbreakdown voltage ( V Z ), the currentabruptly increases. Two phenomena :

1. Zener Breakdown.2. Avalanche Breakdown.

Zener is predominant for V Z less than 4

V. Avalanche is predominant for V Z greaterthan 4 V.


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Zener Breakdown

The field across depletion region becomesso high (about 10 7 V/m) that largenumber of covalent bonds break.

Hence large current starts flowing.


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Avalanche Breakdown

Increased field causes large increase inthe velocities of minority carriers.These carriers cause impact ionization.Cumulative multiplication of chargecarriers occur.


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Junction Capacitances

Two kinds :1. Transition or Depletion Capacitance, C T :

This is predominant in reverse bias. Itdecreases with applied voltage.

2. Storage or Diffusion Capacitance, C D :This is predominant in forward bias. Of

the order of several hundred pF.


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Diode Characteristics 202 July 2013

V-I Characteristics


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V T is cut-in, knee, offset, turn-on, orthreshold voltage.I s is reverse saturation current.


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Si Diode versus Ge Diode


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Temperature Effects

I s almost doubles for every 10 C rise intemperature.V Z increases with temperature.

The width of the depletion layer decreaseand hence barrier potential decreases withrise in temperature.

As a result, V T decreases by 2mV foreach celsius degree rise in temperature.


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DIODE RESISTANCE

Two kinds :1. Static or DC Resistance (High Value) : It

is the resistance offered by the diode to a

dc current.2. Dynamic or AC Resistance (Low Value) :

It is the resistance offered by the diodeto ac current.


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Static or DC Resistance


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Dynamic or AC Resistance


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Dynamic Resistance of a Diode


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Ideal Diode

Ideally, a diode should haveR F = 0 and R R =

Draw the V-I characteristics of an ideal

diode.


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Ideal Diode Characteristics


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Simplified Circuit Model


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(07) PN-Junction.ppt