Bel 03 PN Junction Diodes

8/3/2019 Bel 03 PN Junction Diodes

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P N Junction Diodes

The semiconductor diode is formed by simply bringing p- andn- type semiconductors without disturbing the crystalline

continuity across the junction

Simplified physical structure of

junction diode

Cross section schematic

S. Kal, IIT-Kharagpur


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Holes diffuse from p to n, while electron diffuse from n to p

A negatively charged layer of un-neutralised acceptor ions

is formed in the p-side and a positively charged layer of un-

neutralised donor ions is formed in the n-side. It creates a

built-in potential, vbi across the junction. In the thin layer,

there cannot be any mobile carrier due to the internal built-

in electric field. It is called a depletion layer

Built-in field opposes further flow of electron and holesacross the junction. Thus, in equilibrium, the net electron

and hole currents are zero respectively.

P N Junction Diodes



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Forward Bias Condition

A forward-bias or on condition is established by applying

the positive potential to the p-type material and the negative

potential to the n-type material.



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Application of forward bias (V) will pressure electrons in

the n-type material and holes in the p-type material to

recombine with ions near the junction and reduce the

depletion layer width. Electrons in n-type material and holes

in p-type see a reduced barrier at the junction and strongattraction at the other side of the junction. As V increases,

depletion region continues to shrink until a flood of majority

carriers can pass through the junction resulting in an

exponential rise in current.

IS is minority carrier current and is negligible ( ~ QA ) incomparison to Imajority ( mA )

Forward Bias Condition



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A reverse-bias or off condition is established when +ve

terminal of the external potential is connected to the n-type

material and the ve terminal is connected to the p-type

material.

Reverse Bias Condition



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Reverse Bias Condition

The number of uncovered +ve ions in the depletion region of

the n-type material will increase due to the large number of

free electrons drawn to the +ve potential of the applied

voltage. Similarly, the number of uncovered ve ions will

increase in the p-type material. The net effect is widening ofdepletion region this will establish a great barrier for the

majority carrier to overcome and effectively Imajority becomes

zero.

The minority carriers at both sides of the junction will cross

the barrier easily resulting in minority carrier current and isknown as reverse saturation current ( represented by Is ). The

term saturation comes from the fact that it reaches its

maximum level quickly and does not change with V.



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VDO(= VK , VT) p Cut in voltage (upto which the IF remainsnegligible)

For Ge, VDO} 0.2V 0.3 V

For Si, VDO} 0.6V 0.7V

VDO decreases with temperature at a rate 2.5 mV/0 C

Current-voltage Characteristics of a PN junction Diode



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Diode current equation is

ID = IS [ {exp(qVd / LkT) 1} ] L = ideality factor} 1or ID = IS [ {exp(Vd / VT) 1} ] VT = (kT/q) = thermal voltage

= 0.026V at 300K

IS = reverse saturation current

IS is contributed by minority carries and increases with

increasing temperature and decreasing band gap. RoughlyIS doubles for every 10

o C rise of temperature

A simple diode circuitS. Kal, IIT-Kharagpur


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mA1DIfor26(mA)DI

26er

300KTatAmp)(in

D

I

0.026v

I

V

/dVdI

1r

V

Ie.I

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dI

e.IIVVwithBiasForwardFor

dI

dVr

)(rResistanceDynamicForwardorAC

I

VR,ResistanceStaticorDC

D

T

DD

e

T

D/VVS

TD

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/VVSDTD

D

Dd

d

D

DD

TD

TD

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Graphical analysis of the circuitS. Kal, IIT-Kharagpur


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The ideal diode: (a) Diode Circuit Symbol

(b) i v CharacteristicS. Kal, IIT-Kharagpur


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Approximating the diode forward characteristics with two

straight lines

Current-Voltage Drop Model of a p-n Junction Diode



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Piecewise linear model of the diode forward characteristic and

its equivalent circuit representation

Piecewise-linear Model of p-n Junction Diode


Bel 03 PN Junction Diodes

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