Fundamentals of PN Junction Diode

Why P-N Junction Diode

• PN junction diode is nonlinear circuit elements and many signal processing function

need it e.g. signal rectification .

• PN junction is an important semiconductor device in itself and used in a wide variety

of applications such as rectifiers, Clipper and Clamper circuits, Photo detectors, light

emitting diodes (LED) and laser diode (LD) etc.

• PN junctions are an integral part of other important semiconductor devices such as

BJTs, JFETS and MOSFETs.

P-N Junction

• When a p-type semiconductor is brought in contact with n-type semiconductor, the

contact surface is called p-n junction.

• Diffusion of electrons and holes from majority carrier side to minority carrier side

until drift balances diffusion.

• Region near to the p and n junction depleted from free carriers because of the

majority carriers diffusion ( leaving only fixed –ve and +ve ions in p and n region

respectively).

• This internal electric field produces built-in-potentail which gives rise to the drift

current to the minority carriers and balances the diffusion current.

• Forward Biasing of P-N Junction diode

A p-n Junction is said to be in Forward Bias when the P-type region is made positive

with respect to the N-type region.

• Reverse Biasing of P-N Junction diode

A p-n Junction is said to be in Reverse Bias when the P-type region is made negative

with respect to the N-type region.

See the direction and magnitude of of drift current carefully. Drift current almost remains constant as it depends mainly on number of minority carriers

Rapid increase in ID when reverse bias voltage exceeds a break down voltage VZ Breakdown mechanism is either Avalanche or Zener

Avalanche break down:-

• If both p-side and n-side of the diode are lightly doped, depletion region at the

junction widens. Application of a very large electric field at the junction may rupture

covalent bonding between electrons. Such rupture leads to the generation of a large

number of charge carriers resulting in avalanche multiplication.

• In avalanche breakdown, VZ increases with temperature.

Zener break down:-

Breakdown Mechanism:

If both p-side and n-side of the diode are heavily doped, depletion region at the

junction reduces. Application of even a small voltage at the junction ruptures

covalent bonding and generates large number of charge carriers. Such sudden

increase in the number of charge carriers results in zener mechanism.

• In Zener breakdown, VZ decreases with temperature

• If the maximum specified power dissipation is not exceeded, breakdown is not

a destructive process.

In the forward bias region, the characteristics of Si diode shift to the left

(lower voltage) at a rate of 2.5mV/oC increase in temperature.

Reverse saturation current is approximately doubles for every 10oC rise

in temperature. If Is= Is1 at T = T1, then at temperature T2, Is2 is given by,

I = I x 2(T –T )/10

• Diode Approximation:

Ideal diode

When diode is forward biased, resistance offered is zero, When it is reverse biased resistance offered is infinity. It acts as a perfect switch

• 2nd approximation:

const. voltage drop model

When forward voltage is more than 0.7 V, for Si diode then it conducts and offers zero resistance. The drop across the diode is 0.7V. When reverse biased it offers infinite resistance.

• 3rd approximation:

Piecewise linear model

When forward voltage is more than 0.7 V, for Si diode then it conducts and offers resistance. When reverse biased it offers very high resistance but not infinity.

• The straight line represented by the above equation is known as the load line.

The load line passes through two points, I = 0, VD = V and VD= 0, I = V / RL.

• The intersection point of load line and diode characteristics curve gives the

operating point.

Important terms used for a p-n junction diode:

Breakdown Voltage: It is the minimum voltage at which p-n junction breaks down with sudden rise in reverse current.

Knee Voltage: It is the forward voltage at which the current through the junction starts to increase rapidly.

Maximum forward Current: It is the highest instantaneous forward current that a p-n junction can conduct without damage to the junction .

Peak Inverse voltage (PIV): It is the maxium reverse voltage that can be applied to the p-n junction without damage to the junction .

Maximum power rating: It is the maximum power that can be dissipated at the junction without damaging it.

• If the maximum specified power dissipation is not exceeded, breakdown is not a destructive process.

Conclusion of the Course, you gate the Certificate of the Course.