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ETE411 :: Lecture 11

Dr. Mashiur Rahman

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Review of last class

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• During the formation of junction, 2 phenomena take place:1. A thin layer is set up on both sides of the junction.2. A junction/barrier potential is developed across the

junction• Formation of depletion layer:

1. Greater concentration of holes in P than N region 2. Greater concentration of electrons in N than P region

• The difference in concentration establish a density gradient across the junction resulting in carrier diffusion and terminate the existence by recombination

OBJECTIVES

Up on completion of this topic the student will be able to know

• How to forward bias the diode, under forward biasing how the diode works.

• How to reverse bias the diode, under reverse biasing how the diode works.

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PN-JUNCTION FORWARD BIAS

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• As soon as battery connection is made, the holes in P region are repelled by the positive battery terminal & electrons in N region are repelledby the negative battery terminal. Thus both holes & electrons are driven towards the junction where they recombine.

• This allow easy current flow through the semiconductor as the crystal offers low resistance. FB reduces the thickness of the depletion layer.

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Forward characteristics

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• At room temperature, a potential difference is require before a reasonable amount of forward current starts flowing in a junction. This is known as threshold voltage, Vth.For applied voltage less than Vth, current flow is negligible. As applied voltage increase beyond Vth, forward current increase sharply. If forward voltage increase beyond a certain safe value, it will produce an extremely large current which may destroy the junction due to overheating.

PN-Junction with forward Bias

• Positive terminal of the battery is connected to anode and negative terminal is connected to the cathode.

• Increasing the applied voltage the potential barrier decreases.

• Potential barrier disappears when applied voltage exceeds barrier potential.

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• The resistance of junction becomes nearly zero and current starts flowing in the junction.

• Now the current increases sharply with increase in applied voltage.

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PN-JUNCTION REVERSE BIAS

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• During the RB, holes are attracted by the negative battery terminal & electrons are attracted by the positive battery terminal. Thus, both holes & electrons move away from the junction. Hence, no electron-hole combination, no current flow and the junction offers high resistance.

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Practically no current flow, yet there is a small current due to minority carriers which is known as reverse current or reverse current, Isor Io. It also referred as leakage current of PN junction. If reverse bias applied to a junction is increase, a point is reached when the junction breakdown and revere current rises to value limited only by the resistance connected in series with junction. This critical value of voltage is known as breakdown voltage.

PN-JUNCTION WITH REVERSE BIAS

• Negative terminal of the battery is connected to the anode and positive terminal is connected to the cathode.

• The resistance is very high in the order of mega ohms.

• Current is almost zero in reverse bias.

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• Small reverse leakage current flows due to minority carriers present in P & N regions.

• Reverse voltage exceeds breakdown voltage the conductivity of the junction rapidly increases causing the diode goes to damage.

• Operate the diode below break down voltage.

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Zener breakdownWhen reverse bias is increased, the electric field at the junction also increases.

High electric field causes covalent bonds to break.

Thus a large number of carriers are generated. This causes a large current to flow.

This mechanism of breakdown is zener breakdown.

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Avalanche breakdown• The increased electric field causes increase in the velocities of

minority carriers.• These high energy carriers break covalent bonds, thereby

generating more carriers.• These generated carriers are accelerated by the electric field.• They break more covalent bonds during their travel. • Small reverse leakage current flows due to minority carriers

present in P & N regions.• Reverse voltage exceeds breakdown voltage the conductivity of

the junction rapidly increases causing the diode goes to damage.• Operate the diode below break down voltage.

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SUMMARY

We have discussed about

• Forward biasing of diode

• Reverse biasing of diode

• Avalanche and zener breakdowns.

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QUIZ When we apply reverse bias to a junction diode, it

(a) Decreases the potential barrier

(b) Increases the potential barrier

(c) Greatly increases the minority carrier current

(d) Greatly increases the majority carrier current

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2. When forward bias is applied to a junction diode, it

(a) Increases the potential barrier

(b) Decreases the potential barrier

(c) Reduces the majority carrier current to zero

(d) Reduces the minority carrier current to zero

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3. The reverse saturation current in a junction diode is the current that flows when

(a) Only majority carriers are crossing the junction

(b) Only minority carriers are crossing the junction

(c) The junction is unbiased

(d) The potential barrier is zero

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Frequently asked questions

• 1) Discuss the behavior of PN junction under forward biasing

• 2) Explain the working of PN junction diode under reverse biasing

• 3) Define avalanche and zener break down

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