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SEMICONDUCTOR

Diodes PN junction and diode

biasing

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SEMICONDUCTORS

• CRYSTAL DIODE

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SEMICONDUCTORS

DIODES: NOTE STRIPE AND SYMBOL FOR ANODE AND CATHODE

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SEMICONDUCTORS

A type of semi-conductor, simple in construction used for rectification, wave-shaping, circuit protection, and logic operations.

A two terminal device that allows current to flow in one direction, but not the other.

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SEMICONDUCTORS

Formed by combining a P-type and N-type semi-conductor materials together.

At this junction the mobile charges in the vicinity of the junction are strongly attracted to their opposites and drift toward each other.

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SEMICONDUCTORS

Some of the electrons move across the junction and fill the holes near the junction in the p-type material.

In the N-type material, the electrons become depleted near the junction.

This region is called the depletion region, and extends only a short distance on either side of the junction.

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SEMICONDUCTORS

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SEMICONDUCTORS

The opposite charges that build up on each side of the junction create a difference in potential or voltage, referred to as the barrier voltage.

The barrier voltage created at the junction is very small, 0.7 volts for silicon and 0.3 volts for germanium.

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SEMICONDUCTORS

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SEMICONDUCTORS

When a voltage is applied to a diode, it is referred to as a bias voltage.

Current does not normally flow across the P-N junction, but when just the right voltage is applied using the proper polarity, current will begin to flow.

This is called forward biasing.

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SEMICONDUCTORS

What is the purpose of the resistor in this diagram?

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SEMICONDUCTORS

The negative terminal of the voltage source is connected to the N-type material.

This forces the electrons away from the terminal, toward the PN junction.

The free electrons accumulate on the P side of the junction by the positive terminal.

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This action cancels the negative charge on the P side and the barrier voltage is eliminated and current is able to flow.

Current flow occurs only if the external voltage is greater than the barrier voltage.

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SEMICONDUCTORS

Forward bias: Negative terminal of the battery connected to N side (negative)

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SEMICONDUCTORS

Once a current is applied to the diode, a voltage drop occurs.

This voltage drop is equal to the barrier voltage of the diode.

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SEMICONDUCTORS

In a forward bias diode the negative terminal is connected to the N-type material and the positive terminal is connected to the P-material.

If the terminals are reversed, the diode does not conduct and is referred to as reverse bias.

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SEMICONDUCTORS

NOTE THAT NO CURRENT AT JUNCTION

REVERSE BIAS

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Reverse bias increases the depletion zone, hence increasing the barrier voltage.

No current will flow unless the applied voltage is greater than the break over voltage.

This in turn will cause excessive heat and burn up the diode.

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SEMICONDUCTORS

When forward-biased, there is a small amount of voltage necessary to get the diode going. In silicon, this voltage is about 0.7 volts. This voltage is needed to start the hole-electron combination process at the junction.

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SEMICONDUCTORS

The operation of the PN junction in a diode is unidirectional since it conducts current in one direction.

When the diode is forward biased, current flows through it freely.

When the diode is reverse biased, current doesn’t flow through it.