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Diode ratings and construction

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Diodes have two active electrodes between which the signal of interest can flow

The word diode comes from the Greek words meaning “two paths”

They are usually used for unidirectional current properties

This property is called the rectifying property

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When we combine these two types of semi-conductors, we create a PN Junction (the space between the two).

Recall that positive charges develop in the N-type and Negative charges develop in the P-type.

The result is a “barrier” voltage being developed across the PN junction

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Germanium diodes have a typical barrier voltage of 0.3 Volts and Silicon diode barrier is typically about 0.7 volts.

The main characteristic of diodes is their ability to either pass or stop current flow.

Note that a diode will eventually reach a voltage level where it will begin to breakdown (the breakdown voltage).

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Biasing is a function of diodes.

Forward biasing involves allowing current to flow in one direction where reverse biasing blocks current in the opposite direction.

Diodes are like check valves in a water system.

Note that diodes do no behave in a LINEAR fashion when it comes to current and voltage.

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Diode characteristics can be plotted on a graph, which is referred to as the V-I curve

The diodes forward and reverse bias voltages Vc & Vr are plotted to the right and left on the horizontal axis of the graph

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The diodes forward and reverse bias Currents If & Ir are plotted to the top and bottom on the vertical axis of the graph.

The vertical and horizontal crossing point is the zero reference point.

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Germanium/Silicon diode V-I characteristics The point which current

will start to flow after the knee

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When reverse-biased, an ideal diode would block all current, a real diode lets perhaps 10 micro amps through -- not a lot, but still not perfect.

If you apply enough reverse voltage (V), the junction breaks down and lets current through.

Usually, the breakdown voltage is a lot more voltage than the circuit will ever see, so it is irrelevant.

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Germanium/Silicon diode break down voltage

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Diode ratings are specified by manufactures to insure that they do not become damaged or allow unsafe reverse voltages to flow through a circuit.

Temperature is also a factor that has to be taken into consideration, the diode characteristic that is most adversely affected by temperature is the reverse current.

Extremely high temperature allows more reverse current flow than low temperatures.

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Diodes can become damaged by excessive forward current so manufactures usually specify the maximum forward current (If max) that each type of diode can safely handle.

Diodes can also be damaged by excessive reverse voltages that cause it to break down and allow dangerously high reverse voltages, this reverse breakdown voltage is specified as PIV (peak inverse voltage).

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The breakdown voltage tends to increase as temperature increases and the forward voltage drop decreases allowing to pass current sooner, this is true for both germanium and silicon

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Diode symbol showing current flow (If)

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How forward and reverse biased diodes are represented in schematic form.

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Diodes are made by a technique known as the growth method.

PN junctions are grown by placing an intrinsic semiconductor a P type impurity into a quartz container and then heated until the two materials melt.

A small semiconductor crystal (seed) is then lowered into the molten mixture and the seed is rotated withdrawn from the mixture, this forms a P type semiconductor.

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The growth technique under an electron microscope

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As the seed is withdrawn the molten mixture can be doped with N or P type impurities to create either type of layer within the crystal as it grows.

The resulting crystal can then be cut into many PN junctions.

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PN junctions can also be constructed using the alloyed method.

This is done by placing a small pellet of indium on N type semiconductor crystal and then it is heated until the pellet fuses with the crystal.

Since indium is trivalent impurity it produces a P type semiconductor.

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The preferred method is the diffusion technique.

A thin section of N or P type material (wafer) is exposed to an impurity element which is in a gaseous state.

The impurity atoms penetrate or diffuse through the exposed surfaces of the wafer.