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1
SEMICONDUCTORS
Diode ratings and construction
2
SEMICONDUCTORS
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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SEMICONDUCTORS
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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SEMICONDUCTORS
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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SEMICONDUCTORS
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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SEMICONDUCTORS
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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SEMICONDUCTORS
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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SEMICONDUCTORS
Germanium/Silicon diode V-I characteristics The point which current
will start to flow after the knee
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SEMICONDUCTORS
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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SEMICONDUCTORS
Germanium/Silicon diode break down voltage
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SEMICONDUCTORS
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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SEMICONDUCTORS
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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SEMICONDUCTORS
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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SEMICONDUCTORS
Diode symbol showing current flow (If)
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SEMICONDUCTORS
How forward and reverse biased diodes are represented in schematic form.
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SEMICONDUCTORS
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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SEMICONDUCTORS
The growth technique under an electron microscope
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SEMICONDUCTORS
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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SEMICONDUCTORS
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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SEMICONDUCTORS
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.