Electronics >> Diodes
Junction Diodes are two terminal semiconductor devices that are made out of P type and N type (doped) material. One terminal is connected to P material and the other one is connected to N material. The common connecting points where these materials are joined is called the junction.
Diodes permit the current to flow in one direction and block the flow of current in the opposite direction.
The other types of diodes in use are:
[Zener Diodes] and [light emitting diodes (LEDs)]
Diode is a nonlinear device. This is demonstrated in the following graph, and the fact that its current versus voltage is not a straight line.
In the forward region of the graph, the voltage at which the current starts to increase quickly is called the knee voltage of the diode.
Study of a diode circuit usually comes down to determining if the diode voltage is higher or lower than the knee voltage.
- If the knee voltage is higher, the diode conducts easily
- If the knee voltage is lower, the diode conducts poorly
(The knee voltage for a silicon diode is approximately 0.7 volt and for a germanium diode 0.3 volt)
In this circuit the diode is forward-biased because the positive battery terminal drives the P side (Anode) through a resistor and the negative battery terminal is connected to N side (Cathode).
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Breakdown voltage:
Diodes have a maximum voltage rating, and also there is a limit to how much reverse voltage a diode could withstand before it is damaged. If you continue to increase the reverse voltage, you will ultimately reach the breakdown voltage of the diode, at which the component will loose its characteristics and will allow the current to flow in either directions.
(For most diodes the breakdown voltage is at least 50 volts.)
The exact amount of reverse voltage for any diode could be found on its manufacturer's data sheet.
Maximum DC forward current:
If the current in a diode is too large, the excessive heat can destroy the component. For this reason the manufacturer's data sheet indicates the maximum current a diode can safely handle without shortening its life or affecting its characteristics.
Power dissipation:
You can calculate the power dissipation of a diode the same way as you can do for a resistor.
The power rating of a diode is the maximum power that it can safely dissipate without shortening its life or affecting its properties.
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Zener Diodes:
Zener diodes are silicon diodes that are designed, manufactured and optimized for operation in breakdown region. Zener diodes are produced with breakdown voltages of 2 to over 1000 volts. These diodes are capable to operate in forward, leakage and breakdown region.
The difference between Zener diode and a regular junction diode is that, their operation in breakdown region, while kept within the preset manufacturer's specification does not damage the component.
Zener diodes are essentially connected and used within a circuit in reverse biased mode.
Zener diodes are designed to begin allowing the flow of the current once the voltage reaches the predetermined breakdown voltage.
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Light emitting diodes (LEDs):
The energy within a standard diode is radiated in the form of heat, however in LEDs most of this energy is converted to light.
Light emitting diodes use low voltage, have long life, and have fast on-off switching.
A few examples of Light emitting diodes, and the current they use with respect to their diameter:
- 4mm - 5mm diameter » 15 to 20 mA (milliAmp)
- 3mm diameter » 5 to 10 mA (milliAmp)
- 2mm diameter » 1mA (milliAmp)
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Doping of semiconductors:
By adding impurity atoms to a pure base crystal, we alter its electrical conductivity in a controlled fashion and produce semiconductors with specific characteristics.
In semiconductors, n-type material has a larger number of free electrons, and p-type material has a larger amount of holes in their atomic compositions.
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