The current flowing through the diode does not vary linearly with increase in the applied voltage. The V-I characteristics of the diode has the exponential relationship. The resistance of the diode varies with increase in the temperature that is why the current does not vary linearly with voltage. The diode is non ohmic and non linear semiconductor device.
Let us understand on what factors the diode current depends.
Reverse Saturation current:
The current flowing through the p-n junction diode when it is reversed biased is called reverse saturation current.The minority carriers are responsible for this current.In a PN junction diode, the reverse saturation current is due to the diffusive flow of minority electrons from the p-side to the n-side and the minority holes from the n-side to the p-side.The reverse saturation current of the diode in the range of μA to nA. The reverse saturation current gets doubled for every 10 degree centigrade rise in temperature.
η, the (exponential) Ideality Factor
Ideality factor is a way of measuring how accurately the diode follows the ideal diode equation. if the diode under consideration behaves exactly as that of an ideal diode, then η will be 1. Its value increases from 1 as the difference between the behaviors of the ideal diode and diode under consideration increases: greater is the deviation, greater is the value of η. The value of η is considered to be 1 for germanium diodes and 2 for silicon diodes. The ideality factor depends on the following parameters of the diode.
- Electron Drift
- Carrier Combination in the depletion region
- Doping Level
- Manufacturing Process
- Purity of the material
Diode Equation in Forward Biased Condition
Diode Equation in Reverse Biased Condition
When the diode is in reverse biased condition , the exponential term becomes negligible and diode current is equal to the reverse saturation current.
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