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Diode Current Equation

The diode current equation shows relationship between the current flowing through the diode as a function of applied voltage. The mathematical expression of the diode current is as given below.

Where,
  I is current flowing through the diode
  Iis reverse saturation current
   q is the charge of electron
  V is the voltage applied across diode
  η is the exponential ideality factor of diode
  K is the boltzmann constant and K = 1.38 x 10-23   J/K
  T is the absolute temperature in Kelvin

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
  • Diffusion
  • Carrier Combination in the depletion region
  • Doping Level
  • Manufacturing Process
  • Purity of the material
The value of ideality factor η is generally lies between 1 and 2.

Diode Equation in Forward Biased Condition

When the diode is in forward biased condition , the large forward current flows through the diode and value of the exponent term is larger and the diode equation becomes

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.

I = – Io

Diode Equation at Room Temperature

Let the room temperature is 27°C.
Temperature in Kelvin = 27 +273 =300 K

K =1.38 x 10-23  JK-1
q =1.6 x 10-19  C
The ratio KT/q is called the thermal voltage of the diode.