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

Where,

I = current flowing through the diode

Io = reverse saturation current

q = the charge of the electron

V = the voltage applied across the diode

η = the exponential ideality factor of the diode

K = 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 an increase in applied voltage. The V-I characteristics of the diode have an exponential relationship. The diode resistance varies with an increase in the temperature. As a result, the current does not vary linearly with voltage. The diode is a non-ohmic and nonlinear semiconductor device.

Let us understand what factors the diode current depends on.

**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 is 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 precisely like an ideal diode, then η will be 1. Its value increases from 1 as the difference between the behaviors of the ideal diode and the diode under consideration increases. The greater the deviation, the greater the value of η. The value of η is 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 generally lies between 1 and 2.

**Diode Equation in Forward Biased Condition**

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

**Diode Equation in Reverse Biased Condition**

When the diode is reverse-biased, the exponential term becomes negligible, and the diode current is equal to the reverse saturation current.

**I = – Io**

**Diode Equation at Room Temperature**

Let the room temperature be 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.

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