- Diode Current Equation Definition: The diode current equation is a formula that describes how the current through a diode responds to the voltage applied across it.
- Key Components: The equation includes the dark saturation current and the ideality factor, which are critical for understanding diode behavior.
- Forward vs. Reverse Bias: In forward bias, the diode conducts a large current, while in reverse bias, current flow is minimal due to the negligible exponential term.
- Temperature Impact: At standard room temperature, the diode’s behavior is influenced by the thermal voltage, which is around 25.87 mV.
- Diode Current Equation Derivation: Understanding how to derive and apply this equation is essential for effectively using diodes in electronic circuits.
What is the Diode Current Equation?
The diode current equation expresses the relationship between the current flowing through the diode as a function of the voltage applied across it. Mathematically the diode current equation can be expressed as:
Where,
- I is the current flowing through the diode
- I0 is the dark saturation current,
- q is the charge on the electron,
- V is the voltage applied across the diode,
- η is the (exponential) ideality factor.
is the Boltzmann constant- T is the absolute temperature in Kelvin.
This equation primarily focuses on two critical parameters.
They are I0, the Dark Saturation Current, and η, the (exponential) Ideality Factor.
Dark saturation current (I0) indicates the leakage current density flowing through the diode in the absence of light (hence, ‘dark’).
This parameter is the characteristic of the diode under consideration and indicates the amount of recombination which occurs within it.
The dark saturation current, I0, increases with a higher recombination rate and varies directly with temperature and inversely with material quality.
η, the (exponential) Ideality Factor
Ideality factor (η) indicates the nearness with which the considered diode behaves with respect to the ideal diode.
That is, if the diode under consideration behaves exactly as that of an ideal diode, then η will be 1. The ideality factor, η, starts at 1 and increases as the diode’s behavior deviates more from an ideal diode.
The value of η is typically considered to be 1 for germanium diodes and 2 for silicon diodes.
However, its exact value for the given diode depends on various factors like electron drift, diffusion, carrier recombination which occurs within the depletion region, its doping level, manufacturing technique and the purity of its materials.
Additionally, the ideality factor typically ranges from 1 to 2, fluctuating based on current and voltage levels.
In forward biased condition, there will a large amount of current flow through the diode. Thus the diode current equation (equation 1) becomes
On the other hand, if the diode is reverse biased, then the exponential term in equation (1) becomes negligible. Thus we have
Now let us examine the mode the diode current equation takes its form when we have the diode operating at room temperature. In this case, T = 300 K, also, and 
. Thus
By calculation, the thermal voltage at room temperature is determined to be 25.87 mV, which modifies the diode equation accordingly.
