- 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 relates the current flowing through the diode to the voltage applied across it. Mathematically, the diode current equation reads:
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.
Two parameters in this equation deserve close attention.
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 characteristic of the particular diode and reflects how much recombination occurs inside 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
The ideality factor (η) measures how closely a diode approaches the behaviour of an ideal diode.
A diode that behaves exactly like an ideal diode has η = 1, and η climbs as the device deviates further from ideal behaviour.
The value of η is typically considered to be 1 for germanium diodes and 2 for silicon diodes.
The exact value for a given diode depends on electron drift, diffusion, carrier recombination inside the depletion region, doping level, manufacturing technique and material purity.
In practice, η typically ranges from 1 to 2, varying with current and voltage levels.
In forward bias, a large current flows through the diode. The diode current equation (equation 1) then becomes
On the other hand, if the diode is reverse biased, then the exponential term in equation (1) becomes negligible. Thus we have
Now consider the form the diode current equation takes when the diode operates at room temperature: T = 300 K, with and 
at their standard values. Thus
The thermal voltage at room temperature works out to 25.87 mV, which gives the final form of the diode equation:
