Question

What is the equation for a pn junction diode?

Answer 1

The equation that describes the current-voltage (I-V) relationship of a PN junction diode is the Shockley diode equation. It is given by:

\[
I = I_S \left( e^{\frac{V}{nV_T}} - 1 \right)
\]

Where:

1. I is the current through the diode.
   

1. I_S is the reverse saturation current (a small current that flows through the diode when it's reverse-biased).
   

1. V is the voltage applied across the diode (positive for forward bias, negative for reverse bias).
   

1. n is the ideal factor, typically close to 1 for a silicon diode (it adjusts for non-ideal behavior).
   

1. V_T is the thermal voltage, which is approximately 26 mV at room temperature (300 K).
   

Key points about the equation:

1. In forward bias (positive voltage on the anode), the current increases exponentially with voltage, meaning the diode conducts more as voltage increases.
   

1. In reverse bias (negative voltage on the anode), the current is very small (the term \( e^{\frac{V}{nV_T}} \) becomes very small), and the current is approximately equal to \( -I_S \), which is the reverse saturation current.
   

1. The thermal voltage (V_T) depends on temperature and is approximately 26 mV at room temperature.
   

This equation shows how the diode behaves in terms of the current that flows through it when a voltage is applied.