The Diode I–V Curve

Lesson notes

Overview

A diode blocks current in reverse and conducts exponentially once forward voltage passes ~0.6 V. This topic is rated intermediate and is number 38 of 93 in Engineering. Builds on the basics with relationships, methods and trade-offs.

What you are seeing

The panel above shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.

In depth

A diode conducts in one direction. In reverse bias, only a tiny leakage current flows, while in forward bias the current rises exponentially once the voltage passes the threshold of about 0.6 V for silicon, I = Is(e^(V/nV_T) − 1). This non-linear behaviour allows diodes to rectify AC into DC, protect circuits from reverse voltage, clip signals and detect radio waves. LEDs are diodes with a higher threshold that emit light.

Reading the relationship

The expression "I = Is(e^(V/nV_T) − 1)" is a compact summary of the idea. It uses the symbols I, s, e, V, n, V_T; name what each one stands for before you start. Notice that it contains a ratio, so increasing the denominator makes the result smaller, and an exponential, which changes by the same factor for every equal step. Then use the sliders to see how changing one quantity changes the others.

A guided exploration

1. Set every slider to its middle value and note the shape of the curve. 2. Move one slider at a time and describe whether the curve shifts, stretches or turns. 3. Find the value where the behaviour changes sharply, then connect it to the formula.

Try this

Predict the outcome first, then test extreme slider values to find where the pattern changes or breaks.

Common pitfalls

Always ask what the safety margin is and which assumption would break the design first. Idealised models hide losses, noise and limits.

How it connects

It builds on “Shannon Channel Capacity”, the topic before it in Engineering. Once you are comfortable, continue with “Op-Amp Amplifier Gain”.

Why it matters

Engineers use this idea to design structures, machines and systems that are safe, efficient and reliable.