What it does
A diode is a crystal that is p-type on one side and n-type on the other. Where they meet, free electrons from the n side fill holes on the p side. That leaves a thin depletion layer with no free carriers, only fixed ions: negative on the p side, positive on the n side. Their electric field makes a barrier (about 0.7 V in silicon, 0.3 V in germanium) that stops more carriers crossing.
- Bias. Slide V. With the p side joined to + (forward bias) the barrier drops and the layer thins; past the knee voltage the current shoots up. With the p side joined to − (reverse bias) the barrier grows, the layer widens and only a tiny leakage current flows.
- The picture. Holes (open circles) on the p side, free electrons (filled dots) on the n side, the depletion layer with its ions in the middle, and the battery the right way round.
- V-I curve. The characteristic curve of the diode with your working point. As in school graphs, the forward and reverse halves of the voltage axis have different scales.
- Zener diode. A heavily doped diode that breaks down at a set reverse voltage (5.1 V here) without damage. In the second card it holds a load at 5.1 V while the input changes: a voltage regulator.
This fits Class 12 Physics: semiconductor electronics (p-n junction formation, the semiconductor diode, forward and reverse bias characteristics, the Zener diode as an extension). It follows on from energy bands, where you make the p-type and n-type crystals; next, use diodes to turn AC into DC in the rectifier lab, or see a diode that makes power from light in the solar cell IV tool.
How to use it
- Pick Silicon, Germanium or Zener 5.1 V.
- Move the V slider, or tap a quick value (−5, 0, 0.3, 0.7 V).
- Read the current, the barrier height, the depletion layer width and the dynamic resistance (ΔV/ΔI at that point).
- In the regulator card, move the input voltage, the series resistor R and the load RL.
- Try the Check yourself questions.
Worked example
Silicon at 0.7 V forward bias: I = 10⁻¹⁴ × (e^(0.70/0.02585) − 1) = 5.75 mA. The barrier is 0.7 − 0.7 = 0 V and the dynamic resistance is only 4.5 Ω. At 0.3 V almost nothing flows. Germanium reaches 1.1 mA already at 0.3 V, because its barrier is lower.
Now give 5 V of reverse bias. The barrier grows to 5.70 V, the depletion layer widens from 0.50 µm to 1.43 µm, and the current is only −10 fA (ten million-billionths of an amp). Switch to the Zener diode and go to −6 V: it has broken down and carries 180 mA.
Regulator: Vin = 12 V, R = 220 Ω, RL = 1000 Ω. I_R = (12 − 5.1)/220 = 31.4 mA, I_L = 5.1/1000 = 5.1 mA, so the Zener carries 26.3 mA and the load stays at 5.10 V. Lower Vin to 6 V and the output drops to 4.9 V: the Zener has stopped regulating.
Tips
- The knee is not a sharp switch: current grows about ten times for every 60 mV, so a diode looks “off” until about 0.6 V and “on” after.
- Real reverse currents are larger than this ideal model (nanoamps for silicon, microamps for germanium) because of surface leakage and heat.
- A Zener diode is always used in reverse bias.
FAQ
Why does the depletion layer change width? Forward bias pushes electrons and holes toward the joint and partly refills it; reverse bias pulls them away and uncovers more ions.
Why do the two halves of the graph use different scales? Forward currents are milliamps over a fraction of a volt; reverse voltages run to many volts. One scale would hide one half.
Is anything saved? No. Everything runs in your browser and nothing is stored or sent.
Is this an official textbook tool? No. It is an independent practice page that follows the same school topics.
Where the numbers come from
- The diode equation I = Is (e^(V/ηV_T) − 1), V_T = kT/e (CODATA k/e = 8.617 × 10⁻⁵ V/K), Zener breakdown and the Zener regulator are standard physics. Barrier potentials of about 0.7 V (silicon) and 0.3 V (germanium) are the usual school values.
- The saturation currents (10⁻¹⁴ A and 10⁻⁸ A), the 0.5 µm zero-bias depletion width, the 5.1 V Zener with 5 Ω slope resistance and the regulator parts are made-up teaching values, not data for any real part. The maths was checked in the tests against an independent Python version.
- Drawings are fibub's own. Nothing you enter is stored or sent anywhere.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.