Inside the diode and LED
A one-way valve for current, built from the p-n junction: the idea behind every transistor, solar cell and chip. Light it up and it is an LED. Every picture below is computed while you watch.
What it is
Pure silicon barely conducts. Add a trace of boron and it becomes p-type: it has “holes”, missing electrons that act as positive carriers. Add phosphorus and it becomes n-type, with spare electrons. Join the two and electrons and holes near the junction cancel each other out, leaving a thin depletion region whose built-in field holds the rest back. Push the right way (forward bias) and the barrier comes down: current flows. Push the other way and it grows: almost nothing flows.
I = Is · (e^(V / n·Vt) − 1) Vt = kT/q ≈ 26 mV at 25 °C λ ≈ 1 240 / Eg nm
How it’s made
The p-n junction, live
Holes (○) on the p side, electrons (●) on the n side, the fixed ions of the depletion region between them. Slide the bias: forward, the region shrinks and carriers flood across and recombine (in an LED each recombination is a photon of the band gap’s colour); reverse, it widens. Push reverse far enough and it breaks down. The curve is the diode equation at the temperature you set.
The carriers’ motion is illustrative; the current and the curve are computed. Breakdown is drawn at a typical voltage for each (LEDs are only rated for about 5 V reverse).
The LED and its resistor
An LED’s current climbs exponentially with its voltage, so it must not be fed a voltage; it must be fed a current. The simplest way is a series resistor: R = (Vsupply − Vf) / I.
What goes wrong
- Too much current: an LED without a resistor, or a rectifier diode past its rating, overheats in seconds.
- Reverse voltage: LEDs are only rated for about 5 V reverse; rectifiers have a PIV rating (1N4007: 1 000 V).
- Thermal runaway: a diode’s forward voltage falls as it heats, so diodes in parallel without resistors let the hottest one hog the current.
- Static: small-signal and LED junctions can be damaged by electrostatic discharge from your fingers.
Use it on the bench
- Diode forward and reverse bias — forward and reverse bias with a meter (about it)
- LED and its series resistor — an LED and its series resistor (about it)
- Zener shunt regulator — a Zener diode holding a voltage (about it)
- Half-wave rectifier — a half-wave rectifier (about it)
- Bridge rectifier and filter — a bridge rectifier and its filter (about it)
Check yourself
Exam-style questions (ECE, EE and NC-II reviewers ask these). Each answer links to the scene that shows it.
1. In a forward-biased silicon diode, the voltage across it at a few milliamps is about:
Silicon’s barrier makes current rise steeply from about 0.6 V; germanium’s from about 0.3 V. Try both in the junction scene. See the scene ▸
2. Reverse bias does what to the depletion region?
The applied field adds to the built-in one, pulling the carriers further apart. Slide the bias negative. See the scene ▸
3. What decides an LED’s colour?
An electron falling across the gap gives a photon of that energy. 1.9 eV is red (650 nm); 2.7 eV blue (460 nm). See the scene ▸
4. A red LED (Vf ≈ 2 V) on 5 V should have about 15 mA. The resistor is about:
R = (5 − 2)/0.015 = 200 Ω; the nearest standard values are 220 Ω or 180 Ω. Try it in the LED scene. See the scene ▸
5. Why must an LED never be connected straight to a supply without a resistor?
The diode equation: a few tenths of a volt more and the current goes up tenfold. A resistor (or a constant-current driver) sets the current. See the scene ▸
6. How does a white LED make white?
Most white LEDs are blue InGaN chips with a cerium-doped YAG phosphor. See “How it’s made”, LED, step 4. See the scene ▸
7. What is the depletion region made of?
Near the junction electrons and holes recombine, leaving the fixed ions (− on the p side, + on the n side). Their field is the barrier. See the scene ▸
8. A Zener diode is meant to:
Heavily doped junctions break down at a sharp, low reverse voltage. The breakdown is safe as long as the power is limited. See the scene ▸
Inside the other parts
The resistor · the capacitor · the inductor · the transformer · the transistor · the battery · all the parts