The p–n Junction Diode, Zener Diodes and LEDs

1Three junctions in one lamp

A small solar lantern has three jobs: collect sunlight by day, store the energy in a battery, and give light at night. It does all three with the same structure, a junction between p-type and n-type semiconductor. In the solar panel, light falling on junctions drives a current. A diode between the panel and the battery lets charge flow into the battery by day but stops the battery from discharging back through the panel at night. And the light itself comes from LEDs, junctions that turn current back into light. This lesson explains how one junction can do all of these.

2Forming the junction

Suppose one part of a silicon crystal is doped p-type and the neighbouring part n-type. Near the boundary, electrons from the n side diffuse across into the p side, and holes diffuse the other way; where they meet they recombine (an electron fills a hole) and both carriers disappear. Left behind is a thin region, about a micrometre wide, with no mobile carriers: the depletion layer. It is not neutral. It contains fixed positive donor ions on the n side and fixed negative acceptor ions on the p side.

Figure 2.1: The p–n junction in equilibrium.

These fixed charges create an electric field from the n side to the p side, which pushes back any more electrons trying to cross into the p region (and holes trying to cross into the n region). Diffusion stops when this field is strong enough. The potential difference across the depletion layer, the barrier potential, is about 0.7 V for silicon and 0.3 V for germanium. It cannot be measured with a voltmeter, because contact potentials at the metal connections cancel it.

3Forward and reverse bias

• Forward bias: the p side (anode) is connected to the positive terminal. The applied p.d. opposes the barrier, the depletion layer narrows, and once the applied p.d. approaches the barrier potential, majority carriers cross in large numbers: a large current flows for a small increase in voltage.
• Reverse bias: the p side is connected to the negative terminal. The applied p.d. adds to the barrier, the depletion layer widens, and majority carriers cannot cross. Only a tiny leakage current flows, carried by the few minority carriers (nanoamperes for silicon), until at a large enough reverse p.d. the junction breaks down and conducts heavily.

Figure 2.2: Diode symbols (IEC 60617). On a real diode a band marks the cathode.

Figure 2.3: The current–voltage characteristic of a silicon diode (not to scale: the reverse current is far smaller than shown).

The forward current of an ideal junction follows the Shockley equation, $I=I_s(e^{\frac{eV}{kT}}-1)I\; =\; I\_s\backslash left(e^\{\backslash frac\{eV\}\{kT\}\}\; -\; 1\backslash right)$, where $I_{s}I\_s$ is the tiny reverse saturation current. Because $\frac{kT}{e}=0.026\mathrm{V}\backslash frac\{kT\}\{e\}\; =\; 0.026\backslash ,\backslash mathrm\{V\}$ at room temperature, the current is multiplied by about ten for every extra 60 mV, which is why the curve rises so suddenly and why a diode in a circuit is often treated as "a fixed 0.7 V drop when conducting, an open switch when not".

4Experiment 2.1: diode and LED characteristics

Figure 2.4: Circuit for measuring a diode characteristic. The resistor limits the current if the supply is turned up too far.

Increase the supply in small steps from zero and record the diode p.d. and the current, taking closer readings once the current starts to rise. Then reverse the diode and repeat (a microammeter is needed for the reverse current). Plot $II$ against $VV$. Replacing the diode with LEDs of different colours gives a second experiment: note the p.d. $V_{\text{on}}V\_\{\backslash text\{on\}\}$ at which each LED just begins to glow (viewed through a tube in a dim room). An electron crossing the junction gains energy $e{V}_{\text{on}}eV\_\{\backslash text\{on\}\}$, which is roughly the energy $\frac{hc}{\lambda }\backslash frac\{hc\}\{\backslash lambda\}$ of the photon it produces.

5Light-emitting diodes

In a forward-biased junction, electrons crossing into the p side fall into holes. The energy released is about the band gap $E_{g}E\_g$. In silicon it becomes heat (vibrations of the lattice), but in compound semiconductors such as gallium arsenide phosphide (red) or indium gallium nitride (blue and green) it is emitted as a photon of energy $hf\approx E_{g}hf\; \backslash approx\; E\_g$. That is a light-emitting diode (LED). The colour is set by the material, and a larger band gap gives shorter wavelengths and a larger forward voltage: about 1.8 V to 2.0 V for red, 3 V or more for blue. White LEDs use a blue LED coated with a phosphor that turns part of the blue into yellow. An LED needs far less power than a filament lamp for the same light, and lasts much longer.

An LED must always have a current-limiting resistor in series: because of the steep characteristic, connecting it straight across a supply even slightly above its forward voltage would let a destructive current flow.

6The Zener diode

A Zener diode is heavily doped so that its depletion layer is very thin and its reverse breakdown happens sharply at a chosen, stable p.d., the Zener voltage $V_{Z}V\_Z$ (from about 2 V to over 100 V). In breakdown, the p.d. across it stays almost exactly $V_{Z}V\_Z$ over a wide range of currents. It is used reverse-biased, with a series resistor, to hold a voltage steady: a voltage regulator (Figure 2.5). If the input rises, the extra p.d. appears across the series resistor and the Zener simply takes more current; if the load draws more current, the Zener takes less. Either way $V_{\text{out}}\approx V_{Z}V\_\{\backslash text\{out\}\}\; \backslash approx\; V\_Z$.

Figure 2.5: A Zener voltage regulator.