The semi-conductor diode

The p-n junction

If a region of p-type material and a region of n-type material are formed side by side in a piece of silicon, a p-n junction is formed. It is important that this is made in one piece of the material so that the crystal lattice extends across the boundary. It is not sufficient just to have two pieces in contact.

Both the p-type and the n-type material are electrically neutral, but they both contain an imbalance of conduction electrons or holes. At the boundary, holes drift from the p-type towards the n-type material and electrons drift from the n-type to the p-type to reduce this imbalance. This diffusion of holes and electrons across the boundary sets up a potential barrier which prevents further change, the p-type region becoming slightly negative and the n-type becoming slightly positive. The barrier has a potential difference across it of about 0.1 V, although the exact size of the potential barrier depends on the material. These effects only occur over a very small region (about 10^-3 mm on either side of the boundary) known as the depletion layer. They are summarised in Figure 1.

The p-n junction forms a semiconductor diode. If current is to flow through the diode, then the latter must be connected in a circuit in such a way as to reduce the height of the potential barrier, that is, the p-type material must be made positive to attract more electrons and the n-type must be made negative.

Connecting it in such a way is called forward bias, and round the other way is known as reverse bias. A reverse bias tends to increase the width of the depletion layer while a forward bias will reduce the width of this layer (see Figure 2).

Characteristics of a p-n junction diode

The characteristics of a germanium junction diode are shown in Figure 3. When in the forward biased direction the current increases almost linearly as the potential difference across it is increased, a potential difference of a volt giving a current of a few milliamps. In the reverse direction the current is only a few microamps until the diode breaks down, this occuring at a reverse potential difference of some 30V.

Diodes must be protected from excess temperature, otherwise their structure will be destroyed. The limit for germanium is about 100 ^oC, and that for silicon about 200 ^oC.

Semiconductor diodes have several advantages over thermionic diode valves. They are smaller, require less voltage to operate, do not waste so much energy as heat and are quick and cheap to manufacture.