8

Characteristic of diodes

When a diode is used instead of a metal wire in a simple circuit, the characteristic curve looks quite different (Figure 1; the symbol of a diode is shown in Figure 2). The current is very small for small voltages. At a relatively well defined voltage (here, around 0.7 V), the current rises rapidly and is more or less independent of voltage. For negative values of the voltage (which would lead to a reversal of the charge current) the current is effectively equal to zero.

Interpretation

Here, the conductive properties are clearly different from those of ohmic transports. A diode lets charge pass only in one direction, not in the other. It functions similarly to a valve in a hydraulic circuit.

A diode is made of so-called semiconductors containing different impurities (small quantities of substances different from the main material). Semiconductor materials such as silicon have conductivities that lie between those of good conductors and good insulators (the conductivity of copper is some 10 billion times that of silicon whose conductivity is about 10 billion times higher than that of glass). Usually, the substrate is doped with small quantities of other substances which makes combinations of such “impure” semiconductors perfect materials for important devices (diodes, transistors, light emitting diodes, photovoltaic cells, thermoelectric devices, or more).

In contrast to metals where there are many free charge carriers (negatively charged electrons) related to the transport of charge, there are no free electrons in semiconductors. However, there is always a small number of pairs of positively charged holes and negatively charged electrons that can transport charge. (Such hole-electron pairs can be produced, for example, by light absorbed by the semiconductor.) Doping of the substrate with impurities changes the conductive properties of a sample. If two differently doped semiconductors are assembled into a single unit, a diode may result that lets charge flow only in a single direction across the interface of the parts (see Chapter 4 for a detailed description).