5 Optoelectronic Devices and Sensors
5.1 Introduction

Optoelectronic devices and light sensors are fabricated by using semiconductor materials.

A semiconductor material has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass.

The electrical conductivity of a semiconductor material increases with increasing temperature, which is behaviour opposite to that of a metal. Nowadays, Si (silicon) is the semiconductor more used in electronic applications. Intrinsic semiconductors or i-type semiconductors are pure semiconductors without any dopant species present in the material (undoped). The number of charge carriers, electrons and holes, is therefore determined by the properties of the material itself. In an intrinsic semiconductor the number of excited electrons and the number of holes are equal:

n = p = ni (carriers cm-3), the value of ni depends on the gap of the semiconductor, Eg, and varies with temperature as follows:

(016)

Where T is the temperature in K, kB it the Boltzmann constant: kB = 8.62·10–5 eV/K and A is a constant.

Semiconductor devices can display a range of useful properties such as passing current more easily in one direction than the other, showing variable resistance, and sensitivity to light or heat. The introduction of specific impurities into pure crystal i-semiconductors allows obtaining extrinsic semiconductors: Semiconductors in which the concentration of one type of carrier, electrons or holes, is much in excess of the other type. When the electron concentration is much larger than the holes concentration the semiconductor is called n-type. By opposite, if the density of holes is much larger than the one of electrons the semiconductor is a p-type semiconductor.

Some sensors and devices described in this chapter are based on p-n junction principles.

A p-n junction is a boundary or interface between two types of semiconductor material, p-type and n-type, inside a single crystal of semiconductor. Diodes are semiconductor devices formed by a semiconductor material with a p–n junction connected to two electrical terminals.

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Fig. 8. Electronic symbol of a diode.