Light-Emitting Diodes

Light-emitting diodes (LEDs) are electrolu­minescent semiconductor p-n junction devices that emit optical radiation when operated under forward-bias conditions.

When a semiconductor diode is forward-biased, carriers are injected from the emitter region into the base region. For example, the injected electrons recombine with the holes of the p -region which are the majority carriers in the circumstances.The energy of the released photon is nearly equal to the energy gap Eg, that is,

(7.1)

On substituting the constants in Eq. (7.1), we can determine the energy gap, or the width of the forbidden band, Eg, in electron-volts re­quired for a photon to be emitted with any desired wavelength (in micrometres):

. (7.2)

It follows from the relation in (7.2) that for the emitted radiation to fall within the visible region, that is, at wavelengths from 0.38 to 0.78 um, the semiconductor should have an energy gap of Eg > 1.7 eV. State-of-the-art LEDs are mainly fabricated from gallium phosphide (GaP), silicon carbide (SiC).

Optocouplers

The combination of a miniature light source and a photodetector in the same package has led to a very useful family of devices commonly referred to as optocouplers or optoisolators.

Photodiode optocouplers (Fig. 7.6 a) usually consist of a silicon photo­diode and an GaAs LED.

Fig. 7.6. Several combinations of light sources and photodetectors into optocouplers

Phototransistor optocouplers. (Fig. 7.6 b) usually employ a GaAs LED as the light source and a n – p – n bipolar silicon photo­transistor as the photodetector. Phototransistor optocouplers are mainly used as switches and relays in switching circuits.

A further version of the phototransistor optocoupler is the device using a photo-FET as the photodetector (Fig. 7.6c). The output cur­rent-voltage characteristic is then highly linear.

Дата добавления: 2015-11-14