y doping both types of the material. This leads to a degenerative p-n junction which means that at equilibrium this Fermi level lies within the valence and conduction bands and the Figure 53.9 Double heterojunction LED state between the Fermi level and the end of the band are filled. With strong forward bias there exist at the depletion layer a region which is doubly degenerate, that is contain degenerate electron and Devices operating on the principle of stimulated emission require, holes. This corresponds directly to the population inversion condi- in addition to population inversion, the presence of radiation. In a tion. In this region, called the active region, electrons are available laser this 'radiation' is obtained through optical feedback. Optical for either spontaneous or stimulated emission. Which of the two feedback is established by embedding the active material into a predominates depend on the structure. cavity whose walls are mirrors, (Figure 53.11). Then the operation An LED relies on the spontaneous emission for operation and the is like that of any other oscillator. When heavy forward bias is structure of typical devices are shown in Figures 53.9 and 53.10. applied the population is inverted and spontaneous emission fol- The device consists of three layer two Al Ga As and one GaAs lows. Some of the photons of the spontaneous emission are reflected (x=0). Since the bandgap of AlGaAs is larger than that of GaAs the from the end mirrors and playing the role of the radiation are spontaneous emission generated in the active layer (GaAs) is not irradiating the active material. Due to the stimulated emission effect absorbed and it is available for collection and launching into a fibre. the reflected radiation is amplified and laser action commences. This structure is known as double heterojunction because of the p-p From both the theoretical and practical points it is extremely and p-n junction. important to know the optical wavelength (frequency) at which Contact Stripe AIGaAs Optical guiding layers 