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654_ch11 - ECE 654 Prof S Mohammadi Solid State Devices II...

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ECE 654 Solid State Devices II Prof. S. Mohammadi - 158 - Chapter 11 Light Emitting Diodes In this chapter we discuss theory and implementations of light emitting diodes (LEDs). LEDs are the basis for the development of lasers. In a junction LED photons of near bandgap energy are generated by the process of spontaneous emission, in which a large population of electrons, injected into a normally empty conduction band by a small forward bias, recombine with the holes in the valence band. Spontaneous emission is different from stimulated emission that is used in junction lasers. Spontaneous emission occurs at low forward bias voltages as opposed to the stimulated emission that occurs at much higher forward bias and much higher current densities. LEDs do not require an optical cavity and mirror facets to provide feedback of photons. The emitted photons have random phases, and therefore an LED is an incoherent light source. The linewidth of the spontaneous emission is approximately equal to the photoluminescence linewidth which is a few kT (typically 30-50nm at room temperature). Thus many optical modes are supported, thus LED is a multimode optical source suitable for use with multimode fibers. Advantages of LEDs as a light source are simpler fabrication process, lower cost, higher lifetime and reliability and simple drive circuitry when compared to laser diodes. Ideally, the LED exhibits a linear output light – current characteristics thus is suitable for analog modulations. The light –current characteristics is less sensitive to temperature than that in a laser diode. Main disadvantages of LEDs compared to laser diodes are lower output light power (makes them only suitable for short distance telecommunications), smaller modulation bandwidth (less data capacity), and harmonic distortion due to multimodal output. Superradiant LEDs have output light power comparable to that of lasers. Basic Principles of LEDs: In a forward biased PN junction of a direct bandgap material which has high doping concentration on both P and N side, excess minority carriers (electrons in the P-side and holes in
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ECE 654 Solid State Devices II Prof. S. Mohammadi - 159 - the N-side) recombine with the majority carriers through a radiative recombination process. The resulting photons emitted through the radiative recombination process has an energy of about the direct bandgap of the material g h E ν = (11.1) Transitions of band to band, impurity level to band, donor to acceptor and excitonic transitions all contribute to the spontaneous emission. The rate of the radiative recombination is normally proportional to the forward bias injection rate, and hence to the diode current given by 0 0 1 Forward qV p n n p kT h e D p D n I qA e L L = + -  (11.2) where A is the area of the LED junction, p n0 and n p0 are, respectively, the concentration of the minority carriers at the edge of the depletion region in N-type and P-type materials, and L h and L e are the Debye length of holes and electrons as the minority carriers.
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