Lecture 16: LED Types Semiconductor Optical Communication

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Lecture 16: LED Types Semiconductor Optical Communication

Semiconductor Optical Communication Components and Devices Lecture 16: LED Types Prof. Utpal Das Professor, Department of Electrical Engineering, Laser Technology Program, Indian Institute of Technology, Kanpur http://www.iitk.ac.in/ee/faculty/det_resume/utpal.html Heterostructure Light Emitting Devices h p n Double Heterostructure p-n junction enables the +V -V electrons and holes to spend more time together Holes Electrons in the active layer. Hence Active Layers the probability of (eg. AlGaAs) recombination increases. Cladding Layers (eg. AlGaAs) Moreover optical wave guiding effects can be Conduction incorporated. Band Energy h or E (eV) = 1.24/λ (μm) EFn h Refractive Index EFn Valence Band LED’s Types of LEDs - I - - Energy Spontaneous Emission Metal Contact + + P-Contact Light-emitting Region SLED ELED Light Metal Output Contact N-Contact Edge Emitting LED has only vertical Surface Emitting LED (Burrus LED): confinement of Photons: Easier to Easy to couple high power light into couple into single mode fibers a multimode optical fiber. Metallization Edge emitting LED structures GaAs Substrate 1μm (p) Al0.3Ga0.7As : Sn or Te Recombination 0.1μm to 0.3μm (p) Al0.1Ga0.9As Region 1μm (p) Al0.3Ga0.7As : Ge 1μm (p) GaAs : Ge Oxide Metallization 10-50 μm Solder Metal Strip Contact Epitaxial Cu Heat Sink Layer Strip Width = 13μm SiO2 + Current Distribution P Recombination N Region Cleaved Facet Types of LEDs - II Super luminescent LED (SLD) End facet not to be a good 4 reflector such that standing o T=0 C waves are not generated. 3.5 5oC 3 10oC 2.5 15oC SLD 2 20oC There is gain of Photons in 1.5 o the waveguide due to 25 C Light OutputLight (mW) stimulated emission but it is 1 30oC 35oC not large enough to self 40oC sustain oscillation. Due to a 0.5 large number of photons present they are guided in a 0 rectangular waveguide. 0 50 100 150 200 250 Current (mA) LED Characteristics 3 i = 200 mA 10 2.5 To 80 145K 2 Surface Emitting LED 1.5 SLED 1 1 Edge Emitting Optical(a.u.) Power LED TT 1 ELED 0.5 SLD ILD T SLD Optical OutputPower (mW) e T0 0 T 0 1 2 3 4 5 1 Current (a.u.) 0.1 0 10 20 30 40 50 60 70 80 Temperature (oC) ILD : Injection Laser Diode, to be dealt with in later lectures Review Questions 1. How does a double - heterostructure LED help in increasing the efficiency ? 2. Why is it necessary to take precautions that in an SLD standing modes do not develop? How is the facet handled for that purpose? 3. Qualitatively compare the same for the SLED and SLD when the junction temperature rises to 200oC above room temperature. 4. Explain why the modulation bandwidth of an SLD is larger than that of a SLED..

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