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General_opto - Basics of Diode-Based Optoelectronics(S...

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1 Basics of Diode-Based Optoelectronics (S. Tewksbury) Contents A THE BASIC DIODE ....................................................................................................................................................... 1 A.1 B ASIC S TRUCTURE OF D IODES ................................................................................................................................. 1 A.2 C URRENT -V OLTAGE E QUATIONS OF D IODES ........................................................................................................... 3 B LIGHT PROPERTIES ................................................................................................................................................... 6 B.1 P HOTON E NERGY AND W AVELENGTH ...................................................................................................................... 6 B.2 E NERGY AND W AVELENGTH ..................................................................................................................................... 6 B.3 R ELATIONSHIP B ETWEEN P HOTON F LUX AND P OWER OF O PTICAL B EAM ............................................................. 7 B.4 O PTICAL A BSORPTION AND A BSORPTION C OEFFICIENT ........................................................................................ 10 C DIODE PHOTODETECTOR ..................................................................................................................................... 14 D LIGHT EMITTING DIODE ....................................................................................................................................... 18 This set of notes provides an overview of diode-based optoelectronics. Major equations are included in text boxes, with basic derivations included to help understand those basic equations. A The Basic Diode A.1 Basic Structure of Diodes The diode is illustrated in Figure A1 as a rectangular silicon sample doped with acce P tors on the left and do N ors on the right. The orientation of the standard diode symbol relative to the sample is also shown in the Figure. V diode Ideal Diode P-region resistance R P N-region resistance R N P-side depletion region with width x p N-side depletion regionwith width x n Active Region of Diode V diode Depletion width W Figure A.1: Basic semiconductor diode (Lecture structure)
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2 P-type N-type P-type substrate P-side depletion region N -side depletion region P-side contact N-side contact Current (a) More realistic diode structure (side view showing depletion layer depth P-type N-type V dd V dd (c) Simplified lecture model P-type (b) More realistic diode structure (top view showing diode area) N-type P-type substrate Figure A.2: Vertical structure of realistic diodes We have used the illustration in Figure A.1 to show the basic structure of a diode. However, they are not fabricated in this way. Instead, the more realistic structure is roughly illustrated in Figure A.2. Figure A.2a illustrates the P-type substrate on which a deep N-type layer has been fabricated (we'll see how later). This N-type layer corresponds to the right side N-type layer in Figure A.1. In this N-type layer, next a shallow P-type region is created. This corresponds to the P-type region on the left side of Figure A.1. Forward current flows (as shown by the green colored line) from the P-side contact to the contact on top of the N-type layer in Figure A.2a (a rather circuitous path but this is basically what happens). The depletion layers between the P- type and N-type layers in Figure A.2a are shown in light gray color.
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3 Figure A.2b shows the view looking down from the top, showing the two contacts (one to the P- type and the other to the N-type layer. The area of the blue-colored P-type region is basically the cross sectional area of the diode. Figure A.2c shows the same simplified picture as shown in Figure A.1 but rotated 90 degrees to show the relationship to the more realistic structure in Figure A.2a.
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