Effect of Absorption Region Thickness on InP/InGaAs PIN Photodiode

Abstract

Optical receiver front-end modules are key components in every light wave communication system. The very first part of this module is PiN photo-diode which plays important role in sensitivity of the system in optical communication. This sensitivity and selectivity is governed by tailoring the geometry and material selection of the device. Light sensitive area of PiN photodetector is its intrinsic absorption region. In this thesis 2D simulation work of InGaAs/InP photodiode is presented using Sentaurus Synopsys Technology Computer Aided Design (TCAD) software. The absorption region thickness is varied from0.2µm to 4um with a step of 0.2µm to check for different dark currents and Photon absorption densities. The dark current varies from 0.4nA/µm to 6nA/µm for i-region thickness of 0.4µm to 3.8µm respectively at 12 volts reverse biased. The device possesses high breakdown voltage below -25 volts which makes it safe to use within low voltage range. Ray trace optical model is used to calculate photon absorption rate in the device. Photon absorption density increases with increase in absorption region thickness. The results satisfy the above mention statement and photon absorption density turned out to be 1.8x1022 /µm and 4x1022 /μm for i-region thickness of 1μm and 4μm respectively. Similarly responsivity increases with increase in absorption region at biased voltage of -12 volts and wavelength of 1.6µm. The capacitance is the major source of noise in photo detectors as the frequency is limited by RC time constant factor. The capacitance of the device is simulated 0.38fF and 1.8fF for thicknesses of 1µm and 4µm respectively. Increase in i-region thickness decreases the capacitance but on the other hand it increases the dark current. Literature reveals that increase in absorption region thickness also decrease the band-width of the device which is critical factor when using PiN diode as an optical receiver front end. The band width for 2µm iregion thickness is calculated 13.5 GHz while for 3.8µm thickness is just 7.1 GHz. So there is trade-off between dark current, bandwidth and photon absorption density. The optimum i-region thickness would turn out to be around 3µm. Device can be tailored be keeping these parameters in view to achieve desired results.