ISSN: 2278 – 1323 International Journal of Advanced Research in Computer Engineering &Technology (IJARCET) Volume 2, Issue 11 , November 2013 2788 All Rights Reserved © 2013 IJARCET Modeling of Avalanche Photodiodes Performance under Thermal and Radiation Effects Ahmed Nabih Zaki Rashed 1* , Abd El-Naser A. Mohamed 1 , Imbaby I. Mahmoud 2 , Mohamed S. El_Tokhy 2 , and Osama H. Elgzar 2 1 Electronics and Electrical Communications Engineering Department Faculty of Electronic Engineering, Menouf 32951, Menoufia University, EGYPT 2 Engineering Department, Nuclear Research Center, Atomic Energy Authority, P.O. 13759, Inshas, EGYPT Abstract - Radiation-induced damage in Avalanche Photodiode (APD) was shown to result from the dark current and a change of the effective doping concentration occurring within the photodiodes. In this paper a model to reveals the effect of ionizing radiation and temperature on the performance of APDs is built by using Vissim environment. This proposed model provides a mean to control the properties of APD when they are selected to operate in thermal radiation environments. Efficiency, sensitivity, responsitivity, and signal to noise ratio are modeled. The temperature effects are combined with radiation effects to formulate a rigours treatment for the APD behavior. The results are validated against published experimental work in temperature case and show good agreement. Keywords– Avalanche Photodiode, Neutron Radiation, Thermal Effects, Efficiency, and signal to noise ratio. I. INTRODUCTION Considering the continuous revolutions of optoelectronic and photonic components optical detectors with single photon sensitivity have been developed to achieve the highest per photon efficiency in an optical communications link. One such single photon detector is an avalanche photodiode (APD) [1]. APD is a crucial component in large part because of their use as detectors in fiber-optic communication systems where they not only exhibit higher sensitivities than PIN structures for low light level applications such as long to medium haul fiber networks but also very fast photodetectors [2], [3]. They operate by converting clusters of detected photons, associated with information-carrying pulses of light in a digital communication system, into cascades of electrons. These cascades have sufficiently high charge to be readily detected by the electronics following the APD [4]. Furthermore their high sensitivities have recently led them to be widely used in high-radiation environments [2], [5], which include reactors and the space environments. Therefore the radiation sources influence the APD properties can be neutrons, gamma, or X-rays, or a wide range of charged particles including electrons, protons, and heavy nuclei [6]. Moreover the major degradation in the photodiode properties occurs by neutron irradiation [7]. In view of the fact that in case of irradiated APDs both incident light and ionizing radiation produce electron-hole pairs, which results in a photocurrent. However, the ionizing radiation interact with the bulk material (volume effect), whereas the information carrying photons are merely absorbed in the active region [8]-[11]. Consequently this leads to increase dark current as well as noise and thus raise the minimum detectable light power [12]-[15]. Thus, it is essential to be acquainted with the thermal radiation influences on APDs properties in the radiation environment, and how we can optimize these properties in order to improve APDs characteristics. Consequently, we are concerned with radiation effects on device efficiency, responsitivity, sensitivity that enables us to calculate the degradation that occurs in APDs performance under irradiation environment. In addition, improving the signal to noise ratio contributes in achieving maximum usage of the communication system bandwidth, and to control APDs properties when they selected to operate in radiation fields. The arising effects of radiation induced damage are decisive in designing high-bit-rate optical communication systems. The motivation of this work comes from the need to examine the competence of these devices for exploitation in the neutron irradiation environmental applications and tests. This work is done by using VisSim environment. VisSim is a visual block diagram language for simulation of dynamical systems and model based design of embedded systems. It uses a graphical data flow paradigm to implement dynamic systems based on differential equation. This paper is organized as follows: Section II presents the basic assumptions and modeling of radiation induced performance degradation, section III describes the model results. However section IV is devoted to conclusion. II. BASIC ASSUMPTIONS AND MODELING OF RADIATION INDUCED PERFORMANCE DEGRADATION Avalanche photodiodes have already shown very good performance in various applications than PIN photodetectors. However, their performance is degraded drastically when they are operated in radiation environments. Radiation results in an increase in the photodiode dark (leakage) current and therefore a larger background noise level. The other important permanent damage effect from neutron irradiation is a reduction in optical sensitivity, signal to noise ratio and efficiency. In this model two APDs were used, one of effective thickness =140ȝm and the other is equal to 120ȝm and the photodiodes are assumed to overfill the fiber so that all mode groups are received. The cross sectional areas diameters of the two devices are 0.8 and 0.5 mm. Operating wavelengths of the optical sources used for transmitting data are 850 nm and 1310 nm. To illustrate the performance degradation of avalanche photodiode in radiation environment, the effect of the neutron radiation on the properties of avalanche photodiodes such as efficiency, responsitivity, sensitivity and signal to noise power has been studied under different thermal condition. Moreover neutron radiation effect has two main contributions to the dark current of an APD. The first of these is the surface dark current, generated at or near of the perimeter of the junction.