Photodetectors
The chapter provides an overview of recent research on photodetectors for fiber optic communication systems. Following an introduction on basic properties (responsivity, bandwidth, noise) and material systems, photodiode structures including p-i-n photodi
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Photodetectors Andreas Beling and Joe C. Campbell
Abstract The chapter provides an overview of recent research on photodetectors for fiber optic communication systems. Following an introduction on basic properties (responsivity, bandwidth, noise) and material systems, photodiode structures including p-i-n photodiode (PD), metal–semiconductor–metal (MSM) photodetector and low-noise avalanche PD (APD) are discussed. The third section focuses on the design and performance of high-speed photodetectors and covers uni-traveling carrier (UTC) PDs, waveguide PDs, traveling wave photodetectors, and integrated photoreceivers.
7.1 Introduction In optical communication systems semiconductor photodetectors are used for the optoelectronic conversion of the modulated light signal into the electrical domain. This chapter focuses on junction photodiodes including p-i-n and avalanche photodiodes (APD) and metal–semiconductor–metal (MSM) photodetectors. The basic concepts of the light receiving process as well as advanced high-speed photodetector types are introduced.
7.1.1 Fundamentals The light absorption process in a semiconductor photodetector for photogeneration of electron–hole pairs is based on the internal photoelectric effect and requires the Andreas Beling (¬) Joe C. Campbell Department of Electrical & Computer Engineering, University of Virginia, 351 McCormick Road, PO Box 400743, Charlottesville, VA 22904, USA e-mail: [email protected], [email protected]
H. Venghaus, N. Grote (eds.), Fibre Optic Communication – Key Devices Optical Sciences 161. DOI 10.1007/978-3-642-20517-0_7, © Springer-Verlag Berlin Heidelberg 2012
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photon energy h to be at least equal to the bandgap energy Eg of the absorber material. Only then is the available energy of one photon sufficient to excite an electron from the valence band to the conduction band leaving a hole in the valence band. For this band-to-band transition, the upper wavelength limit for photon absorption is given by: g εm D
1:24 : Eg ŒeV
(7.1)
Under the influence of an electric field, that is established by an applied bias voltage, electrons and holes are swept across the absorber which results in a flow of photocurrent in the external circuit [1]. The external quantum efficiency ext quantifies the ability of the photodiode (PD) to transform light into an electrical current and is defined as the number of charge carrier pairs generated per incident photon: ext D
Ipd h ; q Popt
(7.2)
where Ipd is the photogenerated current by the absorption of the optical input power Popt at frequency , and q is the elementary charge (1:602 1019 C). Ideally ext D 1, that is each photon generates one electron hole pair. However, it will be shown below that in practice photodiodes usually exhibit ext < 1 because of several effects including finite absorber thickness, carrier recombination, optical reflections and coupling losses. A common figure of merit is the responsivity Rpd , defined as the ratio of photocurrent to optical input
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