Ultrafast photodiodes under forward-bias conditions

Driven primarily by the use of wireless mobile data and Internet videos, global network data traffic is continuing to increase. The information and communication technology sector thus takes up an ever-larger portion of global electricity consumption (now at about 10%).1 To minimize the demands of this growth, it is therefore necessary to increase the energy efficiency of high-speed network data processing. To date, a number of processing techniques have been adapted to increase the energy efficiency of high-speed networks. For instance, optical interconnect (OI) techniques2 provide a revolutionary way to reduce the carbon footprint of data centers and their wired networks. The DC component of the high-speed optical data signal at the receiving end of an OI system, however, still produces waste heat energy. This energy is proportional to the product of the DC reverse bias of the photodiodes (PDs) and the output photocurrent,3 and this heating effect could thus be a serious issue for the next generation of OI systems. Such systems have densely packaged integrated circuits, with millions of optoelectronic components and optical channels for high-speed linking (i.e., at >50Gb/s). PDs that could sustain high-speed performance, even under zero (forward)-bias operation, would thus be a potentially effective solution for minimizing the OI thermal issue. In this work, we describe our recently developed unitraveling carrier photodiodes (UTC-PDs).4, 5 We include type-II (i.e., staggered-jump) p-n absorption/collector (A/C) interfaces in these devices to further improve their speed under zero-bias operation.6, 7 In addition, we have designed and demonstrated7 our UTC-PD—with a gallium arsenide/indium gallium phosphide (GaAs/In0:5Ga0:5P) A/C junction—for application at 850nm because this is the most popular optical wavelength for very short reach linking (i.e., <300m) in modern data centers.2 To minimize the increase in the junction capacitance of our Figure 1. (a) Conceptual cross section of the proposed gallium arsenide/indium gallium phosphide (GaAs/In0:5Ga0:5P) unitraveling carrier photodiode (UTC-PD), which includes an undercut mesa structure. S. I.: Semi-insulating. (b) The DC optical–electrical (O–E) power conversion efficiency of the device at different biases.