# Integrated Photodetectors Based on Group IV and Colloidal Semiconductors: Current State of Affairs
## 1 Introduction  
Microphotonics is a branch of microelectronics where devices manipulates light on a microscopic scale and it is used in optical networking. Essentially, it consists in wafer-level integrated devices and systems that allow to emit, transmit, detect, and process light along with other forms of radiant energy with photon as the quantum unit [1]. The immunity of optical signals to external influences, such as the intensification of light beams without interaction and the possibility to realize cheap densely packed components capable of large-scale integration, make microphotonics an area of great interests in research and development [2]. In order to exploit the existing technology in microelectronics, Silicon (Si) is considered the material to be investigated to realize integrated photonic circuits. For this reason, silicon-based microphotonics has gained growing interest over the past decades. Indeed, fabrication technologies employed for micro- and nano-silicon photonics allow the integration of electronic, photonic, and sensing devices on the same chip with a very low cost. In recent decades, considerable efforts have been devoted to the development of new silicon photonic components that have led to innovative solutions with applications in the field of telecommunications and multichip optical interconnections and which promise to improve the performance of the next generation of commercial processors. Such efforts have given rise, for example, to new complementary metal-oxide-semiconductor (CMOS) configurations of optical modulators capable of operating in the gigahertz regime, waveguides and photonic crystal switches (PhC) to direct the light on chips on a sub-micrometric scale [3]. These include the innovative use of nanostructures with a negative effective refractive index capable of guiding light with very low losses [4; 5] and switching its optical path through unprecedented effects [6]. Furthermore, the matching of processing technologies have paved the way for new devices integrated with fluidic, electronic, and photonic circuits for sensing detection and healthcare, which have led tothe development of a new generation of biochips to be applied in fields ranging from biomedical to environmental [7; 8; 9]. On the other hand, for a complete manipulation of the light on a single intelligent microsystem, close integration with an emissive source and a detector capable of receiving the modulated light and switching it into an electrical signal is crucial. In this direction, new experiments on Raman emission in silicon nanocrystals have shown an extremely high increase of the optical gain [10; 11; 12], encouraging the research on this way. Finally, to complete a perfectly integrated optoelectronic multisystem, it is necessary to comply with the need to detect light and convert it into an electrical signal, all using semiconductor basic materials, such as Silicon, and metals compatible with current fabrication technologies. In this sense, considerable efforts have been made to obtain an efficient photodetector that can be easily integrated and processed through standard electronic manufacturing steps. Basically, photodetector (PD) is a device that converts optical signals into electrical signals, thus it is also known as O/E convertor. The most used types of photodetector in optical communication systems are semiconductor-based photodetectors, usually called as photodiodes, because of their high detection efficiency, fast detection speed, and small size [13]. Photodiodes, like the structures of laser diodes, are based on the p-n junctions or p-i-n junction, where the diode is integrated with a wide, undoped intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. This intrinsic region enhances detection speed.