## 1 Introduction  
Silicon photonics is a technology for implementing various optical functionalities in silicon, offering the potential for large-scale integration of multiple optical and electronic functions on a single silicon chip. This should result in revolutionizing applications in microelectronics (through the replacement of metal/dielectric stacks with optical interconnects), in telecommunications (integratedoptical circuits), and in biological and chemical sensing. Significant advantages exist when using silicon as the base material for such devices: in particular, the vast amount of research now available to designers and process engineers on all aspects of the material system, as well as the established, global silicon processing industry that has evolved from almost six decades of silicon-based microelectronics fabrication. Silicon wafers have the lowest cost per unit area and the highest crystal quality of any semiconductor material. The industry is able to produce microprocessors with hundreds of millions of components--all integrated onto thumb-sized chips--and offers them at such low prices that they are increasingly used in consumer electronics. Silicon manufacturing represents the most spectacular convergence of technological sophistication and economics of scale [1-3]. Silicon photonics was pioneered by Soref during the 1980 [4-7]. Creating low-cost photonics for mass-market applications by exploiting the powerful IC industry has been the traditional motivation of silicon photonics researchers. Silicon is used in photonics due to its good optical properties, the low cost of the material and its easy manufacturability; at present, however, it is not suitable for the manufacture of active devices. Despite the success of hybrid technologies in achieving active optical components, many people think that only all-silicon active devices will make silicon photonics a \"killer technology\". Following this line of reasoning, tremendous progress has recently been made in technological processes based on the use of silicon-on-insulator (SOI) substrates, which has permitted the obtention of reliable and effective fully CMOS-compatible optical components such as low-loss waveguides, high-Q resonators, high-speed modulators, couplers, and optically pumped lasers. All these devices have been developed to operate in the wavelength range from C band (1,528-1,561 nm) to L band (1,561-1,620 nm) where defect-free intrinsic bulk silicon has minimal absorption [8-13]. One of the crucial steps toward the integration of microphotonics with silicon-based electronics is the development of efficient chip-scale photodetectors (PD) integrated on silicon. Bulk photodetectors are perhaps the oldest and best understood silicon optoelectronic devices. Commercial products operate at wavelengths below 1,100 nm, where band-to-band absorption occurs. Additionally, they are used in conjunction with scintillators, and are widely used as x-ray detectors applied in medical tomography equipment and airport luggage scanners. One of the most important advantages of silicon is that, due to the quality of its crystalline material and its excellent passivation properties, very low dark-current PD devices can be obtained [14]. For the realization of photodiodes integrated in photonics circuits operating at wavelengths beyond 1,100 nm, silicon was not considered the right material because of its transparency. However, in recent years, in order to take advantage of low-cost standard Si-CMOS processing technology, a number of photodetectors have been proposed based on different physical effects, such as: mid-bandgap absorption (MBA), surface-state absorption (SSA), internal photoemission absorption (IPA) and two-photon absorption (TPA). In this paper, an overview of the state of the art on near-IR (NIR) sub-bandgap all-silicon photodetectors is presented. First, the physical effects and the working principles of these devices are elucidated.