In the past 50 years, the high gain in quantum efficiency of photoconductors is often explained by a widely accepted theory in which the photogain is proportional to the minority carrier lifetime and inversely proportional to the carrier transit time across the photoconductor. It occasionally misleads scientists to believe that a high-speed and high-gain photodetector can be made simply by shortening the device length.The theory is derived on the assumption that the distribution of photogenerated excess carriers is spatially uniform. In this Letter, we find that this assumption is not valid for a photoconductive semiconductor due to the metal-semiconductor boundary at the two metal electrodes inducing carrier confinement. By solving the continuity equation and performing numerical simulations, we conclude that a photoconductor intrinsically has no gain or at least no high gain, no matter how short the transit time and how long the minority lifetime is. The high gain observed in experiments comes from other extrinsic effects such as defects, surface states and surface depletion regions that localize excess minority carriers, leaving a large number of excess majority carriers accumulated in the conduction channel for the photogain. Following the Ohm's Law, a universal equation governing the photogain in a photoconductor is established at the end of this Letter.

Silicon nanowires (SiNWs) have emerged as sensitive absorbing materials for photodetection at wavelengths ranging from ultraviolet (UV) to the near infrared. Most of the reports on SiNW photodetectors are based on photoconductor, photodiode, or field-effect transistor device structures. These SiNW devices each have their own advantages and trade-offs in optical gain, response time, operating voltage, and dark current noise. Here, we report on the experimental realization of single SiNW bipolar phototransistors on silicon-on-insulator substrates. Our SiNW devices are based on bipolar transistor structures with an optically injected base region and are fabricated using CMOScompatible processes. The experimentally measured optoelectronic characteristics of the SiNW phototransistors are in good agreement with simulation results. The SiNW phototransistors exhibit significantly enhanced response to UV and visible light, compared with typical Si p-in photodiodes. The near infrared responsivities of the SiNW phototransistors are comparable to those of Si avalanche photodiodes but are achieved at much lower operating voltages. Compared with other reported SiNW photodetectors as well as conventional bulk Si photodiodes and phototransistors, the SiNW phototransistors in this work demonstrate the combined advantages of high gain, high photoresponse, low dark current, and low operating voltage. Published by AIP Publishing.

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However, the spectral responses of silicon photodiodes are limited to wavelengths shorter than 1.1 µm, significantly shorter than communication wavelengths (≈1.55 µm). Approaches have been explored to extend the spectral range of silicon-based photodetectors to communication wavelengths. One involves incorporating chalcogen dopants into Si through picosecond or femtosecond laser irradiation. [19][20][21][22] Although the room temperature responsivity of 35 mA W −1 at 1550 nm was achieved using this process, it is not suitable for the integrated circuit application due to the complicated laser annealing process and the noncrystalline Si surface formed in the process. [20,23] Recently, pulsed laser treatment using nanosecond laser has been reported to form singlecrystalline surface on gold ion doped silicon. The resultant Si:Au photodiodes have the maximum room temperature EQE of 9.3 × 10 −5 at communication wavelengths. [24] Nevertheless, Au is an important detrimental elements to silicon-based devices and therefore incompatible with the CMOS process. [25,26] Although silicon hyper doped with silver was made into photodiodes, [27] doping silicon with erbium (often with oxygen) [28][29][30] is particularly interesting for photodetection since Er/O doped silicon can be also potentially made into silicon light sources at communication wavelength. [31][32][33][34][35] Traditionally, Er/O doped silicon suffers from Er/O precipitation after standard rapid thermal annealing (RTA), which results in strong nonradiative recombination. [36][37][38] Consequently, the RTA-treated Er/O silicon cannot emit or detect photons at communication wavelengths efficiently at room temperature. [28,36] Recently, we employed a deep cooling (DC) process to treat the Er/O implanted silicon. [31] The processed samples exhibit a strong photoluminescence at room temperature, two orders of magnitude higher than the samples treated by standard RTA process. In this work, we explore the possibility to use Er/O doped silicon treated by the DC process for high-performance photodetection at communication wavelengths. The samples were first annealed at high-temperature (≈950 °C) and cooled Wide band infrared photodetectors have found a wide range of applications in sensing, communication, and spectral analysis. However, the commonly used infrared photodetectors are based on Ge and III-V semiconductors which are not complementary metal-oxide-semiconductor (CMOS) compatible and therefore have limited applications. There is a huge demand for silicon-based infrared photodetectors due to its low-cost and compatibility with CMOS processes. Nevertheless, the spectral bandwidth of Si photodetectors is limited to wavelengths below 1.1 µm. Several approaches are developed to extend Si photodetection bandwidth to communication wavelengths. Er/O doped Si is a promising approach which, however, suffers from low infrared responsivities at room temperature when the samples are treated with the standard rapid thermal annealing (RTA). In this work, a novel deep cooling...

Optoelectronically probing the trap state density of single nanoscale devices is a powerful in situ nondestructive technique that is of significance for developing high gain photoconductors by surface engineering. However, the previously demonstrated optoelectronic methods are based on the exponential transient photoresponse assumption and only trap states in a very narrow bandgap region can be probed. In this Letter, we demonstrate a cryogenic technique that is capable of measuring the density of surface trap states in the full half bandgap without the exponential transient photoresponse assumption. The technique is applied to an array of silicon nanowire photoconductors that are fabricated on silicon-on-insulator (SOI) wafer by the top-down approach. Diethyl 1-propylphosphonate (DPP) and hexadecane molecular monolayers are self-assembled on silicon nanowire surfaces as the passivation layer in comparison with dry oxide passivation. The surface trap state density of the dry oxide passivated nanowires exponentially increases from the bandgap center, reaching a peak of ∼5 × 10 cm eV at 50 meV below the conduction band. The defect state density is significantly suppressed after DPP and hexadecane molecules are grafted onto the nanowire surfaces via covalent bonds. The experimental observations are consistent with the density functional theory calculations.