Pushing the limits of ab-initio-NEGF transport using efficient dissipative Mode-Space algorithms for realistic simulations of low-dimensional semiconductors including their oxide interfaces

We investigate the trade-offs between accuracy and efficiency for several flavors of the dissipative mode-space NEGF algorithm with the self-consistent Born approximation for DFT Hamiltonians. Using these models, we then demonstrate the dissipative self-consistent DFT-NEGF simulations of realistic 2D-material devices including their oxide interfaces with large slab dimensions (up to 1000 atoms) and up to several 100,000 atoms in the full device, pushing the limit of ab-initio transport

Design of Multi Gb/s Monolithically Integrated Photodiodes and Multi-Stage Transimpedance Amplifiers in Thin-Film SOI CMOS Technology

The development of new integrated high-speed Si receivers is requested for short distance optical data link and emerging optical storage (OS) systems, notably for the Gb/s Ethernet standard [1] - [8] and Blue DVD (Blu-Ray, HDDVD) [3], [4], [9]. As requirements on bandwidth, gain, power consumption as well as low read-out noise and cost are quite severe, an optimal design strategy of a monolithically integrated solution, i.e. with on-chip photodetector and transimpedance amplifier (TIA), is required. In optical communication, however, non integrated detectors are usually employed [2] - [8] since the particular indirect energy band properties of Silicon make this semiconductor not very efficient for optical reception at 850nm wavelength. As Si is the most widely used and low cost semiconductor material in electronics and due to the availability of low-cost 850nm transmitters, there is yet a great interest and challenge to integrate such receivers. 1 to 10 Gb/s, high sensitivity and low complexity, low-cost silicon photodetectors for the monolithic integration of optical receivers for short distance applications at 850nm are really an issue as the Si absorption thickness required for high-speed (low transit time and low capacitance) favors thin-film technologies for which the responsivity is low. Some solutions exist but at the price of more costly and complex fabrication processes [10-16]. At the system level, owing to its low dark current (pA range) [17], low capacitor (10fF) for the photodetector [1] and possibility to integrate this detector with high-performance low-capacitance transistors, global thin-film SOI monolithically integrated photoreceivers have potentially higher gain and lower noise performances which in turn, as we will show here, can increase the C-sensitivity and alleviate this requirement on the photodetector itself. Furthermore only SOI photodiodes have so far achieved bandwidth compatible with the 10Gb/s specification and even higher data rate among the "easy to integrate" Si photodetectors [1], [15], [16] and [18]. In the blue and UV wavelengths, these diodes achieve a high responsivity [17] and then combine all the advantages of high speed, low dark current and finally high sensitivity [1]. This makes SOI receivers the best candidate for blue DVD applications and future optical storage generation. This also suggests that blue wavelength for multi Gb/s short reach optical communication could be used in a near future under the condition that the recent progresses in blue emitting sources make them available [17, 19]. We present here a top-down design methodology, fully validated by Eldo circuit simulations [20] and experimental measurements, which allows to predict and optimize, starting from the speed requirements and the technological parameters, the architecture and performances of the receiver. Our approach generalizes the one proposed in [21] to all inversion regimes. In addition our design strategy is based on the gm id methodology [22] and allows one to optimize the diode and the transimpedance in a simultaneous way. Thanks to this modeling and the low capacitance of thin-film integrated SOI photodiodes, we have optimized various monolithic optical front-end suitable for 1 to 10 Gb/s short distance communication or Blue DVD applications that show the potentials of 0.13μm Partially-Depleted (PD) SOI CMOS implementation in terms of gain, sensitivity, power consumption, area and noise. In section 2 (Optical Receivers Basics), the simple resistor system is first presented as well as its limitations. The transimpedance amplifier is then introduced and its basic theory and concepts such as transimpedance gain, bandwidth and stability are derived. Important parameters to compare transimpedance amplifiers are also discussed as well as architectures most often used in the high speed communication area. Then in section "Design of Multistage Transimpedance Amplifiers", we present our top-down methodology to design transimpedance amplifiers in the case where the voltage gain of the voltage amplifier used in the TIA is independent of the feedback resistor Rf. This is usually the case when the TIA bandwidth is not too close to the transistors frequency limit ft of a given technology and leads to a multi-stage approach. Our design procedure is then applied to the design of a 3 stages 1GHz bandwidth transimpedance amplifier in a 0.13 μm PD-SOI CMOS technology. Finally, in section "Single stage Transimpedance Amplifier Modeling", we present a top-down methodology to design transimpedance amplifiers when the voltage gain depends on Rf. This is the case for very high-speed singlestage transimpedance amplifiers. Our design procedure is then applied to the design of a single stage 10GHz bandwidth transimpedance amplifier in a 0.13 μm PD-SOI CMOS technology and to the design of a 1GHz bandwidth single stage TIA in a 0.5 μm FD-SOI CMOS technology

Low temperature tunneling current enhancement in silicide/Si Schottky contacts with nanoscale barrier width

The low temperature electrical behavior of adjacent silicide/Si Schottky contacts with or without dopant segregation is investigated. The electrical characteristics are very well modeled by thermionic-field emission for non-segregated contacts separated by micrometer-sized gaps. Still, an excess of current occurs at low temperature for short contact separations or dopant-segregated contacts when the voltage applied to the device is sufficiently high. From two-dimensional self-consistent non-equilibrium Green's function simulations, the dependence of the Schottky barrier profile on the applied voltage, unaccounted for in usual thermionic-field emission models, is found to be the source of this deviation

Low-Wavelengths SOI CMOS Photosensors for Biomedical Applications

INTRODUCTION : Biological agents may be characterized (in terms of quantity (or concentration), purity, nature) using optical ways like spectrometry, fluorometry and real-time PCR for example. Most of these techniques are based on absorbance or fluorescence. Indeed, many biological molecules can absorb the light when excited at wavelengths close to blue and ultraviolet (UV). For example, DNA, RNA and proteins feature an absorption peak in the deep UV, more precisely around 260 and 280 nm (Karczemska & Sokolowska, 2001). This work is widely focused on those wavelengths. A biological sample concentration measurement method can be based on UV light absorbance or transmittance, as already known and realized with high-cost and large-size biomedical apparatus. But, often, the difficulties come from the limitation for measuring very small concentrations (close to a few ng/µL or lower) since the measurement of such small light intensity variations at those low wavelengths requires a precise light source, and very efficient photodetectors. Reducing the dimensions of such a characterization system further requires a small light source, a miniaturized photosensor and a processing system with high precision to reduce the measurement variations. Some light-emitting diodes (LED) performing at those UV wavelengths have recently appeared and may be used to implement the light source. Concerning the optical sensor, while accurate but high-cost photosensors in technologies such as AlGaN and SiC provide high sensitivities in UV low wavelengths thanks to their semiconductor bandgap (Yotter & Wilson, 2003), the silicon-on-insulator (SOI) layers absorb the photons in that specific range thanks to an appropriate thickness of the silicon. Adding excellent performances of low power consumption, good temperature behavior and high speed (Flandre et al., 1999; 2001), the SOI technology allows the designers for integrating a specific signal processing integrated CMOS circuit to transform the photocurrent into a digital signal for example. This opens the possibility to build a low-cost, complete and portable microsystem, including the light source, the photodetector and a recipient for the sample to characterize […