A low-voltage CMOS-compatible time-domain photodetector, device & front end electronics

During the last decades, the usage of silicon photodetectors, both as stand-alone sensor or integrated in arrays, grew tremendously. They are now found in almost any application and any market range, from leisure products to high-end scientific apparatuses, including, among others, industrial, automotive, and medical equipment. The impressive growth in photodetector applications is closely linked to the development of CMOS technology, which now offers inexpensive and efficient analog and digi-tal signal processing capabilities. Detectors are often integrated with their respective front end and application-specific digital circuit on the same silicon die, forming complete systems on chip. In some cases the detector itself is not on the same chip but often part of the same package. However, this trend of co-integration of analog front end and digital circuits complicates the design of the analog part. The ever-decreasing supply voltage and the smaller transistors in advanced processes (which are driven by the development of digital cir-cuits) negatively impact the performance of the analog structures and complicates their design. For photodetector systems, the effect most importantly translates into a degradation of dynamic range and signal-to-noise ratio. One way to circumvent the problem of low supply voltages is to shift the operation from voltage domain to time domain. By doing so, the signal is no longer constrained by the supply rails and analog amplification is avoided. The signal takes the form of a time-based modulation, such as pulse-width modulation or pulse-frequency modulation. Another advantage is that the output signal of a time-domain photodetection system is directly interfaceable with digital circuits. In this work, a new type of CMOS-compatible photodetector displaying intrinsic light-to-time conversion is proposed. Its physical structure consists of a MOS gate interleaved with a PN junction. The MOS structure is acting as a photogate. The depletion region shrinks when photogenerated carriers fill the potential well. At some point, the anode of the PN structure is de-isolated from the rest of the detector and triggers a positive-feedback effect that leads to a very steep current increase through the PN-junction. This translates into a signal of very high amplitude and independent from light-intensity, which can be almost directly interfaced with digital circuits. This simplifies the front end circuit compared to photodiode-based systems. The physical behavior of the device is analyzed with the help of TCAD simulations and simple behavioral and shot-noise models are proposed. The device has been co-integrated with its driver and front end circuit in a standard CMOS process and its characteristics have been measured with a custom-made measurement system. The effect of bias parameters on the performance of the sensor are also analyzed. The limitations of the device are discussed, the most important ones being dark current and linearity. Techno-logical solutions, such as the implementation of the detector on Silicon-on-Insulator technology, are proposed to overcome the limitations. Finally, some application demonstrators have been realized. Other applications that could benefit from the detector are suggested, such as digital applications taking advantage of the latching behavior of the device, and a Photoplethysmography (PPG) system that uses a PLL-based control loop to minimize the emitting LED-current

Wide and ultra-wide bandgap oxides : where paradigm-shift photovoltaics meets transparent power electronics

Oxides represent the largest family of wide bandgap (WBG) semiconductors and also offer a huge potential range of complementary magnetic and electronic properties, such as ferromagnetism, ferroelectricity, antiferroelectricity and high-temperature superconductivity. Here, we review our integration of WBG and ultra WBG semiconductor oxides into different solar cells architectures where they have the role of transparent conductive electrodes and/or barriers bringing unique functionalities into the structure such above bandgap voltages or switchable interfaces. We also give an overview of the state-of-the-art and perspectives for the emerging semiconductor β- GaO, which is widely forecast to herald the next generation of power electronic converters because of the combination of an UWBG with the capacity to conduct electricity. This opens unprecedented possibilities for the monolithic integration in solar cells of both self-powered logic and power electronics functionalities. Therefore, WBG and UWBG oxides have enormous promise to become key enabling technologies for the zero emissions smart integration of the internet of things

Chip- and System-Level Reliability on SiC-based Power Modules

The blocking voltage, switching frequency and temperature tolerance of power devices have been greatly improved due to the revolution of wide bandgap (WBG) materials, such as silicon carbide (SiC) and gallium nitride (GaN). Owing to the development of SiC-based power devices, the power rating, operating voltage, and power density of power modules have been significantly improved. However, the reliability of SiC-based power modules has not been fully explored yet. Thus, this dissertation focuses on the chip- and system-level reliability on SiC-based power modules. For chip-level reliability, this work focuses on on-chip SiC ESD protection devices for SiC-based integrated circuits (ICs). In order to develop SiC ESD protection devices, SiC-based Ohmic contact and ion implantation have been studied. Nickel/Titanium/Aluminum (Ni/Ti/Al) metal stacks were deposited on SiC substrates to form Ohmic contact. Circular transfer length method (CTLM) structures were fabricated to characterize contact resistivity. Ion implantation was designed and simulated by Sentraurus technology computer aided design (TCAD) software. Secondary-ion mass spectrometry (SIMS) results show a good match with the simulation results. In addition, SiC ESD protection devices, such as N-type metal-oxide-semiconductor (NMOS), laterally diffused metal-oxide-semiconductor (LDMOS), high-voltage silicon controlled rectifier (HV-SCR) and low-voltage silicon controlled rectifier (LV-SCR), have been designed. Transmission line pulse (TLP) and very fast TLP (VF-TLP) measurements were carried out to characterize their ESD performance. The proposed SiC-based HV-SCR shows the highest failure current on TLP measurement and can be used as an area-efficient ESD protection device. On the other hand, for system-level reliability, this dissertation focuses on the galvanic isolation of high-temperature SiC power modules. Low temperature co-fired ceramics (LTCC) based high-temperature optocouplers were designed and fabricated as galvanic isolators. The LTCC-based high-temperature optocouplers show promising driving capability and steady response speed from 25 ºC to 250 ºC. In order to verify the performance of the high-temperature optocouplers at the system level, LTCC-based gate drivers that utilize the high-temperature optocouplers as galvanic isolators were designed and integrated into a high-temperature SiC-based power module. Finally, the high-temperature power module with integrated LTCC-based gate drivers was characterized by DPTs from 25 ºC to 200 ºC. The power module shows reliable switching performance at elevated temperatures