Design and simulation of readout circuit for superconductor nanowire single photon detectors in cryogenic environments

This thesis investigates the development of readout circuits for Superconducting Nanowire Single-Photon Detectors (SNSPDs), with a special emphasis on the cryogenic electronics that facilitate their operation near 4.2 Kelvin. SNSPDs exploit the light-driven superconducting-to-resistive phase transition of ultra-thin wires to achieve high detection efficiency, low dark count rates, and ultrafast response times. Photon absorption in these detectors generates a hotspot, disrupting the superconducting state, which is then converted into a measurable electrical signal. A critical design consideration is the proximity of the first-stage preamplifier to the SNSPD to minimize signal loss and optimize load impedance, a strategy that enhances readout effectiveness. However, the challenge lies in designing integrated circuits capable of reliably reading out these signals at such low temperatures. Traditional silicon-based MOSFETs, commonly used in such circuits, often experience carrier freeze-out at extreme temperatures around 4.2 K, leading to compromised performance. This phenomenon is further exacerbated by issues such as partial doping freeze-out and the freeze-out of parasitic edge transistors, as detailed in recent studies. On the other hand, High Electron Mobility Transistors (HEMTs) have been demonstrated to effectively bypass these limitations, thus proving to be more suitable for use in SNSPD readout circuits. Additionally, the robust performance of GaN HEMT's at temperatures as low as 4.2 K further validates their potential for cryogenic preamplifiers. In this thesis, the suitability of a cryogenic preamplifier circuit designed using GaN HEMTs for SNSPD sensors has been evaluated through SPICE simulations. The ASM-HEMT model was employed to facilitate the SPICE simulation of the GaN HEMTs. Necessary modifications were made to the ASM-HEMT model to ensure that its behavior aligned with experimental results at cryogenic temperatures. The simulations revealed that at the typical SNSPD operating frequency of around 1 GHz, a gain of 31.4 dB was achieved, along with a power dissipation of 1.2 mW. This low-power dissipation is crucial for maintaining the thermal budget constraints of dilution refrigerators, making the design scalable for multi-pixel SNSPDs. Additionally, it was observed that GaN HEMTs exhibit significantly lower on-state resistance at cryogenic temperatures compared to room temperature and achieve up to 75% higher IDSmax values, consistent with experimental results. This confirms the superior performance metrics of GaN HEMTs at low temperatures, as reported in the literature. The findings of this thesis suggest that GaN HEMTs are excellent candidates for use in cryogenic preamplifier circuits, providing a foundational approach to enhancing the efficiency and reliability of SNSPD systems.