arXiv2026
4H-SiC low gain avalanche detectors (LGADs) combine internal gain with the high carrier saturation drift velocity, thermal stability, and low leakage current of SiC. AC-coupled readout (AC-LGAD) adds position information to the precise timing, which enables 4D tracking. This work reports the first experimental demonstration of a 4H-SiC AC-LGAD. Devices with a $1.0\times1.0$~mm$^2$ active area and a $2\times2$ array of AC-coupled pads at a pitch of 500~$μ$m were fabricated with two gain-layer doping concentrations, $4\times10^{17}$ and $5\times10^{17}$~cm$^{-3}$, and characterized with a 375~nm ultraviolet transient current technique (UV-TCT) scanning setup. The hit position and time were reconstructed with the conventional discretized positioning circuit (DPC) and constant fraction discrimination (CFD) methods. They were also reconstructed with a transformer-based neural network, which processes the full waveforms of the four pads through a shared encoder and formulates timing as a per-sample segmentation task. With DPC and CFD, the best position and time resolutions are about 10~$μ$m and 20~ps. The neural network mitigates the nonlinear distortions near the device edges and improves the position resolution to below 5~$μ$m and the time resolution to about 15~ps, improvements of more than 60\% and 30\%, respectively. The higher-doped device reaches a high signal-to-noise ratio at lower bias and keeps good performance at low laser intensities. These results demonstrate the potential of 4H-SiC AC-LGADs for 4D tracking in future collider experiments.