The Single-Electron Sensitive Read Out (SiSeRO) is an on-chip charge detector output stage for charge-coupled device image sensors. Developed at MIT Lincoln Laboratory, this technology uses a p-MOSFET transistor with a depleted internal gate beneath the transistor channel. The transistor source–drain current is modulated by the transfer of charge into the internal gate. At Stanford, we have developed a readout module based on the drain current of the on-chip transistor to characterize the device. In our earlier work, we characterized a number of first prototype SiSeROs with the MOSFET transistor channels at the surface layer. An equivalent noise charge of around 15 electrons root mean square was obtained. In this work, we examine the first buried-channel SiSeRO. We have achieved substantially improved noise performance of around 4.5eRMS− and a full-width half-maximum energy resolution of 132 eV at 5.9 keV, for a readout speed of 625 kpixel / s. We also discuss how digital filtering techniques can be used to further improve the SiSeRO noise performance. Additional measurements and device simulations will be essential to further mature the SiSeRO technology. This new device class presents an exciting technology for the next-generation astronomical x-ray telescopes requiring fast, low-noise, radiation-hard megapixel imagers with moderate spectroscopic resolution.