We present the design and characterization of a large-area Cryogenic PhotoDetector (CPD) designed for active particle identification in rare event searches, such as neutrinoless double beta decay and dark matter experiments. The detector consists of a $45.6$ $mathrm{cm}^2$ surface area by 1-mm-thick $10.6$ $mathrm{g}$ Si wafer. It is instrumented with a distributed network of Quasiparticle-trap-assisted Electrothermal feedback Transition-edge sensors (QETs) with superconducting critical temperature $T_c=41.5$ $mathrm{mK}$ to measure athermal phonons released from interactions with photons. The detector is characterized and calibrated with a collimated $^{55}$Fe X-ray source incident on the center of the detector. The noise equivalent power is measured to be $1times 10^{-17}$ $mathrm{W}/sqrt{mathrm{Hz}}$ in a bandwidth of $2.7$ $mathrm{kHz}$. The baseline energy resolution is measured to be $sigma_E = 3.86 pm 0.04$ $(mathrm{stat.})^{+0.23}_{-0.00}$ $(mathrm{syst.})$ $mathrm{eV}$ (RMS). The detector also has an expected timing resolution of $sigma_t = 2.3$ $mumathrm{s}$ for $5$ $sigma_E$ events.
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