Phase change materials (PCMs) are a promising passive thermal management solution for electronics. However, their integration outside of the microelectronics package introduces additional thermal resistance between the PCM and the heat source, and makes them an non-viable solution for mobile devices due to spatial constraints. In this study, we integrate PCMs within the silicon chip in close proximity to the heat source. We fabricate thermal test vehicles (TTVs) with realistic mobile chip form factors to experimentally assess the performance of a silicon device incorporated with metallic PCM (specifically, a eutectic Bi/In/Sn alloy). The embedded PCM TTV demonstrates a noteworthy 2.3x extension in `on' operational time during temperature cycling compared to an all-silicon benchmark TTV. Furthermore, it achieves a 10\% reduction in the overall maximum temperature rise and a significant 47% decrease in temperature fluctuations during duty cycling. Degradation in the thermal performance of an embedded PCM TTV is observed due to increases in the PCM melt/freeze temperature. This phenomena occurs as result of incomplete phase transitions during the `on' or `off' portions of the duty cycle. However, temperature cycling for 5000 cycles confirms the stability of the embedded PCM for an extended operational period provided that complete melting/freezing occurs during each cycle. The substantial decrease in temperature fluctuations is crucial for enhancing device thermal stability, emphasizing the potential of embedded PCMs in ensuring reliable performance over extended operational periods.