We employed hard X-ray photoelectron spectroscopy in operating devices. This method allows us to investigate bias dependent electronic states while keeping device structures intact. Using this method we have investigated electronic states and potential distribution in a Pt gate metal/high-k gate stack structure under device operation. We have found that a potential gradient was formed at the Pt/HfO₂ interface by analyzing the shifts of the core levels as a function of the applied bias voltage. Angle resolved photoelectron spectroscopy revealed that a SiO₂ layer was formed at the Pt/HfO₂ interface, which is the origin of the potential gradient formed at the Pt/HfO₂ interface.