Aluminum gallium nitride (AlGaN) metal–semiconductor–metal photodetectors were successfully fabricated with different contact materials and structures and were tested with ultrafast lasers. The experimental results were compared with the finite element simulations based on APSYS and showed consistent trend with respect to the device I–V properties and response behaviors. Persistent photoconductivity (PPC) was observed for devices with both gold and aluminum contacts and various structures, and the decay time can be longer than 10 ms. The response time and responsivity were found to be affected by the bias voltage, operating temperature, and incident power. The mechanism behind the long decay time is analyzed from the perspective of the materials properties and factors influencing the decay time are examined. The nature of the metal–semiconductor contact is studied to help understand the PPC effect, and the contact showed ohmic-like behavior.