We report on metal–semiconductor–metal (MSM) photodetectors (PDs) fabricated on InAlGaN/GaN two-dimensional electron gas (2DEG) heterostructures. Electrical and photodetection properties were compared in two structures with and without an AlN spacer between the barrier (InAlGaN) and the GaN. The presence of the spacer hugely reduces the leakage current, allowing biasing at higher voltages. In photodetection, gain is obtained in both structures at a high bias. The photocurrent transient behavior revealed a faster response for excitation energy close to the GaN band edge than for energy above the barrier band edge. The fabrication and improvement of this type of device can lead to integration with the already mature high-electron-mobility transistor (HEMT) technology. # 2013 The Japan Society of Applied Physics