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The electrical and optoelectronic properties of MSM diodes that are based on an AlGaN/GaN HEMT layer system are investigated. Device fabrication uses standard HEMT processing steps, allowing integration in HEMT circuits without the need of sophisticated growth or etching techniques. The CMAX/CMIN ratio can be tuned by electrode geometry in contrast to conventional varactor diode concepts. Capacitance ratios up to 100 have been reached that exceed best values for published heterostructure varactor diodes. RF optimized devices with 0.5 μm electrode length exhibit cut-off frequencies up to 86 GHz. Optoelectronic measurements show the potential of the device as MSM photodetector.