Certain aspects of the present disclosure provide a variable transistor-based capacitive element implemented on a glass or dielectric substrate. Such a variable transistor-based capacitive element may be suitable for use as a tunable capacitor in a passive-on-glass (POG) device, for example. One example device having a tunable capacitance generally includes a glass or dielectric substrate and a transistor disposed above the glass or dielectric substrate. The transistor has a gate region, a drain region, and a source region, wherein a capacitance of the transistor is configured to vary based on a voltage between the gate region and the drain region.