Monolithic HBT with Wide-Tuning Range Varactor

A monolithic semiconductor device, comprising:

• a substrate;
• a transistor structure positioned on the substrate;
• a varactor structure positioned on the substrate and physically separated from the transistor structure on the substrate;
• wherein both the transistor structure and the varactor structure have distinct instances of a common set of layers, the common set of layers comprising:
• a semiconductor base layer comprising a first semiconductor material having a first band-gap;
• a plurality of successive semiconductor layers positioned between the substrate and the semiconductor base layer, the plurality of successive semiconductor layers including a tuning layer comprising a second semiconductor material having a second band-gap larger than the first band-gap, wherein the tuning layer has a non-uniform doping profile with doping concentration that varies in accordance with distance from a surface of the tuning layer proximal to the semiconductor base layer;
• a first contact electrically coupled to the semiconductor base layer;
• and
• a second contact electrically coupled to a respective layer of the plurality of successive semiconductor layers;
• wherein the varactor structure has a tunable capacitance that varies in accordance with an applied voltage between the first contact of the varactor structure and second contact of the varactor structure.