We report the growth, fabrication, and electrical properties of a fully ' epitaxial semimetal-semiconductor Schottky diode materials system: ErAs on InAlGaAs. The coherent, thermodynamically stable, lattice-matched interface eliminates oxide and interfacial third phases, making the Schottky barrier height and associated electrical parameters dependent on the fundamental molecular bonding (e.g., interface dipoles) rather than unintentional extrinsic effects. ErAs:InAlGaAs diodes thus have highly tunable properties; by adjusting composition, doping, and interface type, the Schottky barrier height is tunable from similar to 100 meV to similar to 620 meV, short circuit responsivity is tunable from 0.5 to 19 A/W, differential resistance is tunable from 10(3) to 10(10) Omega mu m(2), and capacitance is tunable from 0.4 to 2 fF/mu m(2). Rectification of a rf source has been demonstrated up to 20 GHz. ErAs:InAlGaAs is thus a very promising materials system for design of an upper mm-wave, zero-bias, square-law detector with a significantly reduced noise floor. (c) 2005 American Vacuum Society.