A unipolar n-n heterostrucuture diode is developed in the InP material system. The electronic barrier is formed by a saw tooth type of conduction band bending which consists of a quaternary In0.52(AlyGa1-y)0.48As layer with 0 < y < ymax. This barrier is lattice matched for all y to InP and is embedded between two n+-InGaAs layers. By varying the maximum Al-content from ymax,1 = 0.7 to ymax,2 = 1 a variable barrier height is formed which enables a diode-type I-V characteristic by epitaxial design with an adjustable current density within 3 orders of magnitude. The high current density of the diode with the lower barrier height (ymax,1 = 0.7) makes it suitable for high frequency applications at low signal levels. RF measurements reveal a speed index of 52 ps/V at VD = 0.15 V. The device is investigated for RF-to-DC power conversion in UHF RFID transponders with low-amplitude RF signals.