We present work on a novel In0.53Ga0.47As/AlAs metamorphic asymmetric spacer layer tunnel (mASPAT) diode structure, which was grown on GaAs by solid source molecular beam epitaxy. mASPAT diodes with different mesa sizes were fabricated and tested following growth under optimal conditions. The measured I–V characteristics of these tunneling devices showed rectifying behavior resulting from the asymmetric design of the epitaxial spacer layers. The extracted curvature coefficient, junction resistance, and leakage currents at −1 V resulted in an estimated theoretical cut-off frequency ft at zero bias exceeding 180 GHz for 4 × 4 μm2 mesa devices. The obtained results demonstrate the potential use of mASPAT devices on GaAs as a low-cost alternative to devices fabricated on InP substrates for high-volume zero bias microwave and millimeter-wave detectors.