The strong dependence of electrical properties of Pt/Ti ohmic contact to p–In0.53Ga0.47 As (Zn: 5 × 1018 cm−3) on the interfacial microstructure formed by rapid thermal processing (RTP) were intensively studied by transmission electron microscopy, Auger Spectroscopy, and transmission line model (TLM) measurements. The rapid decrease of the specific contact resistance with an increase in RTP temperature was correlated with the development of an interfacial reaction zone. Significant interdiffusion of Ti, In and As across the interface occurred at temperature above, 350°C for a 30 second of RTP. A minimum specific contact resistance (3.4 × 10−6 Ω-cm2) was achieved at RTP temperature of 450°C. The corresponding interfacial microstructure revealed a complicated solid state reaction zone with InAs as one of the major interfacial compounds. The low contact resistance is attributed to the carrier conduction through the InAs regions. This is also consistent with the results of Pt/Ti contact experiments to p-type InAs, InP and GaAs binary surfaces, where the lowest contact resistance was achieved on InAs (3.0 × 10−7 Ω-cm2at Zn: 5 × 1018 cm−3). The temperature dependence of specific contact resistance of as-deposited Pt/Ti contact to InGaAs agrees very well with the thermionic emission dominated carrier transport mechanism with an effective barrier height, φb, of 0.13V. The rapid decrease in the contact resistance as well as its reduced temperature dependence after RTP treatment at elevated temperatures suggesting a partial conversion of thermionic emission dominated contact area to field emission dominated regions. A phenomenological theory of multiple parrallel carrier conduction processes was proposed to analyse the temperature dependence of specific contact resistance for contacts with complicated interfacial microstructure. It was found that, for low resistance contacts, majority of the carriers conducted through only a fraction of the contact area via a tunneling mechanism.