The current-voltage (I-V) characteristics of heavily doped n-GaAs metal-thin insulator (102?)-semiconductor barrier diodes can not be described by classical theory of metal-semiconductor contacts. It is necessary to modify the theory by assuming: (1) the electrons pass through the barrier of interface layer by tunnelling, resulting in transmission coefficient P; (2) at reverse bias, the effective barrier height alters due to the presence of interface layer and states and changes with variation of voltage; (3) at forward bias, the effects of interface layer are embodied in ideality factor n. The I-V and I-1/T expressions (table 1) derived from the modified theory are in good agreement with experimental results. The relationship between transmission coefficient and effective barrier height increasing is also discussed.