An analysis of the electrical properties of large surface junction InSb prepared by external diffusion is given. The study of their behaviour at low bias values is carried out starting from current-voltage characteristics. At the temperature of 77°K, the reverse current is simulated to a leakage current. The internal junction current becomes predominant above 140°K. Its variation with temperature shows that it originated from a generation of electron-hole pairs in the charged zone enabling the deduction of the level of recombination centres in question; these are situated at 0·059 eV above the peak of the valence band. The general expression of the internal forward current I′ d as a function of the applied voltage V d is in the form: I′ d∞ exp( qV d βkT ) with 2 < β < 4. The origin of these abnormal values of β is twofold: on one hand, the recombination on the charged zone modifies the ideal rectifier law, by introducing a coefficient β′ situated between 1 and 2; on the other hand, owing to the high values of the superficial resistivity, the bias is not constant all along the junction leading to the introduction of a coefficient β equal to 2β′ in the expression for the total current. The analysis of the junction behaviour at high reverse bias values is effected starting with characteristics, capacity measurements and multiplication factor. Two classes of diodes with distinct properties are singled out. In the first class of diodes at the junction linear profile, avalanche breakdown occurs. On the contrary, the diodes of the second class have an abrupt junction profile and exhibit a tunnel effect. The breakdown introduces a negative slope curve in the characteristic indicating microplasma formation. An experimental study of the surface state shows the influence of atmospheric conditions on the junction properties and enables precise definition of the microplasma trigger mechanism.