Our earlier XPS and UPS studies have shown that etching a silicon surface with 5% HF concentration produces a stable surface with minimum contamination and low density of surface states. This has prompted us to study the I-V and C-V characteristics of MIS diodes fabricated on silicon surfaces treated with 5% HF. Tunnel oxides of ~20 Å are grown on p-type (100) surfaces by a high-pressure (2 atm) and low-temperature (250 °C) method. Aluminum dots are vacuum evaporated through metal masks onto the oxide to fabricate MIS diodes. For photovoltaic measurements the gate electrode is made semitransparent by reducing its thickness to ~120 Å. From the variable-illumination current-voltage characteristics it is observed that the ideality factor n = 1.25, density of interface states D_it = 9 × 10¹¹/ cm^-2 eV^-1, effective barrier height V_b = 0.74 V, open-circuit voltage V_oc = 0.28 V and density of fixed oxide charges Q₀~10¹¹ cm^-2 of 5% HF treated diodes are lower than those of untreated devices where n = 1.54, D_it = 2 × 10¹² cm^-2 eV^-1, V_b = 0.78 V, V_oc = 0.42 V and Q₀~10¹² cm^-2. This is attributed to the removal of the native oxide and passivation of silicon dangling bonds by HF treatment. C-V characteristics of the MIS diodes also confirmed the reduction in barrier height on HF treatment. An increase in negative charges with increasing HF concentration, possibly due to excess of fluorine ions which may be responsible for the reduction in barrier height, is found in the interfacial layer.