Abstract <p>The effect of ion irradiation on the ohmic contacts of Mo/n⁺-Si (111) and Mo/p⁺-Si (111) microwave silicon epitaxial planar n–p–n transistors on their electrical parameters was studied. The leakage currents of the p–n junction of the collector-base, emitter-base, and collector-emitter section, emitter resistance, current gain, time of resorption non-basic carriers, and voltage drop at the direct bias of the emitter-base junction were measured on the transistors. Mo films with a thickness of about 100 nm were deposited by magnetron sputtering. Irradiation of formed Mo/n⁺-Si (111) ohmic contacts to emitters by P⁺-ions and B⁺-ions of formed Mo/p⁺-Si (111) contacts to the base were carried out from the planar side of the Si wafer. Irradiation was performed at a fluence ranging from 1.25 × 10¹⁴ to 6.25 × 10¹⁴ ions/cm² for Mo/n⁺-Si (111) contacts and a fluence of 6.25 × 10¹⁴ ions/cm² for Mo/p⁺-Si (111) contacts and at temperatures of 25°C. It was founded that after irradiation of ohmic contacts to the emitters of the transistor, no change in the electrical parameters, except to a decrease in the value of <i>h</i>_21<i>E</i> (~30%) was observed. At irradiation of ohmic contacts to the base and reduction of the base doping fluence down to 2.5 × 10¹⁵ ions/cm² only an increase in leakage currents of p–n junctions was observed, which at times exceed the maximum permissible values of 1 × 10⁻⁵ A. Irradiation of contacts to the transistors base by B⁺-ions and heat treatment at 500°C leads to a decrease in the value of p–n junction leakage currents.</p>