A new process is described for the diffusion of phosphorus into silicon. The diffusions are carried out in a closed system, using as the diffusant source a glass consisting of phosphorus pentoxide and calcium oxide. This glass is shown to satisfy the various criteria which apply to such a source. The process is shown to be highly controlled. Sheet resistance values between 0.3 (±2%) and ∼10,000 (±20%) ohms/□ can be obtained, and the junction depths are reproducible to about . Surface concentrations as low as about 1016 cm−3 and as high as 1021 cm−3 can be obtained with good reproducibility. The process is compatible with the oxide masking of layers with surface concentrations as high as 1021 cm−3.