Planar p-on-n HgCdTe heterojunction photodiodes have been fabricated using a plasma-induced type conversion process for device junction isolation. The technique is presented as a fully planar alternative technology to the commonly used mesa isolation structure. The starting material consisted of an indium-doped n-type mid-wavelength infrared (MWIR) HgCdTe absorbing layer that was capped by a I-mum-thick wider bandgap arsenic-doped p-type layer. Junction-isolated p-on-n diodes were formed by selectively p-to-n type converting the p-type cap layer using a plasma process. Photodiode dark current-voltage measurements were performed as a function of temperature, along with noise and responsivity. The devices have cut-off wavelengths between 4.8 mum and 5.0 mum, exhibit diffusion-limited dark currents down to 145 K, give R(0)A values greater than I X 10(7) Omega . cm(2) at 80 K and greater than 1 x 10(5) Omega . cm(2) at 120 K, and have negligible 1/f noise current at zero applied bias.