  We demonstrate an InAlN/GaN-on-Si HEMT based UV detector with photo to dark
current ratio > 107. Ti/Al/Ni/Au metal stack was evaporated and rapid thermal
annealed for Ohmic contacts to the 2D electron gas (2DEG) at the InAlN/GaN
interface while the channel + barrier was recess etched to a depth of 20 nm to
pinch-off the 2DEG between Source-Drain pads. Spectral responsivity (SR) of 34
A/W at 367 nm was measured at 5 V in conjunction with very high photo to dark
current ratio of > 10^7. The photo to dark current ratio at a fixed bias was
found to be decreasing with increase in recess length of the PD. The fabricated
devices were found to exhibit a UV-to-visible rejection ratio of >103 with a
low dark current < 32 pA at 5 V. Transient measurements showed rise and fall
times in the range of 3-4 ms. The gain mechanism was investigated and carrier
lifetimes were estimated which matched well with those reported elsewhere.
