Evaluation of a Junction Termination Extension APD for Use with Scintillators

An avalanche photodiode with a new type of ring structure around the active area was built. The junction termination extension (JTE) APD has three diffused rings around the main junction to reduce the electric field and obtain near ideal breakdown voltage. This design has the advantage that it does not need a sharp bevel edge or grooves to avoid early breakdown at the surface. The JTE rings can be obtained by well controlled single ion-implantation through a single mask. Photodetectors with 2 mm diameter active area have been built by implantation of boron with a dose of 2, 3, 4 and 5 times 10¹² cm^-2, followed by deep diffusion to 14 mum. The dark current is strongly dependent on the implantation charge, decreasing with decreasing charge. For the APDs with implanted dose of 5 times 10¹² cm^-2 a gain of 8 is obtained at 1120 V. The energy resolution of an APD coupled to a BGO scintillator from a ¹³⁷Cs gamma-ray source was measured to be 24.4%. We have also performed simulations of the gain and breakdown voltage that correlate well with the results.