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An avalanche photodiode (APD) with a 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 at the surface. 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 a well-controlled ion-implantation through a single mask. The process uses standard planar technology for silicon devices. Several APDs with 2-mm diameter active area have been built by implantation of boron with a dose of 2, 3, 4, and 5 x 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 an implanted dose of 5 x 1012 cm-2, a gain of 8 is obtained at 1120 V, indicating that the devices have premature breakdown. The energy resolution from a 109Cd X-ray source (22.16 keV) was measured to be 4.7-keV full-width at half-maximum, which corresponds to 560 rms electrons noise. We have also performed simulations of the gain and breakdown voltage that correlate well with the results up to a gain of 5.