An avalanche diode including a gain region and a readout structure including an n-type (p-type) region having electrically isolated segments each including implanted regions; a p-type (n-type) region; and a first electrode on each of the segments. The gain region includes a p-n junction buried between the n-type region and the p-type region: an n+-type region having a higher n-type dopant density than the n-type region; a p+-type region having a higher p-type dopant density than the p-type region; and the p-n junction between the n+-type region and the p+-type region. A bias between the first electrodes and a second electrode (ohmically contacting the p-type (n-type) region) reverse biases the p-n junction. Electrons generated in response to electromagnetic radiation or charged particles generate additional electrons m the gain region through impact ionization but the segmented region comprises a low field region isolating the gain region from the first electrodes.