An image sensor includes a semiconductor material having an illuminated surface and a non-illuminated surface. A plurality of photodiodes is disposed in the semiconductor material to receive image light through the illuminated surface. The semiconductor material includes silicon and germanium, and the germanium concentration increases in a direction of the non-illuminated surface. A plurality of isolation regions is disposed between individual photodiodes in the plurality of photodiodes. The plurality of isolation regions surround, at least in part, the individual photodiodes and electrically isolate the individual photodiodes.