The present invention provides a heterojunction photodiode which includes a pn or Schottky-barrier junction formed in a first material region having a bandgap energy E.sub.g1. When reverse-biased, the junction creates a depletion region which expands towards a second material region having a bandgap energy E.sub.g2 which is less than E.sub.g1. This facilitates signal photocurrent generated in the second region to flow efficiently through the junction in the first region while minimizing the process-related dark currents and associated noise due to near junction defects and imperfect surfaces which typically reduce photodiode device performance. The heterojunction photodiode can be included in an imaging system which includes an array of junctions to form an imager.