  We demonstrate, for the first time, a high granularity multi-well solid-state
detector (MWSD) with the unipolar time-differential property for ultrafast
pulse response. Results show a speed improvement factor of up to ~400 for
amorphous selenium MWSD. The significance of the results presented here is the
ability to reach the intrinsic physical limit for detector pulse speed by
transitioning from the slow transit-time-limited response which depends on the
bulk carrier transport mechanism, to the ultrafast dispersion-limited response
which depends on the spatial spreading of the collected carrier packet.
Furthermore, the proposed structure is the practical approach for achieving
stable avalanche multiplication gain in large-area direct radiation detectors.
