  The hard X-ray sky now being studied by INTEGRAL and Swift and soon by NuSTAR
is rich with energetic phenomena and highly variable non-thermal phenomena on a
broad range of timescales. The High Energy Telescope (HET) on the proposed
Energetic X-ray Imaging Survey Telescope (EXIST) mission will repeatedly survey
the full sky for rare and luminous hard X-ray phenomena at unprecedented
sensitivities. It will detect and localize (<20", at 5 sigma threshold) X-ray
sources quickly for immediate followup identification by two other onboard
telescopes - the Soft X-ray imager (SXI) and Optical/Infrared Telescope (IRT).
The large array (4.5 m^2) of imaging (0.6 mm pixel) CZT detectors in the HET, a
coded-aperture telescope, will provide unprecedented high sensitivity (~0.06
mCrab Full Sky in a 2 year continuous scanning survey) in the 5 - 600 keV band.
The large field of view (90 deg x 70 deg) and zenith scanning with
alternating-orbital nodding motion planned for the first 2 years of the mission
will enable nearly continuous monitoring of the full sky. A 3y followup pointed
mission phase provides deep UV-Optical-IR-Soft X-ray and Hard X-ray imaging and
spectroscopy for thousands of sources discovered in the Survey. We review the
HET design concept and report the recent progress of the CZT detector
development, which is underway through a series of balloon-borne wide-field
hard X-ray telescope experiments, ProtoEXIST. We carried out a successful
flight of the first generation of fine pixel large area CZT detectors
(ProtoEXIST1) on Oct 9, 2009. We also summarize our future plan (ProtoEXIST2 &
3) for the technology development needed for the HET.
