We have developed large format CdZnTe pixel detectors optimized for
astrophysical applications. The detectors, designed for the High
Energy Focusing Telescope (HEFT) balloon experiment, each consists of an array of 24x44 pixels, on a 498 μm pitch. Each of the anode
segments on a CdZnTe sensor is bonded to a custom, low-noise
application-specific integrated circuit (ASIC)optimized to achieve low threshold and good energy resolution. We have studied detectors fabricated by two different bonding methods and corresponding anode plane designs---the first detector has a steering electrode grid, and is bonded to the ASIC with indium bumps; the second detector has no grid but a narrower gap between anode contacts, and is bonded to the ASIC with conductive epoxy bumps and gold stud bumps in series. In this paper, we present results from detailed X-ray testing of the HEFT
pixel detectors. This includes measurements of the energy resolution
for both single-pixel and split-pixel events, and characterization of the effects of charge trapping, electrode biases and temperature on the spectral performance. Detectors from the two bonding methods are contrasted.