  We have used diffraction gratings to simplify the fabrication, and
dramatically increase the atomic collection efficiency, of magneto-optical
traps using micro-fabricated optics. The atom number enhancement was mainly due
to the increased beam capture volume, afforded by the large area (4cm^2)
shallow etch (200nm) binary grating chips. Here we provide a detailed
theoretical and experimental investigation of the on-chip magneto-optical trap
temperature and density in four different chip geometries using 87Rb, whilst
studying effects due to MOT radiation pressure imbalance. With optimal initial
MOTs on two of the chips we obtain both large atom number (2x10^7) _and_
sub-Doppler temperatures (50uK) after optical molasses.
