When GaAs is grown via MBE at 200 degrees C (compared to the normal 58
0-600 degrees C), the films manifest a remarkable set of structural, e
lectrical and optical properties. As grown, LT GaAs (as it is commonly
known) has an excess of As, contains a high concentration of point de
fects, displays hopping band conductivity and shows little, if any, ph
otoluminescence. After a 600 degrees C anneal, the layers have a relax
ed lattice constant, are highly resistive and contain a distribution o
f As precipitates. The point defect concentration diminishes by severa
l orders of magnitude. The photo-induced carrier lifetime typically is
of the order of 200 fs. This account will review what is known about
these interesting materials, discuss models to explain the transport p
roperties and mention potential applications.