 Beryllium is a p @-@ type dopant in III @-@ V compound semiconductors . It is widely used in materials such as GaAs , AlGaAs , InGaAs and InAlAs grown by molecular beam epitaxy ( MBE ) . Cross @-@ rolled beryllium sheet is an excellent structural support for printed circuit boards in surface @-@ mount technology . In critical electronic applications , beryllium is both a structural support and heat sink . The application also requires a coefficient of thermal expansion that is well matched to the alumina and polyimide @-@ glass substrates . The beryllium @-@ beryllium oxide composite " E @-@ Materials " have been specially designed for these electronic applications and have the additional advantage that the thermal expansion coefficient can be tailored to match diverse substrate materials . 
