another way to use materials of different lattice constants is to place a buffer layer between them. this is done in the mhemt or metamorphic hemt, an advancement of the phemt. the buffer layer is made of alinas, with the indium concentration graded so that it can match the lattice constant of both the gaas substrate and the gainas channel. this brings the advantage that practically any indium concentration in the channel can be realized, so the devices can be optimized for different applications (low indium concentration provides low noise; high indium concentration gives high gain).