to allow conduction, semiconductors are doped with impurities which donate either mobile electrons or holes. however, these electrons are slowed down through collisions with the impurities (dopants) used to generate them in the first place. hemts avoid this through the use of high mobility electrons generated using the heterojunction of a highly doped wide-bandgap n-type donor-supply layer (algaas in our example) and a non-doped narrow-bandgap channel layer with no dopant impurities (gaas in this case).