a method of improving field - effect transistors , and the product thereof , wherein the resistivity of the upper layer of the source - gate channel region of a gaas field - effect transistor may be selectively raised is disclosed . impurity ions are implanted in the source - gate channel region followed by a much shallower implantation of boron in the same region . the boron ion concentration should exceed the n + impurity ion concentration by a factor of 2 or more .