Metal-Insulator Transition in a Double-Donor System, Si: P, As

Measurements of electrical conductivity $\sigma$ as a function of temperature in the doubledonor system Si: P, As are reported. Data are related to a reduced concentration variable which is defined in terms of space filling by the interwoven impurity systems. The metalinsulator transition, when compared with those in Si: P and Si: As, appears to be unaffected by the additional vertical disorder introduced by the differing ionization energies of the two donors.