  In some semiconductors donor impurity atoms can attract additional electrons,
forming negative donor impurity ions. Thus we have 3 energy levels for
electrons: zero energy levels at the bottom of the conductivity band, negative
energy levels of the bounded electrons of the negative donor impurity ions, and
deeper negative energy levels of the outer electrons of the neutral donor
impurity atoms. So the donor impurity atoms could serve as active centres for a
maser. The maximum achievable relative population is 0.5. Typical wavelength of
the generated oscillation is 0.14 mm; three level scheme could be realized at
rather low temperatures, considerably lower than 6 K.
