Tunneling between two semiconductors with localized electrons: Can it reveal the Coulomb gap?

It is shown that the voltage dependence of the tunneling conductance between two lightly doped semiconductors, which are separated by a large area tunneling barrier, can reveal the high energy part of the Coulomb gap if the barrier is thick enough. At barrier thickness smaller than the average distance between impurities no Coulomb gap feature can be found. This happens because such tunneling is sensitive to very rare shortest pairs of occupied and empty states localized at opposite sides of the barrier, whose density of states in this limit has no Coulomb gap. Small area tunneling contacts are also discussed. It is shown that the tunneling conductance of a point-like contact exponentially grows with the applied voltage. This dependence does not permit a direct measurement of the Coulomb gap.