A unique class of microcrystalline semiconductor materials which can be modulated, within a crystalline phase, to assume any one of a large dynamic range of different Fermi level positions while maintaining a substantially constant band gap over the entire range, even after a modulating field has been removed. A solid state, directly overwritable, electronic and optical, non-volatile, high density, low cost, low energy, high speed, readily manufacturable, multibit single cell memory based upon the novel switching characteristics provided by said unique class of semiconductor materials, which memory exhibits orders of magnitude higher switching speeds at remarkably reduced energy levels. The novel memory of the instant invention is in turn characterized, inter alia, by numerous stable and non-volatile detectable configurations of local atomic order, which configurations can be selectively and repeatably accessed by input signals of varying levels.

Phase transition is controlled by controlling fractal dimension of a fractal-coupled structure overall or locally. For a magnetic material, ferromagnetic phase transition temperature of magnetic particles arranged to have self-similarity is controlled by fractal dimension. For a half-filled electron system confined in a treelike fractal, Mott transition is controlled by fractal dimension of the system. Stronger quantum chaos than conventional ones is generated by adding magnetic impurities to the fractal-coupled structure, and through this process, Anderson transition is controlled.

A phase change memory device includes: a semiconductor substrate having active areas; a pair of word lines formed over the active areas and connected with each other at each end thereof; source areas formed in the respective active areas at both sides of the pair of word lines; drain areas formed in the respective active areas between the word lines of the pair of word lines connected with each other at each end thereof; ground lines and cell selection lines formed so as to make contact with the respective source areas respectively; lower electrodes formed so as to make contact with the drain areas; phase change layers and upper electrodes stacked over the respective lower electrodes; and bit lines formed over upper portion of the active areas so as to make contact to the upper electrodes.

A memory includes: first and second recording layers for recording information by utilizing a reversible phase change between a crystalline phase and an amorphous phase which occurs due to increases in temperature caused by application of an electric current pulse. The crystallization temperatures of the first and second recording layers, T.sub.x1 and T.sub.x2 have the relationship T.sub.x1 <T.sub.x2. The crystallization times of the first and second recording layers, t.sub.x1 and t.sub.x2, have the relationship t.sub.x1 >t.sub.x2. R.sub.a1 +R.sub.a2, R.sub.a1 +R.sub.c2, R.sub.c1 +R.sub.a2, and R.sub.c1 +R.sub.c2 are different from one another where the resistance value of the first recording layer in the amorphous phase is R.sub.a1, the resistance value of the first recording layer in the crystalline phase is R.sub.c1, the resistance value of the second recording layer in the amorphous phase is R.sub.a2 and the resistance value of the second recording layer in the crystalline phase is R.sub.c2.