Later, in 1964, L. V. Iogansen discussed the possibility of resonant transmission of an electron through double barriers formed in semiconductor crystals. In the early 1970s, Tsu, Esaki, and Chang computed the two terminal current-voltage (I-V) characteristic of a finite superlattice, and predicted that resonances could be observed not only in the transmission coefficient but also in the I-V characteristic. Resonant tunneling also occurs in potential profiles with more than two barriers. Advances in the MBE technique led to observation of negative differential conductance (NDC) at terahertz frequencies, as reported by Sollner et al. in the early 1980s. This triggered a considerable research effort to study tunneling through multi-barrier structures.