[1] After several years working as a member of the technical staff at Bell Laboratories (BTL) at Murray Hill, NJ, developing an ultrasonic amplifier, a mechanism invented by D. L. White, Tsu moved to the IBM, T.J. Watson Research Center in Yorktown Heights, NY as an associate to Leo Esaki beginning a well-known collaboration that yielded a theory of man-made quantum materials, superlattices and quantum wells.

From 1985 to 1987, Tsu served as the amorphous silicon program group leader at the National Renewable Energy Laboratory (then known as SERI, Solar Energy Research Institute) in Golden, CO. His theoretical derivation of the relationship between optical absorption and disorder in amorphous silicon and germanium in terms of fundamental constants shows that the slope of the Tauc plot is uniquely determined by the oscillator strength of the transition, the deformation potential, and the mean deviation of the atomic coordinates obtained from the radial distribution function (RDF).

In 1972, Tsu organized a group and was invited by the Chinese Science Academy that resulted in the first report on the technology in China published in Scientific American.

[5] This led to his involvement through establishing the first Chinese Scientific delegation visit to the US invited by the US-China Relations Committee of the US Academy of Science.

Ray's theoretical analysis at IBM led to the important concept of modulation doping for carrier mobility enhancement independently of, and prior to, the work of Dingle, et al. at Bell Labs.

[6] These pioneering contributions have led to many current technologies including terahertz oscillators,[citation needed] negative differential conductance (NDC) in the I-V characteristics of superlattice devices,[7] resonant tunneling quantum well (double barrier) structures,[8] of phonon band folding and the related Raman spectra, and the discovery of forbidden phonon modes.