Over the past decade, we have witnessed a number of spectacular advances in the fabrication of crystalline semiconductor devices due mainly to the pro- gress of the different techni ques of heteroepitaxy. The di scovery of two- dimensional behavior of electrons led to the development of a new breed of ultrafast electronic and optical devices, such as modulation doped FETs, permeable base transistors, and double heterojunction transistors. Comparable progress has been made in the domain of cryoelectronics, ultrashort pulse generation, and ultrafast diagnostics. Dye lasers can generate 8 fs...