In certain semiconductors such as GaAs and InP the average velocity as a function of field strength displays a maximum followed by a regime of decreasing velocity. Hilsum, Ridley, and Watkins postulated that peculiarities in the band structure of semiconductors would lead to the above phenomenon. The conduction band in compound semiconductors such as GaAs has away from the Г point (centre of Brillouin zone) local minima at the X and L valleys (satellite valleys). These valleys are a few hundred meV above the Г valley. The application of a sufficiently strong electric field will accelerate the electrons from the Г valley to the L valley where they may be subsequently scattered. At the L valley the electron velocities are greatly reduced. This drop in velocity was experimentally demonstrated by J. B. Gunn. In the set of questions that follow we will examine the reasons for this reduction in velocity and how this phenomenon has been commercially exploited.