Discussion
In which of the following devices mobility of the electrons decreases as the electric field strength increases over a certain range ?
Answer: C
Electron mobility falls with increasing field in GaAs.
In gallium arsenide, the conduction band has a low-energy valley where electrons are light and highly mobile, and a higher satellite valley about 0.3 eV above it where they are heavy and slow. As the field rises past about 3 kV/cm, electrons gain enough energy to transfer to the upper valley, so their average drift velocity actually falls as the field rises. This negative differential mobility is the transferred electron (Gunn) effect used in Gunn diodes.
In Si and Ge the drift velocity simply saturates at high fields; it does not fall. In metals, mobility is essentially independent of the field.
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