Carrier Transport and Mobility Questions and Answers

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.

Answer: A

The transit time is 7 × 10^-11 s.

The transit time is the thickness of the active region divided by the drift velocity:

\[ \begin{aligned} t &= \frac{L}{v_{d}} = \frac{7 \times 10^{-6}\ \text{m}}{1 \times 10^{5}\ \text{m/s}} \\ &= 7 \times 10^{-11}\ \text{s} \end{aligned} \]

That is 70 ps, corresponding to an oscillation frequency of about \(1/t \approx 14\) GHz, typical of a Gunn oscillator. The value \(7 \times 10^{-6}\) s forgets to divide by the velocity (it is the thickness in metres), and the other options are many orders of magnitude off.