Transit time - Example

A voltage of 1 Volt is applied across a 1 mm thick piece of silicon. Assuming that the electric field is uniform, calculate the velocity of electrons and holes in the semiconductor. How much time is needed for the electrons and holes to cross the 1 mm region?
Answer: If the electric field is uniform, the carrier velocity can be related to the applied voltage by:

so that for electrons with m_n = 1400 cm²/Vs the velocity equals 1.4 x 10⁷ cm/s. Similarly, for holes with m_n = 450 cm²/Vs the velocity equals 4.5 x 10⁶ cm/s.

The corresponding transit times are obtained from:

yielding t_r = 7.14 ps for electrons and 22.2 ps for holes.