Question

The ratio of the concentration of electrons to that of holes in a semiconductor is $\frac{7}{4}$, then what is the ratio of their drift velocities:

• A. $\frac{5}{8}$
• B. $\frac{4}{5}$
• C. $\frac{5}{4}$
• D. $\frac{4}{7}$

Answer 1

Answer: (D)

$\frac{4}{7}$

The current due to holes and electrons in a semiconductor is given by:
$i_h=n_{h}A{v}_h$ and $i_e=n_{e}A{v}_e$ respectively.
Here, $A$ is the cross section area, $n$ is the concentration and $v$ is the drift velocity.
Therefore, $\frac{i_e}{i_h}=\frac{n_e}{n_h}\times \frac{v_e}{v_h}$
Now in an intrinsic semiconductor, $i_h=i_e$.
So, $v_e/v_h=4/7$