# Electrical mobility definition confusion

1. May 26, 2015

### Wminus

Hi! According to this http://en.wikipedia.org/wiki/Electrical_mobility, the definition of electrical mobility $\mu$ is:

$\vec{v} = \mu \vec{E}$. But since electrical mobility is always positive, this means that the velocity is always parallel to the E-field regardless of charge. How can this be?

thanks for all replies.

2. May 26, 2015

### rolotomassi

Actually its not necessarily always positive. It can be negative in semiconductors.

Wikipedia is not a very rigorous source. It is good to get the idea of something and should not be taken too seriously. The mobility is defined as
v = +uE for 'holes.' And v = -uE for 'electrons.' Where v and E are vector quantities

3. May 26, 2015

### nasu

In the Wiki article they don't use vectors. The text explicitly mention that E is the magnitude of the electric field. So it is a scalar relation between the magnitudes of the drift velocity and of the electric field.
And this is how mobility is defined in Kittel's reference book: as ratio between magnitudes. The values of mobility are given as positive for both electrons and holes.

4. May 26, 2015

### Wminus

Thanks guys

5. May 26, 2015

### rolotomassi

I apologise I should have been more specific that the minus sign cancels due to the relative direction of $$\vec v \ and\ \vec E$$]