Mobility of electrons and holes in semiconductors

The term mobility in electronics refers to the ability to move. Electrical mobility of an electron in a semiconductor gives the idea of the movement of the electron when the material is placed in an electric field. The mobility of free electrons and holes is not the same. It depends on many factors like the strength of the applied electric field, the distance of the charge from the field, etc. In this article, we’re going to discuss the electrical mobility of electrons and holes in semiconductors and conductors.

Contents in this article:

  • Definition of Electrical Mobility
  • Formula of Mobility of electron
  • Unit of Mobility
  • Mobility of electron in conductor and semiconductor
  • Mobility of Holes in semiconductor
  • Why the mobility of free electron is greater than the mobility of hole?

Definition of Electrical Mobility of Electrons

The charge carriers move by the influence of an external electric field. So, due to the application of an electric field charge carriers will get some drift velocity to move in the conductors or the Semiconductors. Electrical mobility of charge carriers is defined as the drift velocity of the carriers per unit applied electric field.

Formula of Mobility of charge carriers

Now, what is the electron mobility formula? Let, after applying an external electric field E, the charge carriers get the drift velocity V. Then the formula for the mobility of the charge carriers is,

\small {\color{Blue} \mu =\frac{V}{E}} ………………..(1)

This is the formula of mobility of charge carriers. This is also the electron mobility formula.

Unit of Mobility

The SI unit of drift velocity is m/s and the SI unit of the electric field is V/m.

So, the SI unit of Mobility is m2/V.s

Dimension of mobility of electron

Drift velocity has the dimension of [LT-1] and the dimension of electric field is [MLT-3I-1]. Then the dimensional formula of mobility of charges is [M-1T2I].

Mobility of free electrons in semiconductors and conductors

Free electrons move in the conduction band. The mobility of the electron is the drift velocity of the electron in presence of a unit amount of electric field. One can get the mobility of electrons both in conductors and semiconductors. The value of Electron mobility is different in different materials.

MaterialMobility of electron in cm2/V.s
Metals (Al, Cu, Au, Ag)10-50
Crystalline Silicon 1400
Gallium Arsenide (GaAs) 35000000
Mobility of electrons in Various materials in cm2/V.s

Mobility of Holes in a Semiconductor

We all know that there is no hole in a conductor. So, hole mobility is applicable only for semiconductors. Sometimes it is called semiconductor mobility. Mobility of holes is their ability to move in a semiconductor in presence of an external electric field. The value of the mobility of holes in crystalline silicon is 450 cm2/V.s.

Why is the mobility of free electrons greater than the mobility of holes?

Holes are not the physical objects. They are the absence of electrons. So, the movement of holes is nothing but the movement of electrons in the opposite direction.

Now, free electrons move in the conduction band and the holes move in the valance band. Now, the binding force of the nucleus on free electrons is smaller than that on the holes (or valence electrons) in the valence band as the valence band is closer to the nucleus. Hence, the free electrons in the conduction band require a smaller electric field to move compared to the holes in the valence band. Therefore, the mobility of free electrons in the conduction band is greater than that of holes in the valence band.

This is all from this article on the mobility of free electrons and holes in semiconductors. If you have any doubt on this topic you can ask me in the comment section.

Thank you!

Related posts:

  1. Drift velocity
  2. What is a hole? how do the holes move in semiconductors?
  3. Why do we need doping in semiconductors?
  4. Intrinsic and Extrinsic types of semiconductors
  5. Why do the metals conduct electricity?