positive hole

Drude model

The Drude model of electrical conduction was proposed in 1900 by Paul Drude to explain the transport properties of electrons in materials (especially metals). The Drude model is the application of kinetic theory to electrons in a solid. It assumes that the material contains immobile positive ions and an "electron gas" of classical, non-interacting electrons of density n, each of whose motion is damped by a frictional force due to collisions of the electrons with the ions, characterized by a relaxation time τ.


The Drude model assumes that an average charge carrier experiences a `drag-coefficient' , gamma. Under an applied electric field E this leads to the following differential equation:

mfrac{d}{d t}langlevec{v}rangle = qvec{E} - gamma langlevec{v}rangle,

where langlevec{v}rangle denotes average velocity, m the effective mass and q the charge of the charge carriers.

The steady state solution (frac{d}{d t}langlevec{v}rangle = 0) of this differential equation is

langlevec{v}rangle = frac{q tau}{m}vec{E} = muvec{E},


, tau = frac{m}{gamma} is the mean free time of a charge carrier, and ,mu is the mobility. Now, introducing charge carrier density n (particles per unit volume), we can relate average velocity to current density:

vec{J} = nqlanglevec{v}rangle.

The material can now be shown to satisfy Ohm's Law with a DC-conductivity , sigma_0.

vec{J} = frac{n q^2 tau}{m} vec{E} = sigma_0vec{E}

The Drude model can also predict the current as a response to a time-dependent electric field with an angular frequency , omega, in which case

sigma(omega) = frac{sigma_0}{1 + iomegatau}.

Here it is assumed that

E(t) = Re(E_0 e^{iomega t});
J(t) = Re(sigma(omega) E_0 e^{iomega t}).

In other conventions, , i is replaced by , -i in all equations. The imaginary part indicates that the current lags behind the electrical field, which happens because the electrons need roughly a time , tau to accelerate in response to a change in the electrical field. Here the Drude model is applied to electrons; it can be applied both to electrons and holes; i.e., positive charge carriers in semiconductors.

Accuracy of the model

This simple classical Drude model provides a very good explanation of DC and AC conductivity in metals, the Hall effect, and thermal conductivity (due to electrons) in metals. The model also explains the Wiedemann-Franz law of 1853. However, it greatly overestimates the electronic heat capacities of metals. In reality, metals and insulators have roughly the same heat capacity at room temperature. Although the model can be applied to positive (hole) charge carriers, as demonstrated by the Hall effect, it does not predict their existence.

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