Ballistic Transport

BALLISTIC TRANSPORT

Definition

When the mean free path of the electron is longer than the dimension of the medium through which the electron travels is called Ballistic transport. Explanation

When the length 'L' of the conductor becomes much smaller than the mean free path 'L' the transport is termed 'ballistic' meaning that the electrons do not scatter during the time it takes to travel through the conductor.

For example, ballistic transport can be observed in a metal nano wire. This is because the wire is of the size of a nanometer and the mean free path can be longer than in a metal.

Condition of Ballistic transport

The mean free path [The average distance that the electron can travel freely] can be increased by reducing the number of impurities in a crystal or by lowering its temperature.

Ballistic transport conditions are

(i) L << L_m and

(ii) L << L_ϕ

Where

L → Length of the conductor

L_m → mean free path [length that the electron can travel before having an elastic collision]

L_ϕ → Length over which an electron can travel before having an inelastic collision. This is also called the phase-coherence length, since it is the length over which an electron wave function retains its coherence.

For L << L_m and L << L _ϕ we have ballistic transport.

Ballistic transport occurs over very small length scales, and is obviously coherent.

The electron doesn't hit anything as it travels through the material, and, therefore, there is no momentum or phase relaxation. Thus, in Ballistic material, the electron's wave function can be obtained from schrodinger's equation.

Application

One practical application of ballistic transport is to ultra-short-channel semiconducting FETS or carbon nano tube transistors.