Drift Current

       When a voltage is applied to a semiconductor, the free electrons try to move in a straight line towards the positive terminal of the battery. The electrons, moving towards positive terminal collide with the toms of semiconductor and connecting wires, along its way. Each time the electron strikes an atom, it rebounds in a random direction. But still the applied voltage make the electrons drift towards the positive terminal. This drift causes current to flow in a semiconductor, under the influence of the applied voltage. This current produced due to drifting of free electrons is called drift current and the velocity with which electrons drift is called drift velocity. Thus drift current means the flow of current due to bouncing of electrons from one atom to another, travelling from negative terminal to positive terminal of the applied voltage.
Note : The direction of conventional current is always opposite to the direction of drifting electrons.
       This is shown in the Fig. 1.
Fig. 1 Drift mechanism causing drift current
       The conventional current direction is always from positive terminal to the negative terminal of the battery. But the operating principle of many semiconductor devices is generally considering the direction of flow of electrons rather than the conventional current.
1.1 Drift Current Densities
       In general, the current density is given by,
                           J = n q μ E                          ............... (1)
       But in semiconductors, two charged particles are responsible for the flow of current, which are free electrons and holes. Under the influence of an electron field, free electrons constitute electron current while holes constitute hole current. Both the charged particles move in opposite directions but constitute current in the same direction. The total current is due to the electron current as well as the hole current, in a semiconductor. Hence the total drift current density is the addition of individual drift current densities existing due to the movement of free electrons and holes.
       The current density due to electron current is called electron current density and denoted as Jn. The current density due to hole current is called hole current density and denoted as Jp.
       Now let      μn = Mobility of free electrons
                         μp = Mobility of holes
                          n = Concentration of free electron
                          P = Concentration of holes
                          E = Electric field applied
       Substituting respective quantities is equation (1) we get,
                        Jp = p q μp E                      ................ (2)
       This is drift current density due to holes.
And                 Jn = n q μn E                         ............... (3)
       This is drift current density due to electrons.
       Total drift current density for a semiconductor is give by,
                       J = Jn + Jp = n q μn E + p q μn E
i.e.                  J = (n μn + p μp) q E                 ........... (4)

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