Branch : Computer Science and Engineering
Subject : Fundamental of Electronic Devices
Drift and Resistance of the charge carrier
Following section contains the drift and resistance of the charge carrier in details.
Drift and Resistance:
Let us look more closely at the drift of electrons and holes. If the semiconductor bar of Fig. given below contains both types of carrier, then the
conductivity of the material. The resistance of the bar is then
where p is the resistivity (Ω-cm).
- The physical mechanism of carrier drift requires that the holes in the bar move as a group in the direction of the electric field and that the electrons move as a group in the opposite direction.
- Both the electron and the hole components of current are in the direction of the ξ field, since conventional current is positive in the direction of hole flow and opposite to the direction of electron flow.
- The drift current is constant throughout the bar. A valid question arises, therefore, concerning the nature of the electron and hole flow at the contacts and in the external circuit.
- We should specify that the contacts to the bar of Fig. are ohmic, meaning that they are perfect sources and sinks of both carrier types and have no special tendency to inject or collect either electrons or holes.
- If we consider that current is carried around the external circuit by electrons, there is no problem in visualizing electrons flowing into the bar at one end and out at the other (always opposite to I).
- Thus for every electron leaving the left end (x = 0) of the bar in Fig. above, there is a corresponding electron entering at x = L, so that the electron concentration in the bar remains constant at n.
- But what happens to the holes at the contacts? As a hole reaches the ohmic contact at x = L, it recombines with an electron, which must be supplied through the external circuit.
- As this hole disappears, a corresponding hole must appear at x = 0 to maintain space charge neutrality.
- It is reasonable to consider the source of this hole as the generation of an EHP at x = 0, with the hole flowing into the bar and the electron flowing into the external circuit.