Accumulation of charges in a particular region is referred to as space charge. The space in which the charges concentrate can be either free space or a dielectric. Further, this cloud of charges might be mobile or immobile in nature. Let us try to understand better with the help of examples.
Example 1: Now, consider the case where we have brought a p-type semiconductor in contact with an n-type semiconductor. As is well known, n-type semiconductor material has excess electrons while the p-type material is depleted of them. Thus, when these two kind of materials are brought in-contact, the electrons will start moving from n-type to p-type.
This causes the electrons and holes present near the junction to recombine with each other. As a result, a certain region around the junction will be depleted of mobile charge carriers. This region is nothing but the space charge region which has immobile ions (Figure 1a).
Example 2: Next, let us assume that we have an electron tube supplied with power. In such a situation, the electrons will be ejected from the cathode terminal and will start moving towards the anode. However these electrons cannot reach their destination instantaneously i.e. they will take some amount of finite time to complete their journey. As a result, these electrons can accumulate near the cathode end of the device forming a cloud of negative charges. This leads to the formation of negative space charge region (Figure 1b) which can move under the influence of applied electric field.
Example 2 indicates that the basic reason which leads to the accumulation of charges is the fact that the rate of removal is less when compared to the rate of accumulation. That is, the cathode terminal emits more number of electrons than those which travel towards the anode. However, even trapping of charges, drift and diffusion can contribute for the occurrence of space charge region. Further if the polarity of the charges constituting the space charge is same as that of the electrode associated, then they are called homocharges. On the other hand, if their polarities differ from each other, then they are referred to as heterocharges.