“space charge “The accumulation of charges in a particular region is called a space charge. The space in which the charges are concentrated can either be free space or a dielectric. In addition, this cloud of charge can be mobile or stationary in nature. Let’s try to understand better with examples.
Now consider the case where we bring a p-type semiconductor into contact with an n-type semiconductor. It is known that the n-type semiconductor material has excess electrons while the p-type material is drained. When these two types of materials are brought into contact, the n-type electrons begin to change to p-type.
This causes a recombination of electrons and holes that are present near the junction. As a result, there are no moving charge carriers in a certain area around the junction. This region is nothing other than the region of the space charge with stationary ions (Figure 1a).
Next, let’s say we have a powered electron tube. In such a situation, electrons are ejected from the cathode terminal and begin to move towards the anode.
However, these electrons cannot reach their destination immediately; H. It will take you a limited time to complete your trip. As a result, these electrons can collect near the cathode end of the device and form a cloud of negative charges.
This leads to the formation of a region of negative space charge (Figure 1b), which can move under the influence of the applied electric field.
Example 2 shows that the basic reason that leads to the accumulation of charges is the fact that the removal rate is lower compared to the accumulation rate. That is, the cathode terminal emits more electrons than those moving towards the anode.
However, even charge trapping, drift, and scattering can contribute to the occurrence of a space charge region. In addition, if the polarity of the charges that make up the space charge is the same as that of the associated electrode, they are called homo charges.
On the other hand, when their polarities are different from each other, they are called hetero charges.
Consequences of Space Charge
The space charge effect is a challenge in influencing the conversion efficiency and power output of thermionic converters. This is because such an electron cloud, which is located around the metal surface, represents an additional barrier for the electrons that are supposed to reach their end point.
This inhibition of the movement of electrons manifests itself in the repulsion of the electrons which are emitted by the electrons present in the cloud.
The space charge effect that occurs in dielectrics also leads to the breakdown of electrical components such as capacitors.
Indeed, when high voltages are applied, the electrical charges emitted by the electrode are trapped in the surrounding gas. It can also be seen that the same effect causes the failure of power cables carrying high voltages.
However, in some scenarios the space charge effect is also seen as beneficial. For example, the presence of a region of space charge on some tubes creates a negative EMF, which equates to a negative bias.
Again, this is commendable as the engineers have better control over the reinforcement process and thus improve its efficiency.
Another example worth mentioning is that the space charge tends to reduce the ignition noise. Basically, the spatial charge affects the easy movement of charges on their way. This feedback reduces the number of randomly arriving charges and thus their statistical variation, which is nothing more than the sound of the fire.