What is Space Charge – Definition, Causes, and Consequences

The collection of electrons (electric charge) at a short distance away from a metal surface forming a cloud of electric charge around the metal surface is known as space charge. These electrons are emitted from the metal surface on the application of electric field. In other words, the space charge is defined as the spatial distribution of electric charge within an insulating medium. In this article, we will discuss the definition and consequences of the space charge.

What is Space Charge?

As discuss above, the space charge is the accumulation or collection of electric charge in a particular region within an insulating medium. This region in which collection of electric charge takes place could be either a free space or a dielectric (insulator). The accumulation of electric charge form a cloud of charge, and this cloud of charge may be of mobile or immobile nature.

In practice, there is no material which a perfect insulator. Therefore, when an electric field of high strength is applied to the insulating material, the insulator will start to breakdown. The value of electric field lower than that causes full breakdown of the material, there will be a small number of molecules in the insulating material that get ionized. Under this condition, the free electrons being negative charged will migrate toward the positive terminal, while the ionized molecules bear positive charge and will migrate toward the negative terminal. This distribution of positive molecules/ions and negative electrons creates a region of space charge within the insulating material. Thus, the region near the positive terminal is negatively charged, whereas the region near the negative terminal is positively charged.

In order to understand the concept of space charge better, let us consider the following examples.

Example 1

A P-Type Semiconductor Material in Contact with an N-Type Semiconductor Material

In this example, we considered a p-type semiconductor in contact with an n-type semiconductor. As we known from the elementary semiconductor physics that a p-type semiconductor material has deficit of electrons, while an n-type semiconductor material has excess of electrons. Thus, when these two types of materials are in contact, the electrons start moving from high concentration region to low concentration region, i.e. from n-type material to p-type material.

This movement of electrons causes the electrons and holes present in the region near to the pn-junction to recombine with each other. Consequent, a small region near the junction will be depleted of mobile charge carriers. This depleted region having immobile charge carriers is nothing but the space charge region.

Example 2

An Electron Tube Supplied with Power

Consider another example of an electron tube which is supplied with power. An electron tube consists of two electrodes namely cathode and anode. When, we supply electric power to the electron tube, then electrons will be ejected from the cathode, and move towards the anode. Although, these electrons cannot reach to the anode instantaneously, but they will take a finite amount of time to reach their destination. Consequently, these free electrons are collected in the region near to the cathode of the electron tube, and form a cloud of negative charges. This cloud of electrons is referred to as the negative space charge region. This space charge region can move under influence of the applied electric field.

This explanation of the space charge region formation in an electron tube indicates that the rate of accumulation of charge is more than the rate of removal of charge, i.e. the cathode emits more electrons than those which move towards the anode. However, in actual practice, the drift, diffusion, and trapping of charge can contribute in the formation of space charge region.

Cause of Space Charge

Although, the range of causes of space charge is very wide, but the most important causes are as follows −

• Space charge can result from the combination of the current density and spatially heterogeneous resistance.

• Space charge can also be due to charge injection from electrodes.

• Space charge can also be due to charge injection from a stress enhancement.

• Space charge can also occur due to polarization in structures like water trees.

• Space charge can also be caused due to ionization of species within a dielectric material to form hetero-charges.

Consequences of Space Charge

The following are the important consequences of the space charge effect −

• In thermionic converters, the space charge affects the conversion efficiency and the output power of the device. This is because the space charge region present around the metal electrode creates an additional inhibition for the movement of electrons. Due to this inhibition, the electrons emitted from the electrode experience a repulsion from the electrons which are present in the space charge region.

• The space charge effect may cause the breakdown of dielectric materials in electrical components like capacitors. In this case, when a high electric field is applied, the electric charges emitted from the electrode get trapped within the dielectric medium, and cause breakdown of the material due to excessive electric stress.

• Space charge effect is one of the primary causes of the failure of high voltage power cables.

Advantages of Space Charge

In some scenarios, the space charge effect is also seen to be advantageous. Some examples of advantages of space charge effect are as follows −

• In tubes applications, the existence of space charge region creates a negative EMF that is similar to providing a negative bias to it.

• In amplifier systems, the space charge effect helps the engineers to have a better control over the process of amplification. Also, it helps in improving the efficiency of the amplification process.

• Space charge effect also helps in reducing the shot noise by providing ease in the movement of electrons in their path. This is because, the space charge effect reduces the number of randomly arrived charges, and thus reduces the statistical vibrations of charges, i.e. shot noise.

Conclusion

From the above discussion, it is clear that space charge is simply the accumulation of electrons or charges in a certain region within a free space or a dielectric or an insulating medium. The space charge region was very useful in some tube applications as it can produce a negative biasing emf within the envelope of the electronic tube. However, on the other hand, it significantly limited the uses of triode amplifiers in practical applications which led to the further innovations and developments like vacuum tube tetrode.