What is Diode Capacitance and What are its Effects on High Frequency Circuits

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In the field of electronics, capacitance is always associated with the capacitors. These are electronic devices used to store charges. However, diodes also tend to have capacitance.

We can define diode capacitance as the capacity of a diode to store energy in the form of an electric charge. Well, this is similar to the capacitance of the capacitors. The capacitance of a diode takes advantage of the depletion layer where there exists the pn junction of the diode. This implies the properties of this unction, such s the thickness of the junction will end up determining the junction capacitance.

Diode capacitance has a direct impact on the operation of high-frequency circuits. These are specialized circuits whose signals tend to have a very high range than that of the normal circuits. These circuits are used for running major applications where the frequency of the signal will directly impact the performance of the device. Telecommunication systems are perfect examples of such applications.

There is a close relationship between the operation of the diodes and these high-frequency circuits. In this article, we are going to discuss in detail the impact of the diode capacitance on high-frequency signals.

Overview of high-frequency circuits

As we have already defined, the frequencies in these circuits are quite high to support their respective application areas. Usually, it can range between 5 kHz to as high as 100 GHz or more depending on the purpose of the circuit or where it is used.

Most of these circuits are commonly used in wireless systems whereby high frequency is required for the transmission of data easily and efficiently. Think of mobile phone communication or satellite systems. For these systems to be effective in transmitting data, their frequencies must be of a high range.

When the frequency is high or at its peak, then the wavelength must be short. This is a key feature of these high-frequency circuits. They tend to have short wavelengths, a key feature that secures the quality of signal that is relayed in the circuit.

High-frequency circuits have various features that are deployed to minimize loss of power when relaying signals.  With such levels of frequency, it will be quite easy for power to be lost in the process.

High-frequency circuits also have an issue with matching the impedance of the signals. In as much s the frequency s high, it should not compromise the signals being generated and transmitted in the circuits.

Transition capacitance (Ct)

This is one of the two types of diode capacitance.  Its features and properties are based directly on the capacitance of the capacitors. We know that capacitors store charges in the form of the electric field. They are able to do so using two conducting plates that are positioned close to each other and separated by a dielectric material.

Since the two plates are good conductors of electricity, they will allow current to flow through them without any resistance. The fact that dielectric material is not a 100% conductor means that electrons cannot penetrate through it. However, they tend to allow some current to leak through them. This feature allows them to store significant volumes of electrons when the capacitor is connected to a voltage source. Also, the capacitance will be determined by the size of the plates and the distance between them.

The same analogy works with the diodes when we are analyzing them from the angle of capacitance. Transition capacitance is achieved when we reverse-bias the p-n junction of the diode. This means that the charges will be stored in the depletion layer of the diode as illustrated in the image below.

Image source Electrical workbook.

This layer comprises of the static negative and positive ions which contain charge carriers. When reverse-biased, there is low resistance on the p-type and n-type regions. This means that both the p-type and n-type regions will be good conductors of electric current. However, there will be high resistance at the depletion layer of the diode. This means that the junction will serve as a dielectric material and hence have the full capacity to store electric charges.

At the depletion layers, there will be minimal movement of both the positive and negative electrons thanks to its high resistance. This electrical property will store the electric charges in the region. This is what we call the transition capacitance of the diode.

The electrons will move once you connect the diode to an external voltage source which will trigger the gradual movement of the electrons. This will cause a direct change on the capacitance of the depletion layer.

When you increase the reverse bias voltage on the diode, the width of the depletion layer or the p-n junction will also increase. As it does so, the size of both the p-type and n-type regions will decrease.

Just like in the case of capacitors, the capacitance of a diode will depend on its energy-storage capacity. This means that when the p-n junction is narrow, the p-type and n-type regions will be thicker and hence have high capacitance. On the other hand, the wider the depletion layer means that the diode will have small capacitance. Increasing the supply voltage on a reverse bias p-n junction will decrease its capacitance.

Take note that the diode transition capacitance will still be there even in the forward bias condition, only that it will be significantly small. This is why full attention is given to the reverse bias.

Diffusion capacitance (Cd)

This is the second type of diode capacitance. Its working principle is opposite to that of the diode transition capacitance. Diffusion capacitance works when the diode is forward biased. This way, it will be able to store maximum electric charges hence the name storage capacitance (Cd). It also has a larger capacitance than the transition diode capacitance.

The capacitance of the diffusion diode is due to the presence of holes and electrons near the depletion layer of the diode. In the transition diode capacitance, the storage of charges is due to the presence of electrons and holes in the p-type and n-type regions.

So, how are the electrons and holes stored in the diffusion capacitance?

When the diode is connected to a circuit in a forward-biased mode, electrons that are in the n-type region will start drifting towards the p-type region where they will combine with the holes. Similarly, holes in the p-type region will start drifting towards the n-type region. The combination of the holes and electrons will reduce the width of the depletion layer.

As the size of the depletion layer is minimized, the electrons will get into the p-type region where they will become minority charge carriers. Once the holes move into the n-type regions, they will also become minority charge carriers.

Quite a significant number of the carriers that are trying to move into the opposite regions will recombine near the depletion layer This means that at the end of the process, both n-type and p-type regions will have adequate charge carriers. The depletion layer will act as a dielectric material, just like it is in the case of capacitors. Since this diode works in a forward-biased mode, the width of the depletion layer will keep decreasing as long as there is a supply voltage. This in turn increases the diode capacitance.

Impact of diode capacitance on the high-frequency circuits

Are you building a high-frequency circuit? Diodes are among the top components that you will have to integrated into your circuit. And when it comes to choosing these diodes, you will have to pay attention to their capacitance. Since you know what diode capacitance is and at the same time you have an idea of what high-frequency capacitance is, let’s look at how the two relate.

• Response to the frequency: the capacitance of the diode will affect the speed at which the circuit and its components respond to the high frequency. In most applications, there is a feedback mechanism that is connected to the signals. Diodes with high capacitance are able to sustain effective frequency response.
• Minimize loss of signals: Diode capacitance is designed to facilitate the management of signals in high-frequency circuits. The level of frequency in such circuits puts them at high risk of signal loss. By choosing the right diode capacitance, it will be easy to minimize such losses and, in the end, maximize the reliability of the circuit.
• Impedance mismatch: Diode capacitance can also have an impact on the impedance matching in a high-frequency circuit. The mismatch is likely to occur when the frequency of the signal is not in line with the energy capacity of the diodes and other electronic components that make up the circuit. The mismatch can lead to weakened frequency. To avoid this, you should choose a diode that has the right capacitance.

Conclusion

Knowing the importance of diode capacitance is one of the effective methods of handling high-frequency circuits. You should buy diodes that have the right capacitance as this will guarantee the smooth running of the circuits.

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