Diodes: Forward vs Reverse Biasing Guide


PCB and PCB assemblies often contain hundreds or even thousands of components, each specifically selected by the engineer designing the project. Each component serves a purpose and understanding the basic operation of these parts is essential in having a successful project. Today we dive into diodes and how they operate in electronics.

When an electrical component requires a particular voltage to overcome a PN junction diode (the fusion of a p-type and an n-type semiconductor), it needs a particular voltage to overcome it. The act of overcoming this barrier is referred to as biasing. Application of voltage from two sides of a conductor’s depletion region allows free electron mobility to navigate across that space in diodes.

Biasing comes in two forms: forward and reverse. A diode typically applies current unidirectionally, in a process referred to as forward biasing. However, it can also move in a reverse direction. When the latter is the case, the PN junction diode will block or prevent the current’s flow, resulting in no significant current flow, though the current is still present. This is a typically beneficial current flow condition for when a situation requires the altering of an alternating current (AC) into a direct current (DC), as well as other functions including electronic signal control.

Comparing The Diode Biasing

While we have now stated the high-level definition of diode operability, we can now build on it with a bit more nuance. It can be a challenging concept to grasp without the understanding of quantum mechanics, but in basic terms, diode operation involves the flow of both negative (electrons) and positive (holes) charges. The semiconductor diode, also known as a P-N junction, allows this flow of activity to occur, as well as promotes the photovoltaic cell operation.

Electrons that are easily displaced are termed as a negative region, cathode, or n-type. To facilitate the excess of these electrons is a process called doping and is an essential contributor to a diode’s operation. Conversely, the p-type (also known as a positive region or anode), promotes the easy absorption of electrons by generating excessive positive particles when the semiconductor is doped.

Diode operation is facilitated by the synergy between the P and N elements across a distance measuring less than a millimeter, traveling across the space named the ‘depleted region’ with the merge point of the two known as a P-N junction. To work accordingly, the depleted region must be surmounted by voltage in order to facilitate effective functionality of the diode, with the minimal required voltage measuring at 0.7 volts. The reverse bias also produces a measure of voltage that moves through the diode, however the charge it carries is largely negligible, often referred to as “leakage current.”

If the voltage through a reverse flow is increased significantly, the diode barrier will break down, resulting in the voltage flowing in the opposite direction of the typical forward trajectory.

More About Diode Functionality And Operation

As negative charges move from the n-type region, the p-type region’s holes begin to fill in, facilitating movement via diffusion. When this occurs, the p-type region begins to contain negative ions, while the n-type region begins to retain positive ions. All of this is dictated by the electric field’s directionality. Depending on how voltage is applied (or biased), this can drive potentially beneficial electrical behavior.

Overall, there are two operating regions and three biasing conditions in the standard P-N diode. The three conditions include:

  • Zero Bias Condition: The diode experiences no application of external voltage potential.
  • Forward Bias Condition: The width of the diode is expanded by applying a positive voltage to the p-type element and a negative voltage to the n-type element.
  • Reverse Bias Condition: The width of a diode is increased by applying a positive voltage to n-type material while applying a negative voltage to p-type material.


Comparing The Biasing

Forward biasing allows the current to navigate more freely through the diode allowing for a cleaner, more expedient form of voltage flow through the junction, whereas reverse biasing impedes the charge carrier flow, strengthening the reinforcement barrier in the diode. This means that in a reverse bias condition, the flow of voltage is largely obstructed from the free flow.

In forward biasing the negative terminals are connected to the anode, while the positive is connected to the cathode, while a reverse bias involves connecting the negative terminals to a cathode, and the positive ones to the anode. The voltage applied to the anode exceeds the voltage level applied to the cathode in a forward bias resulting in a substantial forward current flow, while the opposite voltage magnitude applies in reverse biasing, with weaker voltage progression moving forward through the diode.

Because of the increased resistance to voltage flow in a reverse bias, the depletion region is much thicker, whereas in a forward bias it is much thinner. This also means that during the forward bias the current is significantly increased as compared to the near-lack of voltage transition in a reverse bias, and the reverse bias will act as an electric conductor, while the reverse will be an insulator.

Vinatronic’s manufacturing team has extensive knowledge on diodes and their usage in electronics. If you are looking for a skilled assembly house for your next PCB  or PCB assembly project, reach out to our team. We are ready to help you and your team build your design to the highest of standards.

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