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PN Junction Reverse Bias versus Forward Bias and Their Functional Characteristics

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • Learn about the effects of biasing on a diode.

  • Gain a greater understanding of the difference between PN junction reverse bias and forward bias.

  • Learn about the types and characteristics of forward and reverse bias in a PN junction diode.

 PN junction diodes

PN junction diode in reverse bias functionally acts as an insulator.

Bias, in the field of electronics, signifies a direction or ability to flow in a particular direction, primarily when referring to a diode. Also within the area of electronics, we define biasing or bias as a methodology for establishing a set of currents or voltages at various points on an electronic circuit to establish accurate operating conditions within an electronic component(s).  Biasing also affords a circuit designer optimal control over a diode's functionality.

Types of Biasing

In a typical diode, forward biasing occurs when the voltage across a diode permits the natural flow of current, whereas reverse biasing denotes a voltage across the diode in the opposite direction.

However, the voltage present across a diode during reverse biasing does not produce any significant flow of current. This unique characteristic is beneficial for changing AC (alternating current) into direct current (DC).

There are a variety of other uses for this characteristic, including electronic signal control. For the consideration of this article, we will focus on the PN junction diode and its biasing aspects. However, there are three biasing conditions and two operating regions for a typical PN junction diode, and they are as follows:

  • Forward bias: Here, the voltage potential connections are as follows: -Ve (negative) to the N-type material and +Ve (positive) to the P-type material, across the diode. The effect is a decrease in the PN junction diode's width.

  • Reverse bias: During this biasing condition, the voltage potential connections are as follows: +Ve (positive) to the N-type material and -Ve (negative) to the P-type material, across the diode. The result of this is an increase in the PN junction diode's width.

  • Zero bias: In this biasing state, the PN junction diode does not have an external voltage potential applied.

PN Junction Reverse Bias

As you may know, the bias of a diode (PN junction) in an electrical circuit permits current to flow more effortlessly in one direction than another. Forward biasing indicates the application of a voltage across a diode that enables current to flow easily, while reverse biasing means putting a voltage across a diode in the opposite direction.

In other words, when we apply a voltage across the diode in a manner that the N-type (half) of the diode connects to the positive terminal of the voltage source, and the P-type (half) connects to the negative terminal, the electrons from the external circuit will produce more negative ions.

These negative ions are in the P-type region and fill the holes, thus creating more positive ions in the N-type region. This displaces electrons toward the positive terminal of the voltage source. As a result, both the voltage between the P-type and N-type regions and the depletion region will increase. Also, the total charge on either side of the junction will increase in magnitude until the voltage across the diode equals and opposes the applied voltage. Of course, they cancel each other out, thus ceasing the flow of current within the circuit.

PN Junction Diode Characteristics

The following are the vital characteristics of a PN junction region (junction diode):

  • A semiconductor consists of two types of mobile charge carriers: electrons, and holes.

  • Doping can occur in a semiconductor utilizing donor impurities like antimony, and this is called N-type doping. Also, this doping process contains mobile charges that are mainly electrons.

  • The electrons have a negative charge and the holes have a positive charge.

  • Doping can also occur in a semiconductor utilizing acceptor impurities such as boron, and this is called P-type doping. Moreover, this doping process contains mobile charges that are primarily holes.

  • The junction region does not possess charge carriers, and this region is also called the depletion region.

  • The depletion (junction) region's physical thickness will vary with the voltage application.

Forward Biasing versus Reverse Biasing

Here is a list to help further highlight the differences between these two:

  • A forward bias diminishes the potential barrier, thus allowing current to flow effortlessly across the junction. In contrast, a reverse bias reinforces the potential barrier and impedes the flow of charge carriers.

  • With forward biasing, we connect the positive (+) terminal of the voltage supply to the anode and the negative (-) terminal to the cathode. In contrast, with reverse bias, we connect the positive (+) terminal of the voltage supply to the cathode and the negative (-) terminal to the anode.

  • A reverse bias strengthens the potential barrier, whereas a forward bias diminishes the potential barrier of the electric field across the potential.

  •  A reverse bias has an anode voltage that is less than its cathode voltage. In contrast, a forward bias has an anode voltage that is greater than the cathode voltage.

  • A reverse bias has a marginal forward current, while a forward bias has a significant forward current.

  • The depletion layer of a diode is much thicker while in reverse bias and substantially thinner while in forward bias.

  • Reverse bias increases a diode's resistance, and forward bias decreases a diode's resistance.

  • A reverse bias does not permit the current to flow, whereas it flows effortlessly in forward bias through the diode.

  • Current is negligible or minimal in reverse bias; however, in forward bias, current levels are dependent on the forward voltage.

  • In reverse bias, a device functions as an insulator and as a conductor while in forward bias.

In the area of electronics, the diode is one of its more versatile components. Its ability to function as two separate but equally effective components makes it critically adaptive. Furthermore, the effects of biasing on a diode's functionality provide optimum control over what function a diode will play in your circuit design. This type of versatility affords a designer unparalleled control over a circuit's overall functional design.

Electrical circuit schematic of radio device utilizing a resistor, transistor, PN junction diode, capacitor, and an inductor.

Electrical circuit of radio device utilizing a resistor, transistor, PN junction diode, capacitor, and an inductor.

Regardless of what type of PN junction biasing you decide to use in your circuit, having a high-quality PCB design and analysis software package is the best way to ensure successful implementation into your designs. Allegro, by Cadence, is one such software package, and when using it, you can be sure that not only will your designs be successful, but that they will get done right the first time. 

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