Backward Diode : Construction, Working, Characteristics & Its Applications

A diode is a two-terminal semiconductor component that is used in almost all electronic devices. There are different types of diodes available in the market that are used based on the requirement like tunnel, Schottky, zener, LED, PIN, backward, avalanche, laser, varactor, gunn, BARITT, and many more. So, the backward diode is one of the types of diode which operates in reverse bias mode. This diode is a variation on a tunnel diode or a Zener diode with better conduction for little reverse biases than for forward bias voltages. This article provides brief information on the backward diode, construction, working & its applications.

What is Backward Diode?

A diode that works in reverse-biased mode is known as a backward diode or back diode. It is a kind of PN junction diode mainly designed to provide variation within the tunnel diode & zener diode and design characteristics. This diode is a unilateral device because it operates only in one direction. This diode has the same operations as compared with Zener & tunnel diodes, although it has much low operating voltages.

The backward diode function is to rectify low voltage signals which range from 0.1Volts to 0.6Volts. This diode can also be used as a switch within the multiplier & RF mixer due to its fast switching speed. The backward diode symbol is shown below which looks like a PN junction diode. But as compared to the PN junction, the anode side (P-type) is the same but on the cathode side (N-type) it is modified slightly with the bar.

Backward Diode Construction

The construction of this diode is similar to a tunnel diode apart from the light doping concentration because one junction side is doped lightly whereas another side is doped heavily. It assists in aligning the valance & conduction bands so that it performs similarly to a PN junction diode within the forward bias. In forward bias, there is no tunnel current or quantum tunneling whereas, in reverse bias, a huge tunnel current will supply. When the quantum tunneling happens only in reverse bias, then it has better conduction & lower impedance, so it is named as a backward diode.

Working

A backward diode simply works on the quantum tunneling principle. Quantum tunneling is a quantum mechanical method wherever wavefunctions can go through a potential barrier. So the transmission throughout the potential barrier can be fixed and depends on the barrier height & width exponentially.

This diode operation is similar to a tunnel diode apart from the tunneling occurring in reverse bias. It functions as a typical PN junction diode in forward bias and in reverse bias; it has better current conduction as compared to forward bias. So this diode working can be simply explained with the help of an energy band diagram within different biasing.

Unbiased Condition

The backward diode in this condition is not connected to a power source. So the energy level of the conduction band is higher as compared to the valance band. At first, the electron will exist within the valence band. Once they get sufficient energy, then they move into the conduction band by leaving behind holes.

In this condition, there is no current conduction because both the valance & conduction bands are fairly distant from each other. The valance and conduction bands of P-type & N-type for quantum tunneling must be on a similar level.

Forward Bias Condition

The anode in this biasing is connected to a high potential of the back diode whereas the cathode is connected to a low potential. This diode works similarly to a PN junction diode so it does not perform until the voltage applied crosses a certain limit. In P-type, the majority of charge carriers are holes, and in N-type, it is electrons. Because of the potential applied, the charge carriers like electrons will get energy and exist in the conduction band of the N-type layer whereas the holes exist within the valance band of the p-type layer.

The diode in this biasing operates as a normal diode & the flow of current is because of the majority of charge carriers. The electrons will flow from the N-type layer to the P-type layer whereas the holes move from the P-type to N-type. According to the diagram of the energy band, the gap between the two bands of the equivalent layer reduces & the electrons get sufficient energy for crossing the band gap.

Reverse Bias Condition

The anode in this bias condition is connected simply to a low potential of the diode whereas the cathode is connected to a high potential. Usually, it is used in this mode wherever the quantum tunneling happens.

According to the above energy band diagram, the gap between the two bands of both layers will be increased because of the reverse voltage application. But because of an increase within the band gap as well as high doping concentration, the N-type semiconductor’s conduction band & the p-type semiconductor’s valence band will drop at a similar energy level. So, the electrons flow easily throughout the junction which is called quantum tunneling.

Backward Diode V-I Characteristics

The V-I characteristics of the backward diode are shown below. This characteristic shows the main relationship between the current & voltage throughout the device. In this characteristic, the x-axis signifies the voltage across it whereas the y-axis signifies the current throughout it.

The voltage-current (VI) characteristics of a backward diode are unique compared to those of regular diodes. When plotting the current flowing through the diode against the voltage applied across it, the VI characteristics of a backward diode exhibit distinct behavior, particularly in the reverse-biased mode of operation.

Forward-biased region:

• In the forward-biased mode, the backward diode behaves like a regular diode. It allows current to flow with very little resistance, and the VI characteristics follow a similar pattern to that of a standard diode. As the forward voltage increases, the forward current through the diode increases exponentially, indicating normal diode behavior.

Reverse-biased region:

• The most significant deviation from regular diode characteristics occurs in the reverse-biased region. Instead of experiencing avalanche breakdown like regular diodes, the backward diode exhibits a region of negative resistance due to quantum tunneling effects.

In the negative resistance region, the VI curve shows that as the reverse voltage increases, the reverse current decreases. This is the opposite of what happens in regular diodes, where the reverse current exponentially increases with increasing reverse voltage during avalanche breakdown.

The negative resistance region is the key feature of the VI characteristics of a backward diode. It allows the diode to conduct current in reverse bias, making it useful in specific applications, especially those involving high-frequency oscillators, microwave generation, and parametric amplification.

Reverse breakdown region:

• If the reverse voltage is increased beyond a certain threshold, the backward diode will eventually enter a breakdown region, where the reverse current increases rapidly. This region is different from the negative resistance region and is comparable to the behavior seen in regular diodes when they undergo avalanche breakdown.

The VI characteristics of a backward diode show typical diode behavior in the forward-biased mode but exhibit a unique region of negative resistance in the reverse-biased mode due to quantum tunneling effects. This makes backward diodes valuable components in specific applications where negative resistance and unique electrical characteristics are required.

Quantum tunnelling in Backward Diode:

The phenomenon of quantum tunneling in backward diodes occurs at the junction between the P-type and N-type semiconductor materials. When the diode is under reverse bias, the depletion region widens, creating a potential barrier for the charge carriers (electrons) attempting to cross from the N-region to the P-region.

However, due to the wave-like nature of electrons at the quantum level, there is a non-zero probability that electrons can tunnel through this potential barrier, even though they lack the classical energy to overcome it. In other words, some electrons can “borrow” energy from the surrounding electric field and appear on the other side of the barrier, in this case, the P-region, without having enough energy to surmount the barrier classically.

As these electrons tunnel through the barrier and reach the P-region, they contribute to the reverse current flow. This tunneling effect results in a region of negative resistance in the backward diode’s VI characteristics. As the reverse voltage increases, the reverse current decreases due to an increase in the number of electrons tunneling through the barrier.

The presence of this negative resistance region is what distinguishes backward diodes from regular diodes, where reverse current increases exponentially during avalanche breakdown under reverse bias.

How do I select the right backward diode for my application?

When selecting a backward diode, consider factors such as the desired operating frequency, maximum current and voltage requirements, and package size. Review the diode’s datasheet to ensure it meets the specifications needed for your application.

Are backward diodes widely used in electronics?

Backward diodes have niche applications and are not as widely used as regular diodes. They are specialized components designed for specific high-frequency applications.

Can a backward diode be used as a rectifier in power supplies?

Backward diodes are not commonly used as rectifiers in power supplies. They are not optimized for rectification purposes and are better suited for their unique characteristics in high-frequency circuits.

What precautions should I take when using backward diodes?

When using backward diodes, consider their maximum ratings, operating conditions, and the specifics of your application. Ensure proper heat dissipation if using high currents, and follow standard precautions for handling sensitive electronic components.

How can I test the functionality of a backward diode?

You can test the functionality of a backward diode using a multimeter to measure its forward and reverse voltage drop and current characteristics. Additionally, verify its negative resistance region by applying a reverse bias voltage and observing the current response.

Advantages

The advantages of a backward diode include the following.

• The backward diode has low junction capacitance because of its high doping concentration.
• This diode has a high switching speed because of its low junction capacitance.
• The temperature effect is low on the electrical properties of the diode.
• These diodes generate low noise.
• This diode works on extremely low voltage.
• This diode has higher efficiency.
• This diode sensitivity is -0.1 mV/0C for both silicon & germanium materials.

Disadvantages

The disadvantages of a backward diode include the following.

• This diode works only in reversed-biased mode.
• This diode has a low tunneling effect so it has a low tunnel current.
• This diode has an extremely small depletion region & low junction capacitance.
• This diode cannot resist high voltages.
• This diode is suitable only for < 0.6v voltages.

Applications

The applications of backward diode include the following.

• The backward diode is used as a detector to detect up to 40 GHz frequencies within radio receiver circuits.
• The backward diode is used as a switch in a multiplier or RF mixer where it gives an outstanding signal performance.
• This diode can be used as a rectifier for rectifying weak signals with 0.1 – 0.7 V peak amplitudes.
• This diode provides deviation within the design characteristics of tunnel & Zener diodes.

Thus, this is an overview of the backward diode, construction, working, advantages, disadvantages, and applications. This diode is also called the back diode which works within reverse bias. This diode matches with two diodes like the tunnel & Zener because it combines the characteristics of two diodes like the Zener diode reverse bias operation & the tunnel diode’s negative resistance. Here is a question for you, what is a tunnel diode?