Tunnel Diode: From Nuclear Tunnelling to Quantum Electronics
- bpsinghamu
- Jul 3
- 9 min read
In 2025, the world celebrates the “International Year of Quantum Science and Technologies”, as a global tribute to the scientific breakthroughs that are reshaping the technological landscape. From quantum computing to ultra-secure communication, the subject of “Quantum Mechanics” is no longer confined to blackboards and chalk dust. It is becoming real and impactful. Amid these high-tech advancements, it is easy to overlook that one of the earliest manifestations of quantum mechanics has already been part of our electronic systems for decades. This phenomenon is known as quantum mechanical tunnelling (QMT).
In this article we try to trace the remarkable journey of quantum mechanical tunnelling. The QMT phenomenon has once explained alpha decay in radioactive nuclei, almost hundred years back in 1928. Now it enables the working of electronic devices like the tunnel diode. A student-friendly laboratory experiment is presented to explore this very principle in action. We try to give learners especially undergraduate students the rare opportunity to witness quantum effects. This we can see unfolding not in particle accelerators and complicated experimental setups, but in their electronics laboratories.
The Birth of a Quantum Idea
Quantum mechanics began rewriting the rules of physics in the early 20th century, and one of its first successes was in nuclear physics explaining the radioactivity. Scientists observed that alpha particles (helium nuclei) escaped from the compact, positively charged heavy nucleus of radioactive atoms. Classical physics could not explain how these particles had enough energy to overcome the strong repulsive Coulomb barrier.
In 1928, a young physicist of 24 years old, George Gamow, along with Gurney and Condon, proposed a stunning solution of alpha particle emission. The alpha particles could tunnel through the barrier, even if they lacked the classical energy to overcome the barrier. This was not magic, it was quantum mechanical transmission probability at work. The alpha particle was not always “inside” or “outside” the nucleus; rather, there was a finite probability that it could be detected on the other side of the potential barrier. Gamow applied the principles of quantum mechanics to explain how alpha particles could escape the nucleus despite not having enough energy to overcome the nuclear potential barrier classically. By using the time-independent Schrödinger equation and solving it for the case where the particle's energy ‘E’ is less than the potential barrier height , he derived an analytical expression for the transmission coefficient, which represents the tunnelling probability. Specifically, for a rectangular potential barrier of height and thickness ‘a’, he showed that the probability of tunnelling decreases exponentially with both the height and width of the barrier;
As can be seen from this expression, the tunnelling probability of a particle is strongly influenced by both the height and width of the potential barrier it encounters. A higher barrier leads to a lower tunnelling probability, making it less likely for the particle to penetrate through. On the other hand, if the barrier is narrower, i.e., it’s width ‘a’ is smaller, the tunnelling probability increases significantly.
This behaviour is physically reasonable, as a thinner or lower barrier presents a smaller obstacle, thereby enhancing the likelihood that the particle can escape, such as in the case of alpha decay from a nucleus or electron tunnelling in a tunnel diode. This result provided a quantitative explanation of alpha decay and became one of the first successful applications of quantum tunnelling in nuclear physics.
This insight, now known as QMT was a remarkable moment. It gave confirmation to the idea that subatomic particles do not follow the rules of classical mechanics. These particles are governed by wave functions and probabilities. The QMT phenomenon was confirmed by experimental data and is still central to our understanding of nuclear processes.
Let us try to visualize this. For this consider a potential energy diagram where an alpha particle is trapped inside a potential well with a high energy barrier. Instead of waiting for a classical escape, the particle’s wavefunction “leaks” through the barrier, making escape possible over time. This quantum effect is fundamental, not a curiosity, but a kind of governing principle.
From Atom to Circuit: The Tunnel Diode Is Born
Three decades later, quantum tunnelling took a surprising turn. From the atomic nucleus to the field of semiconductor electronics. In 1957, Leo Esaki, while working at Sony, developed a special type of diode with extremely heavy doping on both the p- and n-sides of the junction. This significantly high doping led to a very narrow depletion region. The thin depletion region was low enough to allow electrons to tunnel through the junction at low forward applied voltages.
The device so formed, is known as the tunnel diode. It showed an unusual and fascinating behaviour. When the forward bias increased, the current first increased, then decreased, and then increased again. This intermediate drop in current created a negative resistance region, a phenomenon made possible only by quantum tunnelling.
Basic Physics Summary of Quantum-Mechanical Tunnelling
All tunnel diodes operate on the principle of quantum mechanical tunnelling (QMT). Here charge carriers move through a potential energy barrier, even when they do not have sufficient energy to overcome it classically. This is made possible by the phenomenon of de-Broglie’s wave–particle duality of electrons.
In the regions where kinetic energy of a particle is higher than the potential barrier, its wave function behaves typically as sinusoidal. But when the particle encounters a region where the potential exceeds its kinetic energy, the wave function decays exponentially, according to Schrödinger’s equation. If this decaying wave amplitude does not reduce to zero completely before reaching the end of the barrier, there remains a finite probability that the particle will emerge on the other side and is referred to as tunnelling.
Though this principle applies universally, quantum tunnelling becomes significant only at nanoscale dimensions, e.g., in tunnel diodes, where thin junctions allow electrons to tunnel through. The effect is particularly important in inter-band and intra-band tunnel diodes, where the barrier shapes and carrier energies differ. The illustration in Figure 1 (B), shows a typical potential energy barrier (V₀) and how a particle with energy E < V₀ can tunnel through quantum mechanically.
Unlike ordinary diodes, where electrons must gain energy to cross the junction, in a tunnel diode they can quantum-mechanically tunnel through the barrier even with minimal applied voltage. The result is super-fast switching behaviour, which made tunnel diodes highly attractive in the early days of high-frequency and microwave electronics.
In a tunnel diode, under small forward bias, filled states in the conduction band of the n-side directly align with empty states in the valence band of the p-side, allowing electrons to tunnel through the thin junction barrier. As the voltage increases further, this overlap vanishes, suppressing tunnelling until standard conduction takes over. This is a beautiful case of nature’s principles being turned into engineering solutions. The same mathematics that describes alpha decay also predicts how a tunnel diode will behave in a circuit.
Exploring Tunnelling in the Laboratory
To bridge theory and practice, a simple lab experiment can be performed to study the V-I characteristic of a tunnel diode, making quantum mechanics tangible and visible to students. The experiment’s strength lies in its accessibility—it requires only basic electronic components and yields impressive results that visually confirm the presence of tunnelling.
The objective is to plot the current-voltage curve of the tunnel diode in forward bias, and to identify key features such as the peak current, valley current, and most importantly, the negative resistance region. This region is the hallmark of quantum tunnelling and its engineering application.
The circuit setup is straightforward. A tunnel diode is connected in series with a 33-ohm resistor, and powered using a 5V DC supply through a variable potentiometer. Two voltmeters are used: one across the resistor to calculate current (V₁), and one across the diode (V₂) to measure applied voltage.
In the experiment, the tunnel diode is forward biased through a potentiometer, with V₁ measuring the voltage across the resistor (used to compute current using Ohm's law), and V₂ across the diode itself. This simple configuration captures the essential behaviour of the diode.
As the potentiometer is slowly adjusted, readings of V₁ and V₂ are recorded. The potential difference across the diode is V₂ volt. The current is calculated using the formula; ; where V1 is the voltage across R3 = 33Ω. This procedure is repeated across a sufficient voltage range to capture the initial increase in current, the peak point, the negative resistance region, and the final rise due to conventional conduction.
The measured data is compiled into a table, showing the diode voltage, the voltage across the resistor, and the calculated current.
Table 1: Typical recorded readings of V₁, V₂, and calculated current I.
S. No. | Voltage, V1 (mV) | Voltage, V2 (mV) | I=V1/R3 (mA) |
|---|---|---|---|
1. | 52 | 9 | 1.5 |
2. | 66 | 13 | 2.0 |
3. | 99 | 22 | 3.0 |
4. | 132 | 36 | 4.0 |
5. | 148 | 50 | 4.5 |
6. | 85 | 154 | 2.6 |
7. | 62 | 206 | 1.9 |
8. | 53 | 220 | 1.6 |
9. | 46 | 322 | 1.4 |
10. | 14 | 389 | 0.4 |
11. | 33 | 452 | 1.0 |
12. | 66 | 478 | 2.0 |
13. | 99 | 491 | 3.0 |
14. | 132 | 500 | 4.0 |
15. | 165 | 507 | 5.0 |
16. | 198 | 513 | 6.0 |
17. | 231 | 518 | 7.0 |
18. | 264 | 521 | 8.0 |
As may be observed from this table that, these values clearly show the rise, fall, and rise again of the diode current as voltage increases. When these values are plotted with current on the y-axis and diode voltage on the x-axis, the resulting curve is unmistakable. The current first rises sharply (due to tunnelling), peaks, then dips (negative resistance), and finally increases again (normal diode behaviour).
The plot of the V-I curve (Fig. 4), clearly displays the peak, valley, and the distinctive dip in current, confirming the non-classical nature of the device operation. The unique shape of the curve like the initial rise, sharp fall, and subsequent increase, serves as a strong visual representation of the tunnelling process. Far more than a simple graph, it offers a direct signature of quantum physics at work, illustrating how quantum mechanical principles can govern the behaviour of macroscopic electronic components.
The experimental V-I characteristic curve of the tunnel diode also reveals key parameters that define its tunnelling behaviour. As observed from the plot, the peak current reaches approximately 4.5 mA at a peak voltage of around 50 mV. Beyond this point, as the forward bias increases, the current begins to drop, an unusual phenomenon not seen in conventional diodes. This decline continues until the valley current reaches approximately 0.4 mA, corresponding to a valley voltage of about 389 mV. This region, where increasing voltage leads to decreasing current, is known as the negative resistance region and is a direct consequence of quantum mechanical tunnelling.
In the context of the tunnel diode, negative resistance refers to a peculiar region in its V-I (voltage-current) characteristic where, as the applied voltage increases, the current actually decreases. This is in stark contrast to ordinary resistive behaviour, where increasing voltage always leads to an increase in current according to Ohm’s law.
More specifically, in the negative resistance region of the tunnel diode, the slope of the V-I curve becomes negative, i.e., This implies that the diode absorbs power differently than conventional resistive devices. It can even amplify signals or generate oscillations under suitable conditions. This phenomenon arises due to quantum mechanical tunnelling. In a tunnel diode, when a small forward voltage is applied, electrons from the conduction band of the n-region tunnel into empty states in the valence band of the p-region, leading to an increase in current. As the voltage increases further, these energy bands no longer align properly, and tunnelling probability decreases, causing the current to drop despite increasing voltage. This drop creates the negative differential resistance. So, "negative resistance" does not mean the diode has a negative value of physical resistance, but rather that in a certain region of operation, the differential resistance (change in voltage over change in current) is negative. This is an important and useful property in high-frequency and fast-switching circuits.
The Broader Picture of the Quantum Devices and Tomorrow’s Technology
Tunnel diodes are not common in consumer electronics. However, they remain invaluable in the microwave oscillators, high-frequency amplifiers, and ultra-fast switches. Their greatest legacy, is that they ushered in an era where quantum effects became practical tools.
Modern quantum devices have built upon this foundation. Josephson junctions, used in superconducting quantum computers, operate through tunnelling of Cooper pairs. Resonant tunnelling diodes push performance further by introducing multiple quantum wells. Even scanning tunnelling microscopes which are capable of imaging individual atoms depend on electron tunnelling between a fine tip and a conducting surface.
To understand the working of a tunnel diode with such an experiment, gives students a conceptual way to approach these technologies. It helps to understand that quantum mechanics is not just theoretical, it can be referred to as a design principle.
Touching Quantum Reality as a conclusion
The tunnel diode experiment is a celebration of how abstract physics becomes real and relevant. It shows that quantum tunnelling, once a solution to a nuclear mystery, is now a practical engineering reality. Students who engage with this experiment not only understand a fascinating device but also connect with the core ideas powering the quantum technology revolution.
As the world moves toward quantum-secured communication, quantum computing, and quantum sensing, it becomes ever more important to teach quantum intuition early. The tunnel diode is a perfect place to start. It brings quantum mechanics out of the realm of the invisible and into circuits students can build, measure, and understand.
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