What Is Quantum Tunnelling?
In the world of quantum mechanics, particles can behave like waves. This wave-like nature is crucial for understanding quantum tunnelling.Â
Imagine a tiny particle, like an electron, approaching a hill, represented by a potential energy barrier. In the classical world, if the particle doesn’t have enough energy to overcome the hill, it would simply turn back. But in quantum mechanics, the particle’s wave function, which describes the probability of finding the particle at a specific location, can seep or ‘tunnel’ through the barrier, even though the particle itself doesn’t have the classical energy to overcome it. This is quantum tunnelling.
How Does Quantum Tunnelling Work?
Again, if a ball is thrown at a wall, it will either bounce back (if it does not have enough energy) or fly over the wall (if it has enough energy). However, quantum tunnelling breaks those rules in the quantum world in the following manner:
- Wave-Particle Duality: The key to understanding tunnelling is that tiny particles like electrons exhibit wave-like behaviour. Imagine an approaching electron as a wave instead of a solid ball.
- Potential Energy Barriers: In this scenario, imagine a wall (a potential energy barrier) that the electron wave needs to overcome to reach the other side. Classically, if the wave doesn’t have enough energy to reach the top of the wall, it can’t get over.
- Tunnelling Through The Barrier: According to quantum mechanics, the electron wave can actually bend and slightly penetrate the barrier, even though it classically lacks energy. It’s like the wave leaks through a thin, porous membrane.
- Probability, Not Certainty: This ‘leakage’ doesn’t guarantee the electron will appear on the other side. There’s just a small probability of it happening. The wider and higher the barrier, the less likely this tunnelling is.
Thinking about probability waves helps explain how a particle can be simultaneously in two places (on both sides of the barrier) until its wave function “collapses” and it’s detected in a specific location.
What Are Some Possible Applications Of Quantum Tunnelling In Semiconductors?
Quantum tunnelling, though sometimes an unwanted side effect in semiconductors, has some applications. Here are a couple of areas where it plays a key role:
- Tunnelling Magnetoresistive Random Access Memory (TMRAM): This type of memory uses tunnelling to store data. In a TMRAM, a thin insulating layer separates two ferromagnetic layers. The orientation of the magnetic fields in these layers (parallel or antiparallel) determines the resistance to current flow through the insulator. Data (0 or 1) can be written and read using tunnelling by applying voltage and manipulating the magnetic fields. This type of memory offers advantages like faster read/write times and lower power consumption compared to traditional dynamic random access memory (DRAM).
- Resonant Tunnelling Diodes (RTDs): These are special-purpose diodes that exploit a specific energy level within the forbidden band gap of a semiconductor. When a voltage is applied, electrons can tunnel through this energy level, significantly increasing current flow. However, if the voltage is increased further, the electrons can no longer reach that specific level and the current drops sharply. This negative resistance characteristic of RTDs allows for unique functionalities in electronic circuits, such as high-frequency oscillators and detectors.
- Future Transistors: As transistors continue to shrink in size, quantum tunnelling becomes a more significant challenge. However, researchers are also exploring ways to utilise tunnelling for new types of transistors. For instance, transistors based on single-electron tunnelling could offer extremely low power consumption and high density, making them ideal for future ultra-miniaturised electronics.
It is important to note that while tunnelling can be harnessed for useful applications, it can also be a source of leakage current in transistors, leading to increased power consumption and heat generation.
