Quantum Tunneling is a quantum-mechanical phenomenon where a particle tunnels through a barrier that it classically could not surmount. It is the operative mechanism in tunnel diodes. Tunneling can arise with barriers of thickness around 1 to 3 nm and thinner. A simple barrier can be created by separating two conductors with an extremely thin insulator.
Tunneling current drops off rapidly as particles get through the barrier. Consequently, tunnel diodes can be created having a range of voltages for which current drops as applied voltage rises. This peculiar property is used in such applications as high-speed devices where the characteristic tunneling probability changes as rapidly as the bias voltage.
The particles that tunnel through the barrier of a tunnel diode are actually charge particles. There are two types of electric charge (that we know of), positive and negative. Actually these words shouldn’t be taken too seriously. There is nothing inherently negative about an electron in the sense of negative numbers being less than zero. In fact it is a historical accident that the electron was labeled negative, a consequence of the fact that electricity was first thought to travel in the opposite direction than now believed.
Terminology notwithstanding, the basic fact, well known to our ancient ancestors, is that when it comes to polarity, likes repel and unlikes attract. This simple fact makes possible atoms, hence matter as we know it.
Electric charge is a conserved property of many subatomic particles. A proton has a specific positive charge that never changes. On a larger scale, objects may acquire or lose various amounts of electric charge. The usual scenario for a macroscopic object is to be neutral with respect to electric charge, because the constituent atoms for the most part are made up of equal numbers of positive protons and negative electrons, stabilized by the stern presence of neutrons.
When atoms gain or lose one or more electrons, they become negatively or positively charged and they become ions. If this happens to a substantial fraction of the atoms in a macroscopic body, the object acquires a charge and it attracts or repels unlike or like charged objects.
Eventually it was discovered that electric charge is quantized, not able to occupy just any position along a continuum but instead existing at discrete multiples of a basic amount. In fact, the fundamental electric charge, exhibited by all electrons and protons, is –e or +e, and it is approximately equal to minus or plus 1.602 × 10-19 coulombs. An exception is the quark, which seems, like a virus in a human cell, to exist as a constituent part of other subatomic particles. It has a charge that is an integer multiple of e/3.
Quantum tunneling works because of the fact that matter can be completely described considering each particle as a wave function. A particle, encountering a supposedly insurmountable barrier can, in a small number of cases, pass from one side of the barrier to the other.
Earnest Rutherford and others anticipated quantum mechanics in the early years of the twentieth century, describing half-life and decay. George Gamow was one of the first to solve the Schrödinger equation, showing that the half life of a particle and energy of emission give rise to tunneling as a probabilistic event.
The view promulgated by classical mechanics maintains that particles lacking sufficient energy to cross a barrier will in fact never do so. Since the energy of a particle, or more properly of a wave function, has energy across a certain bandwidth, a portion of this energy may complete the crossing.
Needless to say, the Heisenberg Uncertainty Principle provides a way of understanding what is going on. It states that position and momentum of a particle cannot both at the same time be known. It is important to see that this is not some kind of psychological impediment to human knowing but instead a measurement issue.
Quantum tunneling takes advantage of the multiple-energy level nature of a particle’s wave function, so it can move in ways that would not be possible in classical physics. There are many practical applications besides tunnel diodes that make use of the phenomenon. Additionally, it has one unfortunate consequence, which is that it sets the stage for current leakage in Very Large Scale Integration (VLSI), resulting in power loss and heat rise in dense IC’s. This may place an ultimate limit on how small microchips can be made.