The Tunneling Transistor
Quantum tunneling is a limitation in today’s transistors, but it could be the key to future devices
Our always-on world of PCs, tablets, and smartphones has come about because of one remarkable trend: the relentless miniaturization of the metal-oxide-semiconductor field-effect transistor, or MOSFET. This device, which is the building block of most integrated circuits, has shrunk a thousandfold over the past half century, from the tens-of-micrometers scale in the 1960s to tens of nanometers today. And as the MOSFET has become tinier, generation after generation, the chips based on it have become much faster and less power hungry than their predecessors.
This trend has given rise to one of the longest and greatest winning streaks in industrial history, bringing us gadgets, capabilities, and conveniences that previous generations could scarcely have imagined. But now this steady progress is under threat. And at the heart of the problem lies quantum mechanics.
The electron has a pesky ability to penetrate barriers—a phenomenon known as quantum tunneling. As chipmakers have squeezed ever more transistors onto a chip, transistors have gotten smaller, and the distances between different transistor regions have decreased. So today, electronic barriers that were once thick enough to block current are now so thin that electrons can barrel right through them.
Chipmakers have already stopped thinning one key transistor component—the gate oxide. This layer electrically separates the gate, which turns a transistor on and off, from the current-carrying channel. Make this oxide thinner and you can induce more charge in the channel, boost the current, and make the transistor faster. But you can't reduce the oxide thickness to much less than roughly a nanometer, which is about where it is today. Beyond that, too much current will flow across the channel when the transistor is “off," when ideally no current should flow at all. And that's just one of several leakage points.
It has long been hard to pin down the precise year when size reductions will end. Industry road maps now project the miniaturization of the MOSFET out to 2026, when gates will be just 5.9 nanometers long—about a quarter the length they are today. This timeline assumes that we'll be able to find better materials to stanch leaks. But even if we do, we'll need to find a replacement for the MOSFET soon if we want to continue getting the performance enhancements we're used to.
We can't stop electrons from tunneling through thin barriers. But we can turn this phenomenon to our advantage. In the last few years, a new transistor design—the tunnel FET, or TFET—has been gaining momentum. Unlike the MOSFET, which works by raising or lowering an energy barrier to control the flow of current, the TFET keeps this energy barrier high. The device switches on and off by altering the likelihood that electrons on one side of that barrier will materialize on the other side.
Back or Through: In classical electrodynamics, an electron (blue) would bounce back from an energy barrier (orange) if its energy did not exceed the barrier height. In fact, electrons have a finite probability of passing through the energy barrier. The thinner the barrier, the higher the probability that such a tunneling event might occur.
That's a huge departure from the way traditional transistors work. But it might be just the thing to pick up where the MOSFET leaves off, paving the way for faster, denser, and more energy-efficient circuits that will extend Moore's Law well into the next decade.
It wouldn't be the first time the transistor has changed form. Initially, semiconductor-based computers used circuits made from bipolar transistors. But only a few years after the silicon MOSFET was demonstrated in 1960, engineers realized they could make two complementary switches. These could be combined to make complementary metal-oxide-semiconductor (CMOS) circuits that, unlike bipolar transistor logic, consumed energy only while switching. Ever since the first integrated circuits based on CMOS emerged in the early 1970s, the MOSFET has dominated the marketplace.
In many ways, the MOSFET wasn't a big departure from the bipolar transistor. Both control the current flow by raising and lowering energy barriers—a bit like raising and lowering a floodgate in a river. The “water" in this case consists of two kinds of current carriers: the electron and the hole, a positively charged entity that's essentially the absence of an electron in the outer energy shell of an atom in the material.
There are two allowable energy ranges, or bands, for these charge carriers. Electrons with enough energy to flow freely through the material are in the conduction band. Holes flow in a lower-energy band, called the valence band, and they move from atom to atom, much as an empty parking space might migrate around a nearly full parking lot as neighboring cars pull in and out.
These bands are fixed, but we can shift the energies associated with them up or down by adding impurities, or dopant atoms, to alter the conductivity of the semiconductor. N-type semiconductors, which are doped to contain an excess of electrons, conduct negatively charged electrons; p-type semiconductors, which are doped to produce a deficit of electrons, conduct positively charged holes.
If we put these two semiconductor types together, we get a junction with bands that are misaligned, thus creating an energy barrier between them. To make a MOSFET, we insert one type of material between two of the complementary type, in either an n-p-n or a p-n-p configuration. This creates three regions in the transistor: the source, where charges enter the device; the channel; and the drain, where they exit.
The two p-n junctions in each transistor provide electronic barriers to the flow of charges, and the transistor can be switched on by applying a voltage to the gate on top of the channel. A positive voltage applied to an n-channel MOSFET gate makes the channel more attractive to electrons, because it decreases the amount of energy an electron needs to have in order to move into the channel. A negative voltage applied to a p-channel MOSFET gate will perform the same task for holes.
This simple barrier-lowering strategy is the most widely used current-control mechanism in semiconductor electronics. Diodes, lasers, bipolar transistors, thyristors, and most field-effect transistors all take advantage of it. But it has a physical limit: Transistors need a certain amount of voltage to be switched on or off. This arises from the fact that electrons and holes are in constant motion due to their thermal energy, and the most energetic among them spill over the barrier. At room temperature, the current flowing over the barrier increases by a factor of 10 when the energy barrier is lowered by 60 millivolts; every “decade" of current change requires a change of 60 mV.
All this current leakage occurs below the device's threshold voltage, which is the voltage needed for the transistor to turn on. Device physicists call this barrier-lowering region the subthreshold region, and 60 mV per decade is known as the minimum subthreshold swing. To keep power consumption down, subthreshold swing should be as low as possible. The device will then need less voltage to be switched on, and it will leak less current when it's off.