All of modern electronic devices work with binary language of just zeroes and ones. Transistors--tiny switches employed in most of these gadgets--play a crucial role in these devices. Over the years, transistors have gone from the size of a small phone, to millions of them fitting in a smartphone! They are ubiquitous and improving their form and performance is an active area of research. Now, scientists from the Indian Institute of Science (IISc) have designed a novel transistor by combining two different types of transistors in to one.
The functioning of a typical transistor is analogous to a dam. There is a ‘source’ from where the water, or in this case, electrons are generated, and a ‘drain’ where the electrons flow into. A gate controls the flow of these electrons between the source and drain. When the gate is closed, a barrier is presented to the electrons and their flow is blocked. When a small voltage is applied to the gate, the height of the barrier reduces, allowing the electrons to hop across, causing a current to flow between the source and the drain. Lesser the amount of voltage required to open the gate, more efficient the transistor becomes.
“The transistor is a gate controlled switch, where the voltage value on the gate decides whether the switch is ‘on’ or ‘off’. For example, in modern transistors, when you apply 1 Volt to the gate, the transistor is turned ‘on’ and when you apply 0 V to the gate, the transistor is turned ‘off’. For an ideal switch, you need large ‘on’ current and ideally zero ‘off’ current. But, all transistors used in today's electronics have a non-zero ‘off’ current, the value of which depends on the ‘subthreshold swing’”, explains Prof. Navakanta Bhat, Chairman of the Centre for Nano Science and Engineering at IISc, and a co-author of the study.
The ‘subthreshold swing’ is the minimum voltage required for opening or closing the gate. Much like the dam, opening or closing the gates requires a minimum amount of energy to do so. “Steeper the subthreshold swing, or lower its value, the faster will be the fall of current and hence, lower will be the ‘off’ current for a given value of ‘on’ current,” remarks Prof. Bhat. The efficiency of such transistors, commonly called thermionic transistors, depend on the amount of voltage required to activate the gate. Thus, in the present the efficiency of such transistors has hit a fundamental bottleneck, since no matter how small they get, the required gate voltage remains fairly large.
“In the today's thermionic transistors, the best (i.e. minimum) value of subthreshold swing is 60mV/decade. In order to produce a ‘on’ current, the electrons from the source of the transistor have to overcome the energy barrier (controlled by the gate) and then go to the drain. Only those electrons that have energy more than the barrier height can contribute to the current. The energy distribution is governed by Boltzmann statistics and hence the limit of 60mV/decade”, explains Shubhadeep Bhattacharjee, a PhD student at CeNSE, who is also one of the researchers in this study.
Researchers over came this bottleneck with another type of transistors called tunnel field effect transistors (tunnel-FET). This type of transistors use a fundamental property of quantum particles, like electrons, called tunneling. According to Shubhadeep, “with this new method of transport (tunneling), the electrons need not attain energies higher than the barrier. Even if the energy is lower than the barrier height, they can ‘tunnel’ through the barrier, provided the tunneling distance controlled by gate is small. Hence we are no longer limited by Boltzmann statistics.” Thus tunnel FETs can increase the efficiency of transistors by decreasing the amount of gate voltage that is required.
However, tunnel FETs too have their own disadvantages, as the amount of current that flow between the source and the drain is very low compared to their thermionic counterparts. To overcome the shortcoming of both these technologies, the researchers of this study built a new device which combines the functioning of both thermionic and tunneling components into one transistor. To achieve this, the scientists used two gates, which could be controlled independently, in a single transistor, along with a new switching element – tunable Schottky barrier, a type of energy barrier to electrons formed at a metal-semiconductor junction.
The new hybrid device can switch between a conventional thermionic transistor and a modern tunnelling transistor, thus providing the benefits of both worlds, with very little of the disadvantages. “By having an independent control of the tunnel barrier height and the tunnel barrier width, we can combine both tunneling and thermionic components in single transistor”, remarks Shubhadeep. The new transistor also consumes much less power than the thermionic ones, and can produce much higher currents than many modern tunnelling transistors.
So where could these new transistors find their applications? “This work will open up the possibility of using new kind of transistor for building future electronic circuits that will have ultra-low power consumption. For instance, a smart watch a few years down the line, could potentially use transistors like this, and provide performance which is far superior compared to the state of the art smart phones available today”, says Prof. Bhat. The union of the transistors has really made the future of electronics exciting!