Understanding electrical conduction in quantum electronic devices

Figure 1: A two-dimensional (2D) metal-oxide-semiconductor field effect transistor (MOSFET).

Transistors form the basis of many high-speed electronic systems, from computers to mobile telephones. These semiconductor devices rely on the ability to conduct electricity, controllably and reproducibly, such that they can switch between metallic (ON) and insulating (OFF) states. Significant improvements in the speed and quality of transistors can be obtained by quantum mechanically confining the charge carriers to move in a thin two-dimensional (2D) sheet. Despite the fact that transistors play such a crucial role in electronic devices it is remarkable that the question of how they conduct electricity has been a subject of considerable controversy over the last 4-5 decades, and many fundamental questions remain unanswered.

Figure 1 shows a 2D silicon transistor, which forms the building block of all modern computers. In this device applying a positive voltage to the gate metal attracts the electrons in the semiconductor up towards it. However the electrons cannot reach the metal due to the insulating oxide layer, so instead they form a two-dimensional (2D) sheet of electrons just below the oxide layer. With no applied voltage to the gate metal there are no carriers under the oxide layer and the transistor is OFF. Appling a voltage to the gate metal attracts these electrons to form a highly mobile sheet of charge carriers to conduct electricity and the transistor is ON.
The act of confining the electrons to a 2D sheet causes unusual quantum phenomena that determine the way in which the electrons conduct electricity. In particular, confining the electrons causes the interactions between them to become a lot more important. Using the highest quality transistors from Bell Laboratories in the USA (where the first transistor was invented in 1947) we investigate these unusual quantum phenomena in order to gain a deeper understanding of how modern high-quality transistors work. In particular we address the question of whether you can have a metallic state in a 2D semiconductor.

In addition the 2D systems in these advanced transistors also exhibit other unexpected quantum phenomena when placed in a perpendicular magnetic field, such as the two quantum Hall effects that were awarded Nobel Prizes in 1980 and 1998. Our research program aims to relate the mechanisms that cause the Quantum Hall effect to the unusual metallic-like behaviour that has recently been observed in zero magnetic field. Figure 2 shows how these devices are measured at UNSW, with figure 3 demonstrating some of our recent results.

(left) Honours student Carlin Yasin and supervisor Michelle Simmons getting ready to measure an ultra-high quality transistor at milli-Kelvin temperatures in a helium dilution refrigerator at UNSW. (right) Detailed resistivity measurements of the lowest density 2D electron system ever fabricated are used to relate the Quantum Hall Effect in strong magnetic fields to unusual behaviour observed at zero magnetic field.

Dr. Michelle Simmons, Dr. Alex Hamilton and Carlin Yasin