In this story, there are two protagonists: A Photon and an Electron. The signal/optical communication devices uses Photons (light) and signal processing devices uses Electrons. The conversion of optics into electronics and vice versa slows down the effective communication. Recently, a new transistor has been proposed and experimentally demonstrated by Leonid V. Butov and Arthur C. Gossard (University of California) which processes signals by emitting light.
Signal Processing is mainly carried out before transmitting and receiving any information. It is mainly done using semi-conductor integrated circuits. These miniature integrated circuits are built using transistors which currently uses electrons for signal processing. These electrons and photons don’t interact directly with each other. This presents a major bottle neck in the modern day communication and signal processing. The direct use of light, without its conversion, would speed up both computation and communication.
The transistor proposed is based on Gallium Arsenide (GaAs) and processes signals using indirect EXCITONS instead of electrons. These excitons are controlled by gate electrodes just like in silicon transistors (standard field effect transistors i.e. FETs) and can be easily coupled with the photons. This results in faster signal transmission to other optically connected on chip and off chip devices. The computation power advantage is not great as compared to the communication one.
Excitons are electron-hole pairs, bound by the attractive force between negatively charged electrons and positively charged holes. Because of this force, excitons tend to recombine fast, releasing a flash of light. The lifetime can be increased by up to ten microseconds when confining electrons and holes in spatially separated layer forming and indirect exciton. The excitons exhibits stable characteristics at low temperature operation (below 40K) but dissociate easily at higher temperatures. The real time applications require stable operation at room temperature and above. This question still needs to be addressed. There is no inorganic semiconductor material available so far that allows stable exciton population at room temperature. Although materials such as ZnSe, CdTe and GaN may survive exciton populations at room temperature but that requires extremely narrow spatial separation between electron hole pairs of the excitons. This stable operation of excitons remains a bottle neck for the fabrication of high quality exciton based ICs (EXICs).
The processing of excitons require transfer of energy before their decay. This limits the number of transistors integrated on a chip which is a crucial condition for computation. The coupling excitons don’t have a long lifetime (few nanoseconds) and a small propagation distance. The excitons having larger lifetime reveals poor coupling with light. This results in another obstacle in the fabrication of real time operating devices.
The success of this technology depends on how these open questions are to be addressed. Optical Exciton based transistors can be a reality and a paradigm shift if we would be able to find materials and methods for room temperature operation. It could easily pave the way for technological revolution. All I would say is Conventional Solid-State Optoelectronics still has a huge intrinsic potential for further development.
Extracted and Summarized from “Will Excitonic Circuits Change Our Lives?” in Optics and Photonics Focus published in August 2008