........ published in NEWSLETTER # 57
[ NATO-PCO Home Page ] [ Table of Contents of NEWSLETTER # 57 ]
........ published in NEWSLETTER # 57
QUANTUM TRANSPORT IN ULTRASMALL DEVICES
by Professor D.K. Ferry, Arizona State University, Tempe (U.S.A.)
The operation of semiconductor devices depends upon the use of electrical potential barriers (such as depletion) in controlling the carrier densities (electrons and holes) and their transport. Although a successful device design is quite complicated and involves many aspects, the device engineering is mostly to devise a "best" design by defining optimal control of the carrier flow through the device. This becomes increasingly difficult as the device scale becomes smaller. Since the introduction of integrated circuits, the number of devices on a single chip has doubled approximately every three years. With this, the critical dimension of the smallest feature, (typically the gate length) has consequently declined as well. The reduction of this design rule is approximately a factor of 1.4 each generation (every three years). If the reduction continues, extrapolation of current technology will require 30 nm design rules by the year 2020.
New problems keep hindering the high-performance requirements for smaller devices. Well-known, but older problems include hot carrier effects, short-channel effects, etc. A potential problem, which illustrates the need for quantum transport, is caused by impurity fluctuations. As devices become too small, the number of impurities is countable and their placement cannot be controlled. Impurities in semiconductor devices are randomly distributed as a result of the nature of the processing. Although electron transport in the device always experiences the effect of the random distribution of the impurities, the statistical contribution of these effects in the performance of a large device is negligible, due to the ensemble averaging that occurs over the large device area. In devices with a small size, however, this averaging does not occur, and these devices are susceptible to a large percentage fluctuation in performance (current, conductance, etc.). This is because devices with gate lengths below 0.1 micron contain only a few hundred impurities (or carriers). It is well known from the study of mesoscopic devices that this impurity fluctuation can lead to significant fluctuation in the local density of carriers and to phase interference effects in the transport. One then needs to provide better understanding of this effect on device design.
The transport of carriers in semiconductor devices has long been a subject of much interest, especially in the realm of device modeling. With the need for new and better CAD tools, it will become necessary to incorporate full quantum mechanical treatments in the modeling of devices. Hence, it appears that more detailed modeling of quantum effects needs to be included in device modeling for future ultrasmall devices. Unfortunately, we do not currently know how to handle these transport systems due to the far-from- equilibrium state of the open quantum system the devices represent.
As a method of discussing these modeling issues, and as a method for training new young people in the approaches that have so far been used, this NATO ASI was organized by D.K. Ferry (Arizona State University), H.L. Grubin (SRA Inc.), A.P. Jauho (Technical University of Denmark), and C. Jacoboni (University of Modena). The topic of the ASI was approached from two directions: semi- classical device physics and mesoscopic device physics, in order to illustrate the two end points of the modeling analysis. With each approach, an attempt was made to identify the relevant physics appropriate to each limit, and to identify the physics that will carry over to the ultrasmall device. Lectures on both the theoretical transport physics (and device modeling) and the experimental effects in mesoscopic devices were included in the program. We hope that the resultant volume (NATO ASI SERIES B342) will prove useful to future workers in the area, as we feel it brings together a seminal amount of material crucial for future directions in the field.
Reference books: 3-4, B52, B180, B251, B340, B342, E234, E270, E289