TABLE OF CONTENTS In this chapter discussed a simple theory of ballistic nanowire MOSFETs. In comparison to traditional MOSFETs with 2D channels, nanowire MOSFETs can display some distinct characteristics, such as a drain conductance that is independent of gate voltage and, in the quantum capacitance limit, a transconductance that is equal to the drain conductance. Because of the low density of states and the ability to make use of high-k gate dielectrics, the quantum capacitance can play an important role in carbon nanotube FETs.
Our scientific understanding and the technological performance of nanowire FETs are progressing rapidly, but many open questions and issues remain. The role of scattering in these devices is one issue. For low biases, symmetry arguments show that elastic backscattering is suppressed, so mean free paths of several hundred nanometers are possible, but under high bias, optical phonon emission occurs and limits the current in a long nanotube to about 25 ?A [5.25]. We are just beginning to understand scattering in short nanotubes [5.26, 5.27] and in CNTFETs [5.24]. In addition to potential applications in digital circuits, other possibilities are also of interest. For example, carbon nanotubes have a direct bandgap, so electrically-pumped optical emitters are a possibility [5.28]. Because of their excellent transport properties, CNTFETs are interesting as high-frequency transistors [5.29]. The possibility of using nanowires to achieve room temperature single electron transistors is also interesting [5.30]. Finally, nanowires are interesting as high-sensitivity sensors for bio-medical applications [5.31, 5.13]. Where this will all lead is hard...
