• Electron Scattering Length - Mean Free Path – le - Avg. distance between scattering

• Si - ~ 5nm; GaAs - ~ 100 nm

• Electrical Resistance is closely related to le

Macroscopic Devices

Mesoscopic Devices

• Active device length is smaller than the Scattering Length

• Electrons may travel without encountering scattering from the randomly distributed scatterers

• Electrons are scattered only at the device boundaries

• Newtonian billiard-ball model

• Electrons free to move in two dimensions but tightly confined in the third

• In some heterostructure semiconductors with offset conduction bands

• Triangular “well” formed at the interface goes slightly below the Fermi energy so that electrons can collect there

• Reduces impurity scattering - increasing the electron mobility

• No random scattering - very high intrinsic working speed and a quick response

• No temperature dependent phonon scatterings - temperature independent

• Electron transport can be, to a large extent, modified and controlled by designing the device boundaries

Ballistic Devices

• Ballistic Rectifier

• Mesoscopic Ballistic Detectors

• Ballistic Y - Branch Switch

• Conventional Ballistic Transistors

• Ballistic Deflection Transistor

• The asymmetric triangular structure, deflects the electrons downward – rectification

• AC or RF source is connected across the left and right contacts

• DC voltage is developed across the top and the bottom contacts

• Mesoscopic solid-state structures as both quantum systems and as detectors

• Operation: The ability of a measured system to control the transport of some particles between the two reservoirs

• Most direct form - Quantum Point Contact

• Output is the electric current I due to ballistic electrons driven by the voltage difference V between the electrodes

• Current depends on the electron transmission probability – state of the system

• “Y” configuration with one source and two drain terminals

• Electric field steers the injected electron wave into either of the two output drain arms

• Electrons need not be stopped by a barrier - fast switching times and low power consumption

• The 2DEG channel increases the mean free path of the electrons, making YBS to operate in the terahertz ballistic regime

• Used as ballistic logic gates with possibility of cascading

Heterojunction Bipolar Transistor (HBT)

• The main difference between the BJT and HBT - Different semiconductor material for emitter and base regions, creating heterojunction

• High doped base, forming 2DEG layer - higher electron mobility while maintaining gain

• Applications: Optoelectronic integrated circuits and mixed signal circuits such as analog-to-digital and digital-to-analog converters

• Heterojunction Bipolar Transistor (HBT)

• High Electron Mobility Transistor (HEMT)

High Electron Mobility Transistor (HEMT)

• Field effect transistor with the 2DEG in heterojunction layer as channel

• Hence channel has low resistance or high electron mobility

• Applications: Microwave and millimeter wave communications, radar and radio astronomy

• Ballistic transport in the 2DEG layer provides the actual transistor nonlinearity

• Vdd accelerates electrons from Vss towards the central junction of the BDT

• A small gate voltage modifies the path of the electrons towards the right or the left

• These electrons are then ballistically deflected from the central triangular feature into one or the other output channels

• Review of a novel and unique concept of electronic devices capable of working at high frequencies

• Devices with ballistic transport

• Low power consumption and produces low noise levels

• Multitude of applications like high speed processors, RF identification, wireless fidelity

[1] A. M. Song, “Room-Temperature Ballistic Nanodevices”, Encyclopedia of nanoscience and Nanotechnology, X, 1 (2004)

[2] S. Datta, Electronic Transport in Mesoscopic Systems (Cambridge University Press, Cambridge, 1995)

[3] D. V. Averin, “Mesoscopic Quantum Measurements”, available online at http://arxiv.org/abs/cond-mat/0603802

[4] E. Forsberg: "Reversible logic based on electron waveguide Y-branch switches", Nanotechnology 15, S298 (2004)

[5] Wikipedia contributors at http://www.en.wikipedia.org

[6] Q. Diduck, M. Margala, and M. J. Feldman, “A Terahertz Transistor Based on Geometrical Deflection of Ballistic Current”, IEEE Microwave Symposium Digest, 345 (2006)