Transcript Ballistic Devices

• 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)
• High Electron Mobility Transistor (HEMT)
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-toanalog converters
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)