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Ballistic Devices - Vikram.ppt

  1. • 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
  2. 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
  3. • 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
  4. • 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
  5. • 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
  6. • 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
  7. • “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
  8. 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)
  9. 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
  10. • 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
  11. • 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
  12. [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 [4] E. Forsberg: "Reversible logic based on electron waveguide Y-branch switches", Nanotechnology 15, S298 (2004) [5] Wikipedia contributors at [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)