Birck Nanotechnology Center

Counter Intuitive Physics of Ballistic Transport in the State-of-the-Art Electronic Devices

In a small enough semiconductor devices, the electron mean free path for collisions with impurities or lattice vibrations greatly exceeds the device size. Hence, the electrons travelling with the thermal or Fermi velocity leave the active region of the device before they experience scattering. Such collisionless electron transport is called “ballistic”. The electron mean free path in silicon at room temperature (on the order of 30 nm) is much greater than the 10 or 12 nm feature size of modern silicon CMOS used, for example, in the recent generations of iPhones or android phones. The current-voltage characteristics of such devices look similar to those of much longer transistors. However, the physics of the ballistic transport hiding behind this misleading similarity is very different and counter intuitive. It has important qualitative consequences for the design of the advanced transistors and integrated circuits. One new ballistic concept the concept of a “ballistic mobility”. A mobility is the coefficient of proportionality between the effective drift velocity in the device channel and applied electric field. Since electrons hit contacts more often in short channel devices, the ballistic mobility is proportional to the device length, as was confirmed by numerous experimental data. At high frequencies, the electron inertia starts playing an important or even dominant role. The high frequency impedance is strongly affected by the electron inertia and by the phase delays of the opposing electron fluxes in the device channel. The waves of the electron density (plasma waves) enable the device response well into the terahertz (THz) range of frequencies. At high excitation levels, these waves are transformed into the shock waves. The rectification and instabilities of the plasma waves enable a new generation of THz and sub-THz plasmonic devices. Ultra-wide band WIFI, advanced homeland security, VLSI testing, and cancer detection are but examples of applications of this plasmonic technology.

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