Chapter 4 . Ultralow Temperature Studies of Nanometer Size Semiconductor Devices

A variety of novel lithographic techniques have been used' to create quasi-onedimensional (Q1D) inversion layers in Si MOSFETs to study how their conductance depends on carrier density. It is remarkable that, despite the differences in the structure of the devices, the results are qualitatively similar: one observes random, but timeindependent, fluctuations in the conductance G as a function of electron density, which are exponentially large at small G and of order e2/h at large G. The latter are the universal conductance fluctuations2 seen in Q1 D metals as well,3 and the former result from one dimensional variable-range hopping.4 Among the inversion layers studied to date, those with smaller width w generally5 had lower mobility ( y 3000cm2/V-s is typical for w 40 nm). In Phys. Rev. Letters 62:583 (1989), we reported studies of narrower inversion layers ( w 25 nm) with higher y (15000 cm 2/V's) than previously reported that revealed qualitatively new behavior. The variations in G are still exponentially large at small electron density and very small at high density, but the variations are periodic in the density of electrons in the inversion layer, even in the absence of a magnetic field.