I recently received this very interesting work about the Gunn effect in Si devices sent to me by Daryoush Shiri:

Imagine an experiment in which you increase the voltage across a piece of semiconductor solid like GaAs and you record the increasing current as a result. After passing a threshold voltage you notice roll off and current decrease. Why? Because some electrons gained enough energy to move to a higher energy band which has higher effective mass or in another language a band in which electrons have lower velocity. This transfer of electrons from a fast band to a slow band reduces their average velocity. Hence increasing the voltage causes a drop in current or in technical terms negative differential resistivity (NDR). This phenomenon is known as Gunn or Gunn-Hilsum effect and it was discovered independently by Gunn and Hilsum in GaAs and other related III-V compounds during 1960’s. This effect was exploited in devices called Gunn diodes for many decades as an essential component in 10-100 GHz oscillators for radar applications. The electric loss in LC or cavity resonators is compensated by NDR of Gunn diode.

Unfortunately this useful effect is lacking in Silicon, the fundamental material of mainstream CMOS technology. The reason lies in the large energy difference between the energy bands ( > 1 eV) within Silicon which means initiating NDR (or current drop) requires impractical amount of voltage. In contrast the distance between energy bands in GaAs is about 300 meV, which explains why microwave Gunn diodes are so far made of GaAs but not Silicon.

In this work, using extensive quantum mechanical computations of all electron-phonon scattering mechanisms and Monte Carlo method, we showed for the first time that Silicon can host Gunn Effect if it is shaped as nanowire and mechanically strained. Making a nanowire out of Silicon modifies the energy bands in such a way that the distance between two electron energy bands can be much less that the already unwanted amount of 1 eV. Interestingly this low energy difference (~ 100 - 280 meV) is also adjustable using mechanical strain. We observed that applying tensile strain of +3% helps the nanowire to show NDR at the applied electric field of 5000 V/cm. For typical lengths of a few micrometer this means a few volts for the applied voltage.

This study suggests that by embedding biased Silicon nanowires on a deformable substrate, it is possible to initiate or inhibit the oscillation at will by turning the applied mechanical strain on or off. Change of nanowire resistivity by more than 100% as a result of strain application also suggests application of this device in nano-mechanical force/stress/pressure sensors. The article promises the entrance of silicon in the realm of microwave devices. It is available at the following link including the detailed computational methods: https://www.nature.com/articles/s41598-018-24387-y

This work was collaboration between Chalmers University of Technology (Sweden), Texas A&M University-Kingsville (TX, USA), University of Waterloo (Canada) and University of Washington (WA, USA). For further information contact Daryoush Shiri at (shiri@chalmers.se).