Silicon-on-insulator (SOI) technology is under active study as a candidate for future device technology. Some images obtained from a study of hot electron effects in a 0.18 µm SOI nMOSFET are shown below, to highlight the ability of DAMOCLES to model potentially important, "realistic" device structures.

SOI nMOSFETs differ from the bulk counterparts (e.g., the preceding nMOSFET and pMOSFET examples) by the lack of a substrate contact. Instead, the active layer sits on a "floating" oxide substrate. The image below shows the electron and hole particle densities in the SOI device cross section.

The device bias is Vgate=2 and Vdrain=0.8 V, with the source grounded. The temperature is 300 K. Electrons and holes are colored according to their kinetic energies. Electrons range from cyan (0 eV) through green and yellow, up to red (1.5 eV). Holes span a much smaller energy range, and are primarily magenta in color (0 to 0.3 eV, labeled -0.3 eV in the figure as hole energy is negative by convention). As electrons flow from source to drain, they impact ionize near the drain end of the channel. The created electrons flow out of the drain, but the created holes remain in the substrate. There is no substrate contact, and hence, no direct path to permit the excess holes from leaving the device. Instead, the holes pool near the source. This alters the substrate potential, and can have important effects on SOI device behavior. Note how the (poly-)silicon gate has been modeled by electron particles in (single crystal) silicon, in order to allow the possibility of observing gate depletion effects.

The conduction band potential corresponding to this SOI device and operating condition is shown below.

The semiconductor potential is shown in shades of blue, while the oxide regions jump to shades of yellow and red due to the 3.1 eV discontinuity between the potential in the two regions.

Here is a plot of the total energy in the conduction band, showing the position of electrons in the Si active layer of the SOI device.

The Si-substrate oxide interface corresponds to the front edge of this figure, while the rear edge corresponds to the Si-gate oxide interface. Electrons move from the source (left) to the drain (right), and are colored according to their kinetic energy. There is a pillar of hot carriers near the drain: this is an artifact of the manner by which DAMOCLES exaggerates rare electron states in order to probe highly improbable portions of the distribution function (like the high energy tail near the drain, in this case).

The plot below shows a similar image, except holes in the valence band are plotted.

The valence band axis is inverted, so that hole energy increases from the bottom to the top of this plot. The holes are quite low in energy, and congregate in the potential energy well in the active layer near the source junction.

Quantitative carrier distribution information is readily available from DAMOCLES simulations. Below is plotted the kinetic energy distributions of electrons near the drain in this SOI device, for two different modeling conditions.

Of note of how different the high energy "tails" of the two distributions functions are. Above 1.25 eV, electron-electron scattering is seen to add many more energetic electrons, compared to the modeling result without electron-electron scattering. Physically, as electrons traverse the channel and then enter the drain, electron-electron scattering provides a mechanism for carriers to gain energy, as well as lose it. While the majority will lose energy, a few (maybe 1 out of 1 million, or so) will gain energy. Quantifying this tiny population of hot electrons is important in estimating device reliability. It may seem counterintuitive that electrons with an energy of 1.5 eV and more exist in a device with only 0.8 V drain bias...but its true! The page discussing low-bias effects provides more details on the physical origin of these unexpected hot electrons.