Sidewall current flow

Figure 6.8 shows a simplified cross section of a unit cell of a 4H-SiC power BJT. The cell pitch, P, is given hy the sum of width of the emitter mesa, width of the p base implant, and the total base-to-emitter spacings. An example of the device layout is shown in Figure 6.9. The goal of unit cell design is to minimize the cell pitch, since most of the current flows along the sidewalls of emitter mesas. It is important to maximize the density of emitter mesa sidewall density without compromising the performance of the transistor.

The current flow through molecules is discussed. We report on some recent calculations of current-voltage (I-V) curves for molecules between two metal surfaces. The change in the density of states of carbon nanotubes when molecules are allowed to interact with their sidewalls is also discussed. We speculate on the future of this area. 

The sidewall of the tip must be isolated to reduce a background electrochemical current flowing through the tip. Soft glass, organic polymers, and Apiezon wax have been used. Details of tip coating methods have been described by Siegenthaler [5]. 

Figures 17.5 and 17.6 show the effect of electron-beam-mediated shrinking of 160-nm contact holes. By irradiating the patterned wafer with electron-beam accelerating voltage 20 KeV, current 4 mA, dose 750 p,C/cm and 20 KeV, current 5 mA, dose 1000 p.C/cm, the hole size was shrunk down to 115 nm and 90 nm, respectively. The nearly circular, doughnut-shaped electron-beam-treated features in the top-down SEM picture of Fig. 17.5, with a lighter outer annulus surrounding a dark interior circle, is indicative of sloped resist sidewall, caused by the flow of the electron-beam-heated and melted resist, which subsequently flows down due to gravity. In contrast, the uncured features have fairly straight sidewall proflIes.

Ohmic overpotential is the loss associated with resistance to electron transport in the gas diffusion layers. For a given nominal current density, the magnitnde of this overpotential is dependent on the path of the electrons. The potential field in the cathodic and the anodic gas diffusion electrodes are shown in Figure 3.17. The potential distribntions are normal to the flow charmel and the sidewalls, while there is a gradient into the land areas where electrons flow into the bipolar plate. The distributions exhibit gradients in both x and y direction due to... 

---