This selection allows you to specify the doping densities for donors and acceptors in the current region.
This commands opens a dialog box where you can specify the doping density and the doping type (n-type or p-type) for background doping that is present uniformly throughout the layer. The mobilities at 300 K for both electrons and holes at this doping density are displayed for reference, as are the diffusivities. The mobilities are either fixed or variable with doping, as selected for this region's material). The majority-carrier mobility shown is used to calculate the resistivity. If you want the doping density that corresponds to some desired 300-K resistivity, then you can just type the desired resistivity directly into this field, then hit the Tab key to set the doping density accordingly.
Doping profiles can be specified for both the front and rear surfaces of the current region. These profiles can be generated using internal models or can be imported from an external disk file.
The internal model allows for two diffusions at each surface. After selecting either the first or second diffusion for the surface, a dialog box is opened for specifying the diffusion details. The dialog box for the first diffusion on the front and rear surfaces can also be opened using a button on the toolbar. Each diffusion must be Enabled within its dialog box before it becomes active. The toolbar button will appear to be depressed if that diffusion has been Enabled. Diffusions can be specified as n-type or p-type. Each diffusion can take one of four shapes, defined as follows:
| Uniform | |
| Exponential | |
| Gaussian | |
| Erfc | |
where No is the Peak Doping, xd is the Depth Factor, and xp is the Peak Position, all of which are adjustable. Note that N(x) = No for x<xp for the Erfc shape.
Also shown in the dialog box are the calculated sheet resistance (ohms/square) and junction depth. The junction depth is based on the currently specified background doping density for the region. The sheet resistance is calculated based on the diffusion profile up to this junction depth, using a temperature of 300 K and the majority-carrier mobility (either fixed or variable, as specified for this region's material). The calculation of the sheet resistance ignores conductance due to dopants from the background or from other diffusions.
To obtain a diffusion with a given junction depth and sheet resistance, you can type the desired sheet resistance in the space provided, then press the Tab key. The Peak Doping is adjusted to give the requested sheet resistance. If the junction depth does not match the desired value, type the desired value in that field, then press Tab to adjust the Depth Factor to obtain the desired junction depth. If the sheet resistance is affected, you can re-enter the desired value in that space, and repeat the cycle until convergence is obtained. Rarely are more than a few iterations required to get quite close to the desired combination.
External diffusion profiles are standard ASCII files with a filename suffix DOP. These can be created using a text editor or generated by another computer program. Each line in the file should contain three numeric values, each separated by one or more spaces or a tab. The first value on each line is a value of distance inward from the surface of the region, with units of mm. The second value on each line is the donor doping density at that position, with units of cm-3. The third value on each line is the acceptor doping density, with units of cm-3. The position values must start at 0 and increase monotonically. The maximum number of positions that may be defined in the file is 500.