PSpice Application Notes

APPLICATION NOTE 2 Figure 1: Photocurrent generator placement Figure 1A and Figure 1B show the proper placement for a diode and a bipolar transistor respectively. These devices could represent discrete components or devices on a bipolar integrated circuit. Figure 1C is an example of the parasitic bipolar diodes and transistors that exist in a CMOS technology. Note that the polarity is such that current flow is from the n-type material to the p-type material. It is easiest to use a current-controlled current source for photocurrent modeling. For example, assume that you have a small circuit containing only two discrete diodes, both of type DMOD. Further assume that the rest of the circuit consists only of passive components. A portion of the PSpice circuit description is given below. VPDMOD 1 0 EXP (0 40MV 100NS 8NS 150NS 50NS) RPDMOD 1 0 1 D12 10 2 DMOD FD12 10 2 VPDMOD D20 7 6 DMOD FD20 7 6 VPDMOD For an integrated circuit design, the photocurrent modeling process is actually somewhat simpler. Since there are relatively few p-n junction types in a typical integrated circuit technology, the number of controlling sources required will be very small. In this case, define the magnitudes of the controlling sources as current densities. By doing this, the parameter in the definition of the current-controlled current source becomes the area of the specific p-n junction being modeled. The following example demonstrates how PSpice can be used to determine the dose rate sensitivity of an integrated circuit. Assume that a die contains a number of diodes all fabricated with the same diffusion and differing only in diode device area. Also assume that dosimetry measurements on this diffusion type have shown a peak current density of 4E-2 amps/cm 2 at a dose rate of 1E7 rad(Si)/sec. A small section of a typical circuit description is shown in the following text. *Radiation effect using ionizing radiation. .PARAM DOSE_RATE = 1E7 ;initialization .PARAM JP_MEAS = 4E-2 ;peak measured current density .PARAM DOSE_MEAS = 1E7 ;at measured dose rate * perform simulations at 1e6, 1e7, and 1e8 rad(Si)/sec .STEP PARAM DOSE_RATE LIST 1E6 1E7 1E8

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