PSpice Application Notes

APPLICATION NOTE 1 Introduction The design of electrical systems for military and space applications requires a consideration of the effects of transient and total dose radiation on system performance. Simulation of radiation effects using PSpice helps to identify critical circuit components which could cause system failure and gives guidance for radiation hardening of the system. The information in this application note provides a starting point for those interested in using PSpice for radiation effects analyses. Dose-Rate Effects When semiconductor devices such as diodes, transistors, and integrated circuits are exposed to ionizing radiation, such as gamma-rays or X-rays, hole-electron pairs are generated within the semiconductor material. These free carriers result in the generation of photocurrents as they are swept through the depletion regions of the p-n junctions of the device or integrated circuit. The magnitude of these currents can be orders of magnitude greater than normal signal levels and can result in temporary or permanent system failure. PSpice can be easily used to model this type of transient effect. Generation of photocurrent The magnitudes of the photocurrents are often obtained from experimental data, but can be obtained from theoretical expressions if information on the fabrication process is available. Usually, if an integrated circuit process has been developed for use in a radiation environment, dosimetry measurements have been made on each of the junction types (for example, source, drain and well-to-substrate diffusions in a CMOS technology). Therefore, the photocurrent data is directly available. If no experimental data is available, it is possible for the integrated circuit designer to use a theoretical expression for photocurrent generation. The following equation represents a rectangular pulse of ionizing radiation: where: Ipp is the photocurrent in amperes, gamma is the dose rate in rads(Si)/sec, q is the electron charge (1.6E-19 coulombs), g is a constant whose value is 4.2E13 carriers/(rad·cm3), A is the junction area in cm2, W is the depletion width in cm, L is the diffusion length in cm, tau is the minority carrier lifetime in sec, tp is the radiation pulse width in sec, t is time in sec, and U(t) is a unit step function of time. For the circuit designer using discrete components, no information is usually available on the process used to fabricate the devices. Dosimetry measurements have been made, however, on a large number of commercial discrete components. This information is often available within aerospace, military and government organizations. When setting up the circuit description for this type of analysis, photocurrent generators should be placed across all p-n junctions in the design. This must include all parasitic junctions as well as the junctions that make up the explicit design components. Examples of photocurrent generator placement are shown in Figure 1.

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