Diffused resistors characteristics at high current density levels-analysis and applications

Both heavily (source-drain) and lightly (well) doped diffused resistors used in modern CMOS integrated-circuit technologies are studied under high current density levels. The effects of high-field mobility degradation, space-charge limited-current, and charge-depletion narrowing (pinchoff) are discussed, and the related analytical solutions are derived and characterized. The applicability of the study to electrostatic discharge (ESD) protection networks, latchup protection circuitry, and output noise suppression resistors is addressed. Velocity saturation due to mobility degradation is found to be the prevailing mechanism, while space-charge limiting current and depletion narrowing have only limited effect on the lightly doped ones. The resulting saturation current densities allow proper resistor designs which can effectively limit ESD and latchup current into highly susceptible nodes and thus-improve overall circuit reliability, potential suppression of I/O switching noise is also discussed and shown to be more sensitive to the ratio between peak noise and steady-state currents. The experimental data confirm the theoretically predicted saturation velocity and critical field for the lightly doped case but show substantial discrepancy for the heavily doped layer.<
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