sheldon
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Richard,
In older processes, the bulk substrate concentration is different for the n-channel and p-channel transistors. Usually one device is built into well, while the other exists in the substrate. The well doping needs to "counter-dope" the substrate doping and is typically ~10x higher in concentration. As a result, the depletion regions change at different rates with applied bias for n-channel and p-channel transistors. The difference in the rate of change of the Drain-Bulk depletion region is a significant contributor to the difference in the channel length modulation.
Best Regards,
Sheldon
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