An asymmetrically doped buried-layer (ADB) structure forlow-voltage mixed analog-digital CMOS LSI's

A new CMOS structure has been developed that is distinguished by its asymmetrically doped buried layer (ADB). This structure makes it possible to achieve high drain output resistance and high transconductance necessary for high-performance analog circuits with a low-voltage power supply. The ADB structure has a high-impurity-concentration “pocket” layer near the channel edge of the buried layer only on the source side and a low-impurity surface region through the channel. The source-side channel region determines the threshold voltage and the drain-side channel region absorbs the drain potential. The low-impurity surface region reduces impurity scattering and enables high transconductance. The fabricated ADB CMOS structure increased the drain output resistance, transconductance, and saturation current down to a 0.3-μm channel length, as compared to a control structure. Furthermore, the drain junction capacitance was reduced because of the low impurity concentration beneath the drain region