Effects of lightly doped drain structure with optimum ion dose on p-channel MOSFETs

Hot-carrier (HC) degradation phenomena in p-channel lightly doped drain (LDD) MOSFETs and the effects of an LDD structure in minimizing HC degradation are described. Hot-electron injection in p-channel LDD MOSFETs causes effective channel length shrinkage. This phenomenon is also observed in conventional p⁺-drain MOSFETs but differs greatly from that in n-channel LDD MOSFETs. This is because hot electrons trapped in gate SiO₂ accumulate carriers (holes) along the channel region near the drain and decrease the pinchoff region. On the other hand, the LDD region in p-channel MOSFETs reduces HC generation to one-tenth of that in p⁺-drain MOSFETs under the same applied voltage. Moreover, the lifetimes of the LDD MOSFETs defined as how long it takes the transconductance deviation to reach 10% are longer by two orders of magnitude than those of the p⁺-drain MOSFETs under the same HC generation conditions. To obtain the best lifetime, it is shown that the optimum boron dose in the LDD region is between 1 and 3×10¹³ cm^-2