20 V LDMOS optimized for high drain current condition. Which is better, n-epi or p-epi?

This paper discusses whether n-epi or p-epi substrates are better for 20 V range LDMOSFETs. We present four optimized 20 V LDMOSFETs and compare them. The best compromise is the LDMOS on n-epi with a high dose n-implant layer which achieves a sufficiently low on-resistance of 17.2 m/spl Omega//spl middot/mm/sup 2/ and a high static breakdown voltage of 24.0 V without breakdown voltage degradation under large drain current flow conditions. The device on-state breakdown voltage for a 5 V gate voltage is 24.5 V.