Electrical properties of MOVPE grown HgCdTe layers on KOH pretreated GaAs substrates, were investigated. Rinsing the GaAs substrates with KOH, or NaOH in water solution before the growth have been used to reduce surface morphological defects (hillocks) density. It was found that the electrical properties depended on out diffusion of potassium during the growth. Ten micrometer thick annealed Hg0.7Cd0.3Te layers are measured to be p-type with p(77) similar to mid 10(15) cm(-3) and this is believed to be related to the average potassium concentration in the HgCdTe layer. SIMS analysis has shown that considerable amounts of potassium are incorporated at the interface of HgCdTe/CdTe and CdTe/GaAs. But a typical potassium concentration of as grown HgCdTe at surface is measured to be around low 10(14) cm-'. We could effectively reduce the residual potassium concentration and obtain HgCdTe with p-type carrier concentration of mid 10(14) cm(-3) by inserting additional CdTe/HgCdTe layer before the growth of HgCdTe layer. This result significantly differed from the case of sodium pretreated sample, which shows the sodium concentration of mid 10(16) atoms cm(-3) throughout full layer. Our result shows that MOVPE grown HgCdTe on KOH treated GaAs substrates could be used to fabricate large area forcal plane arrays which need hillock free surface as well as good electrical properties. (C) 2004 Elsevier Ltd. All rights reserved.