In order to explore a possibility for controlling an electron concentration in phosphorus-doped n-diamond, electron and ionized-donor concentrations in n-diamond/cBN and n-diamond/AlN heterojunctions are analyzed by self-consistently solving Poisson and Schrodinger equations. Although the electron concentration is an order of 10(11) cm(-3) at room temperature for single n-diamond with a donor concentration of 5 x 10(18) cm(-3) and a compensation ratio of 0.01, a modulation-doping technique predicts to provide full ionization of phosphorus donor in the n-diamond/cBN heterostructure and generation of an electron concentration larger than 10(18) cm(-3) at room temperature. (c) 2004 Elsevier B.V. All rights reserved.