Hellwig, Martin Max Planck Institute for Research on Collective Goods, Max Planck Society;
http://www.econometricsociety.org/publications/econometrica/2010/07/01/incentive-problems-unidimensional-hidden-characteristics (Any fulltext)
Hellwig, M. (2010). Incentive Problems with Unidimensional Hidden Characteristics: A Unified Approach. Econometrica, 78(4), 1201-1237.