To aid in the design of low-temperature high-efficiency thermophotovoltaic (TPV) systems, based on low-bandgap photovoltaic (PV) diodes and selective thermal emitters, we developed a detailed model of Gash and GaInAsSb PV diodes. An equivalent circuit model describes the electrical behavior of the diode as a function of the operating point defined by a photocurrent and a junction temperature. The single diode equivalent circuit model has five variable parameters: photocurrent, dark current, ideality, and series and shunt resistance. The model accurately predicts the circuit parameters for a given operating point. In addition, parameterized quantum efficiency is used to calculate photocurrent while accounting for temperature induced bandgap narrowing. Models show very good agreement with experimental results obtained with calibrated blackbody Source. Although we focus oil TPV systems, these methods and results are applicable to solar-thermophotovotaic and concentrator photovoltaic systems. (C) 2009 Elsevier B.V. All rights reserved.