The dependence of reverse-biased leakage current on both voltage and temperature for InP-InxGa1-xAsyP1-yDH (double heterostructures) has been analyzed. We find that at the whole of the temperature range and at a wide range of reverse bias voltages, the reverse current varies exponentially with applied voltage, indicating that the band-to-band tunneling current mechanism prevails. An agreement is obtained between theory and experimental results. The tunneling current becomes substantial at peak junction electric fields as low as 10(5) V/m due to the small direct energy gaps and small effective masses of the structures tested. The process of breakdown in the investigated structures was of the avalanche type. The impact ionization coefficients in InxGa1-xAsyP1-y have been experimentally determined for composition x = 0.68.