Photoelectrical properties and the spectrum of the avalanche multiplication factor versus the reverse voltage at room temperature have been studied for LPE grown for InP-In(x)Ga(1-x)AsyP(1-y) heterostructures. 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 results found for the lowering of the tunneling current in the pre-breakdown region. The temperature coefficient of the breakdown voltage beta = (1/V-B).(dV(B)/dT) > 0 was determined in the temperature range 77-300 K and its value was found to be beta = 5.78 x 10(-4) K-1.