Abstract Heterojunctions of threshold-type chalcogenide glasses deposited onto both crystalline n-GaAs and crystalline n-InP have been fabricated and studied. Once the glass is switched into the on state, very asymmetric behavior is observed in both cases. Under forward bias, the devices exhibit low resistance, but strong current saturation occurs under reverse bias. No microwave gains or losses have been detected. The properties are explained by a consideration of the expected results when an N-type negative-differential-resistance device is placed in series with an S-type negative-differential-resistance device. A band diagram for the heterojunctions has been determined. The investigation of these heterojunctions has led to the new conclusion that a high field near the anode exists in the on state of the glass.