A new measurement technique employing a positively sloping load line has been used to probe the region of apparent bistability near a tunneling resonance in the electrical characteristic of a resonant tunneling diode. This technique is equivalent to using a voltage source and negative series resistance. The appearance of bistability is an artifact of the conventional measuring technique which uses a load line with negative slope. The complete characteristic is found to be a continuous Z shaped curve between 20 and 150 K, corresponding to tristability and in accordance with theoretical models based on the effects of charge accumulation in the central quantum well of the diode. The width of the tristable region passes through a maximum at 40 K and, at 150 K, disappears as the resonance broadens. Above this temperature the resonance develops a region of negative differential resistance (NDR). As the device is cooled below 20 K additional structure develops in the central arm of the Z, with some portions of the characteristic exhibiting five stable current states at temperatures below 15 K. At 4.2 K, the effect of an in plane magnetic field mimics that of increasing temperature. © 1994.