Heterostructure devices have thin layers of alternating materials. Some layers act as barriers to electron flow, but if the layers are thin enough, electrons can "tunnel" through them. If two or more barriers are placed closely together, electrons can reflect between the barriers and resonate at particular energies, allowing complete transmission through the barriers, as if they were not there! This gives rise to negative differential resistance--current that goes down as voltage goes up--an interesting behavior that can be harnessed to form new devices. Use this tool to explore the effects of tunneling through one or more material layers. Change the doping density, material properties, and layer thicknesses, and examine transmission coefficients and current-voltage relationships.