Negative resistance heterojunction devices

A heterojunction optical detector responsive to radiant energy exceeding a predetermined magnitude to exhibit a change in its impedance characteristic comprising:

• first and second semiconducting layers having predetermined energy gaps between their conduction and valence bands, said layers exhibiting low mobility and being doped to have charge carriers;
• potential means connected to said first and second layers and adapted to enable a current to flow through said first and second layers when said layers are in a low impedance state;
• a third high mobility semiconducting layer sandwiched between said first and second layers and having a lesser energy band gap than said first and second layers, said charge carriers from said first and second layers residing in said third layer in the presence of radiant energy less than said predetermined magnitude, the application of radiant energy exceeding said predetermined magnitude causing sufficient energy to be imparted to said charge carriers in said third layer to move them into said first and second layers causing said first and second layers to attain said low impedance state.