What is the Esaki diode and how does it work?
The Esaki diode, or tunnel diode, is a semiconductor device that exploits quantum mechanical tunneling. Let us consider the evidence: in a heavily doped p-n junction, the depletion region becomes extremely thin—on the order of 10 nanometers. This allows electrons to tunnel directly from the valence band of the p-side to the conduction band of the n-side when a small forward bias is applied. The tunneling current initially increases with voltage, but then decreases as the bands misalign, producing a region of negative differential resistance. This is a direct consequence of the Pauli exclusion principle and the band structure of the semiconductor. The data suggest a remarkable phenomenon: the current can decrease while voltage increases, enabling the diode to act as an oscillator or amplifier at frequencies up to 100 GHz. It was a serendipitous discovery—I was simply studying narrow junctions and found something unexpected.