Read the text. A tunnel diode is a two-terminal negative resistance device which can be employed as an amplifier, an oscillator

The tunnel diode

A tunnel diode is a two-terminal negative resistance device which can be employed as an amplifier, an oscillator, or a switch. Because of its very fast response to inputs, it is almost exclusively a high-frequency component. Tunnel diodes require smaller bias voltages and lower load resistances than most other electronic devices.

The width of the depletion region at a pn-junction depends upon the doping density of the semiconductor material. Lightly doped material has a wide depletion region, while heavily doped material has a narrow region. In the case of the tunnel diode, the junction is formed of very heavily doped material, and consequently the depletion region is very narrow.

The depletion region is an insulator since it lacks charge carriers, and usually charge carriers can cross it only when the external bias is large enough to overcome the barrier potential. Barrier potentials are approximately 0.7 V for silicon and 0.3 V for germanium. However, because the depletion region in a tunnel diode is extremely narrow, it does not constitute much of a barrier to electron flow. Consequently, a small forward or reverse bias (not large enough to overcome the barrier potential) can give charge carriers sufficient energy to cross the depletion region. When this occurs, the charge carriers are said to be tunneling through the barrier.

When semiconductor material is very heavily doped with holes (i.e., p-type) there is a shortage of electrons and the valence band cannot be regarded as filled. The result is that at the top of the valence band there is a layer of empty energy levels. With very heavily doped n-type material, there is an abundance of electrons. Consequently, electrons fill the valence band and create a layer of filled levels at the bottom of the conduction band.

The depletion region is very narrow and that the filled levels on the n-side are exactly opposite those on the p-side. In this condition, no tunneling occurs because there are no empty lower energy levels to which electrons from either side might cross the depletion region. Note also that the conduction and valence bands on the p-side are higher (negatively) than those on the n-side. This is a result of the depletion region and barrier potential being created by electrons crossing from the n-side to the p-side. The n-side lost negative charges and the p-side gained them. When the junction is reverse biased (negative on the p-side, positive on the n-side), filled energy levels on the p-side are opposite empty energy levels on the n-side. The result is that electrons tunnel through the banner from the higher-energy levels on the p-side to the lower levels on the n-side. Despite the fact that the junction is reverse biased substantial current flows. With increasing reverse bias more electrons tunnel from the p-side to the n-side and a greater current flows. Therefore, the reverse characteristic of a tunnel diode is linear, just like that of a resistor.