When studying the mechanism of rectification of alternating current in the contact area of two different media - a semiconductor and a metal, the hypothesis was put forward that it is based on the so-called tunneling effect of charge carriers. However, at that time (1932), the level of development of semiconductor technologies did not allow us to confirm the guess experimentally. Only in 1958, the Japanese scientist Yesaki was able to brilliantly confirm it by creating the first tunnel diode in history. Due to its amazing qualities (in particular, speed), this device has attracted the attention of specialists in various technical fields. It is worth explaining here that a diode is an electronic device, which is a combination of two different materials with different types of conductivity in a single case. Therefore, an electric current can pass through it in only one direction. A change in polarity leads to the "closure" of the diode and an increase in its resistance. An increase in voltage leads to a breakdown.
Consider how the tunnel diode works. A classic rectifier semiconductor device uses crystals with an amount of impurities of not more than 10 to the degree of 17 (-3 degree centimeter). And since this parameter is directly related to the number of free charge carriers, it turns out that the latter can never be greater than the specified boundary.
There is a formula that allows you to determine the thickness of the intermediate zone (transition pn):
L = ((E * (Uk-U)) / (2 * Pi * q)) * ((Na + Nd) / (Na * Nd)) * 1050000,
where Na and Nd are the number of ionized acceptors and donors, respectively; Pi - 3.1416; q is the value of the electron charge; U is the summed voltage; Uk is the potential difference in the transition section; E is the dielectric constant.
A consequence of the formula is the fact that the pn junction of a classical diode is characterized by a low field strength and a relatively large thickness. For electrons to fall into the free zone, they need additional energy (communicated from the outside).
The tunnel diode uses in its design such types of semiconductors that change the content of impurities to 10 to the power of 20 (-3 degrees of a centimeter), which differs by an order of magnitude from the classical ones. This leads to a drastic decrease in the thickness of the transition, a sharp increase in the field strength in the region pn and, as a consequence, the appearance of a tunnel transition, when the electron does not need additional energy to get into the valence band. This is because the energy level of the particle does not change when passing through the barrier. The tunnel diode is easy to distinguish from the usual ones by its current-voltage characteristic. The indicated effect creates a kind of surge on it - a negative value of the differential resistance. Due to this, tunneling diodes are widely used in high-frequency devices (reducing the thickness of the pn gap makes such a device fast), accurate measuring equipment, generators, and, of course, computer technology.
Although the current with the tunneling effect can flow in both directions, with the direct connection of the diode, the voltage in the transition zone increases, reducing the number of electrons capable of tunneling. An increase in voltage leads to the complete disappearance of the tunneling current, and the effect is only on the ordinary diffuse current (as in classical diodes).
There is also another representative of such devices - a reversed diode. It is the same tunnel diode, but with modified properties. The difference is that the conductivity value at the reverse connection, in which the ordinary rectifier device “closes”, is higher than with the direct one. The remaining properties correspond to the tunnel diode: speed, low intrinsic noise, the ability to rectify variable components.