Amazing semiconductor device - tunnel diode

When studying the mechanism of rectification of alternating current in the area of contact between two different media - a semiconductor and a metal, a hypothesis was advanced 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 by experience. Only in 1958 the Japanese scientist Esaki managed to confirm it brilliantly, creating the first-ever tunnel diode. Thanks to its amazing qualities (in particular, speed), this device attracted the attention of specialists of various technical fields. Here it is worth explaining that a diode is an electronic device, which is a combination of two different materials in a single case, with different types of conductivity. Therefore, the electric current can pass through it in only one direction. The polarity reversal leads to a "closing" of the diode and to the growth of its resistance. Increasing the voltage leads to "breakdown".

Consider how the tunnel diode works. The classic rectifier semiconductor device uses crystals with an amount of impurities of no more than 10 to the power of 17 (-3 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 limit.

There is a formula that makes it possible to determine the thickness of the intermediate zone (transition pn):

L = ((E * (Uk-U)) / (2 * Pi * q)) * ((Na + Nd) / (Na * Nd)) *

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 input voltage; Uk is the potential difference in the transition section; E is the value of 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. In order for electrons to enter a free zone, they need additional energy (communicated from outside).

The tunnel diode uses in its design such types of semiconductors that change the impurity content to 10 to the power of 20 (-3 centimeter), which is an order of magnitude different from the classical ones. This leads to a drastic reduction in the thickness of the transition, a sharp increase in the field strength in the region of the pn region and, as a consequence, to the appearance of a tunnel junction, when the electron does not need additional energy to enter the valence band. This is because the energy level of the particle does not change when the barrier passes. The tunnel diode can be easily distinguished from the ordinary ones by its current-voltage characteristic. This effect creates a kind of splash on it - a negative value of the differential resistance. Due to this, tunnel diodes are widely used in high-frequency devices (reducing the thickness of the pn gap makes such a device high-speed), accurate measuring equipment, generators and, of course, computer technology.

Although the current in the tunneling effect is capable of flowing in both directions, with direct connection of the diode, the intensity in the transition zone increases, decreasing the number of electrons capable of tunneling. An increase in voltage leads to the complete disappearance of the tunnel current and the effect is only on the ordinary diffuse (as in classical diodes).

Also there is another representative of similar devices - a reversed diode. It is the same tunnel diode, but with changed properties. The difference is that the conduction value at the reverse connection, in which the ordinary rectifying device "closes", is higher than for the direct one. The remaining properties correspond to the tunnel diode: speed, small intrinsic noise, the ability to rectify the variable components.