Dielectric Electronics

a field of physics concerned with the study and practical application of phenomena associated with the passage of electrical currents through dielectrics. The concentration of conduction electrons or other free charge carriers (holes or ions) is negligible in dielectrics; because of this, until recently, dielectrics were used in electrical and radio engineering only as insulators. Research on thin dielectric films showed that upon contact with metals, electrons or holes pass into the dielectric, resulting in the appearance of an appreciable number of free charge carriers in a thin layer of the dielectric near the contact. If a thin dielectric film (usually 1–10 microns) is placed between two metal electrodes, the electrons emitted from the metal fill the entire thickness of the film, and when a voltage is applied to such a system, a current is created through the dielectric.

In 1940 the English physicists N. Mott and R. Gurney predicted theoretically the possibility of controllable emission currents flowing through a dielectric. Dielectric electronics studies the flow in dielectrics of currents that are limited by a space charge in cases of thermionic emission from metals and semiconductors and tunnel emission.

The simplest dielectric electronic device is the dielectric diode, which consists of a metal-dielectric-metal sandwich (Figure 1). In many ways it is similar to an electrical vacuum diode and is therefore called the analogous diode. Its rectifying action is due to the difference in the work functions of electrons from electrodes made of different metals. A metal that has a small work function for electrons passing from the metal into the dielectric (a fraction of an electron volt [eV]) is used for one electrode, the source (the analogue of the cathode); a metal with a large work function (1-2 eV) is used for the other electrode, the drain (the analogue of the anode). Consequently, substantial currents occur in one direction, but they are infinitesimally small in the other. The rectification factor of a dielectric diode reaches values of 10⁴ and higher.

Figure 1. Dielectric diode known as a sandwich structure

The construction of a dielectric triode involves the technological problems of positioning a control electrode— the gate (the analogue of the grid in an electrical vacuum triode)—in the thin dielectric layer between the source and the drain. In one type of triode emission takes place from an n-type semiconductor with electron conductivity into a high-resistance p-type semiconductor with hole conductivity that acts as a dielectric (Figure 2). The low-resistance regions formed from P⁺-type semiconductor with high hole conductivity play a role much like that of the metallic cells of the grid in an electrical vacuum triode. The external voltage applied to these regions controls the amount of current flowing between the source and the drain.

Figure 2. Horizontal section of a dielectric triode with a built-in grid: n is a semiconductor with electron conductivity, p is a dielectric (high-resistance semiconductor with hole conductivity) in which the emission of electrons occurs, and P⁺ are low-resistance semiconductor regions with hole conductivity through which electrons do not pass

In another type of triode (Figure 3), the gate is located outside a CdS dielectric; its role is reduced to altering the potential distribution in the dielectric on which the current value is largely dependent. The physical picture in these triodes is considerably more complicated and differs essentially from the flow of emission electrons in a vacuum. Triodes with an insulated metal-oxide-semiconductor or metal-dielectric-semiconductor gate have become commonplace.

Figure 3. Structure of a triode with an insulated gate

In dielectric electronic devices the advantages of semiconductor and electrical vacuum devices have been successfully combined, and many of their drawbacks are absent. They are microminiature devices. The development of emission currents in dielectrics requires no expenditure of energy to heat an emitting electrode, and the problem of heat removal is not encountered. Dielectric devices have quick response, good frequency characteristics, a low noise level, and low sensitivity to temperature changes and radiation.