Junction capacitor

Junction capacitors are ceramic capacitors made of ferroelectric ceramic materials with an extremely high relative permittivity and thus have relatively high capacitances . However, they show a strong non-linear dependence of the capacitance on the temperature and on the voltage , high frequency-dependent losses and severe aging. They were standardized as "Class 3" capacitors for ceramic capacitors in the German standard at the time in the 1950s. A current standard for junction capacitors and "Class 3" has not existed since the 1980s.

construction

Junction capacitors are made of ferroelectric ceramic materials such as. B. barium titanate or strontium titanate . The ceramic is produced in a sintering process at temperatures between 1100 and 1400 ° C. However, after firing, this material is subjected to additional post-treatment. The ceramic, the individual spheres of which are caked together by sintering, is made conductive by suitable measures. This can be done, for example, by doping with trivalent antimony , the ceramic being converted into an n-conducting state. Subsequently, an insulating, i.e., insulating layer is applied to the surface of the sintered ceramic beads. That is, an electrically blocking layer is built up, which acts as a dielectric for mini-capacitors within ceramic material. This layer can be produced on the one hand by doping the ceramic with the acceptors copper or iron , p-type zones being built up which form an electrically insulating dielectric if the thickness of this zone is greater than the free path of the charge carriers . On the other hand, the surface of the ceramic beads can be oxidized, a very thin, electrically insulating barrier layer of oxidized ceramic, which acts as the dielectric of the capacitor, is also produced. Both versions of the electrically insulating barrier layers have extremely high relative permittivities. They can have values ​​up to 50000. They are also extremely thin. Since junction capacitors, like almost all capacitors in electronics, are basically plate capacitors, the capacitance of which is greater, the larger the electrode area A and the permittivity ε and the smaller the electrode spacing d , the high relative permittivity and the thin dielectric lead to Capacitors with fairly high capacitance values.

${\displaystyle C=\varepsilon _{0}\varepsilon _{\mathrm {r} }\cdot {\frac {A}{d}}}$

The junction capacitor is created by creating a large number of mini-capacitors as series and parallel connections within the ceramic. These many individual mini-capacitors are then transformed into a barrier layer capacitor by metallizing the ceramic surface. Disadvantages of the junction capacitors are the very high temperature and voltage dependence of the capacitance, the low insulation resistance and the relatively high loss factor, which can reach a value of 0.15.

Structure and functional principle of a ceramic barrier layer capacitor

application

Junction capacitors were listed as so-called "Class 3" capacitors within the family of ceramic capacitors. This capacitor class was only used in applications in which the loss factor and the stability of the capacitance value play a rather subordinate role, e.g. B. in power supplies as filter and buffer capacitors or in the field of signal processing for coupling or decoupling of signals.

Manufacturing status

Junction capacitors can only be manufactured flat as single-layer disc capacitors or round as tubular capacitors. Until about the mid-1990s, with their relatively high capacitance values, they could definitely be found as an alternative to smaller electrolytic capacitors in many applications. However, since the production technology of junction capacitors is not suitable for the production of multilayer capacitors and because higher capacitance values ​​with better electrical properties can now be produced with the ceramic class 2 multilayer capacitors than with the class 3 junction capacitors, junction capacitors are not used today (from 2007) more manufactured.

standardization

A standard for class 3 ceramic capacitors has not existed since the 1980s.

literature

• Otto Zinke , Hans Seither: Resistors, capacitors, coils and their materials. 2nd revised and expanded edition. Springer, Berlin et al. 1982, ISBN 3-540-11334-7 .
• Tadeusz Adamowicz: Handbook of Electronics. A comprehensive presentation for engineers in research, development and practice. Franzis-Verlag, Munich 1979, ISBN 3-7723-6251-6 .