Microwave Semiconductor Diodes

Microwave circuits widely use low-power point-contact semiconductor diodes. As a rule, microwave semiconductor diodes are coaxial in structure (Fig. 3.19) so that they can conveniently be mated to coaxial lines or waveguides.

Fig. 3.19. Coaxial microwave diodes

Fast switching of microwave circuits that handle high power uses p-i-n junction diodes ordinarily made of silicon. With no external voltage applied, the resistance of the i -region is high, and it grows still more under reverse bias. Forward bias steps up the injection of holes from the p -region and of electrons from the n -region into the i -region. As a result, the resistance of the i -region and of the diode as a whole is abruptly reduced (by a factor of 10³ - 10⁴).

Fig. 3.20. Structure of a p – i – n junction diode Fig. 3.21. Switching circuit using p – i – n diodes

A more recent trend with regard to switching and other functions in microwave circuits has been to use Schottky diodes. Schottky diodes have a high operating speed which solely depends on the transit time of electrons through the high-resistivity region of semiconductor (less than 10^-11s) and on the barrier (or junction) capacitance which can be made very low. Schottky diodes are able to operate at frequencies as high as 15-20 GHz.

Дата добавления: 2015-11-14