Since World War II, the point-contact diode has been the most useful converter of microwave signals to lower frequencies. Now, a new device—the Schottky-barrier diode—is claimed to offer improved electrical efficiency and higher reliability for this and other applications. A comprehensive evaluation of both devices leads to the conclusion that each has its place in microwave design. Here are the facts.
The manner in which the construction of the Schottky diode differs from the point-contact device is shown in Fig. 1.
Both the point-contact and Schottky diodes consist of a die of semiconductor material on which an epitaxial layer is deposited. The point-contact diode uses a metal whisker to make pressure contact against the epitaxial layer, forming the rectifying junction. The Schottky diode has an additional oxide layer deposited over the epitaxial layer. A “window” is photo-etched through the oxide layer to the epitaxial layer through which a metal contact is then deposited. Thus, the main difference between these devices is the pressure contact used in the point-contact diode compared with the deposited contact in the Schottky diode. The pressure contact in the point-contact diode can damage the junction, depending on the amount of pressure exerted on the contact, while the Schottky diode lends itself to better control and more repeatable fabrication.
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At the semiconductor-to-metal interference an energy barrier (Schottky) exists due to the differences in work functions of the two materials. This barrier level decreases with forward bias and increases with reverse bias, thus making rectification possible. During forward bias the majority carriers (electrons) are injected from the semiconductor to the metal where they are at a higher energy level than the metal-free electrons. Once in the metal, the electrons then give up energy very fast, becoming free electrons. This process occurs with very little flow of minority carriers from the metal to the semiconductor. In this form, a fast response to change in bias is achieved.