Theory and Operation of Crystal Diodes as Mixers

The electrical parameters (barrier resistance, barrier capacity, and spreading resistance) of a crystal diode are quantitatively related to its fundamental physical properties and geometrical construction. The effects of these parameters on conversion loss at uhf and microwave frequencies are discussed, with particular reference to diodes made from n-type germanium and p-type silicon. Semiconductor materials may be compared for their mixer sensitivity potentialities, by use of the figure of merit N1/2b/¿l/2 (N is carrier concentration in cm-3, b is carrier mobility in cm2/volt sec, ¿ is the dielectric constant) which is herein derived. It shows that n-type germanium is a better mixer material than p-type silicon. A method for minimizing the conversion loss of any given semiconductor is developed. Application of the above method produced germanium diodes having conversion losses which attained the theoretical minimum and silicon diodes having losses 0.8 db above the minimum. It is pointed out that the noise temperature of a mixer is dependent on the conversion loss, and that it rapidly becomes less important as conversion loss is decreased. The optimum receiver noise figure is shown to occur quite close to the operating conditions which minimize conversion loss. It is shown also that the temperature dependency of germanium and silicon mixer diodes are nearly the same. The experimental application of the information in this paper resulted in an X-band receiver with an over-all noise figure of 6 db.