Double-drift Silicon IMPATT´s at X band

The optimization of double-drift silicon IMPATT\´s at X band has been explored. This work has involved the fabrication and study of Read-like diodes with various finite avalanche widths made by various energy implantations between two epitaxially grown "drift" zones. These diodes are compared to uniformly doped double-drift diodes. As a result of this work, state-of-the-art powers have been obtained in X band (6.8-W maximum). However, the efficiencies are only slightly greater than those obtained with single-drift diodes (12 versus 10 percent). The efficiencies are correlated as a function of avalanche-to-drift-voltage ratio Va/Vd. This correlation is in agreement with the semiquantitative large signal theory of Scharfetter and Gummel. The inability to get higher efficiencies is clearly shown to depend upon the limited range of implanted impurities obtained with 300-kV implantation equipment. A design window, centered around Va/Vd∼ 1.1 for silicon is presented and accounts for the smaller efficiencies. This window shows the tolerances that are required for the doping and its distribution to obtain optimum efficiency. It is predicted that efficiencies of 21 percent can be obtained for a properly made silicon double-drift Read-like diode, but even this value would still be less than the predicted maximum efficiency for a single-drift n-GaAs Read-like diode (38 percent). The higher output powers were obtained with ∼ 12 percent efficient double-drift diodes, with junction areas 4 times larger than utilized in single-drift diodes. This maintenance of relatively high efficiency with large areas is discussed in terms of an application of Gewartowski\´s circuit-diode theory to double-drift diodes.