Comparative Analysis of High Frequency Characteristics of DDR and DAR IMPATT Diodes

IMPATT (IMPact Avalanche ionization and Transit Time) diodes are principal active elements for use in millimetric generators. Semiconductor structures suitable for fabrication of continuos-mode IMPATT diodes have been well known for a long time (Scharfetter & Gummel, 1969; Howes & Morgan, 1976). They have been utilized successfully in many applications in microwave engineering. The possibilities of using the same structures for pulsed-mode microwave generators were realized too. Considering, that the increase of the output power of millimetric generators is one of the main problems of microwave electronics; it is important to optimize the diode's active layer to obtain the generator maximum power output. From the beginning (Read, 1958) the main idea to obtain the negative resistance was defined on the basis of the phase difference being produced between RF voltage and RF current due to delay in the avalanche build-up process and the transit time of charge carriers. The single drift region (SDR) and the double drift region (DDR) IMPATT diodes are very well known (Scharfetter & Gummel, 1969; Howes & Morgan, 1976; Fong & Kuno, 1979; Chang, 1990) and used successfully for the microwave power generation in millimeter region. The transit time delay of both types of diodes is the essential factor of the necessary phase conditions to obtain negative resistance. The typical DDR diode structure is shown on Fig. 1(a) by curve 1, where N is the concentration of donors and acceptors, l is the length of diode active layer. In this type of diodes, the electrical field is strongly distorted when the avalanche current density is sufficiently high. This large space charge density is one of the main reasons for the sharp electrical field gradient along the charge drift path. Because of this field gradient, the space charge avalanche ruins itself and consequently the optimum phase relations degrade between microwave potential and current. This factor is especially important when the IMPATT diode is fed at the maximum current density, which is exactly the case at the pulsedmode operation. The idea to use a complex doping profile semiconductor structure for microwave diode was originally proposed in the first analysis of IMPATT diode by (Read, 1958). This proposed ideal structure has never been realized till now. However, a modern semiconductor technology provides new possibilities for the fabrication of sub micron semiconductor structures with complex doping profiles. This stimulates the search for IMPATT-diode special structure's optimization.