A-J Plane Analysis: A Technique For Active Diode Design

Abstract

1. INTRODUmON The design of nonlinear transit-time microwave and millimeter-wave semiconductor diodes (such as IMPA?T’s) is often based on simple scaling ideas, sometimes assisted by large-signal time-domain computer simulation. It is often found that simple scaling of structures from one frequency to another does not lead to optimum results. In addition, anticipated performance improvements from design changes (e.g. use of different structures, materials, heatsinking, etc.) are often not achieved. Large-signal time-domain simulation can be used to assist the: design of active microwave and millimeter wave diodes. However such simularion prov~des a per unit area characterization of specific diode structures. It yields efficiency data, output power per unit area, and impedance per unit area, but does not, by itself, predict optimum areas and maximum output powers. A-J plane analysis has been developed to overcome the above problems. It is a rapid graphical procedure which provides good physical insight. A-J plane analysis may be used on a stand-alone basis, to permit rapid assessment of the likely effect of proposed design changes (e.g. different structures, different materials, or different heatsinking arrangements). The combination of A-J plane analysis with large-signal time-domain simulation provides a complete CAD capability for design and optimization (of active diodes. The central idea of A-J plane analysis is to establish the limits of allowed combinations of area (A) and DC cumnt density (4. Allowed combinations an: limited by several mechanisms, including: thermal limitations; space-charge-induced field perturbations; and various impedance limitations. Each of these limitations gives rise to a boundary in the A-J plane between allowed and disallowed combinations of area and cimnt density. The location of each boundary line is a function of material, saucture, and frequency. A-J plane diagrams are figures showing all the A-J plane constraints. The area and DC current density combination corresponding to maximum input power, and the factors limiting the input power, are easily established using A-J plane diagrams. The utility of proposed design changes is easily established by constructing a modified A-J plane diagram and seeing whether the new design permits significantly higher input powers. The organization of the paper is as follows. The mapping of individual design constraints on to the A-J plane is described in the next section. Stand-alone interpretation and use of A-J plane diagrams is discussed in section 3. Examples are provided showing how to assess the influence of changes of heatsinking arrangement, structme, and semiconductor material. Complete design procedures using large-signal timedomain computer simulation in conjunction with A-J plane analysis are discussed in section 4.