Effect of separation and depth of N+ diffusions in the quality factor and tuning range of PN varactors

Abstract

Variable capacitors, the varactors, are key components in many types of radiofrequency circuits and thus high quality varactors are essential to achieve high quality factors in these devices. This work presents results of a study on the variation of tuning range and quality factor when varying the depth and separation of N+ diffusions in a PN junction varactor with fixed number of cells. For test needs four types of cells, varying the geometry of N+ and P+ diffusions were designed. The varactors were formed by horizontally and vertically overlapping cells. Based on their implementation structure, the varactors were divided into two groups, each comprising 4 varactors. The varactors belonging to the first group have all N+ diffusions connected to the buried layer. Varactors from the second group use floating N+ diffusions and a buried N+ diffusion to separate pairs formed by two adjacent cells. Post implementation measurements show that the area of varactors from in the first and second group is 1795.74 μm2(51.9 × 34.6) and 1288.92 μm2 (46.7 × 27.6), respectively. The varactors from the 1st group have a high tuning range, whereas the ones from the 2nd group high quality factors and require less area. © 2011 SPIE.