I. Introduction
The market of wireless transmission systems is growing and new industrial, scientific and medical bands will be available for new applications in the millimeter wave (MMW) frequency range. In parallel, low power silicon technologies such as CMOS 28nm FDSOI offer transistors with transition frequency ( greater than 100 GHz and a very high density of integration for digital blocs. This allows the realization of fully integrated radio frequency (RF) analog circuits on silicon, which is a key feature for MMW communication system. In the design of high performance RF transceiver, varactors with high quality factor are requested. Therefore, for higher frequency applications such as the MMW ones, the quality factor of the LC tank in VCOs is now limited by the varactor Q-factor instead of the inductor one. As accumulation MOS varactors are widely adopted for the increasingly popular CMOS-based MMW systems, the improvement of their Q-factor is critically required. However, there have been important efforts reported to improve it [4]–[6]. This letter proposes an accumulation MOS varactor integrated on the ST Microelectronics CMOS 28 nm FDSOI technology targeted as low power to serve battery operated and wireless applications. The varactor is processed on hybrid area [2] (without FDSOI) with high N-well doping density. This implies having naturally a relatively low channel resistance and consequently a relatively high Q factor at MMW frequencies.