Impact of multi-finger MOSFET geometry on the electrical performance of RF circuits

Abstract

The increasing demand of low-cost, low-power and high-efficiency integrated systems is making more complex to design radio frequency (RF) analog circuits. Using multi-finger (MF) Metal-Oxide-Semiconductor (MOS) Field Effect Transistors (MOSFETs) is an attractive technique to optimize the circuit performances. It reduces the silicon area, the gate resistance and parasitic capacitances compared to the single finger MOSFET, which mainly impact the high frequency and noise performances. However, selecting the optimal number of fingers remains a challenging issue. This paper investigates the number of fingers (nf) influence on the transistor parameters and assesses its effect on several key functions in RF transceivers. The study focuses specially on the obtained performances of civil RF circuits, implemented in 130 nm CMOS technology, as a function of nf. First, design of a differential RF bandpass filter (BPF) is presented. The results show that using multi-finger MOSFETs leads to reductions in chip area by 66.5%, in power consumption by 15% and in noise figure by 43% with an improvement in linearity and frequency range compared to the conventional approach. Then, an inductorless LC-Voltage Controlled Oscillator (VCO) and a Low Noise Amplifier (LNA) operating around 2.4 GHz have been designed using three different configurations depending on nf. Obtained results display an improvement in the area, power gain, frequency and noise performances by applying the multi-finger optimization, and show that keep increasing nf can degrade the stability, linearity and power consumption. The proposed circuits are also tested through Monte Carlo simulations confirming their robustness to process and mismatch variations. Detailed analytical comparison between the different proposed circuits and configurations of nf proves that the MF technique is reliable when nf is inferior to 5.