A π Small-Signal Model of MEMS Series Switches Based on the Parameter- Extraction Method

A novel π small-signal model of MEMS series switches is presented based on a parameter-extraction method, which can determine a fringing capacitance (C_f) in the end of each signal line as well as a contact resistance (R_c). For an on-state modeling, the average error of S₁₁ for the π model is 5.1% in the range of 5~35 GHz, whereas that of the conventional model of two-port series-impedance without C_f is 167.2 %. It is noted that the conventional model is not valid anymore for the on-state model and C_f should be taken into account. The small-signal modeling by the proposed π model demonstrates the validity of the approach.

1. Introduction

Metal-contact microelectromechanical-system (MEMS) series switches have demonstrated much better isolation and insertion loss compared to those of solid-state devices [1], [2]. Recently, the MEMS switches have been applied to the dual-path power amplifier for wireless communication systems [3] and to 4-bit true time delay 's for telecommunication systems [4]. For their integrated circuit applications, an accurate electrical model is critical for designing an optimized device with respect to a particular application and evaluating the performance of the whole system, including discrete devices. Numerical approaches for an electrical modeling of MEMS series switches were discussed using electromagnetic simulators [1], [3]. They may predict almost accurately simulated results, but the approaches cannot provide proper equivalent circuits, with which small-signal performances of the devices can be characterized. On the other hand, two-port series-impedance models of MEMS series switches were presented in [2], [4], where off-state upper capacitance (C_op) and on-state resistance (R_on) were mainly used for the model parameters; however, the on-state two-portseries-impedance models could not predict exactparameters due to the difficulty in obtaining physicallybased R_on and C_f. Therefore, other approaches should beused for a proper modeling. One of the methods fordetermining R_on and C_f is the parameter-extraction method(PMD), which has been widely used in analyticalapproaches for equivalent-circuit parameter extractions ofsolid-state devices, such as high electron mobilitytransistors (I1EMTs) or hetero-junction bipolar transistors(HBTs) [5]–[7]. In this paper, a newly proposed π small-signal model of metal-contact MEMS series switches ispresented, which is based on the parameter-extractionmethod. Also, to demonstrate a validity and accuracy of the proposed model. modded S-parameters of the π modelare compared and discussed with those of theconventional model. Figure 1.

Micrograph and schematic cross-sectionalviews of a MEMS series switch with a membrane area of 100 × 400 um2.

Figure 2.

Structure-based π small-signal model of metal-contact MEMS series switches