## 1 Introduction  
With the development of modern wireless communication systems, passive devices, such as balancers, mixers, and power splitters, play an important role in the field of radio frequency (RF)/microwave. Bandpass filters (BPFs) have been vastly studied in the field of microwave filters, as they form an important block of RF/microwave integrated circuits and systems. In addition, the demand for BPF has continued to increase along with an increasing demand for low cost, a wide usage of miniaturization, high performance, and so on [1]. Therefore, researchers have been exploring processing techniques that lead to high precision, mass production, and low cost. Recent studies have demonstrated many advanced manufacturing technologies, including low-temperature co-fired ceramics (LTCC), monolithic microwave integrated circuits (MMICs), high-temperature superconductor (HTS), micro-electromechanical systems (MEMS), and micro-fabrication techniques, which promote the development and implementation of BPF design. LTCC is composed of a mixture of various passive components such as hybrid circuits, strip lines, filters, antennas, sensors, transformers, and resonators. LTCC is attractive for BPF fabrication owing to its inexpensive process, and the processing metals have high electrical conductivity and low conductor loss. The disadvantage of LTCC is that the size of the ceramic and the processing plate shrink after firing. In addition, modules must have heat sinks due to the requirement of heat dissipation after heating [2; 3]. MMICs based on GaAs and other excellent III-V materials can be used to develop devices operating at the millimeter-wave band, which offers higher functional capabilities, improved system stability, and reduced weight, size, and cost [4]. The main disadvantage of MMIC-based devices is that their performance is inferior compared with that of the device using discrete components [5]. A microstrip BPF based on HTS has a significant advantage in terms of low insertion loss compared with the conventional microstrip BPF owing to its ultra-low surface resistance, leading to a compact and miniaturized design. However, the application of HTS is restricted because of its high cost and complexity as a result of the low-temperature operating environment [6]. MEMSs have become an attractive option for the manufacture of electronic equipment as they can be used for batch production of miniature structures ranging from micrometer to millimeter sizes. A key feature of MEMSs is the ability to faultlessly integrate mechanical and electronic components into one wafer, resulting in reduced cost, size, and weight. Nevertheless, the unreliability of the power-handling capability and ultimate dynamic range significantly shortens the life cycle of MEMS-based devices [7]. Integrated passive devices (IPDs) represent a matured technology that has been widely used in the past few years. IPDs offer a good trade-off between systems packaging and modular solutions, providing better overall system performance by integrating other functions. The IPD has the advantages of being a simplified model, having a reduced size, and enhanced performance, benefiting from the reduction of parasitic effects compared with standard discrete systems [8; 9]. Moreover, the Q-factor used to evaluate device performances is low for lumped elements but can be enhanced using IPD technology as reported in our previous studies [10; 11; 12]. GaAs substrates have become the second most important semiconductor materials owing to their high saturated electron drift velocity, low field mobility, low parasitics, and good interdevice isolation. Thus, they can be employed in various systems, including medical treatment, information storage, communications, military application, lasing, and sensing [13; 14; 15; 16; 17]. In this study, we designed a quad flat no-lead (QFN)-packaged BPF with an outer intertwined metal track, a center dendritic capacitor, and four air bridge structures, which was fabricated using an IPD processing technique on GaAs substrates.