## 3 Results and Discussion  
The proposed BPF was designed and simulated using the ADS software, fabricated on GaAs substrate with a thickness of 200.1 \(\upmu\)m, a dielectric constant of 12.85, and a loss tangent of 0.006, and packaged using the QFN technique. QFN-packaging technology was used to protect the device from moisture and ensure good thermal and electrical performances. S-parameters are used to describe the electrical performance of linear electrical networks when undergoing various steady state stimuli using electrical signals. In particular, it becomes essential to describe a given network in terms of waves at high frequency instead of voltage or current. An Agilent 8510C vector network analyzer (VNA, Keysight Technologies Inc., Santa Rosa, CA, USA) was used to measure and record the transmission and reflection parameters. Figure 6a shows photographs (Nikon D3300 DSLR camera, Nikon, Tokyo, Japan) of the measurement setup; the magnified view of the aluminum test cube (\(2\times 2\times 2\) cm\({}^{3}\)), the complete PCB (\(2\times 2\) cm\({}^{2}\)), and the packaged chip are shown in Figure 6b-d, respectively. The overall dimensions of the pattern are \(0.8\times 0.8\) mm\({}^{2}\), and the scanning electron microscope (SEM) images were taken to clearly illustrate the manufactured pattern, as shown in Figure 6e. The enlarged air-bridge area and cross-section of the three metal layers are shown in Figure 6f.  
Figure 7a shows the simulated and measured results of the IPD-based BPF. It can be observed that the measured resonant point is located at the frequency of 2.21 GHz, and its 3-dB passband is 1.61-3.20 GHz with a fractional bandwidth (FBW) of 71.94%. The transmission zero is located at the stopband on the right side of the passband, which brings a measured frequency of 6.38 GHz and a magnitude of 30.55 dB. As a considerable figure of merit (FoM) of BPF, the Q-factor is an important indicator to represent the energy lost in the filter, with lower values indicating a smaller loss.  
\begin{table}
\begin{tabular}{c c c} \hline
**Fabrication Objective** & **Technique** & **Material** \\ \hline Passivation layer & PECVD & SiNx \\ Photoresistor & Spin-coating & Negative/positive PR \\ PR removal & Lift-off & Acetone \\ Seed metal & Sputtering & Ti/Au \\ Metal layer & Electroplating & Cu/Au \\ Via & ICP etching & SF\({}_{6}\)/O\({}_{6}\) \\ \hline \end{tabular}
\end{table}
Table 1: Manufacturing techniques used in the IPDs process.  
Figure 6: S-parameter measurement setup of the proposed IPD-based BPF with enlarged views of different parts. (**a**) Measurement setup. (**b**) BPF installed on an aluminum cube. (**c**) Top view of the entire PCB. (**d**) Top view of the packaged chip on the PCB. (**e**) Top view of SEM image. (**f**) Enlarged air-bridge area and cross-section of the three metal layers.  
The Q-factor is 39.17, which can be derived by Y-Parameters [26; 27; 28]. The group delay of the entire passband is always smaller than 0.63 ns, as illustrated in Figure 7b [29; 30].  
Table 2 presents a performance comparison between the proposed BPF and five existing BPFs, indicating that our device has a relatively small chip size, good insertion and return losses, and a relatively wide fractional bandwidth.  
Table 3 presents comparisons with other works using various manufacturing technologies, showing that our GaAs-based IPD BPF possesses an ultra-wide passband and smaller dimensions.