# A 37-40 GHz 6-Bits Switched-Filter Phase Shifter Using 150 nm GaN HEMT
## 1 Introduction  
With the emergence and widespread adoption of the 5 th generation communication (5G), the 3 rd Generation Partnership Project (3 GPP) has established millimeter wave frequency bands, such as n257 (26.5-29.5 GHz) and n260 (37-40 GHz), as standards for 5G New Radio (NR). These defined frequency bands are now being adopted by numerous countries, necessitating the support for mmWave systems to enable cross-network roaming of User Equipment (UE) devices [1; 2]. In high-frequency wireless communication systems, phased arrays play a crucial role in effectively concentrating the transmitter output power. It is crucial to provide appropriate phase and amplitude control signals to individual array elements [3]. Phased array systems utilize beam steering techniques to enhance the signal-to-noise ratio (SNR) and achieve spatial selectivity [4]. Phase shifter (PS) is one the most important elements in a phase array system. To facilitate beam steering, it is essential to have phase shifters at each Radio Frequency (RF) front-end. These phase shifters adjust the phase according to the control signal, enabling coverage of up to 360 degrees of phase adjustment. Fluctuations in gain and input/output reflection within different phase states affect channel quality. Moreover, when considering the number of channels, power consumption, and die size, a phase shifter must achieve a balance among phase resolution, die area, and input/output reflection to ensure optimal performance, efficiency, and cost [5]. The GaN (Gallium Nitride) process finds extensive application in high-power, high-frequency scenarios. Gallium Nitride (GaN), known for its higher breakdown strength, faster switching speed, enhanced thermal conductivity, and lower on-resistance, has led to the development of power devices that outperform their silicon-based counterparts [6; 7]. These attributes make GaN highly versatile in high-power, high-frequency applications. GaN-based phase shifters are currently in development, aiming for seamless integration into other RF circuits, particularly high-power amplifiers (PAs). These GaN-based phase shifters demonstrate exceptional characteristics, including high-power handling capacity, outstanding efficiency, and robust linearity [8; 9; 10]. Phase shifters can be categorized into two main types: passive phase shifters [5; 11; 12; 13; 14; 15; 16; 17; 18] and active phase shifters [19; 20; 21]. Active phase shifters offer a signal gain advantage, but their gain stage must meet stringent linearity requirements in an RF front-end. While this linearity can be improved by increasing DC power consumption, it comes at the cost of reduced power efficiency. Active phase shifters rely primarily on a vector summing technique, and any phase or amplitude mismatches can significantly degrade their accuracy. Refs. describe vector sum phase shifters, which often exhibit high-power consumption [19; 20; 21]. On the other hand, passive phase shifters have distinct advantages, including minimal power consumption and exceptional linearity. When integrated at the transmit/receive (TRx) points within a phased array system, their bidirectional capability can reduce the total number of required phase shifters through efficient sharing [22]. However, it is worth noting that passive phase shifters may introduce some insertion loss and often require multiple bits to achieve the desired phase resolution. In conclusion, passive phase shifters are a suitable choice for high-power RF systems where linearity is critical. The passive phase shifter (PS) can be categorized into several types: transmission line type [16], load-adjusted reflective type [17; 18], all-pass network type [11], passive vector sum modulator [5], and switched filter type [12; 13; 14; 15]. Among these topologies, the transmission line type occupies a larger chip area due to considerations of wavelength. In contrast, the load-adjusted reflective type of passive PS boasts a compact design; however, its single-order structure places limits on achievable phase-shifting ranges.