In this paper, we developed a 2-port dual-band and dual-polarization multiple-input-multipleoutput (MIMO) antenna for fifth-generation (5G) mobile applications at 28 and 38 GHz. The antenna was fabricated using a double-layer Rogers TM RO4003 substrate with a footprint of 3.77 λ 0 × 1.86 λ 0 (λ 0 being the free-space wavelength at 28 GHz). The aperture-coupled patch configuration fed by a coplanar waveguide (CPW) transmission line is adopted as the elementary antenna. The top patch and the coupling aperture are designed to radiate at 38 GHz and 28 GHz respectively for dual-band operation. Further enhancement of the directivity at 28 GHz is achieved through the top patch acting as a parasitic director. A 2-by-4 array antenna for MIMO application was then constructed through physical rotation of the single element at +45 and −45 degrees to create polarization diversities. The 2-port MIMO antenna realized using this configuration demonstrated an impedance bandwidth of S11<−10 dB covering 27. GHz, a gain of greater than 8 dBi, isolation of more than 20 dB and a cross-polarization level of greater than 20 dB. The important MIMO parameters such as ECC, DG, TARC, and MEG have also been characterized and presented.

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In this letter, AlGaN/GaN HEMTs with tall-gatestem structures were realized to improve the power performance of Ka-band devices, and a film thinning process is adopted in the fabrication process to reduce the parasitic capacitance caused by the thick silicon nitride film. According to the S-parameter measurement results, devices owning a tall-gate-stem structure and undergoing the film thinning process have higher cut-off frequency (fT) and maximum oscillation frequency (fmax) values with lower extracted parasitic capacitance. For the load-pull measurement result, the AlGaN/GaN HEMT with a tall gate stem has improved output power density (Pout) and power added efficiency (PAE) at Ka-band. The device with the elevated stem shows a steady-state current density of 883 mA/mm and a maximum transconductance of 323 mS/mm at 20 V bias, and it achieves fT of 39.5 GHz, fmax of 112.9 GHz with the maximum PAE of 24.6 % and the maximum Pout of 6.6 W/mm at 38 GHz.Index Terms-AlGaN/GaN HEMT, tall-gate-stem structure, output power density, power added efficiency I. INTRODUCTIONAN-BASED high electron mobility transistors (HEMTs) have become popular microwave power devices in recent years. The GaN material has several remarkable properties, such as a wide bandgap of 3.4 eV, high breakdown electric field, high electron mobility, and high saturation electron drift velocity. These features enable GaN devices to provide high output power under high-frequency operation. With the development of fifth-generation communication (5G), which requires high-power devices for Ka-band, GaN material will be essential because it can operate at high frequency with excellent power performance [1] - [13].When operated at high frequency, device performances such as cut-off frequency (fT), maximum oscillation frequency (fmax), output power density (Pout), and power-added efficiency (PAE) suffer from parasitic effects. The parasitic capacitance is mainly

This paper presents the bandwidth improvement for dual-band power divider using complex isolation network while maintaining physical port separation. The conventional port-extended power dividers suffered from narrow system bandwidth. A rigorous analysis revealed that such problem was mainly due to the limited impedance bandwidth caused by the odd-mode bisected network. Moreover, the isolation bandwidth provided by the parallel L-C topology in the conventional approach was also limited. To overcome such technical issues, a serial L-C topology was proposed. Derivations of the impedance bandwidth through even- and odd-mode network analysis have been performed and optimal system bandwidth could be achieved when the reflection coefficients of the corresponding bisected networks exhibited minimum frequency dependence. Based on the theoretical analysis, simultaneous achievement of bandwidth broadening, size compactness, and physical port extension at both frequencies is possible with optimum combinations of the design parameters. The experimental results evidenced that other than the improvement in system bandwidth, the fabricated prototype featured low extra insertion loss, good isolation across the bands, and compactness in size while maintaining physical separation between the split ports compared with previously published works.

This study reports AlGaN/GaN high-electron-mobility transistors (HEMTs) fabricated by the Stepper Lithography on a 4-inch wafer for Ka-Band applications. Small gate length (L G ) of 100 nm was achieved through a 2-Step Photolithography Process and the gate region of the AlGaN/GaN HEMT was defined by using two lithography steps to form gamma-shaped gates. The 4-inch AlGaN/GaN HEMT wafer demonstrated high electrical performance uniformity with respect to the maximum drain-source current density (I DSS ), the peak extrinsic output transconductance (G m ), and the threshold voltage (V th ). At V DS = 20 V, the AlGaN/GaN HEMT exhibits an I DSS of 1004.2 mA/mm, a Gm value of 363.6 mS/mm, a maximum output power density (P OUT (MAX) ) of over 10 W/mm, and a power gain of 8.8 dB with a maximum 51.1% Power-added efficiency (PAE) at 28 GHz in Continuous Wave (CW) mode. The results show the potential of AlGaN/GaN HEMT fabrication with high yield and outstanding RF performance using Stepper Lithography for 5G applications.INDEX TERMS 5G, high uniformity, output power, stepper lithography, small gate length.