# A 32-GHz Eight-Way Power Amplifier MMIC in 150 nm GaN HEMT Technology
## 3 Simulation and Measurement Results  
The proposed 32 GHz power amplifier was designed in 150 nm GaN HEMT technology with an \(f_{T}\) of 34.5 GHz. Figure 6 shows a photograph of the implemented amplifier and the chip on board (CoB) for the measurement purpose. The implemented MMIC occupies an area of 4.3 mm \(\times\) 4.5 mm (19.35 mm\({}^{2}\)), including all the RF, ground, and DC pads. In order to construct a reliable CoB, there are various technicalities to consider. While epoxy is the most widely used die-attach material during packaging, special attention should be paid to the removal of voids or fumes generated during the hardening process of the epoxy, which can cause instability of the MMIC amplifiers. We used eutectic bonding as the die-attachment material to avoid undesired instability due to the parasitic capacitance from the voids formed in the epoxy. Eutectic bonding utilizes the properties of alloys that can melt at low temperatures to help handle sensitive products at high temperatures, such as integrated circuits. In this work, a preform with a composition of 80% gold and 20% tin (80 Au20 Sn) was used. The high solubility between the materials of the eutectic agent enables strong binding and stable connections. It also possesses excellent electrical and thermal conductivity, making it suitable for applications that involve a large amount of heat generation and dissipation. Figure 5: Load-pull simulation result of the 8 \(\times\) 50 \(\upmu\)m OSV HEMT. The external bias circuitry in the board comprises single-layer capacitors, ferrite beads, chip capacitors, and resistors. A large inductance generated by a relatively long bonding wire also heats the stability of the HPA under test due to the increased reactance in the path of the bias lines. To mitigate this effect, single-layer ceramic capacitors were placed in the vicinity of the DC pads of the packaged MMIC. In the implemented board, single-layer capacitors of 100 pF and 300 pF were employed for external gate and drain bias lines. To ensure the stability of the bias line and reduce noise from the power supply, ferrite beads were used to provide high series resistance at high frequencies, which dissipates the noise power in the form of heat. In implementing a reliable external biasing circuitry, multiple chip capacitors with a different self-resonance frequency (SRF) are used with de-Qing resistors in series (a combination of a 0.1 \(\upmu\)F shunt capacitor, a 1 \(\upmu\)F capacitor with a 3 \(\Omega\) de-Qing resistor, a 50 \(\upmu\)F capacitor with a 20 \(\Omega\) de-Qing resistor). While improving the stability of the MMIC HPA by adding series resistors in the gate bias line, it is important to note that an excessively high series resistor in the gate bias line may lead to a more positive gate bias voltage due to the gate leakage current. Thus, a proper resistor value should be selected. Here, we placed a 50 \(\Omega\) resistor in series for the gate bias line. We measured the power amplifier's small signal (S-parameters: S11, S12, S21, and S22) and large signal performance. DC bias was supplied by a Rohde & Schwarz (R&S) HMP4040 device. The S-parameters were measured with the vector network analyzer R&S ZNB40. During large-signal measurements, the waveform generator Keysight 33250A generates a pulse-modulated signal with a 10% duty cycle and 1 ms pulse. The input signal was generated from a signal generator, R&S SMA100B, and for measurement at a high-input-power level, a driving amplifier was used. To prevent returned signals from the reflection, an isolator, Fairview Microwave FMIR1036, was used between the driving amplifier and the designed power amplifier. The output power was measured with a spectrum analyzer, R&S FSV3044. A block diagram of the measurement setup for the small signal is illustrated in Figure 7. Figure 8 shows a block diagram of the setup for large-signal measurement.