# A Review of Ku-Band GaN HEMT Power Amplifiers Development
## 2 Ku-Band GaN HEMT HPA Technology
### High Power Amplifier Design
As shown in the schematic of Figure 4b, an RC stabilization circuit with resistors and capacitors connected in parallel is often employed to enhance the stability of the input side of the circuit. Due to the large size of the transistor, substantial DC currents and RF signals are input and output. This increases the risk of oscillation, particularly if the signal diverges to one side. To mitigate even-mode and odd-mode oscillations, large resistors are typically connected to the gate and drain (see the beige boxes in Figure 4b) [48]. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Reference** & **Gate Length (\(\mu\)m)** & **Substrate** & \(\mathbf{V_{DD}}\) **(V)** & **Pout (W)** & **PAE (\%)** \\ \hline
[23] & 0.25 & – & 24 & 20 & 16 \\
[24] & 0.15 & SiC & 28 & 47.5 & 36.2 \\
[25] & 0.25 & SiC & 25 & 7.2–9.5 & 35 \\
[26] & 0.25 & – & 30 & 25 & 26–30 \\
[27] & 0.15 & SiC & 25 & 7.9 & 35 \\
[28] & 0.25 & SiC & 40 & 63 & 30 \\
[29] & 0.15 & SiC & 28 & 8.3–13.2 & 35.7–45.4 \\
[30] & 0.15 & SiC & 28 & 16–25 & 30–40 1 \\
[31] & 0.20 & SiC & 28 & 79–93 & 28.7–31.5 \\
[32] & 0.25 & SiC & 28 & 40 & 17 \\
[33] & 0.1 & Si & 11 & 8.9 & 27 \\
[34] & 0.1 & Si & 12 & 8.9–15.8 & 30–41 1 \\
[35] & 0.1 & Si & 9 & 10 & 35 \\
[36] & 0.15 & – & 28 & 40–50 & 36 \\ \hline \end{tabular}
\end{table} 
Table 4: Summary of the key performance characteristics of reported Ku-band GaN HEMT HPA MMICs. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Reference** & **BW (GHz)** & \begin{tabular}{c} **Output** \\ **Power (W)** \\ \end{tabular} & \begin{tabular}{c} **\# of GaN HEMTs** \\ **in the Final Stage** \\ \end{tabular} & \
\begin{tabular}{c} **Total Gate Width** \\ **(mm)** \\ \end{tabular} &
\begin{tabular}{c} **\# of Stages** \\ \end{tabular} \\ \hline
[23] & 13.75–14.5 & 20 & 8 & 9.6 & 3 \\
[26] & 13–18 & 25 & 8 & 8.64 & 3 \\
[28] & 12.7–13.25 & 63 & 16 & – & 3 \\
[30] & 13–17 & 16–25 & 8 & 7.68 & 3 \\
[31] & 14–18 & 79–93 & 16 & 10.88 & 3 \\
[32] & 15.25–16.25 & 40 & 16 & 25.6 & 4 \\
[36] & 13–15.5 & 40–50 & 16 & 15.36 & 3 \\ \hline \end{tabular}
\end{table} 
Table 5: Comparison of reported HPA MMICs above 20 W from a circuit design perspective. larger and more numerous ground source vias reduce the parasitic inductance caused by the source via structure, which improves load-pull characteristics and enhances device performance by dissipating heat generated by self-heating more effectively. However, the ISV structure can excessively increase the size of the main power cell, posing challenges for MMIC manufacturing. Therefore, it is common to use an ISV structure for the drive stage and an OSV for the main stage [32]. Additionally, careful design is essential for the bias circuit. As shown in Figure 4b, a virtual bias mimic circuit with open stubs and RCs in series on the opposite side of the actual bias input is used to ensure that each connected transistor presents the same impedance to the gate bias supply (see the green boxes in Figure 4b). The drain bias circuit also requires careful design to ensure that each transistor receives a drain voltage through a line that is in phase with the others (see the gray boxes in Figure 4b). Table 6 compares Ku-band GaN HEMT HPAs implemented using the MIC approach. Compared with the MMIC schemes in Table 4, more 0.25 \(\upmu\)m processes were utilized, with the average output power ranging from 50 to 120 W, which was higher than that of the MMIC schemes. However, the PAEs fell within the 20 to 30 percent range. All MIC power amplifiers in Table 6 were implemented in the IM-PAM type. Table 7 compares the HPA MIC references of Table 6 from a circuit design perspective. The GaN dies used in the surveyed papers were primarily the CGHV1J070D, sold by MACOM (formerly Wolfspeed), and one developed in-house by Mitsubishi, Japan. The number of GaN HEMTs in the main power stage varied from 12 to 64, depending on the output power, with a total gate width approximately in the range of 15 to 30 mm. It is evident that MIC power amplifiers are primarily developed to achieve high output power in a narrow band.