# A Review of Ku-Band GaN HEMT Power Amplifiers Development
## 2 Ku-Band GaN HEMT HPA Technology
### High Power Amplifier Design
Due to the size constraints imposed by PCB implementation of input-output matching circuits, a single-stage design was often utilized, which necessitates a separate high-input driving power in the system configuration. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reference** & **Gate Length (\(\upmu\)m)** & **Substrate** & **V\({}_{\text{DD}}\) (V)** & **Pout (W)** & **PAE (\%)** \\ \hline
[39] & \(-\) & \(-\) & 24 & 63 & 32 \\
[40] & \(-\) & \(-\) & 24 & 50 & 29 \\
[41] & 0.25 & SiC & 24 & 63 & 25 \\
[42] & 0.25 & SiC & 40 & 50 & 23 \\
[43] & 0.25 & SiC & 40 & 57–66 & \(-\) \\
[44] & 0.25 & SiC & 50 & 50 & \(-\) \\
[45] & 0.15 & \(-\) & 24 & 100 & 32 1  \\
[46] & \(-\) & \(-\) & 24 & 80 & 23–28 1  \\
[47] & 0.15 & \(-\) & 30 & 120 & 31 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Summary of the key performance characteristics of reported Ku-band GaN HEMT HPA MICs. Figure 5: (a) OSV and (b) ISV layouts of a 4 \(\times\) 50 \(\upmu\)m GaN HEMT. Figure 6 illustrates a typical example of a Ku-band MIC power amplifier implemented using an internal matching approach. To compactly implement an impedance transformer that transformed the low impedance of a large GaN HEMT die into a high impedance, the PCB connected to the GaN HEMT die was made from a material with a very high dielectric constant, as shown in Figure 6. This design prevented the feeding line widths from becoming excessively large, keeping the overall module size manageable. The power dividing and combining components that followed were implemented using materials with lower dielectric constants. In [41], PCBs with \(\varepsilon_{r}\) values of 38.5 and 9.8 were used, while [43] also utilized PCBs with \(\varepsilon_{r}\) values of 40 and 9.8. When transforming impedance, it is common to employ a multi-step impedance conversion to achieve a gradual transition and avoid a high Q-factor on the Smith chart [43]. Additionally, the GaN HEMT die and PCB were connected via wire bonding, and this was advantageous to minimize the bonding length and maximize the number of bonds to reduce parasitic inductance. Figure 7a shows an example photograph of an actual fabricated Ku-band GaN HEMT IM-PAM, and Figure 7b presents a photo of a GaN HEMT power amplifier module featuring extensive wire bonding. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reference** & **BW (GHz)** & **Pout (W)** & **\
\begin{tabular}{} **\# of GaN HEMTs** \\ **in the Final Stage** \\ \end{tabular} &
\begin{tabular}{} **Total Gate Width** \\ **(mm)** \\ \end{tabular} & **\# of Stages** \\ \hline
[41]1 & 13.75–14.5 & 63 & 24 & 28.8 & 1 \\
[42] & 13.75–14.5 & 50 & 12 & 14.4 & 2 \\
[43]2 & 16.2–16.8 & 57–66 & 12 & 14.4 & 1 \\
[44]2 & 12.4–13.8 & 50 & 12 & 14.4 & 1 \\
[45]1 & \(-\) & 100 & 48 & 28.8 & 1 \\
[46]1 & 13.75–14.5 & 80 & 48 & \(-\) & 1 \\
[47]1 & 11.7–12.2 & 120 & 64 & 30.72  \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Comparison of reported HPA MICs from a circuit design perspective. Figure 6: Design example of a Ku-band MIC HPA implemented with an internal matching approach. Figures reproduced with permission from ref. [43]. Copyright 2018 MDPI. On the other hand, packaging and die attach technology that effectively dissipates heat is critical for achieving large output powers of 50 W or more. Figure 8a,b display the temperature distribution of a 20 W class GaN HEMT HPA bare die, with only DC power applied and with both DC and RF power applied, using a high-resolution infrared (IR) scope. It is evident that the maximum temperature nearly doubled from 90 \({}^{\circ}\)C to over 180 \({}^{\circ}\)C when RF power was applied near the center channel of the transistor, highlighting the necessity for effective heat dissipation techniques during the packaging process. Table 8 compares the thermal conductivity of heat spreader materials commonly used in die attach. Table 9 summarizes the thermal conductivity of materials utilized as heat sinks or thermal interfaces.