# A Review of Ku-Band GaN HEMT Power Amplifiers Development
## 2 Ku-Band GaN HEMT HPA Technology  
### GaN HEMT Process  
To realize a GaN HEMT power amplifier, it is essential to develop a GaN HEMT process that provides the necessary design kits and fabricates the circuits or devices. Figure 3 shows the cross-sectional structure of a typical GaN HEMT [13]. GaN HEMTs with high cutoff frequency (\(\mathrm{f_{T}}\)) and maximum oscillation frequency (\(\mathrm{f_{MAX}}\)) are required, as obtaining gain becomes more challenging with increasing operating frequency. Therefore, the gate length of the transistor needs to be reduced according to the scaling rule. In this case, the breakdown voltage is also reduced proportionally, which can be a disadvantage in power amplifier design. The substrate material is also an important factor in the choice of process. Generally, GaN HEMT processes are divided into two types: those using silicon carbide (SiC) substrates and those using silicon (Si) substrates. SiC substrates have high thermal conductivity, which is beneficial for dissipating heat generated by self-heating due to the high power density of GaN HEMTs. Therefore, they are often used in the process for GaN HEMT HPAs. However, SiC is hard and difficult to process during post-processing tasks such as dicing. In contrast, Si substrates are easier to produce using mature silicon-based semiconductor technology, are simpler to process, and are more amenable to mass production, providing significant economic advantages. However, they have relatively poor thermal conductivity and are more susceptible to self-heating effects. Figure 2: Comparison of breakdown voltage and cutoff frequency among various high-speed semiconductor devices [9; 10]. (Data from [9; 10]). The most commonly used processes for GaN HEMT HPAs are the 0.15 \(\upmu\)m and 0.25 \(\upmu\)m GaN HEMT processes. Here, 0.15 \(\upmu\)m and 0.25 \(\upmu\)m refer to the gate lengths of GaN HEMT devices. To achieve the required gain for power amplification in Ku-band, GaN HEMTs with gate lengths of 0.25 \(\upmu\)m or less are preferred. Recently, 0.15 \(\upmu\)m processes have become more popular due to their ability to achieve a high gain and high power added efficiency. Table 2 compares the device and process information for the commercial 0.25-\(\upmu\)m GaN HEMT process, while Table 3 compares the device and process information for the commercial 0.15-\(\upmu\)m GaN HEMT process. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Foundry**} & \multirow{2}{*}{**V\({}_{\text{DD}}\) (V)**} & \multirow{2}{*}{**Substrate**} &
\begin{tabular}{c} **Breakdown** \\ **Voltage (V)** \\ \end{tabular} & \multirow{2}{*}{**f\({}_{\text{T}}\) (GHz)**} & \multicolumn{2}{c}{**P\({}_{\text{out}}\)**} & \multicolumn{1}{c}{**PAE**} \\  & & & **Voltage (V)** & & **(W/mm)** & **@10 GHz (\%)** \\ \hline Qorvo & 40 & SiC & 75 & 32 & 6 & \textgreater{}60 \\ MACOM (Wolfspeed) & 28/40 & SiC & \textgreater{}84 & – & 4.2/6.6 & \textgreater{}55 \\ GCS & 28/48 & SiC/Si & 200 & 23 & 4 1/10.8 & 45 1/65 \\ UMS & 30 & SiC & \textgreater{}120 & 25 & 4.52 & — \\ WIN semi & 28/40 & SiC & 120 & 23 & 5/10 & 65/60 \\ NXP & 50 & SiC & \textgreater{}150 & – & – & — \\ \hline \hline \end{tabular} \({}^{\dagger}\) The data were measured at 15 GHz. \({}^{2}\) The frequency was unknown. \({}^{3}\) The data were estimated from the graphs.  
\end{table} 
Table 2: Comparison of device and process information for commercial 0.25-\(\upmu\)m GaN HEMT process [14; 15; 16; 17; 18; 19; 20]. Figure 3: Cross-sectional structure of a typical GaN HEMT [13]. Most commercial foundries still use SiC substrates, with the exceptions of MACOM/France (formerly OMMIC) and GCS. The 0.25 um process is based on a drain voltage of 28-30 V. Recently, GaN HEMT processes have been developed to operate at higher voltages of 40-50 V, depending on the foundry, leading to a nearly twofold increase in power density. However, the fT is only 23 to 32 GHz, which can limit power gain at high edge frequencies in the Ku-band.