# Review of Ka-Band Power Amplifier
## 2 Technology  
There are several semiconductor processes that have been demonstrated for power amplifiers at the Ka band. They are gallium arsenide pseudomorphic high electron mobility transistors (GaAs pHEMT) [11], gallium nitride high electron mobility transistors (GaN HEMT) [12], indium phosphide heterojunction bipolar transistor (InP HBT) [13], silicon complementary metal-oxide-semiconductor (Si CMOS) [14], silicon and germanium bipolar complementary metal-oxide-semiconductor (SiGe BiCMOS) [15] and silicon on insulator complementary metal-oxide-semiconductor (SOI CMOS) [16]. In the late 1970s and early 1980s, the HEMT structure was proposed and developed [17; 18]. During this period, a number of research groups around the world proposed the AlGaAs/GaAs crystal structure, collectively known as HEMT. The traditional GaAs HEMT structure profile is shown in Figure 2a [19]. However, there exists a deep donor state trap DX center in the AlGaAs/GaAs heterojunction, which causes device degradation at low temperatures [20]. To solve this problem, Phemt was invented. In 1986, [PERSON] first fabricated the AlGaAs/InGaAs/GaAs Phemt structure on a GaAs substrate [21]. The GaAs Phemt structure is shown in Figure 2b. GaAs Phemt solves the above problems by introducing the AlGaAs isolation layer, which is used to isolate the two-dimensional electron gas (2 DEG) layer and doping layer [22]. Compared to HEMT devices, Phemt greatly improves electronic transport characteristics. The electron mobility velocity of GaAs is 5 or 6 times that of Si and has higher drift velocity and better high frequency characteristics [23]. In recent years, Phemt's cut-off frequency fT and maximum frequency fMAX reached 360 GHz and 400 GHz, and they are widely used in millimeter-wave frequencies [24]. However, GaAs has high thermal resistance and low breakdown voltage, which limits its application in high voltage and high power. In 2018, [PERSON] presented a PA targeting more than 25 Db gain and 27.3 dBm output power between 96 and 98 GHz adopting 0.1 \(\upmu\)m GaAs Phemt [25]. In 2020, [PERSON]. [26] proposed a 3-stage CS PA in 65 nm CMOS, achieving 13.4 dBm saturated output power (Psat) with peak PAE of 11.3% in the 85-101 GHz range. Compared with CMOS process, the GaAs process has the advantage of high output power at higher frequency. GaN HEMT and GaAs Phemt devices operate on a similar principle and are both high electron mobility transistors operating through 2 DEG [27]. However, due to GaN's special spontaneous polarization, GaN HEMT can also produce 2 DEG without doping. The profile of GaN HEMT is shown in Figure 3a. Figure 2: (a)Traditional GaAs HEMT structure profile, (b) GaAs pHEMT structure. GaN and GaAs have similar frequency characteristics, but due to the special spontaneous polarization of GaN, the concentration of 2 DEG is one order of magnitude higher than that of GaAs [28]. Therefore, GaN has high current density and outputs high power at high frequency. In 2018, [PERSON] presented a GaN MMIC Pas using Qorvo's QgaN15 released process. The output power of the reported PA is approximately 46.2 dBm at 27.5-29.5 GHz [29]. The substrate selection of GaN is a key problem. GaN devices grown on a SiC substrate can obtain higher performance, but the cost is very high. Therefore, several research groups regard GaN on the Si substrate as a feasible solution for the large-scale market application, and massive commercial products have been designed and manufactured. However, compared with the SiC substrate, the Si substrate has worse electrical and thermal properties [30]. The cost and performance must be considered comprehensively to select a more appropriate scheme. In a word, the GaN process can bring some advantages of III-V technology (III-V compound semiconductor technologies include GaAs, GaN, InP, etc.) and its price is lower than the GaAs technology due to the possibility of integration on the Si substrate. In the research of high frequency electronic devices, the transistors using InP materials are adopted in HEMT and HBT technology.