# A 32-GHz Eight-Way Power Amplifier MMIC in 150 nm GaN HEMT Technology
## 1 Introduction  
Deep space exploration has been developed since the 1960s, and unmanned probes are exploring the far reaches of outer space and transferring data back to Earth. Early data transfer for deep space exploration like the Apollo Program, which was aimed at lunar exploration, required low data transfer rates with several kilobits per second (kbps) [1, 2]. However, the demand for higher data rates has grown over time. It is essential to provide a large amount of communication capacity for spacecraft-to-Earth links to achieve this aim. As a result, deep space radio communication systems have moved toward higher frequencies, from the L-band to the S-band, which was followed by the S-band to the X-band [3]. Moreover, the exponentially rising amount of investigated data and the need for higher transmission capacity are driving satellite communications to higher bands, such as those above the X band [4]. The millimeter-wave frequency band allows deep-space satellite communication systems to reach high data transmission rates, with low communication delays. The Ka-band (27-40 GHz) provides relatively wide bandwidth and high throughput. The ITU radio regulations assigned the downlink frequency ranges for deep space exploration to the X-band at around 8.4 GHz (8.4-8.45 GHz) and the Ka-band at around 32 GHz (31.8-32.3 GHz). Ka-band downlink deep space exploration systems can downlink data at a rate approximately six times higher than X-band systems [5]. However, because of the Earth's troposphere, radio meteorological factors, convective rainfall, nonprecipitating clouds, stratiform rain events, atmospheric noise temperature, and other conditions, the attenuation is much higher at the Ka-band [6]. Since signal attenuation increases during propagation at high frequency, high-performance amplifiers in the system are vital in improving the link distance and the quality of communication [7]. For more than fifty years, under Moore's Law [8], the silicon-based CMOS integrated circuit has been the backbone of the information society. Currently, silicon semiconductor scaling has reached the nm level, facing technical limitations and performance limitations due to the inferior performance of silicon. To develop such a high-performance transceiver at a millimeter-wave regime, high-power, low-cost, and highly reliable solutions for solid-state power amplifiers have been investigated in III-V compound semiconductor technologies like gallium arsenide (GaAs), indium phosphide (InP), alumina nitride (AlN), and gallium nitride (GaN). III-V compound semiconductor technology has excellent transistor characteristics in the high-frequency range, and the semi-insulating substrate facilitates high-quality integrated passive components with low parasitic elements [9; 10; 11]. Table 1 shows the properties of various semiconductor materials. Specifically, GaN has a high-breakdown voltage from a wide energy gap of 3.4 eV, a high-carrier concentration, high-electron mobility, high-saturation velocity, and high-thermal conductivity. Because a high-breakdown voltage transistor (HEMT) is available with a high-current density and robustness at high temperatures in GaN technology, the PA can achieve a higher output power with improved power-added efficiency (PAE) at the millimeter-wave regime. Two-dimensional electron gas (2-DEG) channels of the GaN HEMT are created by AlGaN/GaN heterojunctions, in which a wide bandgap semiconductor (AlGaN) is placed to contact with a narrow bandgap semiconductor (GaN). The impact of scattering by dopants is reduced with a 2-DEG channel created at an undoped pure GaN layer so that high-electron mobility can be achieved [12]. The operation of GaN power devices at high-current densities and voltages causes high temperatures. This self-heating may result in a drain current drop and limit its output power and shorten its lifetime [13]. Hence, the heat generated during the operation of the device should be emitted efficiently. One good way to cool down the device is to emit heat to the substrate.