Difference Between Gallium Arsenide/GaAs and Gallium Nitride/GaN RF Power Amplifiers

              

Radio waves were first mapped out and harnessed back in 1873. Since that time, radio frequencies and microwaves have been used in numerous applications from the telegraph and the first radio broadcasts making their way into people's homes to radar technology and musical instruments. They also paved the way for military drones, satellites, and the internet among other modern-day essentials. GaAs, or gallium arsenide and GaN, or gallium nitride are the 2 most important semiconductor material that are used for building high-power microwave switches and amplifiers, more specifically TR module-based radar assemblies in fighter jets. While GaN has emerged as the leading semiconductor material for high-power microwave switches and amplifiers, GaAs is still the preferred material of choice for low noise.

What and Why Are These Used For

It's important to understand what semiconductors are and why they're important for RF power amplifiers. Conductors are materials that allow heat or electricity to flow through them. Insulators, on the other hand, block the flow of heat and electricity. Semiconductors fall in the middle of this spectrum. They offer electrical and thermal conductivity that's lower than conductors but higher than insulators.

RF amplifiers essentially take in radio signals, break them down, transmit them over long distances, and reassemble them. Semiconductors facilitate this process in a number of ways. They basically bridge the gap between other components in the amplifier to generate more power, send the signals further, provide greater clarity, and other functions. They also help disperse heat generated by the amplifiers and regulate the flow of electricity without blocking it entirely.

GaAs or Gallium Arsenide

GaAs, or gallium arsenide, semiconductors came into play during the 1980s, and they quickly became the go-to solution for RF and microwave applications. They offer several advantages over their limited counterparts. GaAs devices are capable of functioning at frequencies above 250 GHz because of their higher electron mobility and saturated electron velocity. Due to their wider energy band gap in comparison to those of other semiconductors, they don't overheat as easily. They also create less noise interference.

These semiconductors are available in several forms with diameters of up to six inches. Types of GaAs microwave-integrated circuits are used in an array of devices, including tablets and smartphones. Of course, those aren't their only practical uses. They're often the best choice for lower-voltage applications and devices in which reduced noise and interference and high resistance to radiation and heat are required.

The important material properties of these two semiconductor materials are electron mobility, thermal conductivity, energy bandgap, and bandwidth. For power amplifiers, a higher energy bandgap allows the device to be run at higher voltage as the breakdown field in the device will be higher for a given device size. The thermal conductivity is also important from a device lifetime perspective. When the thermal conductivity is larger, the device will run cooler, which will provide longer lifetime. In addition, since a device with higher thermal conductivity runs cooler, physically smaller heat sinks can be used with the device. Although cost of producing GaN on 4H-SiC is greater than the cost to produce a GaAs wafer, the overall amplifier cost is lower (both for integrated circuits and larger amplifier units) due to lower packaging/housing and heat sink costs.

The bandwidth is a function of crystal structure and the construction of the device itself. Bandwidths can reach about a decade in both devices, although the center frequencies are different for GaN vs. GaAs. Finally, the electron mobility is related to the conductivity of the device in the ON state. A device with higher electron mobility will have higher conductivity in the ON state, thus a power amplifier or RF amplifier will be more efficient. In the right frequency ranges, each type of device will have different efficiencies.

The semiconductor compound gallium arsenide (GaAs) has been used to manufacture monolithic microwave integrated circuits (MMICs) since they were first developed in the mid-1980s. MMICs use compound semiconductors to enhance data communications in radio frequency (RF) systems across the microwave and mmWave frequency spectrum.

GaAs was chosen as an alternative to silicon in integrated circuits because of its improved performance at higher frequencies. Silicon-based devices experience greater losses at high frequencies and can deliver less power than their compound semiconductor counterparts. GaAs is now well-established in commercial and military applications and is used extensively in RF devices ranging from consumer electronics such as smartphones to radar systems.

GaN or Gallium Nitride

GaN, or gallium nitride, semiconductors are relative newcomers to the field. They were developed during the 1990s and commonly used in devices that operate at high frequencies and require significant amounts of power. This type of semiconductor is highly resistant to radiation, making it ideal for military drones and satellites among other devices that could potentially end up in strenuous situations.

Although GaAs devices can withstand considerable levels of heat, GaN transmitters have even higher tolerances. They can also operate at higher voltages than their counterparts. They're more suitable for use with microwave and THz frequencies. Under certain circumstances, GaN transistors provide more efficiency than GaAs varieties. They potentially use less energy and allow less energy loss than other types of semiconductors. At the same time, they tend to have a higher energy output. GaN has higher electron mobility than GaAs and other semiconductors, so it has a broader amplification range. Despite typically coming in a much smaller package than GaAs, it can rapidly disperse heat to avoid burnout even at extremely high voltage levels.

To meet even more demanding performance requirements, often driven by the needs of the defence sector, gallium nitride (GaN) emerged as an alternative to GaAs. It offers greater power density, efficiency and operating temperature all properties of GaN’s wide bandgap.

GaN has been around for almost three decades, but the lengthy processes of research, trialling and development mean that it has only more recently become established as a viable alternative to GaAs at higher frequencies. Crucially, the cost of fabrication using GaN has now been reduced to commercially acceptable levels, as a result of scaling up small prototype devices onto bigger wafer sizes. GaN is now fabricated on the same wafer sizes as GaAs, meaning the costs of the two technologies are more comparable. That gives device manufacturers the opportunity to embrace GaN MMICs to achieve the power and efficiency gains required for cost sensitive applications. Though GaN technology is more expensive than other options, those who require high-power and frequency capabilities, as well as greater efficiency, insist it's well worth the extra cost.

A Comparison – GaAs Vs GaN

Neither GaAs nor GaN power RF amplifiers are right for all situations, but each one certainly has plenty of uses. Simply stated, those who require less noise and interference with ample heat resistance often turn to systems with GaAs semiconductors. In cases where extremely high voltage is required and large amounts of heat are generated, GaN may be the better choice. GaN is also the most suitable alternative when efficiency is of the utmost importance.

Either type of semiconductor can be used to make a wide range of amplifiers. They can also be combined with other materials and components for additional versatility. GaAs is typically used in larger types of circuits and applications whereas GaN is usually more condensed, making it the solution for smaller setups. Still, the most effective option, combination of components, and system layout depend on your unique needs and expectations.

Here, we see that GaN has a bandgap in the UV, making it an ideal material for integrated UV photonic/electronic circuits. This makes the breakdown field an order of magnitude larger than that of Si and GaAs. We see that GaN and Si have similar thermal conductivity values, which are much higher than that of GaAs. For power amplifiers, both for DC and at RF frequencies, one should note that GaN has higher electron mobility in the inversion layer than in the bulk crystal, while the opposite is true in Si. Charge carriers in the inversion layer will move easier through the active region in a GaN device in the ON state as the ON state resistance is lower. This means GaN has higher efficiency and available power output than Si and GaAs in the ON state. In general, this extends up to higher frequencies than Si amplifiers.

The Gallium Arsenide (GaAs) and Gallium Nitride (GaN) are versatile materials for such applications but with relative merits and demerits. GaAs transistors are suitable for both narrowband and wideband applications due to very wide operating frequency range (30 MHz to millimetre-wave frequencies as high as 250 GHz). They are highly sensitive, generate very little internal noise and have power density typically around 1.5 W/mm. But low break down voltage (5x105V/cm), low output power (5-10W) and inability to withstand higher temperatures are the main limitations.

On the other hand, GaN possess the improved physical and chemical characteristics, with high output power, high operating temperature (1000°C in vacuum), fast heat dissipation, high breakdown voltage (4x106V/cm), high power density (5-12W/mm), high frequency characteristics and large band gap (3.4eV) which allow significant reduction of devise size. Also, high breakdown voltage increases the overall impedance which make it suitable in matching process and enables efficient operation in broad band region.

GaN is considered a wide-bandgap material compared to GaAs, with a bandgap of about 3.4 eV for GaN compared to 1.4 eV for GaAs. A material’s bandgap related to the amount of energy required to shift an electron from the top of the valence band to the bottom of the conduction band within a semiconductor formed on that material. A wide bandgap typically refers to a material with bandgap of greater than 1 or 2 eV.

GaN typically exceeds GaAs in material parameters relating to higher energy and power, and in the speed of achieving higher-energy states. For example, the saturation velocity of GaN, at 2.7 × 107 cm/s, is somewhat higher than the 2.0 × 107 cm/s of GaAs. The critical breakdown voltage field determines the highest voltage that can be safely applied to a solid-state device, and the breakdown electric field of GaN, at 4 × 106 V/cm, is much higher than the 5 × 105 V/cm of GaAs.

 

GaN has certain traits that support smaller circuits for a given frequency and power level, allowing the higher power densities and efficiencies much sought after by designers of power-efficient wireless base stations and microcells. For one thing, he higher-voltage capacities of GaN allow the fabrication of much smaller devices for a given power level than on GaAs materials. For example, the defect density of any semiconductor wafer will limit the practical size of circuits that can be manufactured repeatably and reliably on that wafer, implying that device area be minimized for best production yields.

 

Because the power density of GaN materials is much higher than GaAs or even silicon semiconductor materials, thermal conductivity is an important material parameter for characterizing how well a device will dissipate heat due to dielectric and conductor losses as well as basic device inefficiencies. The thermal conductivity of GaN, at 1.7 W/cm-K, is more than three times the thermal conductivity of GaAs, at 0.46 W/cm-K. High thermal conductivity translates into the lowest temperature rise at conduction, a characteristic that enables GaN devices to handle higher power levels than GaAs devices using the same device structure, such as a field-effect transistor (FET).

GaN devices are currently fabricated on different substrate materials, such as GaN on silicon (Si) and GaN on silicon carbide (SiC) wafers, with some debate about which process offers the best performance. Some larger companies, such as Raytheon Co., maintain both GaAs and GaN foundries as part of their in-house capabilities in support of military applications. Many commercial foundries will offer details on the benefits of each process with some foundries, including WIN Semiconductors Corp., Global Communication Semiconductors LLC, and Qorvo, offering different forms of GaN processes along with GaAs fabrication services as well.

Choosing a GaN vs. GaAs power amplifier for RF applications and power electronics applications is all about balancing the relevant frequency range against efficiency and cost. As GaN is normally deposited on SiC, it will have much higher efficiency at high frequencies while also having longer lifetime. This takes advantage of the high thermal conductivity of 4H-SiC (4H-SiC is 490 W/m•K), which easily dissipates heat down to a die-attached paddle.

The primary high frequency application in automotive, defense, and aerospace is mmWave W-band radar (for automotive) and M-band radar (NATO’s band). GaN devices can support these higher frequencies thanks to their flat dispersion. W-band radars are moving away from Si and GaAs in favor of GaN devices as the larger current output equates to higher total power output. This, in turn, provides longer range for an mmWave radar module. There is another aspect of GaN that is not often considered, which is the high-power input rolloff in power amplifiers. Compared to GaAs, GaN has a more gentle rolloff as the input power increases and eventually drives the device into saturation (see the corresponding graph). This sets the driving power corresponding to the 3OIP (third-order intermodulation point, read more here) to a higher value.

This, in turn, lowers the 1 dB compression point. Although a 1 dB compression point is viewed as desirable from the standpoint of distortion, the real important value is the linearity in the input-output curve. This will limit the input power at which intermodulation products in FM signals become noticeable in the sidebands. This will then put pressure on filter design to suppress these intermodulation products on the amplifier output. During amplifier design, both for ICs and PCBs, amplifier behavior can be determined using circuit simulations with the right GaN SPICE models. Circuit models have been difficult to build in the past, although the current industry-standard circuit model for GaN power transistors is the Angelov model. Without a circuit model, analytical equations will need to be used with standard values for material properties to analyze device behavior in a simulation.

Final Word

At frequencies between 6GHz and 80GHz, GaN offers valuable benefits over GaAs. Firstly, it offers significantly greater power density. For the same-sized device, GaN offers around six to eight times the power of GaAs. It means GaN can be used to achieve the same power as GaAs within a much smaller die area. The other major advantage of GaN is efficiency. At lower frequencies, GaN is around twice as efficient as GaAs. GaN is also easier to match over wide bandwidths when compared to GaAs.

These benefits make GaN the ideal choice when size, weight and power (SWAP) are critically important, as they are in many defence, space and aerospace applications. There are opportunities to replace GaAs MMICs directly with GaN MMICs in some RF systems. When it’s important to retain the existing size and dimensions of a device, GaN MMICs can be used to boost power and efficiency. Such applications include military radar systems, jammers and electronic countermeasure systems, in which greater power means greater range. There are huge benefits to increasing the reach of these assets, meaning a wider area can be covered by a single asset and enabling operatives to be located further away from potential threats.

Exchanging GaAs for GaN MMICs within existing devices, the power and efficiency of hardware can be significantly enhanced, without the need to re-engineer the housing or structure of the overall module. That is especially important for RF systems that are housed in fixed locations, such as in the nose cone of an aircraft, inside ground-based radar stations, or on-board military vehicles or ships. Replacing GaAs MMICs with GaN alternatives here is an effective way to deliver major upgrades to these devices within their pre-defined footprints.

GaN is also beginning to displace GaAs in telecommunications base stations. Here, the efficiency gains achieved by GaN can significantly reduce running costs. These facilities consume a huge amount of electricity to power both the RF devices they house and the air-conditioning systems required to cool them. Energy efficiency is a major concern for the telecoms industry, particularly since new 5G base stations consume up to twice the energy of 4G stations. Any technology that can improve energy efficiency is highly prized in this sector. Some of the best-quality GaN developments today are happening in Asia, where semiconductor expertise and resources are well established. This is where vast majority of low node size silicon fabricators are based.