by Liam Devlin, CEO, PRFI
GaN technology is the leading technology for the realization of microwave Solid State Power Amplifiers (SSPAs). It benefits from a high breakdown voltage leading to high power density. Smaller transistors can therefore be used for a given RF output power requirement, resulting in higher device impedances and lower matching network losses as well as better suitability to broad band operation.
In many ways GaN technology is similar to GaAs MESFET and PHEMT, but simply assuming that GaN devices are just higher power GaAs parts would mean that the full performance potential of GaN is unlikely to be realized and can result in performance shortfalls. Fortunately, help is on hand; PRFI has been designing GaN PAs for many years and can help in navigating the GaN technology options and implementing the most effective design.
GaN PAs can be realized using a variety of implementation options, which will be described in more detail below:
- PA modules using commercially available discrete transistors
- PA components using discrete GaN transistor die and Integrated Passive Devices (IPDs)
- PA modules using commercially available GaN PA MMICs
- Full custom GaN PA MMICs
Option 1 — PA Modules Using Commercially Available Discrete Transistors
This option offers modest Bill Of Materials (BOM) costs and can yield high performance PAs. It is better suited to lower microwave frequencies as the package parasitics of the discrete transistors become more difficult to incorporate into the amplifier matching networks as the operating frequency increases. The resulting PAs will also be larger than a more integrated solution.
PRFI has designed many RF GaN PAs based on packaged discrete GaN transistors at frequencies up to X-band. Lower power parts tend to be supplied in SMT compatible plastic packages with higher power transistors being housed in metal-based ceramic packages. At L-band, PRFI designs have demonstrated power levels as high as 125W with an associated Power Added Efficiency (PAE) of 70% for a PA covering 0.96 to 1.215 GHz. This performance was for the complete PA including all package losses, bias and matching network losses and the losses of the RF connectors (Figure 1). It clearly demonstrates the excellent performance that can be achieved with high quality GaN technology and the right design approach.
The practical power levels that can be achieved from a discrete packaged transistor reduce as frequency increases. Ultimately, the bonding inductance at the input of the transistor becomes resonant with the transistor’s input capacitance. Operating close to this point is impractical, with all attempts at matching resulting in very narrow band operation. This can be addressed with the inclusion of passive matching within the package; this approach is discussed later. With GaN technology it is possible to design PAs at frequencies up to X-band using packaged, discrete transistors. PRFI has used a plastic packaged discrete transistor (Figure 2) to design a 5W X-band GaN Power Amplifier. The amplifier is optimized for the 9.3 to 9.5GHz band, has 11dB small signal gain, and provides more than +37dBm output power at 3dB gain compression with a corresponding drain efficiency of greater than 55%. At frequencies and power levels above this, the use of discrete packaged transistors becomes impractical unless combining multiple smaller transistors is adopted or some form of in-package matching is used.
Option 2 — PA Components Using Discrete GaN Transistor Die and Integrated Passive Devices (IPDs)
This approach can be used to realize input matched transistors to help mitigate the problems of packaging parasitics or can be used to develop complete PA components with lower costs than a fully integrated GaN PA MMIC.
These components can be as simple GaN power transistors with an input matching die in the same package to produce a power transistor that is already input matched to 50 Ω (Figure 3). This is more advanced than a pre-matched transistor, which is not fully matched to 50 Ω. An example of this type of component is the QPD1020, which PRFI designed in collaboration with Qorvo. The internal input matching is designed to cover the 2.7 – 3.5 GHz frequency range and is well suited to S-band radar applications. Such applications commonly use only part of the frequency band and the output matching network can therefore be implemented on the PCB to allow large-signal performance to be optimized for output power or efficiency (or a compromise between the two) over the desired frequency band.
PRFI designed the internal input matching die and optimized the performance of the composite component. A PA reference design using the component was also developed to demonstrate the achievable performance and the benefits of the internal input match (Figure 4). The output matching network of the PA was designed for optimum efficiency between 2.7 to 3.1 GHz. The resulting connectorized PA demonstrated a small signal gain of 17.5dB with a minimum RF output power of 21W and an associated efficiency of 57%.
As RF power and operating frequency increase, internal matching of discrete packaged power transistors becomes essential to their practical operation. It is also possible to develop multi-chip SMT packaged components comprising multiple GaN transistor die and lower cost IPD (Integrated Passive Devices) incorporating matching, biasing and power combining circuitry. This offers the potential to develop a single packaged component with the benefits of GaN at a much lower cost than a fully integrated GaN MMIC. It is an attractive option at RF and lower microwave frequencies where the passive circuitry can occupy a large die area and the impact of die to die bonding parasitics is more easily tolerated.
Option 3 — PA Modules Using Commercially Available GaN PA MMICs
There is an increasing number of commercially available GaN MMIC components. These can be used to quickly develop PA modules incorporating bias sequencing, temperature monitoring, driver stages and reflected power monitoring circuitry, as required. The speed of development can be rapid and the design risks modest.
Fully integrated GaN MMICs allow operation at higher frequencies than PAs using discrete transistors or even the internally matched transistors described above. GaN PAs operating at mmWave frequencies are now commercially available and GaN technology is continuing to extend the capabilities of solid state PAs. Careful design of the PCB is essential to get the most from a packaged GaN MMIC. The PCB must ensure low grounding inductance, good thermal performance, adequate current carrying capability for DC bias tracks and RF tracks that can handle the high RF powers that can be generated with GaN technology.
PRFI has designed a variety of PA modules containing GaN MMICs. These have included novel applications such as a microwave PA module that formed part of a propulsion system for an in-space transportation service company. The module, shown in Figure 5, included a GaAs driver MMIC and GaN output device to provide high gain and high output power. Bias conditioning circuitry was also included. The module was successfully launched into space in 2019 and is operating as intended.
Option 4 — Full Custom GaN PA MMICs
A full custom GaN MMIC offers a means of developing a compact component that precisely matches your requirements. Development costs are obviously higher than using commercially available GaN PA MMICs, but high volume production costs can be lower and the functionality and performance can be tailored to precisely match the requirements.
There are a growing number of commercially available GaN ICs processes offered on a foundry basis. The foundry provides a Process Design Kit (PDK) containing simulation and layout models and a design guide. The MMIC designer uses these tools to design, simulate and lay out an IC, which the foundry will fabricate for a fee. Successful designs can be moved to production and wafers of ICs can be purchased from the foundry. The availability of powerful and accurate EM simulation tools allows all proximity and layout effects to be taken into account and improves the accuracy of the simulation. EM simulation is an essential tool for ensuring first pass design success and becomes increasingly important with increasing frequency and with layouts which are compacted to reduce die area and unit cost.
Figure 6 shows a photograph of an X-band PA for phased array radar applications designed by PRFI. It covers 9 to 11.5GHz and delivers an output power of 7W (38.5dBm) from a 29dBm drive with a Power Added Efficiency (PAE) of 42%. The design was realized on the 0.25µm gate length GaN on SiC process of UMS (GH25). The compact nature of the layout is evident and accurate EM simulation was essential to allowing this. The result of this compact layout is a small die size of just 1.5mm x 2mm, which means around 2,300 PAs can be fabricated on a single 4” diameter wafer.
Other GaN MMIC designs undertaken by PRFI include 10W DC to 18GHz switches and a 3.5GHz Doherty PA (Psat of 45dBm with a peak PAE of 50%; PAE at 8dB power back-off, 31.5%). GaN processes offering good performance to mmWave are now available on a foundry basis and PRFI recently completed the design of a 28GHz GaN PA on Wolfspeed’s 0.15µm process.
GaN technology has rightly gained popularity as the preferred choice for high performance microwave SSPAs. With the right design skills it can provide excellent performance and reliable operation.
The practical examples included in this article were designed by the following members of the PRFI design team: Andy Dearn, Stuart Glynn, Graham Pearson, Tony Richards, Robert Smith