Learn more about GaN technology

GaN: a reliable technology

GaN is a proven compound semiconductor technology. Since the 1980s, compound semiconductors have been the dominant microwave integrated circuit (IC) technology in high-performance applications. This is because they enable a superior combination of speed and power compared to simple silicon-based semiconductor devices.

Compound semiconductors are composed of two or more different element groups in the periodic table, while simple semiconductor devices are composed of single elements such as silicon (Si). As shown in Figure 2-1, GaN is one of the compound semiconductors that combines elements from the third and fifth columns of the periodic table, so it is called a III-V compound semiconductor.

These III-V semiconductors can be used in a variety of applications. Over the past four decades, gallium arsenide (GaAs) has been the most widely used, with billions of GaAs ICs operating around the world. GaN offers a superior combination of speed and power handling compared to GaAs. GaN's superior power performance for a given transistor speed allows it to displace other technologies in thousands of applications across a wide frequency range.

GaN monolithic microwave integrated circuits (MMICs) and discrete transistors first entered production in the late 2000s, targeting solid-state applications at the highest power levels. In millimeter wave (mmWave) applications, GaN has replaced GaAs at higher power levels, enabling tens of watts of power in the Ka-band compared to what MMICs offer in competing technologies. At lower frequencies, such as L-band, GaN transistors can achieve over 1,000 watts of power!

GaN can use a variety of substrate materials, such as silicon, silicon carbide (SiC), GaN and diamond. GaN is compatible with high thermal conductivity substrates such as SiC, enhancing its advantages in high-power applications.

How the inherent material properties of GaN in a transistor create superior RF crystals

Conceptually, field-effect transistors (FETs) built using GaN are similar to transistors built using other semiconductor materials such as GaAs, indium phosphide (InP) or Si that use gate contacts or nodes. In the case of GaN radio frequency (RF) devices, the implementation is usually a depletion mode high electron mobility transistor (HEMT).

Depletion-mode HEMTs apply a negative bias to the gate electrode. This cuts off the current flow between drain and source. A depletion-mode FET is designed to be on when the applied gate voltage is zero; it can be turned off by pulling the gate below the threshold voltage.

GaN devices consist of a longitudinal material structure that defines many inherent properties and a lateral structure that enables contact with the material structure and control of charge flow (see Figure 2-2). As with other FETs, the lateral structure includes source, drain and gate contacts. Often, there are other structures nearby that provide magnetic field control, such as the source field plate shown in Figure 2-2.

Figure  2-2  : Basic GaN FET geometry.

The following is shown in Figure 2-2: 

» The barrier provides two key functions: isolation between the gate and channel, and charge capacity to support the flow of electrons. It is usually made of aluminum gallium nitride (AlGaN).

» Channel is pure GaN. It provides a conductive path for current flow between the drain and source contacts. GaN's high saturation speed and mobility enable high-speed transfer and current levels between the drain and source of the device.

» Buffers are used to limit the flow of charge within the channel to avoid leakage to the substrate and ensure isolation between transistor devices.

» The substrate determines the mechanical and thermal performance of the device. Devices with higher power consumption can benefit from substrates with higher thermal conductivity. SiC substrate material is easy to use, provides excellent heat dissipation performance, and is compatible with GaN material growth and MMIC preparation.

The following are important functions of horizontal structures: 

»  The gate of the device controls the current flowing through the device from the drain to source contacts. The length of the gate determines the speed of the device and the time it takes for electrons to flow through the control area.

» Source and drain contacts provide low-impedance access to the intrinsic device. Isolation between the gate and these contacts not only creates unnecessary parasitic access resistance, but also increases the breakdown voltage required to support intended operation.

GaN process options demystified

FETs can be optimized for the target application by making trade-offs between transistor speed, current capability, breakdown voltage, efficiency, and reliability. To meet the needs of different GaN applications, manufacturers offer multiple process technologies across a wide range of frequencies and power levels. With multiple GaN process options available, circuit designers can optimally match a specific GaN process technology to their application, simplifying and speeding up designs. Figure 2-3 illustrates Qorvo's family of GaN process technologies designed to accommodate a variety of applications across multiple market segments.

Figure  2-3  : Qorvo GaN process technology options for Class AB performance.

For example, very high power applications such as 1 kW transistors operating at 2 GHz will benefit from the GaN process with higher breakdown voltage because it increases operating voltage and RF power density. Increasing the operating voltage will also increase output efficiency. This is a trade-off between increasing access resistance and reducing transistor speed. The Qorov GaN50 process is capable of operating at 65 V and offers these advantages.

Millimeter-wave power amplifier (PA) applications such as 20 W MMIC operating at 30 GHz require the use of high-speed devices that can provide high gain at high frequencies. Device design tradeoffs will favor shortening gate length, minimizing access resistance, and maximizing current capability. This can reduce breakdown voltage and power density. The Qorov GaN15 process is capable of operating at voltages up to 28 V and offers these advantages.

In both examples, GaN devices offer higher operating voltages than other technologies, demonstrating the technology's inherent speed and voltage advantages. The advantages of higher operating voltage are not limited to the PA circuit, but can also bring benefits to the entire system.

For example: Phased array antenna systems (a common application for GaN PAs) may require hundreds or thousands of individual power amplifiers.

DC power distribution in these antenna array systems has always been a challenge because the power supplies take up space, add weight, and cause DC power losses. But GaN's higher operating voltage enables lower DC currents and excellent size, weight, power and cost (SWaP-C) performance to address the DC power distribution challenges faced by these systems.

Reliability Assessment of GaN Devices

Reliability is extremely important in all electronic systems, so it is a key consideration when selecting a semiconductor. A key advantage of GaN is that it can operate at higher voltages and power densities than other semiconductors. GaN meets these stringent requirements with proven reliability at high junction temperatures while achieving a mean time to failure (MTTF) of over 107 (10 million) hours at 200°C and 225°C at 225°C. Achieve a mean time between failures of more than 106 (1 million) hours under temperature conditions. GaN's higher safe operating channel temperatures and longer lifetime enable system designers to advance their applications and products.

GaN manufacturers use different approaches to failure analysis: some rely on thermal imaging, while others use a combination of thermal imaging, product packaging testing and modeling. But all manufacturers and standards bodies agree: GaN is more reliable in high-power, high-temperature applications than other technologies. As shown in Figure 2-4, GaN's reliability far exceeds that of GaAs-based transistors.

Figure  2-4  : Using Qorvo MTTF curve

Reliability Comparison Example of GaN vs. GaAs Technology Devices GaN applications typically subject devices to higher stress operating conditions such as higher current densities, higher ambient temperatures, and higher electric fields. Whether a result of device design or device usage, these problems can be caused by piezoelectric effects, thermal mismatch, or packaging.

There is another inherent device characteristic of GaN devices that requires attention: stress caused by the inherent piezoelectric properties of GaN. Figure 2-5 shows the peak stress region of a GaN FET. In GaN devices, however, this behavior is well characterized and easy to understand. Therefore, with current GaN process technology, this is no longer an issue.

Figure  2-5  : High electric field region of FET.

Today, GaN devices are used in a variety of applications with the most stringent and challenging reliability requirements, including mission-critical systems and aerospace applications. GaN's reliability and stability transcend transistor and MMIC processes and is optimized to address the electrical, thermal and environmental challenges facing GaN's expanding range of applications. Its environmental stability enables die-level Highly Accelerated Stress Test (HAST) compatibility for all today's GaN processes. GaN packaging and interconnect technologies are also advancing to keep pace.

For example: Today's Qorvo GaN technology is available in high-volume, Manufacturing Readiness Level 10 (MRL 10) mature processes. MRL is a metric developed by the U.S. Department of Defense (DOD) to assess manufacturing maturity. MRL 10 is the highest level of manufacturing maturity, indicating that full-rate production and lean manufacturing practices are in place.

GaN technology continues to advance to support a wider range of applications. These advancements include supporting higher operating frequencies and increasing power levels in a broadband frequency range. As with most other technological advances, low-volume production capabilities are moving toward high-volume, mature production processes.

A key advancement in GaN is the ability to operate at extremely high frequencies (100 GHz or higher) by shortening the GaN gate length. Another advancement is in output power levels: GaN can achieve higher power densities at lower frequencies as the operating voltage increases.

Today, GaN PA designs typically follow the principle of several kilowatts at 1 GHz, hundreds of watts at 10 GHz, and tens of watts at 100 GHz. This rough figure of merit has tripled over the past five years and continues to improve.

GaN technology will continue to evolve while further expanding the frequency range of GaN PAs and increasing their power levels. Other parameters are also being explored, such as increasing high power amplifier (HPA) bandwidth and improving efficiency. Improvements in GaN device performance and advances in circuit design technology will enable continued advancement in these areas.

GaN has come a long way over the past 20 years and is now being further improved to enable wider deployment. What we can be sure of is that GaN technology will be further improved in the future and its application range will be wider.