New directions for GaN electronics

Advances in epitaxy and processing are opening up new opportunities for GaN in handsets, computation, and the delivery of RF signals in the X-band

BY RICHARD STEVENSON, EDITOR, CS MAGAZINE

GaN is, without doubt, the most important and pervasive material within the family of compound semiconductors. It’s initially enjoyed tremendous success within the optoelectronics domain, where it’s been used for several decades to produce countless LEDs, as well as blue and green lasers that are deployed for a variety of tasks, including material processing and colour projection.

While some may argue that the best days of GaN optoelectronics are now behind us, the same cannot be said for the electronics sector. Here GaN is increasingly employed for RF power amplification and fast-charging, with sales sure to increase with improved performance – and there are also lucrative opportunities in handsets and computation.

Key to fulfilling all these promises are technological breakthroughs, which may take the form of improvements in device design or processing. Many advances on these fronts are reported at global conferences, and at the most recent International Electron Devices Meeting held in early December in San Franscico three significant milestones were unveiled: the first integration of a GaN low-voltage power amplifier into a handset, where a three-stage III-N MMIC provided an efficiency of over 50 percent; the development of a GaN chiplet technology based on 300 mm GaN-on-silicon that has much appeal for high-performance, high-density efficient power and high-speed/RF electronics; and the fabrication of an X-band GaN-on-SiC-based HEMT that delivers a substantial hike in power density, realising 41 W mm^-1.

Handsets: From GaAs to GaN?
Since the mass adoption of the mobile phone at the turn of the millennium, the critical task of RF amplification has been performed with a GaAs-based device, typically a HBT. One of its strengths is that it’s well-suited to being driven by a battery, which provides just a few volts.

Offering an attractive alternative on several fronts is the GaN-on-silicon PA. This technology delivers a superior power performance from a smaller form factor, a key advantage, given the limited space in smartphones. What’s more, GaN-on-silicon HEMTs have the potential to be integrated with on-chip power supplies, and can be manufactured in high volume in silicon lines, ensuring cost-competitive chip production.

However, for deployment in handsets, the GaN-based PA has an Achilles heel – its high operating voltage, with devices typically driven at 28 V. But there is no longer a need to target a reduction to just a few volts to develop a handset-friendly solution, thanks to the introduction of higher supply voltages of up to 10 V, realised with the introduction of advanced power management ICs.

Taking advantage of this and delivering a step-change in the performance of PAs that can serve in handsets is a collaboration led by Dynax Semiconductor and involving engineers at Xiaomi Communications and The Hong Kong University of Science and Technology.

Speaking on behalf of this partnership at IEDM, Haochen Zhang from Dynax highlighted a HEMT efficiency of over 80 percent at a drain voltage below 10 V, and a three-stage GaN MMIC evaluated in the main board of a smartphone with a power-added efficiency of more than 50 percent.

“We believe that this marks the dawn of a wireless communication era defined by gallium-nitride-based radio-frequency technologies,” remarked Zhang.

Dynax, which has constructed an advanced GaN manufacturing centre that includes a 4,500 m² cleanroom, has devoted many years to pursuing the deployment of GaN in handsets. Efforts have been supported by an R&D team with over 150 staff that has helped to file over 500 patent applications, both domestically and internationally.

Zhang explained that one of the three key challenges the collaboration faced in developing a GaN PA technology for mobile phones was to ensure that the knee-voltage is as small as possible. “Secondly, gallium nitride devices suffer from gain soft-compression, which undermines linearity. Thirdly, unlike gallium arsenide HBTs, which are voltage-driven D-mode devices and work comfortably at 4.5 volts, gallium nitride devices are voltage-driven D-mode devices with a relatively high operating drain voltage.”

To address the latter concern, Zhang and co-workers introduced a power-management IC, and considered the voltage supply to both the gate and the drain.

Figure 1. Engineers at Dynax Semiconductor, Xiaomi Communications and The Hong Kong University of Science and Technology have developed a GaN HEMT for providing RF amplification in handsets.

Fabrication of the collaboration’s devices (see Figure 1) began by loading high-resistivity silicon substrates into an MOCVD reactor, applying an in-situ substrate treatment, and adding an AlN nucleation layer with an optimised thermal budget to regulate the distribution of impurity atoms at the AlN-silicon interface. This approach is claimed to reduce aluminium diffusion into the silicon substrate and suppress the formation of a parasitic channel in silicon, which would lead to an RF loss associated with the substrate.

“The substrate loss and interface loss is optimised, to be around 0.2 dB at 10 gigahertz, which is a level comparable to gallium-nitride-on-silicon-carbide devices,” remarked Zhang.

Figure 2. Substrate loss for III-N-on-silicon heterostructures is reduced with an AlN nucleation layer that has an optimised thermal budget and regulates the distribution of impurity atoms at the AlN-silicon interface.

Using these epiwafers, the team deposited a hard mask for re-growth of heavily doped GaN. Dry etching removed the SiO_x/SiN_x hard mask prior to GaN re-growth, involving a pre-dose of indium atoms that acted as a surfactant, improving interface morphology and promoting doping efficiency through the suppression of silicon self-compensation. Fabrication of the HEMTs was completed with hard mask removal, surface passivation through low-pressure CVD of SiN_x, and the addition of source and drain electrodes and a T-shaped gate. Resulting devices have a contact resistance of 0.09 Ω mm and a sheet resistance 251 Ω/sq.

Measurements of the DC characteristics of these devices – which have a gate length of 0.25 µm, and gate-to-source and gate-to-drain distances of 0.4 µm and 0.8 µm, respectively – determined an on-resistance of 0.76 W mm^-1, a knee voltage of 1.6 V, and a saturated drain-source current of 1.5 A mm^-1. It is said that these impressive figures provide the foundation for the high performance of the GaN HEMTs.

The breakdown voltage for these transistors is 98 V, a value claimed to fully guarantee device performance, ruggedness and reliability.

Small-signal characteristics at a drain-source voltage of 5 V revealed a peak cut-off frequency (f_T) of 31.2 GHz and a maximum oscillation frequency (f_max) of 66.2 GHz, and large-signal power sweeps determined a power-added efficiency of 84.2 percent and a maximum output power of 2.84 W mm^-1.

Zhang pointed out that even higher output powers have been realised by other teams, using devices with an InAlN barrier layer. “However, the reliability of such indium-incorporated devices remains an issue.”

To determine the reliability of their devices, Zhang and co-workers subjected 15 devices to high-temperature reverse bias tests (HTRB) and high-temperature operating lifetime (HTOL) tests.

“After the HTRB and HTOL stress, the devices show negligible performance degradation,” said Zhang. For example, the change in output power is below 0.1 dB.

The engineers also determined a mean-time-to-failure of 2,500 years at a junction temperature of 225 °C.

These GaN HEMTs provided the key building block for a three-stage PA MMIC that features a shunt power structure with high-pass and low-pass networks to increase bandwidth and linearity.

Zhang told delegates that to optimise linearity, it is critical to consider the inter-stage matching network between the driver PA and the final-stage PA. And he revealed: “To counteract gain compression, the gain curve of the driver stage was deliberately designed to expand at power saturation, and thereby enhance gain flatness over the entire bandwidth.”

Measurements of this MMIC under modulated signals at 2 GHz revealed a gain of 39.3 dB and a power-added efficiency of 60.1 percent. Benchmarking against GaAs HBTs, using values in publications and for a commercial product (see Table 1), led Zhang to claim: “This work demonstrates significantly higher output powers, due to a higher supply voltage, and especially optimised device fabrication and epi processes.”