Gallium nitride is the undisputed technology for achieving high-efficiency operation in high-frequency applications, such as those at X-band (8–12 GHz).
And GaN on SiC devices can deliver the much-needed high temperature reliability and power density for these applications due to unbeatable thermal conductivity of the materials as well as a lattice match between them.1
But device selection for X-band applications doesn’t end with choosing the material technology, because turning the bulk material characteristics into high-performance GaN on SiC devices is quite another matter.
For X-band application designers, Wolfspeed, the vertically integrated GaN on SiC device manufacturer and provider, foundry services brings to the table not only over 30 years of experience in wide-bandgap materials and development but device design success.
Process is the secret sauce
Wolfspeed counts several processes in its portfolio, each designed to best meet a different set of application requirements (Figure 1). For instance, G28V5 is a high-performance 28-V process that targets high-frequency applications as well as lower-frequency operation for the highest efficiency or wide bandwidth requirements.
The effect of gate dimensions on gain, frequency, and breakdown voltage is well-established. A key parameter to consider is gate length, which influences gate resistance and the gate-to-drain capacitance. Generally, a shorter gate decreases capacitance which enables higher frequency operation, while a longer gate length increases gate capacitance. Gate length is therefore inversely proportional to the maximum frequency (FMAX) and unit-gain cutoff frequency (FT) exhibited by a GaN HEMT.
Wolfspeed has achieved various process characteristics shown in Figure 1 with carefully considered tradeoffs, among other parameters, between maximum frequency, breakdown voltage, and gate length.
Process selection is therefore an important step in the device design that the company undertakes when fabricating GaN on SiC parts.
Selecting the process
In designing GaN on SiC MMICs for X-band, several preliminary design choices must be made, including selecting a process technology. Wolfspeed’s latest GaN on SiC process, the G28V5, addresses the needs of operation beyond X-band to 40 GHz. A key factor that enabled this was the decrease in gate length to 0.15 μm.
It features two gold RF interconnect layers, two varieties of capacitors, thin-film and bulk GaN resistors, and dielectrically supported bridges for connections to circuit elements, such as capacitors and inductors. The SiC substrate is just 75 μm thick and has the smallest substrate via sizes available in a GaN on SiC MMIC process. This allows for the FET footprint to be very small for X-band applications.
The G28V5 features:
- • 0.15-μm gate length
- • Threshold voltage (VP) ~–2 V
- • 28-V bias with >84-V breakdown
- • FMAX >120 GHz
- • 12-dB gain @ 30 GHz
- • 3.75-W/mm power density @ 30 GHz
- • Power-added efficiency (PAE) >40% @ 30 GHz
- • Metal 1 = 3 μm; Metal 2 = 3 μm
- • Metal-insulator-metal standard density capacitance 180 pF/mm2
- • High-density capacitance 305 pF/mm2
- • Thin-film resistance 12 Ω/sq
- • GaN resistors 66 Ω/sq and 410 Ω/sq
The process offers mean-time-to-failure of over 1 million hours at 225°C and has been fully qualified.
More considerations for MMICs
Other preliminary design considerations include transistor size and bias, the number of transistors required to meet output power requirements, matching considerations, and load-pull simulations.
Load-pull simulations must also be conducted for the process to understand how PAE varies at the target frequency over a range of load impedances (Figure 2). Wolfspeed uses these measurements and describes devices with physical equations. The resulting device models are so accurate as to make first-pass design possible.
Figure 2: Measured G28V5 capability data
PA designed for X-band
With these considerations taken into account, Wolfspeed has developed the CMPA801B030S, a packaged, 40-W PA for the 7.9 to 11.0 GHz application range. It is part of the CMPA801B030 series of MMICs that offers peak outputs ranging from 30 W to 40 W, with gains from 16 dB to 28 dB.
The CMPA801B030S MMIC utilizes two stages of gain to deliver 20 dB of large signal gain (Figure 3). Its 40% efficiency supports lower system DC power requirements, and together with a junction temperature Tj rated to 225°C, the MMIC simplifies the cooling subsystem.
Furthermore, the part is offered in a 7 × 7 mm plastic overmold QFN to meet space constraints and high-throughput manufacturing requirements.
Process to device to reference design
X-band applications, such as phased-array radar, including synthetic aperture and active electronically scanned array radars, are important equipment for a very wide range of markets, including airspace monitoring and weapons targeting for defense, commercial aviation and maritime navigation, air and maritime traffic control, fire control systems, weather monitoring, and even high-resolution urban monitoring and vegetation mapping.
Market researcher Strategy Analytics estimates that just the X-band radar segment will grow from $6.3 billion in 2018 to over $8.7 billion in 2028.2
1GaN on SiC: The Substrate Challenge (https://www.wolfspeed.com/knowledge-center/article/gan-on-sic-the-substrate-challenge/)
2Strategy Analytics, Defense Radar Market and Technology Forecast: 2018–2028