technology GaN electronics
there is not as much software available for aiding the design of GaN HEMTs as there is for silicon devices, Titizian says that what exists is adequate.
Outsourcing epiwafer growth Integra decided against developing in-house, GaN-on-silicon growth expertise. Instead it evaluated material from several epiwafer suppliers. In some cases these wafers were too brittle, or yielded devices with leakage currents that were far too high. However, these problems did not plague the material provided by a Japanese manufacturer, which is now Integra’s sole supplier of epiwafers.
Integra has developed two products
operating in the upper S-band (3.1-3.5 GHz). These devices are targeting military radar. The transistor shown here produces 120 W. The other device, which produces over 10 W, is an internally matched part with 50 ohm impedance at the leads
“We were debating whether to go with GaAs or GaN,” reminisces Titizian. “After the information we got at that conference we decided that we were going with GaN.”
Efforts began in earnest in the latter half of 2008. At that point the company set itself the target of sampling its first GaN products in 2010. It is a goal that has been completely fulfilled following the company’s announcement of two GaN HEMT products operating in the S-band at the IEEE MTT-S International Microwave Symposium that was held this May in Anaheim, CA.
Initially Integra’s GaN development involved just two senior staff – a process engineer and a designer. Since then the team has expanded to five, with three employees working on the processing of the device and another two focusing on circuit development. This appears to be a remarkably small number of engineers for driving the development of a transistor employing a material entirely new to the company.
Titizian, however, is not surprised by the tremendous progress: “ We are semiconductor experts, and we understand semiconductor physics. It’s true that GaN presents some different challenges, but we understand the device physics.”
When founded, the company’s expertise lay in silicon bipolar devices. However, more recently it has expanded its knowledge base, first with the development of silicon LDMOS technology, which is not so far removed from GaN HEMTs. “If we had to go from bipolar to GaN, then that may have been a bigger jump, ” says Titizian, who reveals that extensive design iteration was not required for the development of the company’s HEMTs. Although
24
www.compoundsemiconductor.net August / September 2010
By adopting an outsourcing model, Integra slashed its capital expenditure on new equipment needed to provide its fab with the capability to process GaN-on-silicon HEMTs. Its only recent addition is a chorine-based, inductively coupled plasma (ICP) etcher. However, there have also been some adjustments to processing tools on the 6-inch line, so that they are capable of handling 4-inch GaN-on-silicon wafers.
The epiwafers that Integra imports have a silicon (111) foundation, because this orientation is better at accommodating the lattice and thermal mismatches between silicon and GaN than the more widely used (100) cut. Processing this material presents no problem whatsoever for Integra, because the company’s bipolar products are also grown on the silicon (111) orientation.
Integra has also been able to draw on its silicon technology for packaging its GaN HEMTs, which share the housing employed for LDMOS devices. Problems associated with parasitics are negated by taking sufficient care with the assembly process.
The pinnacle of all this effort-to-date has been the launch of products at the MTT-S show: a pair of single-ended devices housed in a ceramic flanged package, which deliver either a 25 W or 50 W peak output in the 2.7-3.1 GHz range. Both HEMTs produce at least 12 dB of gain and have a breakdown voltage in excess of 200 V. These performance figures were recorded using 300 µs pulses, a 10 percent duty cycle, and an operating voltage of 50 V.
What’s the pecking order?
Although the performance of these devices is impressive for a firm that only started developing GaN products two years ago, these HEMTs are not state-of-the-art commercial products.
“We’re getting 3.5 W/mm, and there are devices out there by Cree and Eudyna in the 5 W/mm range,” admits Titizian. “We could design devices with higher W/mm, but we’re focused on offering a reliable device that will not
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