Nikkei Electronics Asia -- August 2009
Insights
GaN-Based Power Devices Improve Power Conversion

With radical improvements in R(ON) x Qsw figure of merit (FOM), leading to an application value proposition - efficiency x density/cost - that is an order of magnitude better than state-of-the-art silicon, gallium nitride (GaN)-based power devices promise a revolution in high efficiency, high density, cost effective power conversion solutions.

By enabling rapid adoption of switch mode power supplies (SMPS), silicon power MOSFETs have been at the forefront of power conversion for last three decades.

From planar HEXFETs, introduced in 1978 by IR, to TrenchFETs and superjunction (SJ) FETs, power MOSFETs have given bipolars a run for their money for nearly 30 years. But now this silicon power device has approached a performance plateau. That means, going forward it does not have the juice to deliver performance/cost ratio demanded by next -generation applications. Consequently, any performance increment will result in unwarranted excessive expenses.

New materials and transistor structures are therefore needed to fill this power conversion performance gap. Even though silicon carbide (SiC) developers have been tackling these issues in the past ten years, it has not made any dent in this market because of cost. Besides the intrinsic cost structure of SiC, the limited supply of quality material also makes this technology very expensive, adding to the non-scalability of the substrate size and the expitaxial deposition throughput shortcomings.

Anticipating a need to look for solutions beyond silicon, scientists and engineers have been researching new technologies. One of these is the proprietary GaN-based power device technology platform developed by IR, which delivers a FOM performance at least 10x better than existing silicon MOSFETs.

GaN-on-Si Benefits

Since bulk GaN substrates are uneconomical, developers have taken the hetero-epitaxial route for building GaN-based power devices. However, until now, major substrates used for GaN epitaxy have been SiC or sapphire. But, both are relatively expensive propositions.

Although silicon was an attractive low-cost alternative, it remained difficult because of defects and deformations due to intrinsic mismatch in lattice constants and thermal expansion coefficients. Leveraging the extensive industry experience in GaN epitaxy and devices, significant engineering efforts have been made to resolve these issues. As a result, GaN-on-silicon technology platform has been developed to offer high epitaxial film uniformity, lower defect levels and higher device reliability. In addition, the device manufacturing process is CMOS compatible, thus allowing high volume deposition of GaN-based material on low-cost silicon wafers with larger diameter substrates. This novel GaN-on-Si design and fabrication technology platform is referred to as GaNpowIR.

As shown in the Fig, the basic GaN-on-Si-based power device is a high electron mobility transistor (HEMT), based on the presence of a two dimensional electron gas (2DEG) spontaneously formed by the intimacy of a thin layer of AlGaN on a high-quality GaN surface. As this device structure is a HFET with a high electron mobility channel that conducts in the absence of applied voltage (normally on), several techniques have been developed to provide a built-in modification of the 2DEG under the gated region that permits normally off behavior.

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