Picogiga Sampling First Compound Substrate Optimized for GaN Devices
Posted date : May 11, 2007

Jean-Luc Ledys explains how SopSiC can solve the GaN substrate dilemma.

Picogiga International, a division of the Soitec Group recently announced pre-production availability of SopSiC, a Smart Cut™ engineered substrate for gallium nitride (GaN) based power-switching and high-frequency devices. Customer response has been very positive.

Innovation leaders have long predicted that GaN transistors, which can handle both high power and high frequencies (in the 1-20GHz range) with minimal distortion, would eventually replace GaAs and LDMOS in a range of wireless (RF) communication systems such as radar, satellite communications and base stations. GaN is also a leading candidate for discrete power devices (Schottky diodes or power switches) utilized in power conversion for everything from hybrid cars to laptop computers.

GaN’s economic challenge

The challenge for GaN has been to make it economically viable – and the starting substrate has been the bottleneck.

GaN is deposited epitaxially—traditionally on a bulk substrate, with the substrate acting as a “seed” for growth.

To ensure low-distortion and optimal power amplification over a high-frequency signal, the lattice structure of the GaN and the seed have to be a nearly perfect match. Until now, this has effectively limited potential substrates to either bulk silicon (very inexpensive), sapphire (low performance, medium cost) or bulk silicon carbide (SiC) (very expensive).

Beating the heat

However, there’s another constraint with GaN transistors – as you push the power, heat becomes an issue. Beyond a certain point, silicon is no longer viable for evacuating the heat away from the transistor. Until now, that has left SiC as the only other substrate option for higher power and frequency applications.
But in addition to being expensive and limited in sources of supply, SiC is only available in very small diameters, preventing III-V device makers from leveraging the economies of scale of bigger wafers, as we have seen in the silicon industry.

Figure 2. Final inspection of a 4” SopSiC wafer. (Courtesy: Picogiga International)

SopSiC: best $/watt

SopSiC (which stands for silicon-on-polysilicon-carbide) offers a significantly better performing solution than silicon—and a considerably less expensive solution than SiC. In terms of dollar/watt, SopSiC is an extremely attractive solution.

While GaN on both silicon and silicon carbide is part of our existing epiwafer product line for high-power applications, Picogiga is uniquely positioned in the industry to deliver a radically different approach to the GaN substrate dilemma.

Smart Cut: for a unique approach

What makes the SopSiC structure so different is that it is engineered using Smart Cut layer transfer and bonding technology (the same technology used by Soitec in high-volume SOI wafer production). SopSiC includes: a bottom layer of polysilicon-carbide, an insulating buried oxide layer, and a high resistivity (1-1-1) silicon top layer (see Figure 1).

SopSiC samples for customers are now available in 3” and 4” diameters (see Figure 2), with the capability of being scaled up to 6” or even 8” versions.

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