This page contains a Flash digital edition of a book.
INDUSTRY ENGINEERED SUBSTRATES


Table 1. Electrical and thermal properties of various commercially available substrates commonly used in RF electronics or that may be used to host GaN.


smaller, due to the introduction of directly deposited diamond substrates.


One reason that the authors advocate a move to diamond is because it is the best commercial heat-spreading material in the world. It can have a room temperature thermal conductivity of more than 1500 W m-1


K-1 , and can be


four-to-five times that of the next best semiconductor substrate, SiC (see table 1 for details of key physical properties of various compound semiconductors). Diamond can be deposited to within hundreds of nanometres of the GaN channel, where it can efficiently extract heat out of the transistor-based device.


Simulations, modelling, and experiments all illustrate the promise of GaN-on- diamond. Researchers from various groups, including those assembled by


the Defense Advanced Research Projects Agency (DARPA), have determined that such transistor-based devices can operate at reduced channel temperatures and are capable of delivering about three times the areal power density of state-of- the-art GaN-on-SiC RF power amplifiers.


Diamond development The first thought of turning to diamond substrates is now more than a decade old, since which there have been several key breakthroughs. They include showing that high-quality diamond substrates can be deposited on 3-inch and 4-inch GaN wafers; demonstrating that it is possible to construct RF power amplifiers incorporating our diamond technology and undertaking numerous mechanical/materials, electrical, thermal, and reliability measurements that clearly indicate the superiority of GaN-on-diamond over GaN-on-SiC


in RF electronics. Unusual for a new semiconductor technology, the authors’ wafer formation process has undergone very few changes during its development (see Figure 1 for a pictorial overview of the progress). However, during that time there has been an exhaustive refinement with optimization of virtually every aspect of GaN-on-diamond wafer technology. These efforts include introducing larger wafers, improving coverage yield of GaN- on-diamond, optimising the thickness of interfacial material between the GaN and diamond, reducing the wafer bow and warp and refining methods for depositing/removing protection layers on top of the GaN epitaxy.


GaN-on-diamond wafers (see Figure 2) are formed by first depositing the AlGaN/GaN HEMT structure by MOCVD on high resistivity silicon. The epitaxial stack includes a 1.2 μm-thick proprietary transition layer, an 800 nm- thick undoped GaN buffer layer, a 17 nm-thick Al0.26


Ga0.74 and a 2 nm-thick GaN cap layer.


This epitaxial structure − GaN buffer, AlGaN barrier and GaN cap − is coated on the bottom of the GaN buffer with a dielectric and then a 100 μm-thick CVD diamond layer. This pair of additional layers is added by first removing the host silicon (111) substrate and transition layers beneath the AlGaN/GaN epitaxy, before depositing a 35 nm-thick proprietary dielectric onto the exposed AlGaN/GaN, and finally growing a 100 μm-thick CVD diamond substrate onto the dielectric adhering to the epitaxial AlGaN/GaN films.


Figure 1. Development of GaN-on-diamond wafers includes the first carrier mounted 3-inch GaN- on-diamond wafer in 2011 and the first 4-inch free-standing GaN-on-diamond wafer in 2012.


The real test for GaN-on-diamond is whether it delivers improvement to transistor performance. This is the focus of the remainder of this article, which details two studies: The first is of a comparison of GaN-on-diamond and GaN-on-silicon HEMTs − note


42 www.compoundsemiconductor.net October 2014 Distribution Statement A (Approved for Public Release, Distribution Unlimited)


N Schottky barrier


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76