GaN on silicon carbide wafers

Modelling and design tools adapting to new materials

The growing use of silicon alternatives such as gallium nitride and silicon carbide is forcing changes in electronic modelling and design software

The use of alternative organic and inorganic materials and compounds to replace traditional silicon within integrated circuits (ICs) and transistors is widely expected to expand over the next few years. As a result, modelling and simulation tools are being upgraded to let design and manufacturing companies work out how best to use them.

Various analyst predictions forecast broader adoption of components based on transistors that use inorganic materials such as gallium nitride (GaN) or silicon carbide (SiC) as a semiconductor band-gap compound to handle the same voltages but in smaller, more reliable packages.

French market research firm Yole Développement predicted that revenue from sales of GaN-based devices would grow from $10m in 2012 to $500m in 2016, for example, with an expected surge in demand for electric vehicles using them for mains battery chargers, DC-DC voltage conversion systems and drivetrains predicted to push that figure to $1bn beyond 2017.

More recently IHS, has forecast that GaN-on-silicon LEDs will increase their market share from 1 per cent in 2013 to 40 per cent in 2020.

Mobile networks lead

Semiconductor company Cree manufactures SiC substrates and group III-nitride epitaxial wafers for use in transistors utilised for light emitting diodes (LEDs), power switching devices and radio frequency (RF) power transistors for wireless communications.

In 2014 Cree introduced a family of high-power GaN RF transistors in low-cost plastic packages, including a 300W model operating at 2.7GHz. They are destined for the fourth generation (LTE, or long term evolution) cellular mobile network infrastructure currently being deployed by operators worldwide.

Laurence Wilson, research director at ABI research calculates that while silicon laterally diffused metal oxide semiconductor (LDMOS) has been the dominant technology in microwave and RF applications for the last 20 years, GaN RF power alternatives have recently 'captured a meaningful market share'. But while adoption of GaN components has so far been limited because of their far higher costs, Cree's plastic-package design brings the difference down to a level that is much closer to parity with silicon.Wilson believes this will make GaN transistors the principal choice for next-'generation wireless networks.

Cree did not reveal which EDA or M&S software it used to create its new plastic-packaged transistors, but the company produced a suite of Verilog-A non-linear device models for a previous family of GaN RF devices using Agilent's Advanced Design System (ADS – the EDA software system now owned by Keysight Technologies' EEsof EDA division) and the Microwave Office design platform owned by AWR (formerly Applied Wave Research).

When combined with envelope simulators, these Verilog-A models allow designers to investigate high efficiency circuit approaches, such as Doherty amplifiers, to improve adjacent channel power ratios, spectral re-growth and error vectors. With the 2.7GHz modules available at power levels of 60, 100, 150, 200 and 300 watts, and at different frequencies ranging between 690-960MHz, 1800-2300MHz and 2300-2700MHz, EDA and M&S tools will have played a huge part in validating Cree designs to ensure required levels of drain efficiency, gain, moisture sensitivity levels (MSL-3) and JEDEC environmental standards.

RF power components

Agilent upgraded its Integrated Circuit Characterisation and Analysis Platform (IC-CAP) widely used for device modelling to include GaN support in 2013. The software was later chosen by Nitronex, a producer of GaN-on-silicon RF power devices that is now part of MACOM, to create GaN design flows able to span both device modelling and circuit simulation. The company said building high-power RF devices needed software that allowed its engineers to experiment with different combinations of bandwidth, efficiency and power to deliver the required levels of reliability, performance and yield.

Used in parallel, Agilent/Keysight's EEsof EDA software and its IC-CAP model extraction application helped to provide more accurate modelling and simulation for GaN FET amplifiers while at the same time streamlining its design process.

Freescale Semiconductor, another supplier of RF power transistors, introduced a 125W continuous wave gallium nitride on silicon carbide (GaN-on-SiC) transistor in December 2014. This component is targeted at wideband amplifiers in scientific equipment and military communications products for the US defence sector, including jammers, radar implementations and electronic warfare systems. Freescale also provides a GaN transistor as part of its Airfast RF power product line for mobile VHF, UFH and 700-900MHz transceivers used in public safety, professional mobile radio and other outdoor machine-to-machine (M2M) communications environments.

The company did not reveal which modelling and simulation or EDA software it used for its MMRF5014H GaN-on-SiC transistor design, but AWR built a Microwave Office 2010 design software library for its RF high-power models which are also available for Agilent's ADS whilst Freescale has previously worked with Mentor Graphics on its silicon products.

TCAD takes up the mantle

Many companies, universities and research institutions are also designing semiconductor devices based on alternative materials using technology computer aided design (TCAD) tools which produce SPICE models that allow engineers to move from generation one to generation two of their prototypes as they start to explore their component design processes in more detail.

One is the Shibaura Institute of Technology (SIT) in Japan which adopted Silvaco's Atlas TCAD device simulator for its research on GaN in October 2013, pledging to use the software to aid its applied research into GaN compound semiconductors for power electronics.

Others have used Atlas to build simulations of GaN Super HFET (super heterojunction field effect transistor) structures which they say will speed up the product design process and shorten the development period.

Silvaco is also working with Arokia Nathan, Professor of Photonic Systems and Displays at the University of Cambridge, who is examining ways of harnessing graphene and other nanomaterials for innovative new applications and usage scenarios.

'The people we are working with in the UK are involved in printed logic and this tends to be simpler type applications but very innovative,' says Silvaco engineer Ahmed Nejim. 'Many printed logic applications will be a very good fit into the Internet of Things – things like NFC, smart logic that will communicate some data [which can] be produced as cheaply as possible on flexible substrates or colourful rendering. Believe it or not there is a lot of intellectual property (IP) riding on this, because the deployment of the technology is so novel and the usage scenarios so unexpected.'

One example is PragmatIC, a start-up developing printed electronic logic circuits, which was one of six companies to launch the Printed Intelligent NFC Game cards and packaging (PING) project in March 2015. This EU-backed initiative will focus on creating a manufacturing flow for flexible, thin-film electronics that can be used to embed wireless identification and power transfer technology into printed materials such as cards, stickers and packaging or printable substrates like paper, cardboard and plastic, initially for IoT and 'Internet of Games' applications. The project will also look at supporting further integrated features such as sensors, displays and sounds.

Another company focused on delivering TCAD tools for power semiconductor design is Synopsys. Its Sentaurus platform includes a multi-dimensional device simulator capable of simulating electrical, thermal, and optical characteristics of a wide range of power devices, including insulated gate bipolar transistors (IGBTs), power MOSFETs, laterally diffused metal oxide semiconductors (LDMOS), thyristors and high frequency high power devices made from wide-band-gap materials such as GaN and SiC.

Elsewhere, Taiwan's Industrial Technology Research Institute (ITRI), which has been instrumental in nurturing semiconductor companies such as TSMC and UMC, began using Sentaurus in 2011 to develop a new generation of SiC devices targeting applications in hybrid and electric vehicles, smart grids and other innovative power systems.

A big selling point for GaN, SiC and other materials is lower production costs given that there is no need for a clean room, whilst new, faster methods of production enabled by printed electronics in particular will put extra pressure on suppliers of existing modelling and simulation tools to adapt.

'New materials lend themselves to a new way of production called reel-to-reel which means [a manufacturer] can flood the market within a week or two. Design software has to cope with that as well, because you are changing technology generation so fast, it becomes really hard to go and agree a standard every time,' says Nejim. 'It also allows a lot of small to medium size enterprises to come into the game; you do not have to be a huge fab with billions of dollars of pre-investment.'

Standardised models

Nejim argues that standardised TCAD models supporting alternative organic and inorganic materials which can be passed onto mainstream silicon foundries have been relatively slow to emerge. He suggests this might be because some manufacturers are reluctant to reveal the innovative designs they are working on, or which tools they are using to perfect their models, but it may also be because components based on GaN, SiC and other materials have been, and remain, a comparatively niche area of expertise.

In the past, this situation led to different vendors implementing their own proprietary models, leaving the Compact Model Coalition (the industry group formed to choose, maintain and promote the use of standardised transistor models) struggling to keep up. The CMC has made some progress in recent years however, particularly around the Berkeley Short-channel IGFET Model (BSIM) for MOSFET integrated circuit simulation, which was created by the Department of Electrical Engineering and Computer Sciences at the University of California, Berkeley.

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