A lump of polycrystalline SiC

From abrasives to general purpose ceramics, silicon carbide (SiC) has been produced since 1893 and is ubiquitous in daily life. SiC is also no stranger to electronics as an active material for power MOSFETs and other high power components. Since the early 2000’s, SiC SPICE model and analysis techniques have been investigated due to early lack of commercialization and support from many manufacturers. These days, all that is changing, and SiC models are becoming more widely available from specific manufacturers and the research community.

Why has it taken so long for the industry to start supplying SPICE models for these important components? Both SiC and GaN-SiC components exhibit interesting material properties that run counter to those seen in Si, and a direct conversion from Si to SiC parameters is difficult. This is also problematic as there are different variants of SiC (4H-SiC, 6H-SiC, and unintentionally doped SiC). In addition, fewer manufacturers are making these components, so there is simply less support for SPICE models.

Despite the difficulties in finding reliable and consistent SPICE models, the research community has taken a leadership role in developing SPICE models for SiC-based components. Let’s look into an easy way to create SPICE models for SiC MOSFETs for use in power conversion, amplifiers, and other power electronics applications.

## Defining a SPICE Model for SiC MOSFETs

Many device manufacturers are still using user-defined math equations with custom math functions (.FUNC syntax) and/or the DDT function to define the behavior of SiC devices. The research community is also using equations to describe device behavior, rather than using circuit components. Although these models are slower in simulations, they work for operating point analysis, DC sweeps, and transient analysis simulations. Be careful when working in AC and always compare your model against some benchmark results.

The common parameters to be input into a SPICE model for 4H-SiC and 6H-SiC MOSFETs are shown in the table below:

These parameters are necessary when working with static (DC) simulations. When working with time-dependent simulations, the following parameters are also required:

## SiC SPICE Models and Analysis

These values can be used in the standard MOSFET equations for static and dynamic behavior, although some models will simplify the relevant equations in some cases. The particular aspects and inputs into some subcircuit models are highly manufacturer-specific. Take a look at this IEEE article for more information on working with analytics models for these components.

Not all SPICE model libraries will contain component models for specialized SiC MOSFETs. Unless you plan to build your own SiC MOSFET model, you’ll need to get a component model from somewhere. Thankfully, SiC MOSFET manufacturers are taking time to develop, test, and release these models for their components. In the event you do not have access to a verified model for your SiC MOSFET, you can still create your own model using standard SPICE codes.

In terms of simulation requirements, these MOSFETs do not require any more sophisticated simulations than would be required for any other amplifiers. Perhaps the most important point in circuit design for any transistor is to determine a load line for your system as this will show you when the device crosses over into nonlinear behavior. If you’re planning to run in the linear regime, you should verify your output is distortion-free with transient analysis simulations. More advanced simulation techniques like load-pull are critical for SiC MOSFETs used as power amplifiers, and harmonic balance allows you to examine how an arbitrary input spectrum is transformed to an output in the linear and nonlinear regimes.

## GaN-SiC MOSFETs and Power Amplifiers

The power electronics and RF amplifier world will increasingly depend on GaN-SiC components compared to SiC or GaAs components. Compared to GaAs, GaN provides higher bandwidth at higher frequencies and it can tolerate a higher channel temperature. GaN is also as a wide bandgap semiconductor (3.4 eV bandgap for GaN compared to 1.42 eV for GaAs), which allows smaller GaN devices to be run at higher voltages than GaAs. The electron drift velocity in GaN also increases at high field strength, whereas it decreases in GaAs at high field strength. This means, at a given high operating voltage, higher current can be elicited from GaN compared to GaAs.

GaN was formerly placed on GaAs substrates, although this confined excessive heat to the GaN device. As SiC has much higher thermal conductivity compared to GaAs (25 W/m°C, 10x higher than GaAs), it acts as an ideal substrate for GaN MOSFET power amplifiers for high frequency applications. When placed on SiC, GaN devices can run at lower temperatures for a given power output, which extends their reliability. A comparison of GaN and GaAs devices for high power RF applications is shown below.

GaN on SiC vs. GaAs reliability curves

The reliability of these devices is clearly proven, but more needs to be done to build a suite of SPICE models for these devices on SiC. These power amplifier models will become more important in circuit and signal chain simulations as 5G rollouts progress. In particular, thermal simulations are quite important given recent news that 5G handsets were overheating and failing during summer months. All linear and nonlinear behavior with lower frequency SiC MOSFETs and higher frequency GaN-SiC MOSFETs can be captured with the right SPICE model and simulation steps. Be sure to check with your component manufacturer for verified SPICE models.

## Circuit Models for 4H-SiC, 6H-SiC, and GaN MOSFETs

Although most commercially available models are not circuit models, the adventurous designer can find plenty of help from the research literature. The Angelov model is seen as the current industry standard for GaN power MOSFETs, although it contains a number of parameters that are difficult to fit in such a nonlinear model. A 2019 article in Electronics (MDPI) presents a useful model for SiC MOSFETs. Any of these models can be brought into your schematic design software as a new component for use in SPICE simulations.

Your next high frequency power electronics system will need one or more SiC or GaN components, and you’ll need to use the best PCB design and analysis software to build your new system. The SiC SPICE model and analysis tools in PSpice Simulator for ORCAD and the full suite of analysis tools from Cadence are ideal for building advanced SiC and GaN PSpice models and other applications. You’ll also have access to verified models directly from manufacturers for simulating circuit behavior.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.

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