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2011 Evince Technology Limited, Netpark, Sedgefield, County Durham, UK, TS21 3FD

Why Diamond Is Better Than Anything Else

Ask any electronics engineer "what is the ultimate semiconductor material?", and the majority will say "diamond". Take a look at any comparative table of properties and the reason is obvious (strangely exponents of gallium nitride and silicon carbide always omit the diamond column from their presentations!):

Property (relative to silicon)

Si

GaAs

GaN

SiC

diamond

Thermal conductivity

1

0.3

0.9

3.1

13.5

Thermal expansion coefficient

1

1.6

2.2

1.6

0.03

Dielectric constant

1

1.06

0.9

0.9

0.5

Electron mobility

1

5.67

0.83

0.67

3.0

Hole mobility

1

0.67

0.42

0.08

6.3

Saturated carrier velocity

1

1

2.2

2

>2.5

Where diamond really scores is that it far outperforms any other material in terms of its ability to insulate very high voltages across very thin layers of the material. The lower the insulation strength the more base material you need to start with (which is a big issue when wide bandgap semiconductors are already at least 10 times the cost of silicon), but more importantly the slower the device operates - which is now why 6,500V appears to be the ultimate practical limit for silicon insulated gate bipolar transistors (IGBTs).

Dielectriccomparison

If you continue to explore further, look at the benchmark factors that have been proposed to compare various wide bandgap semiconductors for their suitability for high power electronic applications and the disparity becomes even more apparent. All these benchmarks are based on the constants relating to the material properties listed above as they are applied in engineering formulae the underpin the design of semiconductor devices:

Benchmark Factor (relative to silicon)

Si

GaAs

GaN

SiC

diamond

Johnson Figure of Merit

1

11

790

410

5800

Keyes Figure of Merit

1

0.45

1.6

5.1

31

Baliga Figure of Merit

1

28

400

290

12500

Baliga High Frequency Figure of Merit

1

16

44

34

1100

Despite its obvious intrinsic advantages, diamond has struggled in the past to make any significant progress as a semiconductor material. The reason for this because of is two widely held perceptions:

Historically there has been limited availability of high quality substrate materials

Over the past 10 years there have been major advances in the production of diamond by Chemical Vapour Deposition. Today there are several leading material growers offering extremely high quality diamond materials in single crystal, polycrystal (m grain size) and nanocrystalline (5nm to 100nm grain size) forms, with wafer sizes up to 200mm available in the non-single crystal forms and 25mm in single-crystal form. These multiple forms of diamond offer a choice of characteristics that mean the most approriate form can be selected for any given application. Further, and more importantly, the scaled production cost of diamond is projected to be on a par with all other major wide-bandgap materials in use today.

Inability to dope diamond in the same way as silicon (in particular n-type) means that it's not possible to make practical electronic devices

This misconception has dogged diamond for decades. While it is highly unlikely that a truly effective compatible n-type dopant will be found for diamond, there is also an unspoken truth about diamond: it's not a semiconductor, rather it's an electrical insulator with some phenomenal charge transport properties. Making good working devices in diamond first requires tearing up the rule book to fully exploit its unique properties.

Misconceptions such as these have even propogated through national strategy documents such as, the Power Electronics Research and Develpment Program Plan published by the US Department of Energy in April 2011 which states "... the diamond manufacturing process is still in its infancy; it is expected that research will yield diamond power devices no sooner than 2–3 decades from now.". This quote is verbatim from a 2007 review paper, that in turn is verbatim from a 2003 review paper that in turn takes it reference from a 1998 paper. Even by the pessimistic reckoning of this statement diamond is already well over halfway there!

The simple fact is that diamond is ideal material to meet the needs of the energy systems of today and tomorrow where the need exists to precisely control the flow of electricity from watts to megawatts. Unlike other wide bandgap materials, diamond has the potential to be able to clearly differentiate itself against existing silicon on cost (per switched watt) and performance. Beyond power electronics, diamond has a wide potential of electronics applications that exploit other facets of diamonds superior capabilities including: bio-compatible and ultrahard wearing MEMs, photovoltaics and extreme environment devices.

For more on diamond read on ...