Wide Band-Gap

The Exciting New World of Wide Band-Gap – Solid State Power:-

Power semiconductor technologies are approaching an important threshold with both technical and commercial implications. The changing performance metrics will have dramatic consequent impact on the requirements of board level interconnect while the market potential will create important opportunities for packaging and interconnect technologies that meet these requirements.

Here at BPA in our publications, seminars, and workshops, we have been examining the advances in power semiconductor technologies and how these are driving the emergence of new types of printed circuit boards providing enhanced thermal and power management- what BPA has called “MiB”- for “Metal in (the) Board”. The well-known IMS (insulated metal substrate)/“MBCCL” (metal backed copper clad laminate) and “MCPCB” (metal core printed circuit board) are subsets of MiB- which includes a number of other technologies from thermal via arrays to embedded bus and low count power lines.

So far this has just been Silicon

DirectFETUntil very recently, advances in power semiconductors have been enabled by a combination of silicon processing and device packaging technologies. Junction resistances and subsequent power losses have been reduced through improved doping, imaging, and process technologies, while packaging has met the challenge with innovative designs that drastically reduce junction-to-case thermal resistance as shown by this example from International Rectifier.



For Progress with Power the Semiconductor material must be different.

Silicon presents electro-physical characteristics which place fundamental limits on how much power the device can manage. Silicon power semiconductor design has developed alternatives in the form of MOSFET and IGBT geometries, but at the end of the day it’s still silicon. Inexpensive, widely available, with high-yield standardized processes- but still silicon.

All this is in the process of changing. Wide band-gap (WBG) semiconductors have been known for some time. The two most common forms are Silicon Carbide (SiC) and Gallium Nitride (GaN). These materials differ substantially from silicon in that the combination of atomic size and lattice structure creates a larger gap between the conductive and insulating (valence) bands typical of semiconducting materials. The 3 eV bandgap of WBG semiconductors is closer to that of an insulator, ie it takes more energy for electrons to jump from the valence to the conduction band than it does in a silicon semiconductor with a 1.1 eV band-gap. This means much higher voltages can be managed on-chip with the right driver electronics.

Semiconductor materials – Compared

table of semiconductor material

This table compares the physical parameters of semiconductor materials showing the bigger Energy Gap (Band-Gap) for Sic and GaN.

WBG means a smaller device

In fact, not only can higher voltages be handled, but the size of the chip itself can be significantly reduced compared to a silicon device running an equivalent voltage.

… that can operate at much higher temperatures

Another characteristic of great interest in particular to Traction applications (electric vehicle drivetrains and transmissions) is that WBG semiconductors can run hot. Very hot- in excess of 250°C.

… well beyond the capabilities of conventional board technologies

The 125 – 150°C junction temperature limitations of existing families of Silicon power devices do not pose particular challenges to the substrate they’re mounted on, other than to get the heat out. A WBG device running at 275°C is over the Tg of even high temperature polyimides.

… and the growing market will demand more cost effective technologies.

The WBG interconnection strategy seems similar to the early days of silicon power design: mount the power devices on a substrate consisting of copper clad alumina (Al2O3) (commonly known as “DBC”- “direct bond copper”), with control logic on a thick film ceramic hybrid.

While functional, this configuration is just waiting to be substituted by more cost/performance efficient technologies. This looks like a real opportunity for MiB.

Ok, so what’s in it for us?

Compared to world power semiconductor demand which is forecast to become a $65 billion market by 2022 (IHS Inc) the share of WBG devices seems like small beer: at $3 billion it is less than 5%. On the other hand, the applications making up this share are not only growing rapidly (CAGR averaging 20% per year with some applications almost 40%), but they are going to be very concerned about heat and power management- otherwise they wouldn’t be paying top dollar for WBG as opposed to silicon.

Therefore board level interconnection technologies which offer the ability to integrate WBG devices and control logic in a hot, high power environment will be in demand, as will be the materials necessary to withstand continuous operating temperatures in excess of 250°C. Right now ceramics can satisfy that demand, but with the trade-offs of relatively high cost (compared to organic substrates), limited form factor, and mechanical fragility.

Opportunity awaits

For more information, contact Bill Burr or Nick Pearne.

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BPA Consulting Ltd

Tel: +44 (0)1306 875500
Skype: bpa-consulting

Company No.: 3709460

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