Displacing the Silicon Power MOSFET
with eGaN^® FETs

As discussed in Issue 2 of Fast Just Got Faster, gallium nitride (GaN) and silicon carbide (SiC) have started to displace silicon (Si) in power conversion due to higher performance and lower cost. It is anticipated that GaN transistors will continue to become the dominant power device.

This issue of Fast Just Got Faster discusses attributes of enhancement-mode GaN transistors that make it a disruptive technology and identifies four factors required for GaN technology to displace the silicon MOSFET as the “power transistor of choice.”

Enhancement-Mode FETs – A Disruptive Technology at a Competitive Cost

EPC introduced the first “off-the-shelf” enhancement-mode GaN field effect transistor (eGaN® FET) in mid-2009. These first products were for voltage ratings from 40 V to 200 V, a range that represents 76% of the $6.3B power MOSFET market! Three fundamental attributes eGaN FETs offer the power designer are switching speed, small size and competitive cost.

For comparison, there are two types of GaN transistors – depletion mode (normally “off”) and enhancement mode (normally “on”). Depletion-mode devices are inconvenient because, at the startup of a power converter, a negative voltage must first be applied to the control electrode (gate of the device) to avoid a catastrophic failure. Compensating for this inherent limitation requires the addition of a MOSFET, which adds to its cost. An enhancement-mode device, on the other hand, does not have this inherent start-up limitation and does not require a MOSFET be included in its design.

Since their introduction, eGaN FETs have proved to be the simplest and lowest cost GaN device to manufacture since they are processed in a standard silicon wafer foundry using standard, existing silicon processing equipment.

Furthermore, eGaN FETs have a lateral structure, which enables chip-level packaging thus eliminating the need for the costly outer plastic package required by standard silicon MOSFETs. The size comparison between an eGaN FET and a power MOSFET is obvious in side-by-side comparison of equivalent power conversion circuits (see figure). In addition to consuming valuable space in a power conversion system, the plastic packages used by MOSFETs are, on average, 50% of the final power MOSFET’s cost.

In addition to this size advantage, the switching speed of eGaN FETs is 10 times faster than the best available silicon transistors. This speed advantage is the reason gallium nitride transistors can increase power conversion efficiency, lower overall system costs, and enable new, exciting applications that are simply beyond the reach of the venerable silicon power MOSFET.

Displacing the Silicon Power MOSFET with eGaN FETs

35 years ago, the silicon power MOSFET was a disruptive technology that displaced the bipolar transistor. The dynamics of this transition taught us that there were four key factors controlling the adoption rate of a new power conversion technology:

1. Does it enable significant new applications?
2. Is it easy to use?
3. Is it VERY cost effective to the user?
4. Is it reliable?

Let’s now address each of these questions individually for the next generation of technology – eGaN FETs compared with silicon power MOSFETs.

1. Does it enable significant new applications?
   
   Some examples of large new applications that are made possible primarily because of the higher switching speed of eGaN FETs include:
    
   1. Envelope Tracking: This is a power supply technique that can double the energy efficiency of RF power amplifiers used to transmit all of our voice and data through satellites, base stations, and cell phones. Envelope tracking is accomplished by tracking the power demand precisely and providing the power to exactly fit the amplifiers signal modulation needs. Today, RF power amplifiers operate at a fixed power level delivering maximum power whether or not the transmitter needs it. Excitingly enough, eGaN FETs are the first transistors capable of tracking power demands at the high data transmission rates used in 4G LTE networks!
   2. Wireless Power: Cut the cord! Wireless power transfer enables cell phone, game controllers, laptop computers, tablets, and even electric vehicles to re-charge without being plugged in. A high frequency standard (6.78 MHz) for power transmission is currently being adopted by an industry consortium (A4WP). MOSFETS do not perform well at speeds this fast, whereas eGaN FETs offer an alternative that switches fast enough to be ideal.
   3. LiDAR (Light Distancing And Ranging): LiDAR uses pulsed lasers to rapidly create a three dimensional image of a surrounding area. This technique is widely used for geographic mapping functions and is technology driving (so to speak) “driverless” cars. The higher switching speed of eGaN FETs drive superior resolution and response time that enable LiDAR applications beyond the mapping functions to applications such as real-time motion detection for video gaming, computers that no longer require touch screens, and fully autonomous vehicles.
2. Is it easy to use?
   
   eGaN transistors from EPC are designed to be very similar in behavior to existing power MOSFETs, and therefore power systems engineers can use their design experience with minimal additional training. To assist design engineers up the learning curve, EPC has established itself as the leader in educating the industry about gallium nitride devices and their applications. As a matter of fact, in addition to publishing over 50 articles and presentations, in 2011 EPC published the industry’s first GaN transistor textbook (in English and Chinese) – GaN Transistors for Efficient Power Conversion. The second edition is being written now and will be published by the world wide textbook publisher, John Wiley, in late 2014. EPC is working with more than 30 universities around the world in order to lay the groundwork for the next generation of highly skilled power system designers trained in getting the most out of eGaN FETs.
3. Is it VERY cost effective to the user?
   
   As noted previously, EPC’s eGaN FETs are produced using processes similar to silicon power MOSFETs and actually have many fewer processing steps than MOSFETs. The only step that is more costly is the growth of the thin “epitaxial” gallium nitride crystal layer on top of a standard silicon wafer. Going forward, improved machine design will mean that this epitaxial growth will not be a significant cost adder. As GaN transistors mature, and taking into account the significant packaging advantage discussed above, the cost of producing eGaN FETs has the potential for being significantly lower than that of MOSFETs – at which point the designer does not even need to take advantage of the higher performance of GaN to realize cost savings in the system!
4. Is it reliable?
   
   GaN on silicon transistors are in the early stages of establishing their reliability. To date, tens of millions of hours of stress testing from several manufacturers suggest this technology is capable of performing at acceptable levels of reliability in commercial applications today.

Thus, fast switching speed, small size and competitive cost give the enhancement-mode GaN transistor the attributes to continue to displace the silicon MOSFET. As a measure of their success, GaN FETs are conquering the four factors required to displace the silicon MOSFET as the “power transistor of choice.”