Powering with Power Semiconductors

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Power semiconductor is a kind of high power electronic power device which is refilled and sealed according to certain function combination. The emerging market for silicon carbide (SiC) and gallium nitride (GaN) power semiconductors is forecast to pass the $1 billion mark in five years, energized by demand from hybrid and electric vehicles, power supplies and photovoltaic (PV) inverters. Worldwide revenue from sales of SiC and GaN power semiconductors is projected to rise to $3.7 billion in 2025, up from just $210 million in 2015, according to IHS Inc., the global source of critical information and insight. Market revenue is also expected to rise with double digit growth annually for the next decade. SiC Schottky diodes have been on the market for more than 10 years, with SiC metal-oxide semiconductor field-effect transistors (MOSFET), junction-gate field-effect transistors (JFET) and bipolar junction transistors (BJT) appearing in recent years, according to the latest information from the latest IHS SiC & GaN Power Semiconductors Report. SiC MOSFETs are proving very popular among manufacturers, with several companies are already offering them, and more are expected to in the coming year. Today, the ongoing major trend in solar designs is the increase of power density based on a reduction of switching losses, enabling smaller heatsinks, and also allowing higher operating frequencies, enabling smaller magnetics. SiC diodes have increasingly become a staple component in modern solar string inverter solutions as well as in micro-inverter applications. Hybrid solutions are a standard part of today’s solar inverters all around the world.

Power semiconductors employed in power management systems include power switches, and rectifiers (diodes). Power switches include MOSFETs, IGBTs, and BJTs (bipolar junction transistors). MOSFETs, IGBTs and BJTs are found in two different forms:

  • Discrete Power Semiconductors- These devices are only a single type housed in a single package.
  • Integrated Power Semiconductors- Integrated with other circuits in a single package, they may be housed in a multi-chip module, or MCM package, that is, interconnected with other devices in the same package.

Detailed descriptions of the key categories of power semicconductors follow.

Power MOSFETs

Power MOSFETs (Metal-Oxide Semiconductor Field Effect Transistors) are three-terminal silicon devices that function by applying a signal to the gate that controls current conduction between source and drain. Their current conduction capabilities are up to several tens of amperes, with breakdown voltage ratings (BVDSS) of 10V to over 1000V.
IGBTs

The insulated gate bipolar transistor (IGBT) is a three-terminal power semiconductor noted for high efficiency and modest switching speeds. It switches electric power in many modern appliances: electric cars, variable speed refrigerators, air-conditioners. IGBTs are usually only discrete devices, or may have an integrated diode. 
 SiC

SiC (silicon carbide) power semiconductors can theoretically reduce on-resistance to two orders of magnitude compared with existing Si devices. The use of SiC device is expected to reduce power loss extensively, when applied to power conversion systems. SiC devices as well as power MOSFETs or IGBTs may be used with rectifier devices such as Schottky barrier diode (SBD). SiC-SBD have been introduced, but SiC power MOSFETs has been difficult to manufacture because usable SiC material has been difficult to produce. However, SiC power transistors are now available.

GaN

Gallium nitride (GaN) is grown on top of a silicon substrate. The end result is a fundamentally simple, cost effective solution for power switching. This device be­haves similarly to Silicon MOSFETs with some exceptions. 
GaN transistors behave in a similar manner to silicon Power MOSFETs. A positive bias on the gate relative to the source causes the device to turn on. When the bias is removed from the gate, the electrons un­der it are dispersed into the GaN, recreating the depletion region, and once again, giving it the capability to block voltage. Among GaN’s features are:

  • GaN offers superior performance compared with both silicon and silicon carbide.
  • Device-grade gallium nitride can be grown on top of silicon wafers.
  • GaN-on-silicon offers the advantage of self-isolation and therefore efficient monolithic power integrated circuits can be fabricated economically
  • Enhancement-mode (normally off) and depletion mode (normally on) GaN devices are available.

Power Semiconductor Reliability

Excessive operating voltage can cause power semiconductor failures because the devices may have small spacing between their internal elements. An even worse condition for a power semiconductor is to have high voltage and high current present simultaneously. A few nanoseconds at an excessive voltage or excessive current can cause a failure. Most power semiconductor data sheets specify the maximum voltage that can be applied under all conditions. The military has shown very clearly that operating semiconductors at 20% below their voltage rating provides a substantial improvement in their reliability.

Power Semiconductor Failures

Another common killer of power semiconductors is heat. Not only does high temperature destroy devices, but even operation at elevated, non-destructive temperatures can degrade useful life. Data sheets specify a maximum junction temperature, which is typically between 100°C and 200oC for silicon. Most power transistors have a maximum junction rating of 125°C to 150°C, the safe operating temperature is much lower.

Transient Effects

Power semiconductors can be destroyed by very short pulses of energy. A major source of destructive transients is caused by turning on or off an inductive load. Protection against these problems involves a careful combination of operating voltage and current margins and protective devices.

The market

Due to limited market volume until recently, there hasn’t been much advancement in manufacturing in this space. The industry was able to meet the demand mostly on 2- and 4-in. wafer diameter processes until the end of the last decade. Industry adopted 6-in. wafers a couple of years back and the limited volume of production using 8-in. wafer process is a sign that demand is picking up. Let’s look at one of the major aspects that involves the so-called extrinsic defects originating mainly from the manufacturing process itself. One of the important processing steps when it comes to the origination of a defect is epitaxial deposition. Bulk drift layers are formed using this step. It’s a long processing step that needs to have high purity, but it has a high propensity to generate more defects. Another subsequent yet critical step is the so-called ion implantation, where the appropriate dopants are added to the bulk layer. The process variation needs to be tightly controlled across the wafer and between the wafer and lots to achieve uniform thickness. Variation outside the tolerance significantly alters the device characteristics.

Advantages of Wide-Bandgap Devices

The most telling graphic that sums up the advantages of the wide-bandgap device compared to its silicon counterpart. The large bandgap and the critical electric field enable high voltages to be blocked with thin layers, resulting in lower resistance and associated conduction losses. Thin layers not only provide low on-resistance, but allow for smaller form factors and reduced capacitance, leading to higher frequency operation.

Wide-bandgap semiconductors offer a number of advantages over silicon. (Source: PowerAmerica, Veliadis)

Future of Power Semiconductors

With its integrated manufacturing concept – manufacturing of SiC chips utilizing the same production lines as the high volume silicon power chips. In addition, this integrated concept brings volume flexibility, a key factor in order to handle needed emerging technologies in fast moving market segments. Based on a deep system understanding and a clear focus on cost performance it has been possible to successfully define products by forming optimized combinations between silicon and silicon carbide based semiconductors. This move, away from a purely semiconductor technology driven definition of products, towards solutions tailor made for the targeted system is seen as a key element for the success of SiC in the future. Based on the experience with the diode technology, a similar roll out of SiC transistors will follow in the next few years. This is an important next step in order to move SiC much closer to the level of a mainstream technology.

As listed above, key elements will be:

  • Proven ruggedness
  • Attractive cost/performance enabling a measureable system advantage
  • Volume production capability
  • Product definition driven by system understanding During the last years, intensive studies have been carried out mainly in order to understand the system benefits of SiC.

The increase of switching frequency for a converter using unipolar SiC transistors can result in dramatically reduced volume and weight of the magnetic components. From an analysis carried out by Infineon, a converter COVER STORY The Future of Power Semiconductors Rugged and High Performing Silicon Carbide Transistors The use of SiC based power semiconductor solutions has shown a huge increase over the last years, it is a revolution to rely on. Driving forces behind this market development are the following trends: energy saving, size reduction, system integration and improved reliability.

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