The development of new technologies in power electronics has directed the industrial market to other resources to optimize energy efficiency. Silicon and germanium are the two main materials used to produce semiconductors today. Limited developments in losses and switching speeds have led technology to new wide bandgap resources such as silicon carbide (SiC).
SiC offers higher efficiency levels than silicon, mainly due to significantly reduced energy loss and reverse charging. This results in more switching power and less energy required during the turn-on and turn-off phases. Lower heat losses also allow for the removal of cooling systems, reducing space, weight and infrastructure costs. Improving the efficiency of energy-intensive IT infrastructure management will become increasingly important as the deployment of IoT and artificial intelligence applications increases and moves to the cloud.
Silicon carbide has a wider bandwidth than pure silicon, which allows the technology to be used even at high operating temperatures.
wide bandgap parameter
Wide bandgap semiconductors have a much wider bandgap than common semiconductors such as silicon or gallium arsenide (GaAs). This naturally translates into a larger breakdown electric field and into the possibility of operating at high temperatures with reduced radiation sensitivity without loss of electrical properties.
As the temperature increases, so does the thermal energy of the electrons in the valence band until they reach the energy necessary to make the transition to the conduction band (at a certain temperature). For silicon, this temperature is around 150°C; however, for WBG semiconductors, these values are much higher.
A high electrical breakdown field provides a higher breakdown voltage. This voltage is the value at which the breakdown body diode is disconnected and an increasing current flows between the source and drain. The breakdown voltage of a PN junction diode is proportional to the breakdown electric field and inversely proportional to the material concentration.
High electric fields provide excellent doping and resistance levels for much lower drift regions. Under the same breakdown voltage, the width of the drift region is inversely proportional to the breakdown electric field.
Another important parameter is the on-resistance in the drift region. Analyzing the previous example of a PN junction diode, we can see that the on-resistance is inversely proportional to the breakdown electric field of the unipolar element.
Thinner semiconductor layers involve a lower density of minority carriers, an important parameter defining reverse recovery current. In fact, all other things being equal, components designed to support higher currents with larger dies will have larger charges that experience transitions between turn-on and turn-off, and thus will have Greater reverse recovery current. The ability of a semiconductor to switch to high frequencies is directly proportional to its saturation drift speed: silicon carbide and gallium nitride drift twice as fast as silicon. As a result, the latter can safely operate at higher frequencies. Additionally, a higher saturation drift rate equates to faster charge removal; this results in shorter recovery times and lower reverse recovery currents.
The possibility to work at high temperatures and wider band gaps also depends on the thermal conductivity of the material. There are several ways to evaluate thermal resistance: you can analyze the thermal resistance between junction and case or junction and ambient.
Junction-to-ambient thermal resistance is a useful parameter when no external heat sink is connected, for example if you want to compare the thermal performance of different packages.
Materials can be compared using a figure of merit that is proportional to the product between on-resistance and gate input charge. These parameters determine conduction losses and switching losses respectively, and are interrelated; generally, components with lower charge values will have slightly higher on-resistance.
Silicon carbide diode
Silicon carbide diodes are mostly Schottky diodes. Classical silicon diodes are based on a PN junction. In Schottky diodes, the metal is replaced by a p-type semiconductor, forming a metal-semiconductor (ms) junction or Schottky barrier. This provides low conduction voltage drop, high switching speed and low noise. Schottky diodes are used to control the direction of current in a circuit so that it flows only from anode to cathode. When the Schottky diode is unbiased, free electrons will move from the n-type semiconductor to the metal forming the barrier. In the forward biased state, electrons can cross the barrier if the voltage is greater than 0.2 V.
SiC diodes have much lower leakage current than normal diodes. As a WBG semiconductor, silicon carbide has much lower leakage current and can be much more highly doped than silicon. In addition, SiC diodes have a higher forward voltage than silicon diodes due to the wider bandgap of silicon carbide.
In an interview with Amine Allouche, a member of System Plus Consulting’s Power Electronics and Compound Semiconductors team, we highlight some of the properties of SiC diodes.
Unlike ordinary PiN diodes, Schottky diodes have no recovery current because they are unipolar elements with majority charge carriers. However, they do exhibit some recovery effects caused by package and circuit parasitic capabilities and inductance. The main application of SiC diodes is in power supply circuits, especially in CCM (Continuous Conduction Mode) PFC (Power Factor Correction) circuits. Silicon carbide (SiC) has found room in industrial charging by endowing diodes with higher fault voltages and higher current capabilities.
“According to Yole Développement, the power SiC bare diode die market was worth $160 million in 2019. This includes various market segments such as automotive, energy, industrial… In fact, SiC diodes are mainly used in medium voltage applications (Automotive, PV, motor control…) to high-voltage applications (smart grid…). In automotive applications, SiC devices, especially SiC diodes, are currently being used in on-board chargers (OBC),” Allouche said.
As with all SiC chips, Allouche emphasized that the main challenges facing SiC diodes can be broken down into three layers:
Material level: SiC wafers are expensive to produce (compared to Si wafers for example). Commercial wafer sizes are still limited (up to 6 inches), while silicon wafers are currently transitioning to 12 inches.
There are a limited number of high-volume suppliers of the high-quality wafers needed to manufacture reliable devices. This is highlighted in our report where we compare raw SiC wafer costs from SiC diode manufacturers/sellers: Infineon, Wolfspeed, Rohm, STMicroelectronics, ON Semiconductor, Microsemi and UnitedSiC.
Device level: Device reliability is challenging in some key process steps, such as SiC epitaxy, SiC doping (requires high temperature), SiC etch… Manufacturing yield still needs to be improved compared to more mature silicon technologies.
Our report details the impact of epitaxy and front-of-wafer fabrication yields on SiC diode production costs.
System level: Packaging is another challenge for SiC diodes. New packaging solutions need to be developed to take full advantage of SiC technology. Yole’s report details the different packaging aspects related to SiC diodes available in the market, from package type, die attach, to wire bonding.
SiC diodes can be assembled into discrete packages, used as anti-parallel diodes with silicon-based transistors in hybrid modules, or as anti-parallel diodes in full SiC modules with SiC transistors.
“For example, in our report we highlighted manufacturers’ die attach options. Of the 11 SiC diodes from seven manufacturers we analyzed, we observed five types of attachment. Most common. However, one vendor uses a specific type of high-performance die attach that hurts manufacturing costs,” Allouche said
The high thermal conductivity of silicon carbide allows for better heat dissipation, offering smaller form factors than silicon. This allows cost reduction and smaller packaging.
The recovery time and electrical recovery charge of silicon carbide Schottky diodes are shallow; importantly and interestingly, recovery time and current are independent of temperature and current transients, unlike silicon diodes, where recovery time and current increase significantly with temperature.
SiC diodes are an excellent replacement in inverters: simply use them as diodes placed in anti-parallel with silicon IGBTs to reduce losses. In a typical hybrid electric vehicle (HEV), replacing silicon components with silicon carbide components can improve traction efficiency by more than 10%. This results in a radiator volume reduction of 1/3.