Hybrid design improves diode robustness and boosts efficiency

Infineon launched the first commercial SiC Schottky diode in 2001, and since then these devices have become a benchmark for virtually lossless operation. They can deliver highly efficient switching at frequencies up to several hundred kilohertz, and have been deployed as the boost diodes in power factor correction units of switched-mode power supplies operating in continuous-current mode.

However, SiC diodes remain more expensive than their silicon equivalents due to higher material costs. In addition, their resistance increases greatly with temperature - known as a positive temperature coefficient for resistance - a typical effect in a majority carrier device. This behavior has significant consequences because current surges cause thermal runaway, a condition that ultimately leads to device failure.

As a result, designers have investigated the surge-current requirements for equipment start-up cycles and fluctuations in the mains supply to set the diode s current rating. So in practice, SiC diodes with current handling capabilities well beyond those required are chosen for regular operation, adding to their cost and hampering deployment in cost-sensitive applications.

In addition, SiC diodes also lack stability at voltages above the diode s specified operating voltage. Ideally, a wide safety margin is needed between its rated voltage and the actual breakdown voltage (the reverse voltage that causes the device to start conducting).

Addressing all of these issues, Infineon has recently launched its second-generation (2G) "thinQ!" 600 V SiC diodes. These devices can operate at voltages well above their specified rating, have a very high surge-current capability, and also deliver the advantages of first-generation diodes, such as zero reverse recovery charge at high breakdown voltages.

The thinQ! 2G design, which we refer to as a "Merged pn Schottky (MPS)", is a combination of a p-n diode and a Schottky diode. This mix of the two technologies allows the device to combine the high-efficiency switching behavior of a Schottky diode with the strong surge-current capabilities of a p-n junction. The idea for unifying the two types of device is not new, and was first discussed by researchers from KTH University in Stockholm, Sweden, at the first international conference on SiC and related materials in 1999. However, up until now this hybrid approach had only been used for reducing the field stress at the Schottky interface. This enabled the combination of higher blocking voltages, low thermionic field emission and small tunneling leakage currents.

The structure of our MPS diodes and our first-generation SiC devices are compared in figure 1. The MPS diodes feature a Schottky interface for low-current operation, a p-n interface for higher current operation and floating p-doped islands. The idea to incorporate islands within the structure originated in low-voltage silicon Schottky diodes, where the approach reduced leakage current through shielding of the Schottky interface. When the technology is applied to SiC, the merged p-doped areas form emitter structures through a low ohmic connection to the p-doped regions and deliver two benefits - improved surge-current handling and breakdown voltages well above the rated value of 600 V. Higher currents can be handled because the device switches from a unipolar to a bipolar current path during surges. This happens when the p-n junction s threshold forward voltage of about 4 V is reached (see figure 2). The p-doped islands pin the breakdown location and lead to homogenous avalanche current distribution over the chip s entire active area, which increases the breakdown voltage.

The improved current handling of the thinQ! 2G diodes is not noticeable under regular operation. This is because the device behaves like a conventional Schottky diode (such as our first-generation IFX SiC Schottky diode) with zero reverse recovery charge and a positive temperature coefficient. However, at high forward currents the thinQ! 2G diodes are essentially temperature-independent, in terms of their I-V characteristics, which prevents the thermal runaway that causes device failure.

Our current-voltage measurements on a 6 A-rated thinQ! 2G MPS diode, using 400 µs pulse widths and initial case temperatures from 55 to 175 °C, are shown in figure 3. At all temperatures the devices switch from unipolar to bipolar behavior at higher voltages, which protects against thermal runaway, thereby reducing device failure and improving equipment reliability. Figure 3 shows that at a case temperature of 25 °C the unipolar to bipolar switch occurs at less than 6 V, but at 175 °C this transition falls to below 4 V.

These measurements also show that our latest diodes deliver a threefold improvement in the maximum value for the surge non-repetitive (isolated) forward current. This means that they can handle higher peak currents, which improves reliability. Product testing also reveals that the "i2t" value, which is a guide to the diodes robustness to pulse currents, is two-and-a-half times higher for the new devices.

The MPS approach also allows thinQ! 2G diodes to withstand substantial avalanche current at breakdown conditions - something that is not possible with our first-generation diode or any other SiC diode on the market (see figure 4). The p-doped islands produce two separate benefits, as they shield the Schottky contact from excessive field peaks, while localizing the breakdown away from the surface. This means that the thinQ! 2G diodes can even deliver highly repeatable avalanche characteristics and a stable performance during voltage spikes that exceed the given operation voltage.

The positive temperature coefficient of the thinQ! 2G diodes that produce these characteristics offers an increase in the reliability, immunity and ruggedness of high-power electrical equipment. Power factor correction (PFC) devices that are normally connected to the mains mostly use bulk capacitors in their PFC stages. However, during transient voltage spikes the performance of these capacitors at voltages above the device s rating is very poor. These applications would be better served by our thinQ! 2G devices, which are less affected by the spikes because of their high actual breakdown voltage.

For most PFC power supplies the diode current rating is determined by the peak current through the PFC diode under non-standard operating conditions, such as start-up or power drop-out. The thinQ! 2G devices provide a surge-current capability that is more than double the regular current, which allows designers to select SiC diodes based on the nominal current in the circuit. This means that in many situations a 6 A SiC first-generation Schottky diode can be replaced by a lower-rated device, such as a 4 A thinQ! 2G diode (see "Second-generation benefits"). This will drive a shift in the entry point for SiC diodes towards lower power levels, and subsequently increase the number of markets that can be addressed by our thinQ! 2G products.

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