FTM / Power & Power Management / Nexperia — The Latest Trench Schottky Rectifiers

Nexperia — The Latest Trench Schottky Rectifiers

Power & Power Management

January 6, 2022
Power & Power Management
Issue 1 2022

Nexperia the Latest Trench Schottky Rectifiers

Higher performance and more attractive trade-offs: the benefits of the latest trench Schottky rectifiers

By Dr-Ing. Reza Behtash, Applications Marketing Manager, Nexperia

Schottky diodes feature a low forward voltage drop and high switching speed. This makes them well suited to a wide variety of applications, such as the boost diode in power conversion circuits.

Traditionally, the trade-offs associated with the use of Schottky diodes have forced the designer to choose between optimizing for forward voltage, leakage current, and the reverse blocking voltage.

Now, a new generation of trench Schottky diodes is helping to reduce the impact of the trade-offs, and to offer greater capabilities than planar counterparts. Designers using trench Schottky diodes can benefit from reduced switching losses, a wider safe operating area (SOA), and lower reverse-recovery charge.

Understanding the trade-offs in Schottky diodes

The ideal rectifier would have a low forward voltage drop, a high reverse blocking voltage, zero leakage current, and a low parasitic capacitance, facilitating high switching speed.

There are two main contributors to the forward voltage drop:

The voltage drop across the junction: the PN junction in the case of PN rectifiers, and the metal-semiconductor junction in the case of Schottky rectifiers
The voltage drop across the drift region

While the forward voltage drop across a PN junction is intrinsically determined by the built-in voltage, and hence mainly by the specified semiconductor material, the forward voltage drop across the metal-semiconductor interface in a Schottky barrier rectifier can be modified by the choice of the Schottky metal: the Schottky barrier is the result of the difference between the metal work (MW) function and the electron affinity of the semiconductor.

By using Schottky metals with a low MW function, the voltage drop across the metal semiconductor interface can be minimized. There is a trade-off between the forward voltage drop across the junction, however, and the leakage current of the Schottky rectifier, as the amplitude of the leakage current is also determined by the Schottky barrier and the electric field across the metal-semiconductor interface. Furthermore, the advantage of the low voltage drop across the junction can disappear when the thickness of the drift region is increased to achieve a high reverse blocking voltage. This is why the reverse blocking voltage of Schottky rectifiers is traditionally limited to less than 200 V.

The advantages of trench technology

The challenge, therefore, is to preserve the low voltage drop across the metal semiconductor interface, given that the power-system designer also wants low leakage current and a high reverse blocking voltage.

Here, trench rectifiers prove very useful. The concept underlying the fabrication of the trench Schottky rectifier is RESURF (reduced surface field). The RESURF effect is illustrated in Figure 1. In a planar Schottky rectifier, the equipotential lines are concentrated close to the top electrode, resulting in a high electrical field near the surface. This results in a large increase in leakage current with increasing reverse voltage, and an early breakdown when the electrical field strength near the surface exceeds its critical value.

Fig. 1: Equipotential lines in a planar Schottky rectifier (left) and in a Trench Schottky rectifier (right) in the reverse direction

By etching trenches into the silicon and filling them with poly-silicon, which is electrically separated from the drift region by a thin dielectric, the trenches act like a field plate in the semiconductor, depleting the drift region in the reverse direction, and resulting in a flattened electrical field profile along the drift region. This means that the trench structure achieves a lower leakage current by reducing the electrical field near the surface and producing a higher breakdown voltage compared to a planar device with the same epitaxial structure.

The Nexperia PMEG*T family of trench Schottky rectifiers, available with voltage ratings between 40 V and 100 V, achieves a well-balanced trade-off between the forward voltage drop and leakage current. Figure 2 shows the leakage current at maximum reverse voltage for 60 V-rated rectifiers plotted against the forward voltage drop at maximum forward current and at 125°C. For comparison, trench and planar Schottky rectifiers from two other manufacturers are also shown. For a given forward voltage drop, the Nexperia device exhibits the lowest leakage current.

Figure 2: Trade-off of forward voltage against leakage current at the maximum reverse voltage and maximum forward current at 125°C

The trench rectifier wider SOA

The lower leakage current of trench Schottky rectifiers compared to their equivalent planar counterparts with a comparable forward voltage drop shows that the trench devices have a wider SOA. SOA plots the maximum reverse voltage that can be applied against junction temperature. Trench rectifiers already feature a wider SOA than an equivalent planar Schottky diode, but Nexperia trench products extend this SOA benefit. Figure 3 shows the SOA at a thermal system resistance of 90 K/W of a PMEG100T080ELPE trench Schottky rectifier, shown in orange, versus a similarly rated trench Schottky device from another supplier, shown in blue. At a junction temperature of 125°C, the maximum allowable reverse voltage of the Nexperia trench device is almost 40 V higher than the competitor product.

Fig. 3: Comparison of the SOA of the Nexperia PMEG100T080ELPE trench Schottky diode with a competitor trench Schottky part

In applications exposed to high ambient temperatures, such as automotive, trench Schottky rectifiers are a good choice as they are more resistant to thermal runaway, the instability which occurs when the increase in dissipated power caused by the leakage current of the rectifier ramps up faster than the heat is dissipated through the system.

The equivalent circuit diagram of a trench Schottky rectifier is shown in Figure 4. Besides the usual parasitic capacitance of the Schottky diode, there is a second parasitic capacitance caused by the electrode and the thin dielectric in the trench structures. This means that per unit area the total parasitic capacitance of a trench Schottky rectifier is higher than its planar counterpart.

Fig. 4: Cross-section and equivalent circuit diagram of a trench Schottky rectifier, highlighting the circuit elements

This parasitic capacitance does not however affect the diode’s switching characteristics or EMI: in fact, trench rectifiers have less stored charge than planar Schottky diodes, and offer excellent switching performance despite the larger parasitic capacitance.

Reverse-recovery behavior and reverse-recovery charge

The switching behavior of a device may be characterized by reverse-recovery measurements. Such measurements are carried out by biasing the rectifier in a forward direction, then switching the device into a reverse condition. Due to the stored charge in the device, represented by the parasitic capacitance in the equivalent circuit shown in Figure 4, which must be first removed before the diode blocks, a so-called reverse-recovery current occurs. The ramp reverse-recovery measurement for a trench Schottky rectifier and its planar counterpart with comparable die size and package is shown in Figure 5.

	Reverse-recovery Charge at 25°C	Reverse-recovery Charge at 85°C	Ramp Reverse-recovery Current at 25°C	Ramp Reverse-recovery current at 85°C
Trench Schottky	8.6 nC	8.5 nC	2.8 A	2.7 A
Planar Schottky	26.3 nC	33.5 nC	5.2 A	5.2 A

Fig. 5: Reverse-recovery behavior of trench and planar Schottky diodes

In this measurement, the current has been ramped down with a di/dt rate of 1 A/ns. The area under the zero line represents the rectifier’s reverse-recovery charge. The blue line is the planar. The graph also shows the lower reverse-recovery current and the shorter reverse-recovery time of the trench rectifier compared to its planar counterpart, despite the higher parasitic capacitance.

The temperature stability of reverse-recovery charge for trench rectifiers is notable, since applications rarely operate at a temperature as low as 25°C. As shown in Figure 5, the reverse-recovery charge of the trench rectifier hardly changes at a high 85°C ambient temperature, while the reverse-recovery charge of a planar Schottky diode increases substantially.

The low reverse-recovery charge of the trench Schottky rectifiers results in significant converter efficiency gains, especially at high frequency, when switching losses are greater. Figure 6 demonstrates this using the example of a 48 V to 12 V power converter.

Fig. 6: Trench Schottky rectifiers increase converter efficiency especially at higher frequencies, when the switching losses are less than those of planar devices

Any ringing that occurs in the trench rectifier during switching does not affect electromagnetic emission levels, as confirmed by the conducted and radiated emission measurements shown in Figure 7.

Fig. 7: Impact on electromagnetic emission: radiated emission (left); conducted emission (right). A 48 V to 12 V buck converter is used as the test vehicle. It uses 3 A CFP3 rectifiers.

Conclusion

In summary, trench rectifiers are a suitable choice if an attractive trade-off between forward voltage drop and leakage current is required. Trench rectifiers should also be selected in high power-density applications in which the ambient temperature is high, since they are more resistant to thermal runaway effects. For applications operating at switching speeds higher than 100 kHz, the reduced switching losses of trench devices are particularly beneficial.

Nexperia offers 60 trench Schottky diodes with voltage ratings between 40 V and 100 V, and current ratings between 1 A and 20 A. These PMEGxxxTx devices are housed in Clip Bond FlatPower packages: CFP2HP, CFP3, CFP5, and CFP15(B). These size- and thermally-efficient packages have become the industry standard for power diodes. The solid copper clip reduces the packages’ thermal resistance and optimizes the transfer of heat to the ambient environment, allowing power-system designers to realize small and compact PCB designs.