1. General description
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.
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2. 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.
3. 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.
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.
4. The trench rectifier wider SOA
The lower leakage current of trench Schottky rectifiers compared to their equivalent planar counterparts with a comparable forwardvoltage 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 futurewafer trench products extend this SOA benefit. Figure 2 shows the SOA at a thermal system resistance of 90-K/W of a TBR10H100HT 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 Futurewafer trench device is almost 40-V higher than the competitor product
5. Reverse-recovery behavior and reverserecovery 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, 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 3.
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.
6. Low forward voltage (VF) and low leakage (IR) balanced Schottky diodes.
The metal to semiconductor diode as laid out by the theories of Walter Schottky in the 1930s is used for its attractive electrical parameters such as a low forward voltage (VF) and fast switching behavior (tRR).
But these advantages also come with a cost. The main complaint about Schottky Diodes is their relatively high leakage current. This leakage current, denoted as ‘IR’ (current in reverse direction), is usually in the μA (10-6A) range for small Schottky diodes and can reach mA (10-3A) for larger power diodes. In comparison, a low leakage PN junction Diode (semiconductor to semiconductor junction) operates in the nA (10-9A) range, such that even large diodes have only μA of leakage.
With battery driven applications such as cell phones, tablets and smart watches, this downside of the Schottky diode can easily become a major concern for battery life. To respond to these concerns, the industry tried to approach this by introducing transistor based ‘Schottky-like’ devices which promise similarly low VF but with significantly lower leakage current. While this approach worked well in some cases, it required a sacrifice of another hallmark parameter of Schottky diodes – namely the fast switching time. It also added more complexity in the fabrication process of such a device, as more complicated CMOS based processes had to be used.
6-1. Is it time to say goodbye to Schottky diodes?
Absolutely not! Futurewafer continues to invest in the research of Schottky diodes and now has trench Schottky solutions with low forward voltage (VF) and low leakage current (IR) that can reach high temperatures of 175-°C, thus providing solutions for power-sensitive applications. Provides true Schottky. Although trench-based Schottky diodes are well-known in the network communication application industry, FutureWafer has indeed expanded the technology to high-end industrial fields to also have applications such as LED boosters, battery management, and wireless charging.
6-2. Advanced Schottky Products for Applications.
This new high-temperature trench diode family delivers Schottky’s renowned low VF and fast tRR while also maintaining low leakage (IR) that is comparable to existing “Schottky-like” products and is Better in most cases. The combination of low VF and low IR optimizes power consumption in energy-sensitive applications, which is one of the hallmarks of these small-signal trench Schottky diodes.
This technology, in turn, enables engineers to take advantage of improvements in applications with high requirements on efficiency and power consumption. Wireless charger applications are an example.
As the energy coupled through wireless transmission into the power receiving Unit (PRU) is relatively small, all further losses in converting the energy should be minimized in order to get the best charging rate. A critical element in this chain is the bridge rectifier which converts the alternating current waveform into a DC power signal. This is then further processed by a DC/DC converter to bring the voltage to compatible levels for charging the battery of the wireless device. The bridge rectifier needs to therefore have the lowest impact on power dissipation, which means that forward voltage and current losses should be minimized as they reduce the precious limited power transmitted by the Power Transmitting Unit (PTU).
6-3. How the VF and the IR impacts the overall effectiveness of the Full bridge.
As an example we look at a positive half wave of the receiving antenna coil. The voltage amplitude of the wave (Vwave ) will be reduced by the forward voltage drop of the Diode D1, resulting in an effective voltage of (Vres = Vwave -VF ) which is then fed into the DC/DC converter. On the other hand, the received current wave, Iwave will be reduced mainly by the leakage (IR2 ) of diode D4 and partially also by the leakage of diode D2.
The resulting current useful for the receiving circuitry is therefore Ires =Iwave –(IR2 +IR4). The new Trench Schottky products are optimized for this case in a way that the product of forward voltage drop (VF) and reverse current losses (IR) are optimized to have the lowest power dissipation (PD).
6-3. Why is this so important?
Consider a Schottky diode with VF 0.2-V and IR 3-mA. The excellent minimum forward voltage drop will mean nothing in a bridge rectifier if the rectification pulse is actually eaten up by the leakage current IR of the other diode in the reverse direction. Conversely, if your leakage current is minimal, 1-uA, but your forward voltage drop is in the 0.8-V range. You lose too much of the initial voltage and suffer when trying to increase the DC/DC voltage conversion. Therefore, the goal is to achieve a balance between IR and VF so that power losses are minimized and the signal voltage is as close as possible to the value on the receiving coil. Futurewafer invests R&D funds in the area of optimizing power losses in its new signal trench Schottky product portfolio
7. Power management efficiency upgrade: Trench Schottky diodes shoulder important responsibilities
7-1. Check losses to control thermal ills
Figure 6 is the basic architecture diagram of a non-synchronous DC-DC buck converter. D1 is the required Schottky diode. The left side shows the current situation when switch S1 is closed (time is T1), and the right side is the current situation when switch S1 is open (time is T2). When time is T2, the output current (lout) flows through D1, and the loss generated is directly related to the forward voltage (Vfw) of D1 and the output current. PT2 is equal to lout×Vfw. Obviously, we want to reduce Vfw as much as possible to control losses and reduce heat generation.
During T1, D1 is in the blocking state, and the only current is the reverse current, which is relatively weak and is mainly determined by the blocking voltage or input voltage Vin. The power consumption generated by the diode in the T1 stage is called PT1, which is roughly equal to Vin × Ir .
There is a dilemma when designing any Schottky diode, which means it can only be optimized for low VF or low IR. Therefore, if VF is chosen low, IR will be high and vice versa. When designing actual applications, it is not only necessary to observe the VF or IR value, but more importantly, the possible results in actual operation must be analyzed. Both VF and IR change with temperature. As the temperature in-creases, VF will decrease, which can actually reduce thermal diffusion while the diode heats up. However, IR will increase as the diode temperature rises. Therefore, when the diode temperature is higher, the leakage current will be relatively higher, and the internal power consumption will be more, which will make the diode temperature higher, thereby increasing the leakage again. Electric current forms an infinite loop.
8. Features of Trench MOS Barrier Schottky TMBS Diodes
Trench MOS Schottky is a diode that uses a Schottky barrier created by the junction of silicon and a metal called a barrier metal (called a Schottky junction) instead of a PN junction. The properties of TMBS vary depending on the type of barrier metal. Also, equipment may or may not be suitable for a given application depending on differences in these characteristics. The table below summarizes the characteristics and suitable applications of different barrier metals. In the table, "×" indicates poor performance or unsuitability compared to other devices.
The SBD using barrier metal A has extremely low VF, but the reverse leakage current IR is slightly higher compared to other devices. As a result, more heat is generated, making these devices unsuitable for situations with higher ambient temperatures. As explained later, these devices tend to suffer from thermal runaway. Due to low VF, low conduction losses and small voltage drop, these components are suitable for use in battery-powered circuits..
TMBS based on barrier metal B balances VF and IR values. They are commonly used in DC-DC converter circuits and are also designed to withstand high temperatures.
SBDs using barrier metal C have extremely low IR values and generate little heat, making them suitable for use at higher temperatures. Therefore, they are favored in automotive equipment applications.
8-1. Thermal Runaway of Si-SBDs
Here, we explain thermal runaway, which is an important phenomenon for study when using Si-SBDs. Thermal runaway is a phenomenon in which heat generation causes the Tj maximum rating of a diode to be exceeded, culminating in device failure in the worst case. As explained above, losses due to the IR of a Si-SBD cannot be ignored. Heat generation is the product of the IR and the VR (reverse voltage), that is, the reverse power loss due to leakage currents multiplied by the thermal resistance. This is the same as in an ordinary heat calculation, and so a metal A SBC, with a large IR value, is particularly disadvantageous. The IR tends to increase as the VR rises, and has a positive temperature characteristic, causing it to rise with the temperature (see the previous explanation). Thermal runaway may occur due to an increase in Tj caused by self-heating (or by a rise in the ambient temperature), causing the IR value to increase, leading to further heat generation and a further rise in IR. Of course, a condition for thermal runaway to occur is that the amount of heat generation is greater than the amount of heat dissipation.
In order to prevent thermal runaway, the thermal design must be adequate to enable heat dissipation even when heat is generated under various conditions. Below are a few important points relating to thermal runaway.
8-2. Features of TMBS Diodes
● Trench uses the mos effect to increase the maximum reverse voltage of VB. It can ensure vb while meeting the characteristics of low VF
● Low surface electric field characteristics can effectively reduce IR
● Compared with planar, tmbs does not have a PN interface and can achieve ideal Schottky characteristics.
● The EAS of tmbs is much larger than MOS, and the IFSM is also very large. It is more resistant to current impact than MOS and can be used at high temperatures.
● Lower Q, smaller switching loss.
9. 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.
Futurewafer offers 113 trench Schottky diodes with voltage ratings between 40-V and 100-V, and current ratings between 3-A and 20-A. These TBR series devices are housed in Clip Bond Flat Power packages: SOD123, SMA, SMB, SMC and TO277. 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.
