Trench Technology Boosts Schottky Rectifier
Planar Schottky diodes serve as output rectifiers and ORing functions in redundant (n+1) power supplies. However, developers are demanding improved forward voltage drop (VF) and reverse leakage current (IR) characteristics, allowing the Schottky diode to operate with lower forward losses and increased maximum junction temperature. This translates into reduced heat dissipation and enhanced safety margin for the diode.
Trench technology provides the answer to improved diode performance. The new optimized trench Schottky diode offers 15% reduction in forward voltage drop (VF) and a four-fold improvement in reverse leakage current compared with a comparable conventional planar device. It also provides a factor-of-two improvement in reverse energy absorption capability, while maintaining equivalent switching characteristics. That means the new trench Schottky can now be rated at a higher maximum junction temperature (Tjmax) of 125°C vs. 100°C for the planar version.
Using ISE software, we optimized the structural parameters of the trench device with 2-D simulations of the basic cell (Fig. 1). The trench width (Wt) and Mesa width (Wm) are kept constant, based on minimum feature sizes of the photolithography process capability. We adjusted the Schottky barrier work function in the electrical simulations to match the actual value extracted from I-V curves of early prototypes.
Device simulations showed that to reduce the forward voltage drop and provide low leakage current characteristics, the trench Schottky should combine a low barrier metal with narrow Mesa width, thin gate oxide, and epi resistivity above a certain minimum value. Under these conditions, the structure offers punch-through characteristics. Also, the distance between the trench bottom and epi/substrate interface (De) determines the breakdown voltage (BV). In addition, simulations indicated that to reduce VF, maintain a high BV, and not incur a surface field-induced leakage increase required a trench deeper than a minimum value.
This newly developed optimal trench structure uses a proprietary sub-micron process that fabricated a 15V trench Schottky diode with maximum average forward current capability of 80A. The device also uses a low barrier Schottky metal. A single TO-247 with a common cathode, center tap configuration houses two die, each with a 40A rating. This new trench-based Schottky diode is the 80CPT015.
At room temperature, its maximum rated VF is 0.39V at 40A, which drops to 0.31V at 125°C junction temperature. The trench Schottky's maximum reverse leakage current (IR) rating is 5mA at room temperature, which goes to 950mA at 125°C junction temperature.
To demonstrate the improvements made with the optimized trench method, we can compare its electrical characteristics with a conventional planar version having the same breakdown voltage and die size. The planar version, labeled 65PQ015, comes in the same TO-247 package using similar bonding and assembly techniques. We can use four key characteristics to compare the two devices: I-V, capacitance-voltage (C-V), switching characteristics, and reverse avalanche energy.
Fig 2(a) shows that, at room temperature, VF for the 80CPT015 trench version is 36 mV lower at 40A than the planar device. As shown in Fig. 2(b), IR (reverse leakage) at room temperature is almost four times lower for the 80CPT015. The improvement is reflected at higher temperature, permitting the trench device to be rated at Tjmax = 125°C vs. a 100°C rating for the planar diode.
Using an LCR meter, we compared the C-V characteristics of the new device with a conventional planar. The junction capacitance is only 30% higher for the trench Schottky at VR = 10V. This increase is due to the MOS-like capacitor formed in the trench electrodes of the new Schottky. Appropriately scaling the oxide thickness (Tox) reduces the contribution of the gate oxide. The switching characteristic of the trench Schottky is comparable to that of a planar diode.
We also examined the reverse avalanche energy (Eas) absorption capability for the two diodes using a conventional inductive discharge test. They were subjected to a wide range of load inductances. The 80CPT015 can absorb twice the energy of the planar before failure. The reason is that each device has its unique field distribution characteristics at breakdown. From device simulations, we observed that the maximum electric field at breakdown is near the trench bottom of each individual cell, allowing the whole active area to dissipate avalanche energy and increase its absorption capability. By comparison, the maximum electric field in the conventional planar device occurs in a limited region at the edge of the P+ guard ring.
We finally examined the new structure using in-circuit evaluation of the trench Schottky diodes in the ORing stage of a 750W commercial power supply with 100Vac to 240Vac input and 12Vdc output at 60A. Here, we monitored the efficiency and device case temperature for the 80CPT015, and compared it with three other conventional diode configurations employed in the same power supply unit: one 65PQ015, two 40L15CTs, and three 40L15CTs. The 40L15CT is a 40A, 15V planar device in a TO-220 package. We replaced the two 40L15CT with one 80CPT015, which increased efficiency by 0.4% and reduced the case temperature by 4°C. Next, we replaced the 80CPT015 with three 40L15CT and observed that one single trench Schottky had the same efficiency as three TO-220s with a temperature increase of only 4.9°C. A single 80CPT015 utilizes 32% less silicon than three 40L15CTs in TO-220 packages.
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