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The principle and method of reducing the on-resistance of high-voltage MOSFET


In power semiconductor devices, MOSFETs play an important role in various power conversions, especially high-frequency power conversions, with high speed, low switching loss, and low driving loss. In the low-voltage field, MOSFET has no competitors, but as the withstand voltage of MOS increases, the on-resistance increases with the power of 2.4-2.6, and its growth rate makes MOSFET manufacturers and users have to reduce it by dozens of times. Rated current to compromise the contradiction between rated current, on-resistance and cost.


Even so, the on-voltage drop generated by the on-resistance of the high-voltage MOSFET at the rated junction temperature is still high. The rated junction temperature and rated current of the MOSFET with a withstand voltage of more than 500V have a high on-voltage, with a withstand voltage of 800V. The above turn-on voltage is surprisingly high, and the turn-on loss accounts for 2/3-4/5 of the total MOSFET loss, which greatly limits the application.


 1. On-resistance distribution of MOSFETs with different withstand voltages.


MOSFETs with different withstand voltages have different resistance ratio distributions in each part of the on-resistance. For example, the epitaxial layer resistance of a MOSFET with a withstand voltage of 30V is only 29% of the total on-resistance, and the epitaxial layer resistance of a MOSFET with a withstand voltage of 600V is 96.5% of the total on-resistance. From this, it can be inferred that the on-resistance of a MOSFET with a withstand voltage of 800V will be almost occupied by the epitaxial layer resistance. To obtain a high blocking voltage, a high resistivity epitaxial layer must be used and thickened. This is the root cause of the high on-resistance caused by conventional high voltage MOSFET structures.


2. The idea of reducing the on-resistance of the high-voltage MOSFET.


Although increasing the die area can reduce the on-resistance, the cost of increasing the cost is not allowed by commercial products. Although the introduction of minority carrier conduction can reduce the on-state voltage drop, the price paid is the reduction of switching speed and the occurrence of trailing current, the increase of switching loss, and the loss of the high-speed advantage of MOSFET.


The above two methods cannot reduce the on-resistance of the high-voltage MOSFET. The remaining idea is how to separate the low-doping and high-resistivity regions that block high voltage and the high-doping and low resistivity of the conduction channel. For example, there is no other purpose except that the low-doped high-voltage epitaxial layer can only increase the on-resistance when it is turned on. In this way, is it possible to realize the conduction channel with high doping and lower resistivity, and try to pinch off this channel in some way when the MOSFET is turned off, so that the withstand voltage of the whole device depends only on the low doping N-epitaxial Floor.


Different from the conventional MOSFET structure, the MOSFET with built-in lateral electric field is embedded in the vertical P region and sandwiches the N region of the vertical conductive region, so that when the MOSFET is turned off, a lateral electric field is established between the vertical P and N, and the N region of the vertical conductive region is The doping concentration is higher than the doping concentration of its epitaxial region N-.


When VGS<VTH, the N-type conduction channel due to inversion by the electric field cannot be formed, and a positive voltage is applied between D and S to reversely bias the PN junction inside the MOSFET to form a depletion layer, and vertically conduct the N region. run out. This depletion layer has a vertical high blocking voltage, and the withstand voltage of the device depends on the withstand voltage of P and N-. Therefore, low doping and high resistivity of N- are necessary.


When CGS>VTH, an N-type conduction channel is formed by inversion by the electric field. The electrons in the source region enter the depleted vertical N region through the conductive channel to neutralize the positive charge, thereby restoring the depleted N-type characteristics, and thus the conductive channel is formed. Due to the lower resistivity of the vertical N region, the on-resistance will be significantly lower than that of conventional MOSFETs.


To sum up, the blocking voltage and on-resistance are in different functional regions. The function of blocking voltage and on-resistance is separated, which solves the contradiction between blocking voltage and on-resistance, and also converts the surface PN junction during blocking into a buried PN junction. At the same N-doping concentration, the resistance The cut-off voltage can be further increased.


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