1. Field of the Invention
The present invention relates to a semiconductor device and, particularly, to a vertical MOSFET.
2. Description of Related Art
A vertical power MOSFET is known as a MOSFET having a high breakdown voltage.
Important characteristics of the power MOSFET include on-resistance (Ron) and breakdown voltage (BVDSS). However, a tradeoff exists between the two characteristics in normal power MOSFETs. For example, reducing the on-resistance results in a lower breakdown voltage; on the other hand, increasing the breakdown voltage results in higher on-resistance.
In the field of high breakdown voltage MOSFETs, superjunction technology which reduces the on-resistance while keeping the breakdown voltage characteristics is proposed.
FIG. 6 is a cross-sectional view of a vertical power MOSFET having the superjunction structure (hereinafter as SJ structure) described in H. Ninomiya, Y. Miura, K. Kobayashi, “Ultra-low On-resistance 60-100V Superjunction UMOSFETs Fabricated by Multiple Ion Implantation”, IEEE Proceeding of 2004 International Symposium on Power Semiconductor Devices & IC's. In the vertical power MOSFET of FIG. 6, a P base layer 203 and an N.sup.+source layer 204 are formed on an N epitaxial layer 202, which is formed on the surface of an N.sup.+silicon substrate 201. Further, a trench 205 is formed in the N epitaxial layer 202, penetrating through the P base layer 203 and the N.sup.+source layer 204. A gate oxide film 206 and a trench gate 207 made of polysilicon are embedded in the gate trench 205.
An interlayer oxide film 208 is formed on the trench gate 207, and a source electrode 210 is formed on its surface. A part of the N.sup.+source layer 204 is exposed from the interlayer oxide film 208, and the N.sup.+source layer 204 and the source electrode 210 come into contact with each other at the exposed part.
A P column region 209 is formed vertically in the N epitaxial layer 202 between adjacent trench gates 207. A drain electrode 211 is formed on the rear surface of the N.sup.+silicon substrate 201.
In the SJ structure, the breakdown voltage characteristics reach their maximum when depletion charge in the N epitaxial layer 202 and depletion charge in the P column region 209 are equal. Thus, when a high voltage is applied between the source and drain of the vertical power MOSFET, if the depletion charge in the N epitaxial layer 202 and the depletion charge in the P column region 209 are in equilibrium, a depletion layer appears uniformly in the N epitaxial layer 202, thereby improving the breakdown voltage characteristics. The depletion charge is determined by the impurity concentration doped into the N epitaxial layer 202 and the P column region 209.
An application of the vertical power MOSFET is recently found in a DC/DC converter of a small personal computer (PC), communication equipment and so on, in which high speed processing is required. It is important in a vertical power MOSFET used in such applications to reduce parasitic capacitance for high-speed switching. Thus, some techniques reduce a total area of the gate oxide film 206 by reducing the density of a cell constituting a transistor without significantly deteriorating the on-resistance characteristics. Reducing the cell density leads to the relatively larger cell size, resulting in a relatively larger distance between the trench gates 207.
In this case, the structure of the conventional vertical power MOSFET, which has the P column region 209 only between the trench gates 207, results in a longer distance between the adjacent P column regions 209. Thus, it is necessary in the conventional vertical power MOSFET to increase the impurity concentration of the P column region 209 in order to keep the equilibrium in the depletion charge between the N epitaxial layer 202 and the P column region 209. However, since there is a limitation in the impurity concentration of the P column region 209, it is difficult to solve the problem by increasing the impurity concentration.
Another approach to solve the above problem is to increase the width of the P column region 209. However, the wide P column region 209 could impede the drain current path, resulting in resistance causing the on-resistance to increase.