Insulated gate power devices with reduced carrier injection in termination area

Abstract

A high power vertical insulated-gate switch is described that includes an active region, containing a cell array, and a surrounding termination region. The termination region is for at least the purpose of controlling a breakdown voltage and does not contain any switching cells. Assuming the anode is the silicon substrate (p-type), it is desirable to have good hole injection efficiency from the substrate in the active region in the device's on-state. Therefore, the substrate should be highly doped (p++) in the active region. It is desirable to have poor hole injection efficiency in the termination region so that there is a minimum concentration of holes in the termination region when the switch is turned off. Various doping techniques are disclosed that cause the substrate to efficiency inject holes into the active region but inefficiently inject holes into the termination region during the on-state.

What is claimed is: 
     
         1 . An insulated-gate switching device formed as a die comprising:
 a p-type substrate having a first p-type dopant concentration, the substrate having a top surface and a bottom surface;   a first electrode formed on the bottom surface of the substrate;   at least one epitaxial layer grown over the substrate;   an array of cells having insulated gates formed in the epitaxial layer;   first n-type regions formed in a top surface of the epitaxial layer, wherein biasing the gates above a threshold voltage creates a current path between the first n-type regions and the substrate;   a second electrode formed overlying the epitaxial layer and electrically connected to the first n-type regions;   a first portion of the device in an area of the array of cells and below the cells being an active region; and   a second portion of the device surrounding the area of the array of cells and surrounding the area below the cells being a termination region,   wherein the substrate, at or near its top surface in the active region, is doped to increase the p-type dopant concentration of the substrate in the active region, but not increase the p-type dopant concentration of the substrate in the termination region.   
     
     
         2 . The device of  claim 1  further comprising the substrate, at or near its top surface in the termination region, being doped to reduce the effective p-type dopant concentration of the substrate in the termination region, thus reducing hole injection efficiency in the termination region. 
     
     
         3 . A method of forming an insulated-gate switching device comprising:
 providing a p-type substrate having a first p-type dopant concentration, the substrate having a top surface and a bottom surface, the substrate having an active region above which a cell array will be formed and having a termination region surrounding the active region;   doping the substrate, at or near its top surface in the termination region, with first dopants to cause the substrate, at or near its top surface in the termination region, to be doped differently from the top surface of the substrate in the active region;   growing at least one epitaxial layer over the substrate after the step of doping the substrate;   forming an array of cells, having insulated gates formed in the epitaxial layer, over the active region, the cells comprising first n-type regions formed in a top surface of the epitaxial layer, wherein biasing the gates above a threshold voltage creates a current path between the first n-type regions and the substrate;   forming termination structures in the epitaxial layer over the termination region, wherein the step of doping the substrate reduces an ability of the substrate in the termination region to inject holes into the at least one epitaxial layer, thus reducing hole injection efficiency in the termination region;   forming a first electrode on the bottom surface of the substrate; and   forming a second electrode overlying the epitaxial layer and electrically connected to the first n-type regions.   
     
     
         4 . The method of  claim 3  wherein the step of doping the substrate comprises doping the substrate with n-type dopants in the termination region. 
     
     
         5 . The method of  claim 3  wherein the step of doping the substrate comprises doping the substrate with oxygen or nitrogen dopants in the termination region to form insulating regions in the substrate. 
     
     
         6 . The method of  claim 3  further comprising doping the substrate in the active region to increase its p-type dopant concentration above the first p-type dopant concentration in the active region. 
     
     
         7 . The method of  claim 3  wherein the array of cells comprises insulated gates that invert areas of the at least one epitaxial layer to control a current flow between the first electrode and the second electrode. 
     
     
         8 . The method of  claim 3  wherein further comprising forming field limiting rings in the termination region. 
     
     
         9 . A method for forming an insulated-gate switching device comprising:
 providing a p-type substrate having a first p-type dopant concentration, the substrate having a top surface and a bottom surface, the substrate having an active region above which a cell array will be formed and having a termination region surrounding the active region;   doping the substrate, at or near its top surface in the active region, with p-type dopants to increase the p-type dopant concentration of the substrate in the active region to above the first p-type dopant concentration, thus increasing hole injection efficiency in the active region;   growing at least one epitaxial layer over the substrate;   forming an array of cells, having insulated gates formed in the epitaxial layer, over the active region, the cells comprising first n-type regions formed in a top surface of the epitaxial layer, wherein biasing the gates above a threshold voltage creates a current path between the first n-type regions and the substrate;   forming termination structures in the epitaxial layer over the termination region;   forming a first electrode on the bottom surface of the substrate; and   forming a second electrode overlying the epitaxial layer and electrically connected to the first n-type regions.   
     
     
         10 . The method of  claim 9  further comprising doping the substrate in the termination region to reduce an ability of the substrate in the termination region to inject holes into the at least one epitaxial layer, thus reducing hole injection efficiency in the termination region. 
     
     
         11 . The method of  claim 10  wherein the step of doping the substrate in the termination region comprises doping the substrate, at or near its top surface in the termination region, with n-type dopants.