Abstract:
In a BSCR or BJT ESD clamp, the breakdown voltage and DC voltage tolerance are controlled by controlling the size of the collector of the BJT device by masking part of the collector.

Description:
FIELD OF THE INVENTION 
   The invention relates to BSCR and BJT clamps. In particular it relates to the breakdown voltage of such clamps. 
   BACKGROUND OF THE INVENTION 
   Bipolar junction devices such as bipolar SCRs (BSCRs) and BJTs as electrostatic discharge (ESD) protection structures experience difficulties when required for high speed output circuits, since in the case of high frequency bipolar structures the subcollector is typically heavily doped in order to provide the requisite short rise times. In fact the rise time may be comparable or even smaller than the typical rise time of an electrostatic discharge (ESD) pulse. Thus dynamic coupling cannot be used to turn on the device since the sensitivity of the device to dV/dt results in the device either having too low a turn-on voltage during normal operation or too high a turn-on voltage during ESD events. Therefore the few available DC breakdown blocking junctions that are currently available in supported devices do not adequately accommodate different breakdown voltage requirements. 
   What is needed is away of tailoring the breakdown voltage to the specific needs. 
   SUMMARY OF THE INVENTION 
   The present invention relies on avalanche breakdown rather than dynamic coupling to control the breakdown voltage of the BJT or BSCR device. This is done by effectively increasing the distance between the BJT emitter and collector, thereby increasing the voltage that is required to sweep carriers across the junction provided by the p-material between the collector and the emitter of the BJT structure. 
   According to the invention, there is provided a method of controlling the breakdown voltage of a BJT or of a BSCR device, which includes an npn bipolar structure with an n-emitter, a p-base, and an n-collector, comprising determining the desired breakdown voltage, selecting a blocking mask for partially blocking the n-collector by an amount that will provide the desired breakdown voltage, and partially blocking the n-collector using the mask. The n-collector is preferably blocked under the n-emitter p-base region. In the case of device formed in a p-epitaxial layer, the result of blocking the n-collector is an increase in the amount of p-epi material lying between the p-base and the collector. The method may further comprise offsetting the p-base doping. For purposes of this application, the use of the term “collector” or “n-collector” when discussing blocking, includes also the sub-collector comprising an n-buried layer (NBL) and n-sinker. 
   Further, according to the invention, there is provided a method of controlling the DC voltage tolerance of a BJT or of a BSCR device, which includes an npn bipolar structure with an n-emitter, a p-base, and an n-collector (which includes a subcollector), comprising partially blocking the n-collector during formation of the of the bipolar structure. 
   Still further, according to the invention, there is provided a method of controlling the breakdown voltage of a BJT or of a BSCR device, which includes an npn bipolar structure with an n-emitter, a p-base, and an n-collector having a subcollector, comprising reducing the number of charge carriers available for avalanche multiplication from the n-collector. The method may further include reducing the number of charge carriers available from the p-base. The reduction in carriers preferably comprises eliminating at least part of the n-collector, e.g., by blocking at least part of the collector during its formation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a cross section through a prior art BSCR, 
       FIG. 2  shows a cross section through one embodiment of an npn bipolar device of the invention, 
       FIG. 3  shows a cross section through one embodiment of BSCR device of the invention, and 
       FIG. 4  shows collector current versus collector-emitter voltage curves for various collector blocking distances. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A typical prior art BSCR with its vertical npn bipolar junction transistor (BJT) structure is shown in cross section in  FIG. 1 . The BSCR includes an n-polysilicon emitter  100  formed on a p-base  102  having external p+bases  104 , and an n-epitaxially grown collector  106  with subcollector in the form of an n-buried layer (NBL)  108  contacted through an n-sinker  110 . The BSCR also includes a p+ region  112 . 
   In contrast, the present invention proposes making some changes to the prior art BJT and BSCR devices. One embodiment of an npn bipolar junction transistor (BJT) with controlled breakdown is shown in  FIG. 2 . The BJT device  200  includes an n-poly emitter  200  formed on a p-base with p+ external bases  202 . The device  200  is formed in a p-epitaxial layer  204 , thus the base with its external bases  202  is also formed in the epi layer  204 . As shown in  FIG. 2 , the collector of the device (indicated generally by reference numeral  210 ) includes a sub-collector in the form of an n-buried layer  216  connected through an n-sinker  218  to a collector contact  220 . However part of the p-collector (which for this discussion includes the sub-collector) below the emitter  200  and below part of the right hand portion of the external base  202  has been eliminated by blocking the collector during formation through the use of a mask (not shown). The result is that the amount of collector/sub-collector has been reduced thereby reducing the number of charge carriers it can contribute during avalanche multiplication. Seen another way, the distance between the emitter  200  and collector  210  has been increased by blocking part of the collector  210 . This is indicated by reference numeral  230 . As shown in  FIG. 2 , the entire device is formed in a p-substrate  240 , and the epitaxial layer  204  is grown on an n-buried layer (NBL)  250 . 
     FIG. 3  shows one embodiment of a BSCR of the invention. Since the BSCR  300  is similar to the BJT  200 , similar structural elements have been depicted by the same reference numerals. In addition to the emitter  200 , base with external bases  202 , and collector  210  with its sub-collector  216 ,  218 , the BSCR includes a p-type region  310 , thereby defining an npnp structure with the n-collector, p-epi/p-base and n-emitter. As shown in  FIG. 3 , The p-type region  310  and the n-sinker  218  are formed in an n-well  260 . Again, the collector  210  has been partially blocked to leave an enlarged p-region, in the form of the p-epi region  204  between the remaining collector  210  and the emitter  200 . 
   The effect of the blocking of the collector is shown in  FIG. 4 , which shows the increasing breakdown voltages for increasing blocking distances from no blocking (curve  400 ), through 3.5 μm blocking (curve  402 ), and 4 μm (curve  404 ), to 4.5 μm blocking (curve  406 ). 
   In addition to selectively blocking the collector in order to achieve the desired breakdown voltage or the desired DC voltage tolerance, the present invention also proposes off-setting the p-base size to account for the reduced number of charge carriers provided by the collector due to the collector having been reduced in size. 
   While the invention has been described with respect to two specific embodiments, it will be appreciated that the invention is not limited to these embodiments but includes other configurations falling within the scope of the claims.