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FET with reduced body effect

IP.com Disclosure Number: IPCOM000016383D
Original Publication Date: 2002-Nov-30
Included in the Prior Art Database: 2003-Jun-21

Publishing Venue



Disclosed is a FET device that has a relatively small increase in threshold voltage with increased source/body potential. Some transistor applications may benefit from a reduced body effect; that is, they operate primarily at a non-zero source/body potential, and the concomitant increase in threshold voltage is undesirable. Two such applications are analog circuits such as current mirrors, and digital circuits such as pass-gate devices (series shutoff switches). The threshold voltage at zero source/body potential is determined by the integrated dopant near to the surface; the threshold voltage at an elevated source/body potential is determined by the doping deeper beneath the surface as well as that near the surface. By a judicious choice of net doping profile a device may be fabricated that has nearly the same zero source/body threshold but differs substantially in the elevated source/body threshold voltage. Accordingly the new device is described as follows: a) the well doping profile should be designed such that: the surface concentration (within the zero-bias depletion region) sets an threshold voltage appropriate to the application; b) the concentration deep below the silicon surface (beyond the elevated voltage depletion region) is appropriate to the junction isolation and sheet resistance requirements, and c) the doping level in the region between the two described above is low enough to minimize the threshold voltage increase under reverse source/body potential. In one particular embodiment the disclosed device and a "standard" device may be fabricated simultaneously and at no additional process complexity. An example of such a well design is illustrated below. The "standard" nfet device has a doping profile designed to be optimized for grounded-source operation and maintenance of a specific minimum threshold. For the additional, reduced body effect device, an additional, compensating implant is added. The resulting doping profile (simulated, Fig. 1) and data (actual, Fig. 2) are shown below. Most conveniently this compensating implant is the same as is provided for the threshold implant for the pfet device. Figure 1