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High Voltage MOSFET Devices

IP.com Disclosure Number: IPCOM000078303D
Original Publication Date: 1972-Dec-01
Included in the Prior Art Database: 2005-Feb-25
Document File: 2 page(s) / 45K

Publishing Venue

IBM

Related People

Spencer, OS: AUTHOR

Abstract

An n-channel metal-oxide semiconductor field-effect transistor (MOSFET) has heavily doped n-type source/drain regions diffused into a lightly doped p-substrate. Voltage drops in the silicon are generally almost exclusively across the wide depletion width of the p-region of the PN junction. With lighter doping in the n-portion of the source/drain areas, more voltage would be dropped across the n-depletion region and thus a higher voltage device would result. The gate oxide can be increased to raise the drain-gate breakdown voltage, to provide a correspondingly higher voltage rating. However, the fabrication of high resistivity n-type diffusions is difficult with standard diffusion technology.

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High Voltage MOSFET Devices

An n-channel metal-oxide semiconductor field-effect transistor (MOSFET) has heavily doped n-type source/drain regions diffused into a lightly doped p- substrate. Voltage drops in the silicon are generally almost exclusively across the wide depletion width of the p-region of the PN junction. With lighter doping in the n-portion of the source/drain areas, more voltage would be dropped across the n-depletion region and thus a higher voltage device would result. The gate oxide can be increased to raise the drain-gate breakdown voltage, to provide a correspondingly higher voltage rating. However, the fabrication of high resistivity n-type diffusions is difficult with standard diffusion technology.

Phosphorous diffusion is slower in silicon dioxide than in silicon at the same temperature. With time, however, phosphorous will.penetrate a silicon dioxide layer and reach the underlying silicon. In this case, the amount of phosphorous reaching the silicon is a function of the silicon dioxide thickness. Thus the thickness of a silicon dioxide barrier can be varied, to achieve a correspondingly graded phosphorous diffusion thereunder.

By growing a thin layer of silicon dioxide 10 over the source/drain regions of the p-silicon substrate 12 prior to phosphorous-deposition 14, higher resistivity diffusions 16 yielding low-voltage gradients and thus higher voltage drops before breakdown are realized. The contact resistance between source/drain and alu...