POWER MOS TRANSISTOR

Abstract: In power MOSFETS, lead resistance loss is an important device design consideration. These devices typically use, for gate electrodes (15), sheets of polysilicon in order to provide low input impedance. Likewise, the source electrodes (17) are integrated together in a sheet-like structure. According to the invention, the sheet electrodes (15, 17) are perforated (18, 19) to improve the high frequency performance of the device by reducing interelectrode capacitance.

Description: MOS TRANSISTOR INTEGRATED CIRCUIT This invention relates to an MOS transistor integrated circuit comprising a plurality of source regions of a first conductivity type formed in the surface of a semiconductor body, a plurality of channel regions adjacent to the source regions, the channel regions having the opposite conductivity type to the first conductivity type, a common drain region of the first conductivity type surrounding the plurality of channel regions, a first insulating layer overlying the channel regions, a sheet of conductive material forming the gate electrode of transistors of the integrated circuit and overlying the surface of the body except for the source regions, a second insulating layer overlying the gate electrode and a sheet of conductive material forming the source contact of the transistors and overlying the second insulating layer. In power MOSFET integrated circuits, lead resistance is an important device design consideration. To minimize this resistance, contact conductors of the normal elongated configuration usually found on integrated circuit chips are replaced by sheets of conductive metal or polysilicon. The individual active regions are typically arranged in an array of triangular, square or other polygonal geometry and the gate conductor is characteristically a sheet-like structure interconnecting the multiple gate regions of the array. Source regions are interconnected via a conducting sheet into & parallel network that results in low distributive power losses. The gate layer is isolated from the silicon below it and the source contacting layer above by thin dielectric layer, typically of SiO2 and/or silicon nitride. While the reductions in power loss have been of significant advantage, an increased complexity of moderately high-power systems has placed a higher premium on switching speed. The improvements described above have increased speed insofar as they have permitted device integration, but further increases of switching speed would be highly desirable. In the invention as claimed the sheet of conductive material forming the gate electrode or the sheet of conductive material forming the source contact is perforated to reduce the capacitance between the source contact and the gate electrode. An embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which: FIG. 1 is a perspective view partly in section of a typical prior art transistor; FIG. 2 is a similar view of a device fabricated in accordance with the invention; and FIG. 3 is a similar view showing the apertured gate conductor with greater clarity. A conventional power MOSFET integrated circuit is illustrated in FIG. 1. It shows substrate 10, collective drain region 11, channels 12 and an array of source regions 13. The structure overlying the substrate includes the gate dielectric 14, the gate conductor 15s upper dielectric layer 16, and the source conductor 17. Each MOSFET comprises a source region, drain region, the intervening channel and the gate conductor overlying the channel. The gate conductor and source conductor both conduct current with low resistive losses, thus increasing power capability. A device with modified electrode configurations is shown in FIG. 2 in which the reference numbers correspond to those of FIG. 1. The FIG. 2 device is capable of significantly higher switching speed than the FIG. 1 device because it reduces the capacitance between the source, gate and drain conductors. The gate conductor 15 appears divided in section but is simply apertured. The apertures result in depressions 18 shown in FIG. 2. The gate conductor apertures significantly reduce the gate to drain capacitance. Apertures 19, formed in the source contacting sheet, reduce the gate to source capacitance. As a consequence of the offset between apertures 18 and 19, both contribute to reducing the gate to source capacitance. Thus, apertures 18 serve a dual role. The device may be made with apertures 18 alone, or in combination with apertures 19 in the source conductor for added benefit. Obviously benefit occurs also from the use of apertures in the source contacting sheet alone. The apertured gate conductor 15, at an earlier stage of fabrication, is shown in FIG. 3. The apertures 18' in the gate conductor do not significantly increase the current density or the resistance of the gate conductor. Alternative locations for the apertures can be used, e.g., those shown at 31 (in phantom). In a preferred embodiment, the gate electrode 30 is formed of polysilicon; or polysilicon alloyed with a metal to form a more conductive layer or combined with a metal (.e.g. Co, Ni, Ta, Ti, Mo, Pt, Pd, etc.) to form conductive intermetallic combinations (e.g., silicides). The gate electrode 30 is also useful as a mask to define the source regions in the substrate. The apertures 18 and the source diffusion windows can be etched simultaneously in the gate electrode structure, thus providing self-alignment. The substrate may include only layer 11. Various additional modifications and extensions this invention will become apparent to those skilled in the art.

Claims: CLAIMS 1. An MOS transistor integrated circuit comprising a plurality of source regions (13) of a first conductivity type formed in the surface of a semiconductor body (10,11), a plurality of channel regions (12) adjacent to the source regions, the channel regions having the opposite conductivity type to the first conductivity type, a common drain region (11) of the first conductivity type surrounding the plurality of channel regions, a first insulating layer (14) overlying the channel regions, a sheet (15) of conductive material forming the gate electrode of transistors of the integrated circuit and overlying the surface of the body except for the source regions, a second insulating layer (16) overlying the gate electrode and a sheet (17) of conductive material forming the source contact of the transistors and overlying the second insulating layer, characterised in that the sheet of conductive material forming the gate electrode or the sheet of conductive material forming the source contact is perforated to reduce the capacitance between the source contact and the gate electrode. 2. An integrated circuit as claimed in claim 1 wherein the gate electrode sheet (15) contains apertures (18') in regions thereof that do not overlie the channel regions (12). 3. An integrated circuit as claimed in claim 2 wherein both the source contact sheet (17) and the gate electrode sheet (15) are perforated. 4. An integrated circuit as claimed in claim 3 wherein the source contact sheet contains apertures (19) in regions thereof that do not overlie the apertures (18') of the gate electrode sheet.

Inventors: CLARKE, PATRICK WILLIAM, HELM, GEORGE DAVID, POTTEIGER, DONALD CARL
Applicants: WESTERN ELECTRIC COMPANY, INCORPORATED
Application year: 1983
Main classification: H01L29
IPC codes: H01L21, H01L29, H01L27