ACCUFET with schottky source contact

An accumulation mode FET (ACCUFET) having a source contact that makes Schottky contact with the base region thereof.

FIELD OF INVENTION

The present invention is in the field of MOSgated power devices, and more particularly relates to accumulation mode FETs (ACCUFETs).

BACKGROUND OF THE INVENTION

Power semiconductor devices such as power MOSFETs are prevalent in power supply applications. For example, power MOSFETs are used as synchronous rectifiers in power supply circuits.

Many power MOSFETs are known to include a body diode, which conducts current under reverse voltage conditions. The body diode of a power MOSFET exhibits relatively high resistance to current and thus dissipates much power. To avoid this undesirable consumption of power it is known to connect a Schottky diode across the body diode of a power MOSFET when, for example, the MOSFET is used as a synchronous rectifier, in order to reduce power loss during reverse voltage conditions. According to one conventional concept, for example, a discrete power MOSFET and a discrete Schottky diode are copackaged. According to another known concept a power MOSFET and a Schottky diode are formed in a single die to obtain a monolithic integrated device. One such device is illustrated by U.S. Pat. No. 6,351,018 ('018 patent). In that device, the source contact and the drift region of the MOSFET form a schottky diode. Thus, in a device according to the '102 patent the drift region has to be configured in order to make a Schottky device. To be specific, factors such as the resistivity of the drift region have to be designed for a Schottky device, which may not be desirable as it may increase the overall resistance of the device.

SUMMARY OF THE INVENTION

A power semiconductor device according to the present invention is a monolithic, integrated device which includes an ACCUFET and a Schottky diode formed in a single die.

As is well known, the base region of an ACCUFET is the same conductivity as its drift region and its source regions. That is, unlike a power MOSFET, an ACCUFET does not include a PN junction.

In a device according to the present invention, the source contact of the ACCUFET makes a Schottky contact with the base region of the device to form a Schottky diode. As a result, the characteristics of the drift region need not be modified to obtain a Schottky diode. Given that the conductivity of the drift region is no longer restricted by the need to make a Schottky contact, the drift can be made more conductive if desired.

According to an aspect of the present invention, a device according to the present invention includes source field electrodes which function to deplete the drift region. Thus, the drift region can be made more conductive without sacrificing the breakdown voltage capability of the device.

DETAILED DESCRIPTION OF THE FIGURES

Referring toFIG. 1, a power semiconductor device according to the first embodiment of the present invention is an ACCUFET which includes drift region10of one conductivity; base region12of one conductivity; source region14of one conductivity; and source contact18ohmically connected to source region14and making Schottky contact with base region12. A device according to the present invention further includes a plurality of insulated gate structures16each disposed within a respective trench26. Each insulated gate16is adjacent base region12and includes gate electrode32, gate insulator30disposed between gate electrode32and base region12, and insulation cap33disposed between gate electrode32and source contact18.

In the preferred embodiment of the present invention, drift region10is disposed over semiconductor substrate20, and includes drain contact22which is electrically connected to substrate20.

Furthermore, a device according to the first embodiment of the present invention includes a plurality of source field electrodes24each disposed within a respective trench26beneath a respective insulated gate16. Each source field electrode24is insulated from drift region10and an adjacently disposed gate electrode32by a respective insulation body28. Insulation body28in each trench26is preferably thicker than gate insulation30at least at the sidewalls and the bottom of trench26.

Referring next toFIG. 2, in which like features have been identified with like numerals, a device according to the second embodiment of the present invention includes a first set of trenches38each for supporting a respective insulated gate16, and a second set of trenches36each for supporting a respective source field electrode24. Second trenches36extend deeper into drift region than first set of trenches38.

Source field electrodes24in the first embodiment and the second embodiment are electrically connected to source contact18, and function to deplete drift region10, thereby improving the breakdown voltage of the device. As a result, drift region10can be made less resistive without adversely affecting the breakdown voltage.

It should be noted that source contact18reaches base region12through recesses34. Specifically, each recess34is formed to reach a depth below source regions14and to terminate in base region12, and source contact18makes Schottky contact with base region12inside each recess34. In the first and second embodiments, each recess34is spaced from insulated gates16, and in the second embodiment each recess34is adjacent a respective insulated source field electrode.

It should be noted that a device according to the present invention could be devised without recesses34, in which case base region12would be configured to reach through source regions14to source contact18.

In the preferred embodiment of the present invention, drift region10, base region12and source regions14are formed in an epitaxially formed silicon, substrate20is a silicon substrate, oxide caps33, gate insulations30, and insulation bodies28are composed of silicon dioxide, gate electrodes32and source field electrodes24are composed of polysilicon, and source contact18and drain contact22are composed of any suitable metal such as aluminum or aluminum silicon.

Also, substrate20, drift region10, base region12and source regions14are of N-type conductivity. Preferably drift region10can be graded to improve breakdown voltage and Rdson.