Active solid-state devices (e.g. – transistors – solid-state diode – Regenerative type switching device
Reexamination Certificate
1998-07-06
2001-07-10
Picardat, Kevin M. (Department: 2823)
Active solid-state devices (e.g., transistors, solid-state diode
Regenerative type switching device
C257S110000, C257S499000, C257S565000, C438S133000, C438S134000
Reexamination Certificate
active
06259123
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to power switching devices. More specifically, the present invention relates to a power switching device for high frequency applications which has a relatively low “on resistance.”
Metal-oxide semiconductor field effect transistors (MOSFETs) have become the standard power switching device because of their fast switching capabilities. Unfortunately, as the breakdown voltages of power MOSFETs increase, a correlative increase in device “on resistance,” R
ON
, is encountered. This undesirable increase is largely a result of the high resistivity of the semiconductor layer which makes the increase in breakdown voltage possible. Increased R
ON
, in turn, translates into conduction losses and increasingly inefficient operation. The relationship between R
ON
and the device breakdown voltage, V
B
, is approximated by the equation:
R
ON
≈aV
B
2.5
(1)
That is, for every doubling of V
B
, R
ON
is increased by a factor of 5.66. Thus, despite their favorable switching characteristics, at some breakdown voltage, standard power MOSFETs become too inefficient for high power operation.
In contrast, insulated gate bipolar transistors (IGBTs) have a lower effective RON than MOSFETs as a result of a four layer structure which facilitates the injection of minority carriers into the high resistivity region. Unfortunately, the injection of these minority carriers results in slower devices which cannot match the switching capabilities of MOSFETs. This is due to the delay required to build up enough minority carriers in the high resistivity region before an IGBT is fully turned on. Similarly, the IGBT experiences a delay turning off because of the time required for the same minority carriers to be removed from this region.
In addition, because the four layer structure of an IGBT is similar to that of a thyristor, if the concentration of minority carriers in the high resistivity region exceeds a certain threshold, the IGBT ceases to behave like a transistor and goes into a latching mode. This behavior is described in detail in U.S. Pat. No. 4,199,774, issued on Apr. 22, 1980, the entire specification of which is incorporated herein by reference. Several techniques have been employed to reduce the susceptibility of IGBTs to latching. One of the most effective techniques involves irradiating the device with electrons after completion of standard semiconductor processing. Other techniques include unique device cell layout, source ballasting, and increasing the doping of the body region of the device. For more detailed descriptions of some of these techniques please see Comparison of 300-, 600-, and 1200-V n-Channel Insulated Gate Transistors, Chow et al., IEEE Transactions on Electron Device Letters, Vol. EDL-6, No. 4, April 1985, pp. 161-163, and The Insulated Gate Transistor: A New Three-Terminal MOS-Controlled Bipolar Power Device, Baliga et al., IEEE Transactions on Electron Devices, Vol. ED-31, No. 6, June 1984, pp. 821-828, both of which are incorporated herein by reference in their entirety. Unfortunately, while these techniques have had varying measures of success in reducing latching susceptibility, the devices remain slower than MOSFETs operating at similar power levels.
A power switching device is therefore desirable which combines the switching speed of a power MOSFET with the low “on resistance” of an IGBT.
SUMMARY OF THE INVENTION
The present invention provides a power switching device which combines the switching speed of a power MOSFET with the low “on resistance” of an IGBT. Moreover, the switching device of the present invention is less susceptible to the above-described latching phenomenon than standard IGBTs. The operational characteristics of the switching device of the present invention are made possible by its unique structure which provides a power MOSFET and an IGBT in parallel in a single device. The devices share a common source/emitter region and a common gate. The drain of the MOSFET comprises a number of island regions adjacent the back side of the device, at which surface the island regions are surrounded by the collector region of the IGBT.
Moreover, by varying the size, shape, number, and alignment of the island drain regions, the operational characteristics of the device of the present invention can be made to be more like either the MOSFET or the IGBT depending upon the application. That is, if in a particular application a fast switching speed is more important than a low “on resistance,” the operational characteristics of the device can be adjusted toward the MOSFET end of the spectrum. This may be accomplished, for example, by increasing the size of the island drain regions. Alternatively, a similar effect may be achieved by aligning the island drain regions more closely with the gate of the device.
Thus, according to the invention, a switching device and a method for fabricating the same are provided. The switching device is fabricated in a semiconductor substrate with a front side and a back side. The switching device includes a first transistor which includes a first region adjacent the front side, a second region within the first region, the semiconductor substrate, and at least one island region adjacent the backside. The switching device also includes a second transistor which includes the first region, the second region, the semiconductor substrate, and a third region coupled to the at least one island region.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.
REFERENCES:
patent: 4066484 (1978-01-01), Moyson
patent: 4564855 (1986-01-01), Van Zanten
patent: 5171696 (1992-12-01), Hagino
patent: 5528058 (1996-06-01), Peng, Jr. et al.
Baliga, B. Jayant et al., “The Insulated Gate Transistor: A New Three-Terminal MOS-Controlled Bipolar Power Device,”IEEE Transactions on Electron Devices(Jun. 1984) Ed—31 (6) :821-828.
Chow, T.P. et al., “Comparison of 300-, 600-, and 1200—V n—Channel Insulated Gate Transistors,”IEEE Electron Device Letters(Apr. 1985) EDL—6(4) :161-163.
Kelberlau Ulrich
Zommer Nathan
Collins D. M.
Picardat Kevin M.
Townsend and Townsend / and Crew LLP
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