Wednesday, August 18, 2004

Patent case on MOSFETS: a new story that's really old

In Power Mosfet v. Siemens (August 17, 2004), the Court of Appeals for the Federal Circuit affirmed the district court judgment that US Patent No. 5,216,275 of Power Mosfet was not infringed.

The decision contains discussion of basic semiconductor technology:

Semiconductor power devices control the flow of electricity through an electronic circuit. They are typically constructed of silicon, which, by itself, is not a very good conductor of electricity. Silicon’s conductivity, however, can be enhanced by a process known as doping. Doping adds impurities to the crystal structure of pure silicon and creates either a surplus or deficiency of free electrons in the silicon material. Both conditions enable the flow of current through the material. When doping results in a surplus of electrons, the material is described as “n-type” because it has a net negative charge. When the result is a deficiency of electrons (i.e., a surplus of “holes”) the material is described as “p-type” because it has a net positive charge. Within the n-type and p-type categories, the material may be further categorized as heavily doped (n+ or p+ regions) or lightly doped (n- or p- regions).
The semiconductor power device described in the ’275 patent is known as a MOSFET. A cross-section from a traditional MOSFET is reproduced above from figure 1 of the ’275 patent. The ’275 patent describes the fabrication process of the traditional MOSFET device as follows: an n- layer 5 is grown on an n+ substrate 4, followed by the growth of a p+ layer 3 on the top of the n- layer 5. The above-described process may also be performed with p-type materials substituted for the n-type materials, and n-type materials for the p-type. ’275 patent, col. 5, ll. 23-29. In the traditional MOSFET design, layer 5 consists of a single conductivity type, either n- or p-type.
Also shown in figure 1 are the electrical connections of the semiconductor device. The terminals labeled “S” are the “source” terminals, where a positive voltage source is connected to the device. The terminal labeled “D” is the “drain” terminal, where the negative voltage connection is made. Terminal “G” is the “gate” terminal, and controls the current flow or, simply put, turns the device on and off. When on, current flows from the source to drain and, when off, current flow is blocked. The ’275 patent refers to region 5 as the “voltage sustaining layer” because, when not conducting current, it sustains a voltage between the S and D terminals.
The on and off states of a MOSFET device are controlled by applying a voltage to the gate terminal. When applied, the gate voltage creates an electric field inside the device that manipulates the electrons in the doped silicon to create conducting channels for current through the silicon material. When the gate voltage is removed, the electrons return to their normal positions and the voltage sustaining layer again prevents current from flowing through the device.
Two characteristics of MOSFETs are relevant to understanding the invention disclosed by the ’275 patent. The “on-resistance” (“Ron”) of the device is the resistance of the conducting channel through the semiconductor material. The higher the on-resistance, the greater the power loss (and accompanying heat generation) resulting from the current flow through the device. The second characteristic is the “breakdown voltage” (“Vb”), which is the maximum voltage that the semiconductor device can sustain between its terminals. In traditional semiconductor devices, there is an exponential relationship between Vb and Ron. See ’275 patent, col. 1, ll. 29-31. Higher Vb values are a desirable characteristic in a semiconductor device, but the resulting benefit must be balanced against the corresponding undesirable increase in Ron values.
The invention of the ’275 patent is a design for a voltage sustaining layer that results in a new relationship between Vb and Ron. According to the ’275 patent, the new relationship allows lower Ron values without the same magnitude of accompanying loss in Vb that results in traditional semiconductor devices. Id. at col. 1, ll. 55-66.

Patent applications for the basic transistor inventions of Bell Laboratories were filed in 1948. Not fully appreciated is that some of the patent applications, particularly those related to FET technology, were rejected over patents to Lilienfeld in the 1930's. (J. E. Lilienfeld, US 1,745,175; filed Oct. 8, 1926; issued Jan. 28, 1930; also US 1,877,140; US 1,900,018). Basic FET technology was not implemented until the 1960's. Thus, although we are litigating MOSFET technology in 2004, the concept of a field effect transistor was described seventy five years earlier.


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