A non-volatile memory structure includes a semiconductor substrate and a first layer of a first dopant type in the semiconductor substrate. The non-volatile memory structure further includes a first well region of a second dopant type over the first layer, a second well region of the second dopant type over the first layer and spaced apart from the first well region, and a third well region of the first dopant type disposed between the first well region and the second well region and extending downward to the first layer.

In a processing method of a stacked-layer film in which a metal film is provided on an oxide insulating film, plasma containing an oxygen ion is generated by applying high-frequency power with power density greater than or equal to 0.59 W/cm.sup.2 and less than or equal to 1.18 W/cm.sup.2 to the stacked-layer film side under an atmosphere containing oxygen in which pressure is greater than or equal to 5 Pa and less than or equal to 15 Pa, the metal film is oxidized by the oxygen ion, and an oxide insulating film containing excess oxygen is formed by supplying oxygen to the oxide insulating film.

An object of the present invention is to provide a semiconductor device combining transistors integrating on a same substrate transistors including an oxide semiconductor in their channel formation region and transistors including non-oxide semiconductor in their channel formation region. An application of the present invention is to realize substantially non-volatile semiconductor memories which do not require specific erasing operation and do not suffer from damages due to repeated writing operation. Furthermore, the semiconductor device is well adapted to store multivalued data. Manufacturing methods, application circuits and driving/reading methods are explained in details in the description.

An object is to provide a deposition method in which a gallium oxide film is formed by a DC sputtering method. Another object is to provide a method for manufacturing a semiconductor device using a gallium oxide film as an insulating layer such as a gate insulating layer of a transistor. An insulating film is formed by a DC sputtering method or a pulsed DC sputtering method, using an oxide target including gallium oxide (also referred to as GaO.sub.X). The oxide target includes GaO.sub.X, and X is less than 1.5, preferably more than or equal to 0.01 and less than or equal to 0.5, further preferably more than or equal to 0.1 and less than or equal to 0.2. The oxide target has conductivity, and sputtering is performed in an oxygen gas atmosphere or a mixed atmosphere of an oxygen gas and a rare gas such as argon.

An object is at least one of a longer data retention period of a memory circuit, a reduction in power consumption, a smaller circuit area, and an increase in the number of times written data can be read to one data writing operation. The memory circuit has a first field-effect transistor, a second field-effect transistor, and a rectifier element including a pair of current terminals. A data signal is input to one of a source and a drain of the first field-effect transistor. A gate of the second field-effect transistor is electrically connected to the other of the source and the drain of the first field-effect transistor. One of the pair of current terminals of the rectifier element is electrically connected to a source or a drain of the second field-effect transistor.

An object is to provide a semiconductor device with a novel structure. The semiconductor device includes a first wiring; a second wiring; a third wiring; a fourth wiring; a first transistor having a first gate electrode, a first source electrode, and a first drain electrode; and a second transistor having a second gate electrode, a second source electrode, and a second drain electrode. The first transistor is provided in a substrate including a semiconductor material. The second transistor includes an oxide semiconductor layer.

A semiconductor device with a transistor having favorable electrical characteristics is provided. The semiconductor device has a memory circuit and a circuit that are over the same substrate. The memory circuit includes a capacitor, a first transistor, and a second transistor. A gate of the first transistor is electrically connected to the capacitor and one of a source and a drain of the second transistor. The circuit includes a third transistor and a fourth transistor that are electrically connected to each other in series. The first transistor and the third transistor each include an active layer including silicon, and the second transistor and the fourth transistor each include an active layer including an oxide semiconductor.

Provided is a memory device having a plurality of memory cells and a refresh circuit. Each of the memory cells is configured to retain multiple data as a potential of a node connected to a gate of a first transistor, one of a source and a drain of a second transistor, and one of electrodes of a capacitor. The refresh circuit is configured to refresh the memory cells. That is, the refresh circuit is configured to determine an interval between refresh operations, estimate a change of the potential of the node due to the leakage of the charge, and provide a refresh potential to the memory cells, where the refresh potential is a sum of the potential read from the node and the potential lost due to the charge leakage.

To provide a memory cell for storing multilevel data that is less likely to be affected by variations in characteristics of transistors and that is capable of easily writing multilevel data in a short time and accurately reading it out. In writing, a current corresponding to multilevel data is supplied to the transistor in the memory cell and stored as the gate-drain voltage of the transistor in the memory cell. In reading, a current is supplied to the transistor in the transistor with the stored gate-drain voltage, and the multilevel data is obtained from the voltage supplied to generate a current that is equal to the current.

An object of the present invention is to provide a semiconductor device in which stored data can be held even when power is not supplied for a certain time. Another object is to increase the degree of integration of a semiconductor device and to increase the storage capacity per unit area. A semiconductor device is formed with a material capable of sufficiently reducing off-state current of a transistor, such as an oxide semiconductor material that is a wide-bandgap semiconductor. With the use of a semiconductor material capable of sufficiently reducing off-state current of a transistor, the semiconductor device can hold data for a long time. Furthermore, a wiring layer provided under a transistor, a high-resistance region in an oxide semiconductor film, and a source electrode are used to form a capacitor, thereby reducing the area occupied by the transistor and the capacitor.