Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Non-heterojunction superlattice
Reexamination Certificate
2000-01-27
2001-04-03
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Non-heterojunction superlattice
C257S030000, C257S262000, C257S263000, C257S280000, C257S364000, C257S365000
Reexamination Certificate
active
06211531
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a controllable conduction device with improved, reduced leakage current characteristics.
BACKGROUND
Integrated circuits have become progressively more miniaturised since their invention in 1959. Initially, their performance was improved by reducing the size of transistors used in the circuits, because the size reduction produced a reduction in parasitic capacitances of the circuit and a reduction in power dissipation. The miniaturisation was achieved by linearly scaling down the size of the features of the integrated circuit, by miniaturising the scale of the lithographic masks used in the manufacturing process.
However, as the scale of the devices was reduced further, it was found that the electrical characteristics of the resulting circuit did not scale linearly, and as a result, the configuration of the individual transistors in the circuit needed to be modified in order to optimise performance.
For example, current leakage from individual transistors of the circuits becomes a prominent factor degrading device performance as the device is miniaturised further and in high capacity dynamic random access memory (DRAM) cells, complex, three dimensional capacitors have been proposed in order to compensate for leakage current. However, the fabrication of such capacitors becomes unduly complicated.
Recently an alternative approach has been demonstrated, applicable to integrated circuits, in which transport of individual groups of electrons, in theory single electrons, is controlled. Reference is directed to “Single-electron memory”, K. Nakazato, R. J. Blaikie and H. Ahmed, J. Appl. Phys. 75, 5123 (1994). A single electron memory is disclosed in WO94/15340. In this device, a small group of electrons e.g. less than ten electrons, is stored at a node, which consists of a island constructed on the nanometre scale by electron beam lithography. The charge that can exist at the node is limited by the so-called Coulomb blockade effect. Once charged with the small group of electrons, no additional electrons can enter the island, due to its charging energy. In order to demonstrate the Coulomb blockade effect, the charging energy of the island needs to exceed the surrounding thermal energy, so that thermal electrons do not swamp the charge of the island. This requires either the device to be cooled to liquid nitrogen temperatures to reduce the thermal energy or, if the device is to operate at room temperature, the scale of the island needs to be of the order of 1 or 2 nm, which is beyond the capability of current e-beam lithographic techniques.
Charge is caused to enter and leave the island by means of a multiple tunnel junction device. In the device disclosed in WO94/15340 supra, the multiple tunnel junction device comprises a side gated structure which gives rise to multiple, stable electron states on the island, which can be used to provide a memory.
It has previously been proposed to improve the characteristics of a conventional transistor, which operates using a conventional current, which comprises many thousands of electrons per second, by associating a multiple tunnel junction device with the gate of the transistor, so that when in the off state, the multiple tunnel junction device minimises leakage current. Reference is directed to our EP-A-0 649 174. In this device, the gate is provided with a finger structure, constructed on the nanometre scale so as to produce e.g. by a field effect, a series of tunnel barriers in the source-drain path of the transistor. The multiple barriers act as a multiple tunnel junction so that in the off state, electron transport through the device is limited by Coulomb blockade, thereby significantly reducing leakage current from the drain to the source. However, this device is difficult to manufacture because the finger members formed in the gate need to be fabricated on a nanometre scale and current technologies do not permit such a device to be constructed readily on a scale small enough for operation at room temperature.
In “Superlattice tailoring to obtain devices with high saturation velocity” IBM Technical Disclosure Bulletin, Vol 29. No 7, December 1986, pp 2931-2, a transistor is described which has a superlattice in its source-drain path, formed of overlying, conductive layers of opposite conductivity type. The superlattice gives rise to energy subbands or minibands which allow the overall electron velocity to be increased.
In our EP 96308283.9 filed on Nov. 15, 1996, and corresponding U.S. Ser. No. 08/958845 filed on Oct. 28, 1997 there is described a memory device which includes a memory node to which charge is written through a tunnel barrier configuration from a control electrode. The stored charge affects the conductivity of a source-drain path and data is read by monitoring the conductivity of the path. The charge barrier configuration comprises a multiple tunnel barrier which may comprise alternating layers of polysilicon of 5 nm thickness and layers of silicon nitride of 2 nm thickness, overlying a polycrystalline layer of silicon, part of which acts as a memory node. Alternative barrier configurations are described including conductive nanometre scale conductive islands which act as a memory node, distributed in an insulating matrix. The advantage of the tunnel barrier configuration is that it reduces leakage current from the memory node without degrading the reading and writing times for the memory. Different types of memory device are described. In a first type, charge carriers from a control electrode pass through the tunnel barrier configuration to the memory node in response to a voltage applied to the control electrode. In a second type of device, an additional gate is provided for the tunnel barrier configuration in order to control the transfer of charge carriers from the control electrode to the memory node.
SUMMARY OF THE INVENTION
In accordance with the invention it has been appreciated that the charge barrier configuration can also be used in a controlled conduction device such as a transistor. The tunnel barrier configuration is used to provide a conductive path between a source and drain. When switched on, charge carriers can flow between the source and drain, but when switched off, the barrier configuration inhibits charge leakage through the path. A large on/off current ratio is thus obtained.
According to the invention there is provided a controllable conduction device comprising: source and drain regions, a conduction path for charge carriers between the source and drain regions, a gate for controlling charge carrier flow along the conduction path, and a barrier structure in the conduction path formed of regions of relatively conductive and non-conductive material such that under a first bias condition charge carrier flow can occur along the path and under a second bias condition the regions present a tunnel barrier configuration that inhibits charge carrier flow along the path, the regions providing an energy band profile that comprises a dimensionally relatively wide barrier component with a relatively low barrier height, and at least one relatively narrow barrier component with a relatively high barrier height.
The multiple tunnel junction configuration may comprise a plurality of alternating layers of relatively conductive and non-conductive material. For example, alternating layers of silicon and silicon nitride can be used, although an oxide of silicon can be used instead of the nitride layer.
The alternating layers of non-conductive material may have a thickness of 3 nm or less, so as to provide the tunnel barrier configuration.
In use, a conventional current may flow along the conduction path between the source and drain regions, which can be switched between on and off states by applying a voltage to the gate. When in the off state, the leakage current is extremely small due to the barrier configurations presented by the multiple tunnel junction configuration in the source-drain conduction path. The layers can be made of sufficiently small thickness that the tu
Itoh Kiyoo
Mine Toshiyuki
Mizuta Hiroshi
Nakazato Kazuo
Shimada Toshikazu
Hitachi , Ltd.
Kenyon & Kenyon
Ngo Ngan V.
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