Static information storage and retrieval – Radiant energy – Amorphous
Patent
1985-07-22
1988-05-24
Moffitt, James W.
Static information storage and retrieval
Radiant energy
Amorphous
357 2, G11C 1142
Patent
active
047470771
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method for determining the state of a memory device.
IEE Proc., Vol 129, Pt I, Solid State and Electron Devices, No. 2, April, 1982, pages 51-54 discloses an electrically programmable non-volatile semiconductor memory device. In its simplest form this contains p and n layers of amorphous silicon deposited onto a conducting stainless steel substrate. The layered structure was formed into a memory device by applying a large forward bias of known polarity which switches the structure into a stable ON state. This step of "forming" permanently modifies the electrical properties of the device. The forming voltage was approximately 20 to 25 volts. After forming the device was then switched to the alternative OFF state by applying a voltage pulse above a low threshold such as 1 V with opposite polarity to the original pulse. The OFF state was stable for reverse bias voltage in excess of 10 V and for forward voltages of approximately 4-5 V. At higher forward voltages it switches to the ON state, i.e. the forward threshold voltage V.sub.ThF is 4-5 V. The ON state was stable for reverse bias voltages up to about 1 volt beyond which it switched to the OFF state i.e. the reverse bias threshold voltage V.sub.ThR was 1 V.
Our copending European patent application No. 83302665 discloses a memory device comprising an electrically conducting substrate and layers of i and p and/or n type amorphous or microcrystalline semi-conducting material which have been conditioned by the application of a voltage sufficiently large to cause the structure to be permanently modified to reduce the electrical resistance of the layers wherein no p and n layers are adjacent in the device.
Both devices have remarkably fast switches times of less than 100 ns.
U.S. Pat. No. 3,530,441 discloses amorphous memory devices which use light to read the memory state of a device by means of a measurement of a light related property e.g. refraction. It also discloses the use of light energy to produce a change in the memory state of a device. The change in memory state is detected by electrical measurements. There is no disclosure of the use of a junction type memory device. U.S. Pat. No. 3,530,441 is concerned with Ovshinsky-effect devices which do not rely on the presence of a junction, but on special properties of a single layer of material.
Japanese examined patent publication No. 55-39915 shows a device having a sandwich structure of a photoconductive layer and a chalogenide memory material (i.e. an Ovshinsky-effect type material). Irradiating the photoconductive layer with light gives a local reduction in resistance so applying a switching voltage to the chalogenide layer. The specification is thus concerned with writing to the device rather than with reading from it.
We have now discovered that memory amorphous or microcrystalline semiconductor junction devices possess a photovoltaic response when irradiated by light, the response differing substantially between the two conductivity levels characterising the ON and OFF states and that the response can be utilised to detect the state of the memory device.
Thus according to the present invention there is provided a process for detecting the conductance state of a non-volatile memory device which is switchable between a high conductance and a low conductance state and which comprises a plurality of amorphous or microcrystalline semiconductor layers at least one of which is a p-type layer in contact with an n or i-type layer, characterised by irradiating the device with light of a wavelength which causes band to band excitation and of an intensity which produces a detectable photovoltaic response and using the photovoltaic response to determine the conductance state of the device.
The photovoltaic response which is measured to determine the conductance state of the device may be the current across the device produced by irradiation, the voltage, or the power (the product of current and voltage).
The wavelength of the light is preferably less than 750 nm.
A laser is a suitable
REFERENCES:
patent: 4545111 (1985-10-01), Johnson
Hockley Peter J.
Thwaites Michael J.
Moffitt James W.
The British Petroleum Company p.l.c.
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