Two-terminal switching devices and their methods of fabrication
Two-terminal switching devices characterized by high on/off current ratios and by high breakdown voltage are provided. These devices can be employed as switches in the driving circuits of active matrix displays, e.g., in electrophoretic, rotating element and liquid crystal displays. The switching devices include two electrodes, and a layer of a broad band semiconducting material residing between the electrodes. According to one example, the cathode comprises a metal having a low work function, the anode comprises an organic material having a p+ or p++ type of conductivity, and the broad band semiconductor comprises a metal oxide. The work function difference between the cathode and the anode material is preferably at least about 0.6 eV. The on/off current ratios of at least 10,000 over a voltage range of about 15 V can be achieved. The devices can be formed, if desired, on flexible polymeric substrates having low melting points.
1. An electronic device comprising:
a substrate;
a plurality of two-terminal switching devices disposed on the substrate, wherein each two-terminal switching device comprises:
a first electrode comprising a layer of first conductive material, having a first work function;
a layer of a broad band semiconducting material physically separate from the layers of semiconducting material on other of the two-terminal switching devices, wherein the band gap of the broad band semiconductor is at least about 2.5 eV and the carrier concentration in the broad band semiconductor is less than about 10 18 cm −3 ; and
a second electrode comprising a layer of a second conductive material, having a second work function, wherein the second conductive material comprises a material having a p+ or p++ type conductivity; wherein
at least a portion of the semiconductor layer resides between the first and second conductive materials, wherein the second work function magnitude is greater than the first work function magnitude.
2. The electronic device of claim 1 , wherein the energy difference between a Fermi level of the first conductive material and the lowest energy level of a conduction band of the broad band semiconducting material is not greater than about 0.3 eV.
3. The electronic device of claim 1 , wherein the carrier concentration in the second conducting material is at least about 10 18 cm −3 .
4. The electronic device of claim 1 , wherein the work function magnitude of the second conductive material is at least about 0.6 eV greater than the work function magnitude of the first conductive material.
5. The electronic device of claim 1 , wherein the energy barrier between the lowest energy level of the conduction band of the p+ or p++ conducting material and the lowest energy level of the conduction band of the broad-band semiconductor material is about 0.3 eV or less.
6. The electronic device of claim 1 , wherein the substrate is flexible.
7. The electronic device of claim 1 , wherein at least some of the two-terminal switching devices of the array are electrically connected to one another by conductive lines.
8. The electronic device of claim 1 , wherein the electronic device is a backplane for a display.
9. The electronic device of claim 8 , wherein the two-terminal switching devices are configured to regulate light from a pixel of the display, wherein the display comprises a plurality of pixel control circuits, and wherein each pixel control circuit of the plurality comprises at least one of said two-terminal switching devices.
10. The electronic device of claim 9 , wherein the switching device is configured to regulate light from a pixel of an electrophoretic or a rotating element display.
11. The electronic device of claim 9 , wherein the switching device is configured to regulate light from a pixel of a liquid crystal display.
12. The electronic device of claim 1 , wherein the broad band semiconducting material is a 2-6 valence compound.
13. The electronic device of claim 1 , wherein the first conductive material comprises a metal selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Ta, Al, In, Nb, Hf, Zn, Zr, Cu, Sn, V, Cr, Mn, Ga, Mo, Ni and Y and wherein the broad band semiconducting material comprises a metal oxide or an inorganic ceramic nanocomposite selected from the group consisting of Mg x O y , Ca x O y , Sr x O y , Ba x O y , Ti x O y , Ta x O y , Al x O y , In x O y , Nb x O y , Hf x O y , Sn x O y , Zn x O y , Zr x O y , Cu x O y , Y x O y , Y x Ba y O z , and Sm x Sn y O z .
14. The electronic device of claim 1 , wherein the first conductive material comprises a metal selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Ta, Al, In, Nb, Hf, Zn, Zr, Cu, Sn, V, Cr, Mn, Ga, Mo, Ni and Y and wherein the broad band semiconducting material comprises a metal chalcogenide.
15. The electronic device of claim 1 , wherein the first conductive material comprises a metal selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Ta, Al, In, Nb, Hf, Zn, Zr, Cu, Sn, V, Cr, Mn, Ga, Mo, Ni and Y; wherein the broad band semiconducting material comprises a metal oxide, an inorganic ceramic nanocomposite, or a metal chalcogenide, and wherein the second conductive material comprises one or more conductive polymers selected from the group consisting of polythiophenes, polypyrroles, polyanilines, polythienothiophenes, and copolymers thereof, wherein each of the conductive polymers is substituted or unsubstituted.
16. The electronic device of claim 1 , wherein the layers of two-terminal switching devices are formed as stacks on the substrate, with the first electrodes located proximate the substrate within the stacks and the second electrodes located distal with respect to the substrate in of the stacks.
17. The electronic device of claim 1 , wherein the electronic device comprises a column-row addressable electric switch matrix.
18. The electronic device of claim 1 , wherein for the two-terminal switching devices I on (V on )/I off (V off ) is at least about 1,000, wherein the forward bias (V on ) is about 2 V, and the reverse bias (V off ) is about −15 V.
19. The electronic device of claim 1 , wherein each two-terminal switching device further comprises a layer of an organic semiconductor between a layer of an inorganic broad-band semiconductor, and the layer of the second conductive material.
20. The electronic device of claim 1 , wherein the electronic device is selected from the group consisting of a Microelectromechanical System (MEMS) device, a field emission device, an electrochromic device, an electroluminescent device, a photodetector, and a biosensor.
21. The electronic device of claim 1 , wherein the electronic device is an electroluminescent device.
22. The electronic device of claim 1 , wherein the electronic device is a photodetector.
23. The electronic device of claim 1 , wherein the electronic device is a biosensor.
24. A method of forming a two-dimensional switching device comprising:
(a) forming a first electrode of the two-terminal switching device on a substrate, wherein the first electrode comprises a layer of a first conductive material, the first conductive material being characterized by a first work functionmagnitude;
(b) forming a layer of broad band semicinductor over at least a portion of the first electrode, wherein the band gap of the broad band semiconductor is at least about 205 eV and the carrier concentration in the broad band semiconductor is less thatn about 10 13 cm −3 ; and
(c) forming a second electrode by forming a layer of a second conductive material having a second work function value, wherein the second conductive material comprises a material having a p+ or p++ type conductivity,
wherein the second wirk function magnitude is greater than the first work functin magnitude.