Thin film transistors are of great interest in the semiconductor industry as they represent a more universally applicable technology than traditional transistor devices. In some cases, thin film transistors also provide new properties that designers may leverage for great advantage. One interesting property is transparency.
The disclosed subject matter concerns transparent channel thin film transistors and a p-type transparent channel thin film transistor. In embodiments, an undoped or lightly doped delafossite material forms a p-type channel in a thin film transistor. In example embodiments, gate, source, drain regions and isolations are also formed from transparent materials to form a completely transparent device. Additional embodiments of the invention concern the integration of a p-type transparent delafossite channel transistor as the basis for a complementary metal oxide semiconductor (CMOS) circuit. A p-type transparent delafossite channel transistor may be integrated with conventional n-channel transparent thin film transistors for forming a CMOS circuit in accordance with embodiments of the invention. A CMOS transparent thin film circuit is thereby formed, and extends the general advantages of complementary circuits to a transparent channel thin film transistor circuit.
Thin film transistors of the embodiments of the invention have the general applicability of thin film transistors. Transparent device embodiments of the invention may be particularly well suited to display applications. Transparent devices are less likely to be affected by light than non-transparent devices, as the transparent devices absorb little to no energy from light.
Thin film transistors of example embodiments of the invention may be solution-processed at low temperatures. Choosing delafossite materials that are either soluble in a solution or capable of suspension in a solution permits processing by a solution technique, e.g., ink jet printing or spin coating. The solution-processed thin film transistors may be fabricated by simple techniques, e.g., direct printing of circuits. Screen printing is an example technique for patterning drain and source regions of solution-processed thin film transistors.
Embodiments of thin film transistor devices will now be illustrated. In the description, particular exemplary devices and device applications will be used for purposes of illustration, but the embodiments of the invention are not limited to the formation of the particular illustrated devices. Dimensions and illustrated devices may be exaggerated for purposes of illustration and understanding of the embodiments. Reference numerals may be used in different embodiments to indicate similar features. The elements of the drawings are not necessarily to scale relative to each other. Rather, emphasis has instead been placed upon clearly illustrating the embodiments of the invention. A device illustrated by a two-dimensional schematic layer structure will be understood by artisans to provide teaching of three-dimensional device structures and integrations.
Exemplary embodiments will now be discussed with respect to the figures. All device layers in the following description are thin film layers.
Embodiments of the transistor 8 include partially transparent devices, e.g., where the p-type transparent channel 10 is the only transparent thin film, as well as completely transparent devices, i.e., where all of the thin films are formed from transparent materials. Additional embodiments include the use of a transparent substrate. The p-type transparent channel 10 is a delafossite film that is undoped or lightly doped. In lightly doped layers, the delafossite has a doping level low enough to maintain its transparency and semiconductor performance. As an example, lightly doped embodiments of the invention include doping levels that result in a carrier (hole) concentration of less than ˜1017 cm−3. The apparent optical band gap of undoped delafossites is in the near-UV range, while heavily doped (and conductive) films may be nearly opaque. Delafossites are the materials having the crystal structure of CuFeO2. Example delafossites include CuScO2, CuAlO2, CuYO2, CuFeO2, CuCrO2, CuGaO2, CulnO2, AgCoO2, AgGaO2, AglnO2, AgScO2, and AgCrO2. Any dopant suitable to provide hole carriers may be used. For example, CuYO2 and CulnO2 can be doped p-type using Ca. As another example CuCrO2 can be doped p-type using Mg. Also, processing that results in a slight surplus of oxygen is often used to obtain p-type conductivity in these materials, and if controlled properly, may produce light doping levels for use as a p-type semiconductor channel in a transistor. Undoped and lightly doped p-type channels will yield an enhancement-mode or weakly depletion-mode transistor device. A negative gate voltage will draw holes from the source and drain contacts 12, 14 to the p-type channel 10 in a region near its interface with the gate dielectric 18. Undoped and lightly doped delafossite films additionally have the advantage of providing a reasonably small positive gate voltage to deplete holes from the channel, thereby producing a relatively low gate voltage turn-off condition.
Any number of materials may be employed for the gate dielectric 18, gate contact 16, source 12 and drain 14. The gate dielectric 18 for example may be a film of SiO2, Si3N4, Al2O3, Ta2O5, HfO2, ZrO2, or the like. The gate contact and source/drain layers may, for example, be formed from a transparent conductor (i.e., a p-type doped wide-band gap semiconductor) such as p-type doped GaN, BaCu2S2, NiO, Cu2O, or various delafossites (CuScO2, CuAlO2, CuYO2, CuFeO2, CuCrO2, CuGaO2, CulnO2, AgCoO2, AgGaO2, AglnO2, AgScO2, AgCrO2), or the like. Gate contact and source/drain layers may also comprise metals such as Au, Pt, Pd, Ni, Cu, W, Mo, Cr, Ag, In, Sn, Ga, Zn, Al, Ti, or the like.
It is beneficial to choose a source and drain contact material to produce efficient hole injection from the source into the p-type delafossite channel 10 at the source/channel interface. Materials may be selected for a desired level of electrical performance. Overall device performance is likely to vary significantly for devices built using various source/drain contact materials. If the source, drain and gate contact films are formed of transparent materials, appropriate gate materials will likely also be transparent, thereby producing a complete device that is substantially transparent.
The delafossite channel 10 and transistor 8 have the capability to provide hole injection in the undoped or lightly doped channel, thereby creating a p-type device. Other transparent semiconductors typically have a high ionization potential (separation between valence band edge and vacuum level), e.g. in the range of 6-8 eV. Hole injection into the transparent channel is achieved when the source and drain contacts are formed of a material (metal or doped semiconductor) having a work function that is nearly equal to or greater than the ionization potential of the channel material. However, even high work function metals, e.g., Au, Pd and Pt, have work functions smaller than 6 eV. The lower ionization potential of the undoped and lightly doped delafossite materials provides the ability meet the conditions for hole injection.
Other exemplary embodiment transistors of the invention are shown in
A variety of techniques are available for the formation of p-type transparent channel thin-film transistors, and circuits that include these transistors. Thin-film deposition techniques such as evaporation (thermal, e-beam), sputtering (DC, RF, ion beam), chemical vapor deposition (CVD), atomic layer deposition (ALD), molecular beam epitaxy (MBE), and the like may be employed. Alternate methods may also be employed, such as solution-based deposition from a liquid precursor (spin-coating, inkjet printing, etc). Film patterning may employ traditional photolithography combined with etching or lift-off processes, or may use alternate techniques such as shadow masking or direct-write patterning (i.e., inkjet printing).
With reference to the transistor 8 of
While specific embodiments of the present invention have been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims.
This application is a divisional application and claims the benefit and priority of U.S. patent application Ser. No. 10/738,690 filed Dec. 17, 2003.
Number | Date | Country | |
---|---|---|---|
Parent | 10738690 | Dec 2003 | US |
Child | 11346029 | Feb 2006 | US |