Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
2002-10-31
2004-10-19
Le, Dung A. (Department: 2818)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S099000, C438S602000, C257S040000
Reexamination Certificate
active
06806124
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method for fabricating a semiconductor device containing at least one first body and a second body, which form a common contact area. One of the bodies is constructed from an organic semiconductor material and the other body is constructed from an electrically conductive contact material. The method according to the invention makes it possible to significantly reduce the contact resistance in the transition of charge carriers from the first body into the second body.
Field-effect transistors are used as switches in electronic circuits. A semiconductor disposed between a source electrode and a drain electrode constructed from electrically conductive material in each case acts as an insulator in the switched-off state of the transistor, while a charge carrier channel forms under the influence of the field of a gate electrode in the switched-on state of the transistor. In this case, electrical charge carriers are injected into the semiconductor layer at the source contact and extracted from the semiconductor layer at the drain contact, so that an electric current flows from source to drain through the semiconductor layer or through the charge channel produced in the semiconductor layer.
Owing to the different Fermi levels of semiconductor material and contact material, an asymmetrical diffusion process occurs at the contact area of the two materials. The different energy of the Fermi levels of the two materials gives rise to an energy difference, which is compensated for by the crossing of charge carriers. As a consequence, an interface potential builds up which, when an external potential difference is applied, counteracts crossing of the charge carriers between the two layers. A potential barrier is thus produced, which has to be surmounted by the charge carriers when entering into the semiconductor material from the electrically conductive contact or when emerging from the semiconductor material into the electrically conductive contact. In this case, the tunneling current produced as a result of the charge carriers tunneling through the potential barrier is smaller, the higher or wider the potential barrier. A low tunneling current corresponds to a high contact resistance.
In semiconductor components based on inorganic semiconductors, an increase in the contact resistance is combated by doping the inorganic semiconductor in a boundary layer oriented toward the contact area. The doping alters the energy of the Fermi level in the inorganic semiconductor, i.e. the difference between the Fermi levels of contact material and semiconductor material decreases. As a consequence, either the potential barrier is reduced, thereby enabling a significantly larger number of charge carriers to surmount the potential barrier and to flood the material opposite, or the potential barrier is narrowed, as a result of which the probability of charge carriers tunneling through the potential barrier increases. In both cases, the contact resistance is reduced.
In the fabrication of field-effect transistors based on amorphous or polycrystalline silicon layers, the contact regions are doped by the introduction of phosphorus or boron into the silicon layer near the source and drain contacts. The phosphorus or boron atoms are incorporated into the silicon network and act as charge donors or charge acceptors, thereby increasing the density of the free charge carriers and thus the electrical conductivity of the silicon in the doped region. This reduces the difference between the Fermi levels of contact material and doped semiconductor material. In this case, the doping substance is introduced into the silicon only in the region of the source and drain contacts, but not in the channel region in which a charge carrier channel forms under the influence of the field of the gate electrode. Since phosphorus and boron form covalent bonds with the silicon, there is no risk of the atoms diffusing into the channel region, so that a low electrical conductivity in the channel region is furthermore guaranteed.
If the doping of the contact regions is high enough, the tunneling probability is already so high in the quiescent state that the junction between the contact material and the inorganic semiconductor material loses its blocking capability and becomes readily conductive in both directions.
Field-effect transistors based on organic semiconductors are of interest for a multiplicity of electronic applications that require extremely low manufacturing costs, flexible or unbreakable substrates, or the fabrication of transistors and integrated circuits over large active areas. By way of example, organic field-effect transistors are suitable as pixel control elements in active matrix screens. Such screens are usually fabricated with field-effect transistors based on amorphous or polycrystalline silicon layers. The temperatures of usually more than 250° C. that are necessary for fabricating high-quality transistors based on amorphous or polycrystalline silicon layers require the use of rigid and fragile glass or quartz substrates. By virtue of the relatively low temperatures at which transistors based on organic semiconductors are fabricated, usually of less than 100° C., organic transistors allow the fabrication of active matrix screens using inexpensive, flexible, transparent, unbreakable polymer films, with considerable advantages over glass or quartz substrates.
A further area of application for organic field-effect transistors is the fabrication of highly cost-effective integrated circuits, as are used for example for the active marking and identification of merchandise and goods. These so-called transponders are usually fabricated using integrated circuits based on monocrystalline silicon, which leads to considerable costs in the construction and connection technology. The fabrication of transponders on the basis of organic transistors would lead to huge cost reductions and could help the transponder technology to achieve worldwide success.
One of the main problems in the application of organic field-effect transistors is the relatively poor electrical properties of the source and drain contacts, i.e. the high contact resistances thereof. The source and drain contacts of organic transistors are usually produced using inorganic metals or with the aid of conductive polymers, in order thus to ensure the highest possible electrical conductivity of the contacts. Most organic semiconductors that are appropriate for use in organic field-effect transistors have very low electrical conductivities. By way of example, pentacene, which is often used for fabricating organic field-effect transistors, has a very low electrical conductivity of 10
−14
&OHgr;
−1
cm
−1
. If the organic semiconductor has a low electrical conductivity, a large difference between the Fermi levels of the electrically conductive contact material and the organic semiconductor material therefore exists at the contact area. This leads to the formation of a high potential barrier with a low tunneling probability for the passage of electrons. Therefore, source and drain contacts often have very high contact resistances, which has the effect that high electrical field strengths are necessary at the contacts in order to inject and extract charge carriers. A restrictive effect is thus brought about not by the conductivity of the contacts themselves, but by the conductivity of the semiconductor regions that adjoin the contacts and into or from which the charge carriers are injected or extracted.
In order to improve the electrical properties of the source and drain contacts, therefore, a high electrical conductivity of the organic semiconductor in the regions adjoining the contacts is desirable in order to reduce the difference in the Fermi levels between the organic semiconductor and the contact material and thus to lower the contact resistances. On the other hand, a high electrical conductivity of the organic semiconductor in the channel region adversely influences the
Klauk Hagen
Kriem Tarik
Schmid Günter
Greenberg Laurence A.
Infineon - Technologies AG
Le Dung A.
Mayback Gregory L.
Stemer Werner H.
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