Patent Publication Number: US-8124444-B2

Title: Method of doping organic semiconductors

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is a Continuation of U.S. application Ser. No. 12/024,484 filed on Feb. 1, 2008 now U.S. Pat. No. 7,821,000, to Kloc, et al. entitled “Method of Doping Organic Semiconductors”, currently allowed, commonly assigned with the present invention and incorporated herein by reference in its entirety. The present application is related to U.S. patent application Ser. No. 11/375,833 to Kloc, et al. entitled “Fabricating Apparatus with Doped Organic Semiconductors”, which is commonly assigned with the present application and hereby incorporated by reference as if reproduced herein in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract No. DE-FG02-04ER46118 awarded by the Department of Energy. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to organic semiconductors. 
     BACKGROUND OF THE INVENTION 
     Organic semiconductors are the subject of intense research interest. Potential benefits of these materials include low-cost, wide area coverage, and use with flexible electronic devices. They have been employed in organic light-emitting diodes (oLEDs) and organic field-effect transistors (oFETs), and in circuits integrating multiple devices. Fabrication techniques such as ink-jet printing have helped reduce the cost of fabrication of these devices and integrated circuits using them. 
     SUMMARY OF THE INVENTION 
     One embodiment is a method that includes forming a contiguous semiconducting region that includes polyaromatic molecules. The method further includes heating the region to a temperature above room temperature in the presence of a dopant gas and the absence of light to form a doped organic semiconducting region. 
     Another embodiment is a method that includes forming an organic semiconducting region that includes a crystalline region of polyaromatic molecules. A dielectric layer is formed over the organic semiconducting region. The method further includes forming an opening in the dielectric layer to expose the organic semiconducting region. The organic semiconducting region is then heated to a temperature above room temperature in the absence of light. 
     Another embodiment is a method that includes forming a crystalline organic semiconducting region that includes polyaromatic molecules. A source electrode and a drain electrode are placed in contact with the organic semiconducting region. A gate electrode is located to affect the conductivity of the organic semiconducting region between the source and drain electrodes. The method further includes forming a dielectric layer of a first dielectric between the organic semiconducting region and the gate electrode. The dielectric layer is substantially impermeable to oxygen and in contact with the organic semiconducting region. An opening is formed in the first dielectric to form a doping channel, wherein a portion of the organic semiconducting region is in contact with a second dielectric via the opening. A physical interface is located between the second dielectric and the first dielectric. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments are understood from the following detailed description, when read with the accompanying figures. Various features may not be drawn to scale and may be arbitrarily increased or reduced in size for clarity of discussion. Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1A-1D  illustrate an embodiments of a FET; 
         FIGS. 2 and 3  illustrate organic semiconducting molecules; 
         FIGS. 4A-4D  and  5 A- 5 B illustrate electrical characteristics of a FET; 
         FIGS. 6A-6F  illustrate an embodiment of a method of forming a FET; and 
         FIG. 7  illustrates an embodiment of a method of doping a FET. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Some polyaromatic semiconductors have been found to have relatively poor stability of electrical properties in the presence of some gases. Oxygen, e.g., can react with a portion of the polyaromatic molecule, thereby altering the electronic properties of the molecule. Such instability may be regarded as undesirable in many circumstances. 
     Some of the embodiments recognize the unexpected benefits of increasing the conductivity of a semiconducting polyaromatic layer by exposure to a dopant gas while heating and excluding light. In some embodiments, the increase of conductivity is substantially due to an increase of carrier concentration without a significant change of mobility of the carriers. Some embodiments stabilize the conductivity of the layer by subsequent exclusion of light and/or further exposure to the dopant gas from the layer using, e.g., a barrier layer or a package. 
       FIG. 1A  illustrates a cross-sectional view of an embodiment of an organic field effect transistor (FET)  100 . The FET  100  includes a channel material  110  having a surface  115 . A source electrode  120  and a drain electrode  130  are in contact with the channel material  110 , and a channel region  140  is shown as a contiguous portion of the channel material  110  between the source/drain electrodes  120 ,  130 . A dielectric layer  150  with a thickness  155  substantially encapsulates the surface  115  of the channel material  110 , with a portion  160  of the channel material  110  exposed to the ambient by virtue of an opening in the dielectric layer  150  that forms a doping channel  165 . In the illustrated embodiment, the channel material  110  is self-supporting. In such embodiments, the FET  100  may optionally be affixed to a substrate. In some embodiments, the doping channel  165  may be formed, e.g., by cleaving the channel material  110 . The channel region  140  is a distance  170  from the doping channel  165 . As discussed further below, in some embodiments the dielectric layer  150  is impermeable to a dopant gas used to dope the channel material  110 . A gate electrode  180  is formed over the dielectric layer  150  between the source/drain electrodes  120 ,  130 . 
     Exposure to the dopant gas increases the majority charge carrier density in the channel material  110  of the organic semiconductor as described herein. It is believed by the inventors that when incorporated into the organic semiconductor, the dopant gas operates to transfer mobile charges to the organic semiconductor, thereby creating charge traps. In some cases, the dopant gas molecules may form covalent bonds with the organic semiconductor. In other cases, the dopant gas molecules may occupy interstitial sites in the organic semiconductor without covalently bonding thereto. Depending on the gas molecule and the organic semiconductor, the charge traps may be positive or negative ions. The exposure to the dopant gas may increase the majority charge carrier density while leaving the mobility of the charge carriers substantially unchanged. As used herein with respect to the charge carrier mobility, substantially unchanged means that the mobility changes less than about 10%. In some cases of substantially unchanged mobility, the mobility may decrease, e.g., by less than 5% or by less than 1%. 
     In some cases, the dopant gas comprises oxygen. In such cases, the gas may be, e.g., a homoatomic source of oxygen, such as O 2  or O 3 , or a heteroatomic source such as H 2 O or N 2 O. In other cases, the dopant gas may comprise a halogen, e.g., F, Cl, I or Br. Doping by the dopant gas may be reversible by non-chemical means, e.g., by exposing the doped channel material  110  to elevated temperature. Such exposure may reverse doping by, e.g., causing outgassing of the dopant gas by the doped organic semiconductor. In some cases, doping may be reversed by exposing the doped organic semiconductor to a reducing gas, e.g., H 2 . 
     The term impermeable as used herein with respect to a dielectric layer or barrier means that the rate of diffusion of the dopant gas through the layer barrier is below a rate that results in a significant change of semiconducting characteristics of the channel region  140  over the operational lifetime of a device employing the channel material  110 . In some cases, an operational lifetime of such a device is about 10 years. A significant change as used with respect to semiconducting characteristics is a change that causes the device, such as a FET, to operate outside its operational specifications. Such a change may be, e.g., a 5% change of conductance at a given gate voltage V gs  from the conductance at a given threshold voltage immediately after manufacture. 
     The channel material  110  is referred to as “exposed” when the portion  160  forms an interface with a second dielectric. The second dielectric may be, e.g., the surrounding ambient or another layer of a solid dielectric material. In the latter case, the solid dielectric material may be the same or a different material as the dielectric layer  150 . 
     The FET  100  may be a component of an apparatus  190 . The apparatus  190  may additionally include other electronic devices, such as resistors, capacitors and transistors, and a power source to operate the FET  100 . The FET  100  may optionally be configured to electrically behave primarily as, e.g. a transistor, resistor, capacitor, or LED. 
       FIG. 1B  illustrates an embodiment of the FET  100  in which the channel material  110  is in contact with a substrate  195 . The dielectric layer  150  is formed over the channel material  110  and the substrate  195 . When the dielectric layer  150  is impermeable to the dopant gas, the substrate  195  is preferably at least as impermeable to the dopant gas as is the dielectric layer  150 . Substrate materials such as glass, silicon and some polymers are sufficiently impermeable to most gases and may also provide mechanical support to the channel material  110 . In the illustrated embodiment, the doping channel  165  may be formed, e.g., by a plasma etch process designed to remove the dielectric layer  150 . Such a process may optionally be designed to stop on the channel material  110 , as shown. The source electrode  120  and the drain electrode  130  are formed on the channel material  110 , and thus lie between the dielectric layer  150  and the channel material  110 . 
       FIG. 1C  illustrates an embodiment in which the source electrode  120  and the drain electrode  130  are formed on the substrate  195 . The channel material  110  is formed over the source/drain electrodes  120 ,  130  and the dielectric layer  150  is formed thereover. The doping channel  165  may again be formed by a plasma etch process. In this case, the etch process may be designed to remove the dielectric layer  150  and the channel material  110  to form the doping channel  165 . In the illustrated embodiment, the exposed portion  160  includes a larger surface area of the channel material  110  than does the embodiment of  FIG. 1B . Thus, the configuration of  FIG. 10  may provide more rapid doping of the channel material  110  with the dopant gas than the configuration of  FIG. 1B . 
       FIG. 1D  illustrates an embodiment in which an optional capping layer  197  is formed over the dielectric layer  150  and the doping channel  165 . The capping layer  197  may serve to prevent exposure of the channel material  110  to the doping gas after doping the channel material  110  as described below. The capping layer  197  may also have other characteristics that are desirable in some circumstances. For example, the capping layer  197  may be chosen to exclude light from the channel material  110  that is capable of forming or breaking molecular bonds or causing molecular excitations therein. 
     The capping layer  197  may be the same or a different material from the dielectric layer  150 . The choice of material for the capping layer  197  may be influenced by, e.g., the ability to fill the doping channel  165 , opacity at a wavelength of interest, and barrier properties. Regardless of whether the dielectric layer  150  and the capping layer  197  are the same or a different material, an interface  198  is formed between the dielectric layer  150  and the capping layer  197 . When the dielectric layer  150  and the capping layer  197  are the same material, the interface  198  may be detected, e.g., by electron microscopy. In some cases, the function of the capping layer may be provided by a package that excludes light and the doping gas, e.g., oxygen. 
     The channel material  110  includes a crystalline or polycrystalline organic semiconductor, which may be a p-type or an n-type semiconducting material. For a p-type material, e.g., when a voltage V gs  of the gate electrode  180  with respect to the source electrode  120  is at or below a threshold voltage, V th , of the FET  100 , the channel region  140  becomes more conductive, and current may flow between the source electrode  120  and the drain electrode  130 . The V th  depends on the dielectric permittivity of the dielectric layer  150  and the dielectric thickness  155 . While not limiting the scope of the invention by theory, in the case of p-type organic semiconductors it is believed that below V th , a charge trap, e.g., an electron trap state or an acceptor state, localizes thermally activated electrons from the valence band, and remaining delocalized holes produce p-type semiconducting properties of the channel material  110 . 
     The channel material  110  is either a single crystal of polyaromatic molecules or a polycrystalline layer of the molecules. The polyaromatic molecules can be members of two broad classes. The first of these classes includes monodisperse compounds incorporating a plurality of aromatic or heteroaromatic units, where the units may be fused to each other and/or linked to each other in a way that maintains conjugation of π-bonds. Conjugated π-bonds provide for delocalization of electrons in the polyaromatic molecules. The second class includes polymers and oligomers having the aforementioned polyaromatic characteristics. Herein, oligomers are polymer chains with less than about 10 repeating units. The polyaromatic molecules in these classes are typically characterized by having p-type semiconducting properties in the solid phase. Numerous such molecules are known in the art. For example, such molecules include acenes, thiophenes, di-anhydrides, di-imides, phthalocyanine salts, and derivatives of these classes of molecules. 
     Acenes are polyaromatic compounds having fused phenyl rings in a rectilinear arrangement, e.g., three or more such fused rings. A subclass of acenes includes those in which the aromatic rings are arranged in a linear fashion, as shown below. Among the linear acenes investigated for semiconducting applications are tetracene (n=2) and pentacene (n=3). 
     
       
         
         
             
             
         
       
     
     Thiophenes are molecules that have a five-member ring containing sulphur. Thiophenes having p-type semiconducting characteristics include those having one or more fused phenyl rings arranged in a linear fashion, with a terminal fused thiophene ring. A general structural representation of thiophenes having two terminal thiophene rings is shown below, for which n=0, 1, 2 . . . . 
     
       
         
         
             
             
         
       
     
       FIG. 2  shows examples of polyaromatic molecules with semiconducting properties that can be used as the channel material  110 . These examples include: pentacene  210  and processable derivatives thereof such as 6,13-bis (triisopropylsilylethynyl) pentacene (TIPS)  220 ; processable derivatives of anthradithiophene  230  and benzodithiophene  240 ; 5,6,11,12-tetraphenylnaphthacene (rubrene)  250  and processable derivatives thereof; naphthalene-1,4,5,8-tetracarboxyl di-anhydride  260 ; and derivatives  270  of N-substituted naphthalene-1,4,5,8-tetracarboxylic di-imide. 
       FIG. 3  shows examples of polyaromatic oligomers and polymers that can be used as the channel material  110 . The examples include: poly(9,9-dioctylfluorene-alt-bithiophene (F8T2)  310 ; poly (3,3′-dioctylterthiophene) (PTT8)  320 ; regioregular poly(3-hexylthiophene) (P3HT)  330 ; poly(9,9-dioctylfluorene) (F8)  340 ; poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT)  350 ; and oligomeric polyaromatic molecule oligothiophene  360 , and derivatives thereof. Those skilled in the pertinent art will appreciate that the above examples of polyaromatic molecules are not exhaustive of such molecules. 
     Additional details regarding semiconducting polymers and applications to FETs may be found in U.S. patent application Ser. No. 11/375,833 to Kloc, et al., previously incorporated by reference. 
     Crystals and films of semiconducting organic molecules do not typically have a significant population of electrons and holes in equilibrium in the absence of an applied electric field. Hence, the conductivity of such molecules is generally low relative to inorganic semiconductors. For example, while intrinsic silicon has a conductivity of about 1.5E-5 Ω −1 cm −1 , intrinsic pentacene may have a conductivity of about 1.8E-8 Ω −1 cm −1  and intrinsic rubrene may have a conductivity of about 1E-9 Ω −1 cm −1 . 
     However, exposure of some organic semiconductor films to a dopant gas can change the conductivity and mobility of charge carriers in the films. In many cases, these changes are detrimental to the purpose for which the film is used. Such exposure may be advantageous, however, if done in a controlled manner as described herein to produce stable semiconducting characteristics. 
     Embodiments disclosed herein benefit from the recognition that an organic semiconductor can be doped with a dopant gas in a manner that results in a substantially stable mobility of charge carriers. Previously known doping methods generally result in a substantial decrease of the mobility of the charge carriers. Without limitation by theory, it is thought that the decrease of mobility in prior art doping methods results from formation of deep charge traps. It is further thought that the embodiments described herein result in shallower charge traps that reduce hole mobility to a lesser degree. When the mobility is constant, or nearly so, a change of conductivity is about proportional to a change of the carrier concentration. Thus, the ability to change carrier concentration without substantially changing mobility, heretofore unknown, simplifies the task of designing circuits using such FETs, and provides the designer with more predictable operational characteristics of the FET. 
     In an embodiment, energy for the doping reaction is provided by raising the temperature of the organic semiconductor above room temperature (e.g., about 25° C.). The minimum reaction temperature will, in general, be related to the dopant gas and the organic semiconductor, as different dopant gases and materials will in general have different activation energies associated with forming charge traps therein by their interaction. A maximum doping temperature will in general be related to the onset of decomposition, or nonuniform or irreversible reactions between the dopant gas and the organic semiconductor. In one embodiment, in which O 2  is the dopant gas and the organic semiconductor is rubrene, the doping reaction may be performed at a temperature ranging from about 90° C. to about 150° C. In some cases, the doping reaction may be performed in a narrower temperature range of about 105° C. to about 115° C. For embodiments which use a dopant gas other than O 2 , the doping temperature range may include temperatures above 150° C. and below 90° C. 
     In addition to exposing the organic semiconductor to elevated temperature, light or other type of electromagnetic radiation may be excluded during such exposure. As used herein, light refers to photons with sufficient energy to induce an excited molecular state of the organic semiconductor molecule. The excited molecule may then react with the dopant gas to produce, e.g., an endoperoxide or other oxygen-related defect when an oxygen-containing dopant gas is used. 
     In the context of the present discussion, the formation of the endoperoxide or other deep charge trap is considered undesirable. Thus, the method described herein excludes photons capable of producing a molecular excitation that may result in formation of such traps. Photons with a wavelength long enough that only heating, e.g., of the semiconductor occurs are not considered “light” in the present discussion. In some cases, light only includes photons having energy greater than about 3 eV, corresponding to a wavelength of about 400 nm or less. This wavelength range corresponds roughly with ultraviolet and higher energy photons. In other cases, light includes visible photons, e.g., having a wavelength up to about 780 nm. Such cases include those in which visible wavelengths are capable of producing molecular excitations in the organic semiconductor molecule capable of reacting with the dopant gas to produce deep charge traps. 
     Doping as used herein includes both additive doping and subtractive doping. Additive doping involves increasing the density of charge traps in an organic semiconductor, while subtractive doping involves reducing that density. It is believed that the dopant gas is relatively weakly associated with the organic semiconductor when the organic semiconductor is doped as described here, therefore making the doping reaction at least partially reversible. The utility of this feature is discussed in greater detail below. 
     When the organic semiconductor is heated in the presence of an ambient including a relatively high partial pressure of oxygen, e.g., the organic semiconductor may be oxidized, and the concentration of oxygen and associated charge traps therein may increase. Conversely, when an organic semiconductor already containing oxygen, e.g., is heated in an ambient including a relatively low partial pressure of oxygen, the concentration of oxygen in the organic semiconductor may be reduced. Thus, the concentration of charge traps associated with the oxygen therein may decrease, resulting in a reduced doping level. 
     In addition to the specific dopant gas and organic semiconductor used, the doping process may further depend on the pressure of the doping gas and the presence of other gases in the ambient. In an embodiment using O 2  as the dopant gas, an oxygen partial pressure of about 50 kPa or greater may be used for additive doping, and a partial pressure of about 10 Pa or less may be used for subtractive doping. As used herein, an ambient with a partial pressure of the doping gas about 10 Pa or less is referred to as a vacuum. In another embodiment, when the doping gas includes, e.g., oxygen, hydrogen in the ambient may act as a reducing agent, facilitating subtractive doping of a previously doped organic semiconductor. 
     In an embodiment, the organic semiconductor includes rubrene. Rubrene may be formed in crystalline form by, e.g., sublimation from a stream of Ar and/or H 2  gas. In a nonlimiting example, the rubrene is heated to about 280-320° C. and the carrier gas is flowed through a horizontal sublimation tube. Rubrene crystals may form spontaneously in a 30 cm zone. A more detailed description of an example process is provided in U.S. patent application Ser. No. 11/159,781 to Kloc, et al., entitled “Purification of Organic Compositions by Sublimation,” which is incorporated by reference as if reproduced herein in its entirety. As described in detail below, FETs may be assembled using rubrene crystals formed in this manner with advantageous results. 
     The inventors believe that the rate of change of semiconducting characteristics of the FET  100  may be limited by the rate of diffusion of the dopant gas into the organic semiconductor and the geometry of the FET  100  formed therewith. For example, for a fixed distance  170 , a higher diffusion rate of dopant gas in the channel material  110  will result in a shorter time to achieve desired doping level. Similarly, for a fixed diffusion rate, a shorter distance  170  will result in a shorter time. As discussed further below, the duration of exposure to the dopant gas or vacuum may be limited to result in a desired conductivity of the organic semiconductor. 
     As described briefly above, in some embodiments the dielectric layer  150  is substantially impermeable to the dopant gas. In such embodiments, the dielectric layer  150 , in combination with the substrate  195  when used, substantially prevents diffusion of the dopant gas into and out of the channel material  110 . The doping channel  165  is formed to provide a path through the otherwise impermeable barrier formed by the dielectric layer  150  and substrate  195 . The dopant gas may diffuse into or out of the dielectric layer  150  through the doping channel  165  and exposed portion  160 . When the channel material  110  is doped to a desired level, the doping channel  165  can be sealed by the capping layer  197  or by a package that excludes the dopant gas. The doping channel  165  exposes a small fraction (e.g., about 10% or less) of the surface  115  of the channel material  110 , and the channel material  110  is considered substantially encapsulated when this fraction remains small. In some cases, the channel material  110  is substantially encapsulated when at least about 80% of the surface  115  of the channel material  110  is covered with the dielectric layer  150 . In other embodiments, about 90% or more of the surface  115  is covered. In yet other embodiments, 99% or more of the surface  115  is covered. 
     The dielectric thickness  155  may be selected with two considerations in mind. First, the dielectric thickness  155  may be chosen to result in a desired V th . Second, the dielectric thickness  155  may be chosen such that the rate of diffusion of the dopant gas through the dielectric layer  150  is low enough to provide stability of the doping level over the lifetime of the FET  100 . The minimum thickness necessary to result in a particular diffusion rate will, in general, depend on the material from which the layer is formed, and will typically be inversely related to the permeability of the dopant gas through a unit thickness of the material. 
     In some embodiments, the diffusion rate of the dopant gas in the channel material  110  is low enough that the channel material  110  need not be substantially encapsulated, or the barrier properties of the dielectric layer  150  may be relaxed. In such cases, sealing the doping channel  165  may also be optional. These embodiments are characterized by the dopant gas having a low enough diffusion rate in the channel material  110  at the operating temperature that changes in doping over the life of the FET  100  may be neglected. 
     In one aspect, the dielectric layer  150  may be deposited in a manner that does not substantially alter the properties of the channel material  110 . In a nonlimiting example, the dielectric layer  150  is a polymer. Polymers may be deposited by, e.g., a spin-on or a chemical vapor deposition (CVD) process. One such polymer formed by CVD is Parylene N, in which oxygen, e.g., may have a permeability of about 1.3E-6 μm 2 ·s −1 ·Pa −1  to about 1.8E-6 μm 2 ·s −1 ·Pa −1  at about 23° C. Parylene N may be deposited from the vapor phase in a highly conformal, pinhole-free form. A thickness of 3-4 μm of Parylene N is often an effective oxygen barrier at a temperature of about 100° C. or less. Substituted Parylenes such as Parylene-C, D, or HT also have dopant gas permeability values comparable to or lower than Parylene N. Parylenes may be deposited at about room temperature (25° C.), thus minimizing risk of heat-induced changes of the channel region  140 . 
       FIG. 4  illustrates, without limitation, the relationship between drain-source current I ds  and gate-source voltage, V gs  of an experimental FET configured as illustrated in  FIG. 1A , using rubrene as the channel material  110 , parylene as the dielectric layer  150 , and O 2  as the dopant gas. The distance  170  from the doping channel  165  to the channel region  140  for this device is about 1.6 mm, and the dielectric thickness  155  is about 3.2 μm. Data were obtained at saturation, V ds =−40 V, to minimize the effect of contact potentials. Under these conditions, I ds, sat ∝μ eff  (V gs −V th ) 2 , where μ eff  is the effective mobility of the holes.  FIGS. 4A and 4B  display I ds   1/2  as a function of V gs .  FIGS. 4C and 4D  display log(I ds ) as a function of V gs .  FIGS. 4A and 4C  are associated with oxidation of the gate dielectric, and  FIGS. 4B and 4D  are associated with reduction. Further details of this experimental work may be found in Woo-young So, et al., “Mobility-independent doping in crystalline rubrene field-effect transistors,” Solid State Communications 483-86 (2007) incorporated herein as if reproduced in its entirety. 
     Two additive doping (e.g., oxidation) steps are shown in  FIG. 4A . The initial I ds   1/2  versus V gs  characteristic is substantially linear below about −20 V, indicating that μ eff  is substantially constant in this range. Doping was done with at a temperature of about 110° C. and an O 2  ambient at about 110 kPa that substantially excluded other gases. After 10 h of oxidation, the transfer curve has shifted significantly to the right, indicating that the FET V th  is lower, and the conductivity of the channel region  140  is higher. Continuing exposure to O 2  and heat for an additional 17 h causes the curve to move further in the same direction, though less so. The 10 h and 27 h curves are shifted without significant change of the slope, indicating that the majority carrier mobility is substantially unchanged after doping. 
       FIG. 4B  shows the effect of subtractive doping (e.g., vacuum annealing) on transport behavior of the experimental FET. Annealing was done at a temperature of about 110° C. with an O 2  ambient at about 1.5 Pa. In this case, I ds   1/2  versus V gs  was measured at 2, 10 and 18 hours of annealing. In contrast to additive doping, in each case, the transfer characteristic shifts left, indicating an increase of V th  and a decrease of conductivity of the FET. Again, the mobility of the charge carriers is substantially unchanged, as inferred from the substantially unchanged slope during annealing. Moreover, after 18 hours of annealing, the FET is almost restored to the initial state, indicating substantial reversibility of the doping process. 
     In  FIGS. 4C and 4D , the transfer curves are replotted on a logarithmic scale. The off-currents, e.g., I ds  taken at V gs =0 V, are used to evaluate the equilibrium state of the channel material  110 , in this case rubrene. After 27 h of oxidation, the off-current is seen to increase in  FIG. 4C  by about two orders of magnitude, while the current at V gs =−60 V is seen to increase about 40% in  FIG. 4A . Conversely, vacuum annealing for 18 h is seen to reduce the off-current by about one order of magnitude  FIG. 4D , and is seen to lower the on-current by about 25% in  FIG. 4B . It is thought that in the on-state, the current of the FET  100  is governed not only by V gs  but also by the materials parameters μ eff , n, and V th . 
       FIGS. 5A and 5B  illustrate the mobility μ eff  (left axis), and V th  (right axis) of the experimental FET using rubrene as a function of annealing time in O 2  ( FIG. 5A ) and vacuum ( FIG. 5B ). The conditions of annealing are as described above in this nonlimiting example. The inset to each panel further illustrates the carrier concentration as a function of annealing time. One sees in  FIG. 5A  that a higher concentration of the dopant species, oxygen in this embodiment, reduces V th . This is believed to be attributable to inducing more holes in the valence band. The μ eff  is also seen to remain substantially unchanged.  FIG. 5B  illustrates that a lower concentration of the dopant species partially restores V th  by removing some of the added holes while id, again remains substantially unchanged. It is believed that the substantially stable nature of μ eff  exhibited by the holes in this example illustrates that the transport mechanism of the channel material  110  is substantially unaffected by the additive and subtractive doping processes. 
     The conductivity a of the channel region  140  may be determined from current characteristics in  FIG. 4  and knowledge of the geometry of the channel region  140 . The observed insensitivity of μ eff  with increased a implies that the carrier density, n=σ/μ, increases with additive doping, as shown in the inset to  FIG. 5A . In this case, the data were obtained from I ds  at V gs =0 V. The doping effect is thus analogous to p-type doping in FETs of inorganic semiconductors where a similar decrease of threshold voltage with increased dopant density leaves μ eff  unchanged. This is consistent with the reported increase of conductivity in rubrene on exposure to oxygen. See, e.g. V. Podzorov, et al., Appl. Phys. Lett. 85 (24)(2004) 6039, in which a light-mediated doping reaction was used. The shift of V th  bears superficial similarity to that observed by V. Podzorov et al., Phys. Rev. Lett. 93 (8) (2004) 086602, wherein V th  was found to increase upon exposure to x-rays. In the latter Podzorov work, however, the shift was associated with creation of deep-level traps, not with a known dopant species as in the illustrated embodiment. 
     Turning to  FIGS. 6A-6F , illustrated is a method of forming a FET  600 . In  FIG. 6A , an organic semiconductor  605  is shown with source/drain electrodes  610  formed thereover. In another embodiment, not shown, the organic semiconductor  605  is formed over the source/drain electrodes  610 . In a nonlimiting example, the organic semiconductor  605  is a rubrene crystal formed by the method described previously. The source/drain electrodes  610  may be formed thereover by, e.g., conductive paint or physical vapor deposition through a shadow mask. The illustrated embodiment shows without limitation a free-standing crystal of the organic semiconductor  605  without a supporting substrate. In other embodiments, not shown, the organic semiconductor  605  may be placed on a substrate. The substrate may include other resistive or semiconductor devices, and may be e.g., an inorganic semiconductor substrate or a flexible organic substrate. 
       FIG. 6B  illustrates the FET  600  after formation of a dielectric layer  615  is formed over the source/drain electrodes  610 . The dielectric layer  615  encapsulates the organic semiconductor  605  and the source/drain electrodes  610 . As described previously, the dielectric layer  615  may be, in one aspect, substantially impermeable at the deposited thickness to the dopant gas to be used. In some cases, the dielectric layer  615  is parylene or a substituted parylene. Connections to the source/drain electrodes  610  may be made before or after the dielectric layer  615  is formed. If the connections are made afterward, openings (not shown) in the dielectric layer  615  may be formed in a manner that does not expose the organic semiconductor  605  to the ambient, such as, e.g., mask and plasma etch. In this embodiment, it is preferred that the source/drain electrodes  610  are formed of a conductive layer impermeable to the intended dopant gas, such as a metal layer. 
     Also illustrated in  FIG. 6B  is a gate electrode  620  formed over the dielectric layer  615 . As for the source/drain electrodes  610 , the gate electrode  620  may comprise a conductive paint or a metal layer. The gate electrode  620  is positioned to produce a channel region (such as the channel region  140 , e.g.) that connects the source/drain electrodes  610  when the FET  600  is operated. In some cases, a preferred configuration of the FET  600  has the gate electrode  620  at least coextensive with a space  622  between the source/drain electrodes  610 . 
       FIG. 6C  illustrates the FET  600  after a doping channel  625  is formed in the dielectric layer  615 . In one embodiment, such as that illustrated in  FIG. 1A , the organic semiconductor  605  may be cleaved and thereby exposed to the ambient. In other embodiments, such as those illustrated by  FIGS. 1B and 1C , the doping channel  625  may be formed by, e.g., a plasma etch process to remove a portion of the dielectric layer  615 . The doping channel  625  formed thereby may be located in any desired position relative to the source/drain electrodes  610 . In some embodiments, the doping channel  625  is located to minimize the required duration of an additive or subtractive doping process in a later step. In other embodiments, the doping channel  625  is formed before the gate electrode  620  is formed. 
     In  FIG. 6D , the FET  600  is subjected to a doping process  630 . The doping process  630  may include an additive doping process, a subtractive doping process, or both. In some cases, the additive and subtractive doping processes are an oxidation process and reduction process, respectively, as previously described. In some embodiments, the doping process  630  is performed before the FET  600  is connected to other electrical components. In other embodiments, discussed further below, the FET  600  is connected to other components before performing the doping process  630 . In a nonlimiting example, the organic semiconductor  605  is additively doped by heating to about 380 K (107° C.) in an O 2  ambient at about 110 kPa. In another example, the organic semiconductor  605  is subtractively doped by heating to about 380 K in an ambient of about 10 Pa of the dopant gas or less. 
       FIG. 6E  illustrates the FET  600  after a first capping layer  635  is formed over the gate electrode  620 . In the illustrated embodiment, the first capping layer  635  may be, e.g., a dielectric layer that is substantially impermeable to the dopant gas. In this case, the first capping layer  635  serves to seal the doping channel  625  to substantially prevent the diffusion of the dopant gas into or out of the organic semiconductor  605 . In some embodiments, the first capping layer  635  is formed from the same material as the gate dielectric layer  615 . In such cases, an interface  636  may typically be detected by electron microscopy. 
       FIG. 6F  illustrates the FET  600  after an optional second capping layer  640  is formed over the first capping layer  635 . The second capping layer  640  may be substantially opaque to ultraviolet (UV) or longer wavelength light to prevent such light from illuminating the organic semiconductor  605 . This result may be desirable where illumination of the doped organic semiconductor  605  by such light causes dopant gas atoms to become disassociated with the organic semiconductor  605 , thus changing the conducting properties thereof. The light is substantially blocked when an insufficient fraction of the light reaches the organic semiconductor  605  to cause a change of doping of the organic semiconductor  605  over an operational lifetime. Substantially opaque means that the second capping layer  640  transmits less than about 5% of incident light. In other cases, the transmission may be less that 1% or 0.1%. In one example, the second capping layer  640  may be a metal or plastic portion of a package containing the FET  600 , or may be an epoxy resin. In another example, the second capping layer  640  is a dielectric mirror, comprising multiple dielectric layers designed to result in reflection of a substantial portion of the light. In another example, functional aspects of the first capping layer  635  and the second capping layer  640  may be combined in a single material layer that blocks both oxygen and light. 
     Turning to  FIG. 7 , illustrated is a flow diagram of a method  700  for adjusting a doping level of a FET such as the FET  600 . In a step  710 , the FET  600  is formed up to the point that the source, drain and gate electrodes are formed, and a doping channel is formed in the gate dielectric layer. (See  FIG. 6C , e.g.) In a step  720 , electrical connections to the FET  600  are made so that the FET  600  may be operated in the manner anticipated by the intended application. 
     In a step  730  and a step  740 , the doping level of the FET  600  is configured in a process referred to herein as “trimming”. The trimming process is analogous to trimming of resistors in certain electronics applications. In step  730  the FET  600  is additively doped, while operating, as previously described to set a desired doping level of the FET  600 . If the operating characteristic of the FET  600  is not within a desired range, the FET  600  may be further additively doped or subtractively doped in step  740  to achieve the desired operating characteristic. In some cases, trimming may include operating the FET  600  in the manner anticipated by the intended application. In some cases, this may include operating the FET  600  in an electrical test system (a “test bed”) operated exclusively for the purpose of determining the doping level. In a nonlimiting example, the test bed may include an oscillating circuit with an operating frequency depending on the doping level of the channel region  140 . In other cases, the operation may be in the actual circuit the FET  600  is to be operated in after establishing the doping level. In such cases, the doping level may be trimmed until the circuit operates within a design value range. Of course, other methods of trimming the FET  600  may be used as appropriate to the application. 
     Trimming of the doping level provides an advantageous means of adjusting transistor characteristics in a device or system where, e.g., it is not convenient or feasible to fabricate the transistor with characteristics within a required operating range. For example, physical dimensions typically are uncertain within a tolerance range. It is generally more expensive to produce devices with a tighter tolerance than those with a looser tolerance. The trimming process provides the means to manufacture with a looser tolerance at lower cost, and then adjust the performance of specific transistors whose performance causes the device or system to operate outside a desired operating range. Also, regardless of the tolerance range, a manufacturing process may occasionally produce statistical outliers with performance outside the tolerance range. The trimming process provides for the adjustment of some outliers, allowing for the recovery of devices or systems that might otherwise be discarded. 
     Heating of the FET  600  may in some cases be done by any means compatible with the operation of the test system or the final circuit, such as, e.g., a heated chamber. In other cases, the heating may be done by a source of spatially confined energy. As used herein, energy is spatially confined when it causes heating of all or a portion of the FET  600  when projected thereon, but does not significantly heat circuit components neighboring the FET  600 , e.g., a distance about the same as the dimensions of the FET  600 . In some cases, the heated area is smaller than the FET  600 . In other cases, the diameter of a region heated by the spatially confined energy source is about the length of the space  622  of the FET  600 . Nonlimiting examples of sources of spatially confined energy include lasers and focused incoherent light. Lasers are commercially available with a spot size on the order of one micron, providing the means to heat only a portion of the FET  600  when the device size is greater than 1 micron. When such a spot size is less than a feature size, e.g., the space  622 , the spot may be scanned to heat the entire feature. Moreover, the wavelength of the light may be chosen in relation to the size of features to be heated to help limit the spatial extent of heating. In some cases, the wavelength may also be limited to avoid producing molecular excitations in the organic semiconductor  605 , as previously described. 
     The spot size, power density, wavelength and illumination time of the spatially confined energy source may be chosen to result in a desired temperature of a particular FET  600  without causing chemical decomposition of the organic semiconductor  605 . In some embodiments, the beam size and power are selected so that only one FET of a plurality of FETs in a circuit is trimmed at a time. In other embodiments, multiple FETS are trimmed simultaneously. After the trimming operation is completed in steps  720 ,  730 , the FET  600  may be sealed with one or more capping layers, or packaged, to exclude a doping gas and/or light in a step  750 . 
     Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.