EP1946382A2

EP1946382A2 - Ambipolar vertical organic field-effect transistors

Info

Publication number: EP1946382A2
Application number: EP06826466A
Authority: EP
Inventors: Yang Yang; Sheng-Han Li; Liping Ma
Original assignee: University of California
Current assignee: University of California
Priority date: 2005-10-21
Filing date: 2006-10-23
Publication date: 2008-07-23
Also published as: WO2007048041A3; WO2007048041A2; JP2009513022A

Abstract

An ambipolar vertical organic field effect transistor has a drain electrode, a gate electrode spaced apart from said drain electrode, a charge injection layer disposed between the drain electrode and the gate electrode, a dielectric layer disposed between the gate electrode and the charge injection layer, and a semiconductor layer disposed between the drain electrode and the charge injection layer. In operation, the charge injection layer balances hole and electron injection barrier heights to provide both N-type and P-type transistor operation modes.

Description

Ambipolar Vertical Organic Field-Effect Transistors

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application No. 60/728,742 filed October 21, 2006,- the entire contents of which are hereby incorporated by reference.

BACKGROUND

1. Field of Invention

The present invention relates to vertical field-effect transistors, devices that incorporate such transistors and methods of manufacture and use thereof.

2. Discussion of Related Art

Organic semiconductor devices such as organic light emitting diodes (OLEDs), field-effect transistors and memory devices have been studied extensively due to their low-cost, flexible, and large-area application advantages. (See C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett. 51, 913-915 (1987); J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns and A. B. Holmes, Nature 347, 539-541 (1990); G. Horowitz, Adv. Mater. (Weinheim, Ger.), 10, 365-377 (1998); and L. P. Ma, J. Liu, and Y. Yang, Appl. Phys. Lett. 80, 2997-2999 (2002).) All references cited anywhere herein are incorporated herein by reference. Organic field-effect transistors (OFETs) have high potential for producing switching devices for active-matrix OLED displays, low-end smart cards and identification tags. However, laterally structured OFETs have two major limitations: 1. device performance, including low current output and high working voltage; and 2. unipolar characteristics. Although in some approaches, such as decreasing channel length or increasing the dielectric constant of the gate material to lower working voltages, the current output is still low (C. D. Dimitrakopoulos, S. Purushothaman, J. Kymissis, A. Callegari, and J. M. Shaw, Science 283, 822-824 (1999); Matthew J. Panzer, Christopher R. Newman, and C. Daniel Frisbie, Appl. Phys. Lett. 86, 103503 (2005)). Ma et al reported a vertical organic field-effect transistor (VOFET), which consists of a capacitor cell stacked with an organic active cell (L. P. Ma and Y.Yang, Appl Phys. Lett. 85, 5084-5086 (2004)). The capacitor cell shares a common electrode with the active cell, which was defined as the source electrode. The VOFET has low working voltage and high current output. This new type of transistor should be able to solve the low current output problem.

Unipolar characteristics are the other problem. Most OFETs show either N or P channel behavior. Integrated circuits systems, especially Complementary Metal-Oxide Semiconductor (CMOS) circuits, typically need both N- and P-type transistors to work. Although some approaches use multiple organic unipolar devices to achieve CMOS design, the use of ambipolar OFETs can simplify the CMOS design and fabrication. Ambipolar OFETs have been fabricated applying blends (J. Meijer, D. M. de Leeuw, S. Setayesh, E. van Veenendaal, B. -H. Huisman, P. W. M. Blom, J. C. Hummelen, U. Scherf, and T. M. Klapwijk, Nat. Mater. 2, 678- 682 (2003)), bilayers of carrier transporting materials (A. Dodabalapur, H. E. Katz, L. Torsi, and R. C. Haddon, Appl. Phys. Lett. 68, 1108-1110 (1996)), and electron injection electrodes (Takeshi Yasuda, Takeshi Goto, Katsuhiko Fujita, and Tetsuo Tsutsuia, Appl. Phys. Lett. 85, 2098-2100 (2004)). However, the operating voltage for these technologies is still not low enough.

SUMMARY

An ambipolar vertical organic field effect transistor has a drain electrode, a gate electrode spaced apart from said drain electrode, a charge injection layer disposed between the drain electrode and the gate electrode, a dielectric layer disposed between the gate electrode and the charge injection layer, and a semiconductor layer disposed between the drain electrode and the charge injection layer. In operation, the charge injection layer balances hole and electron injection barrier heights to provide both N-type and P-type transistor operation modes. An electronic and/or electro-optic devices includes such an ambipolar vertical field effect transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is better understood by reading the following detailed description with reference to the accompanying figures in which:

. "> . Figure l(a) is a schematic cross section diagram of an embodiment of a device structure according to the current invention; and Figure l(b) shows ambipolar operating modes of the device of Figure l(a).

Figures 2(a) to 2(c) show device output characteristics of different structures under various operating conditions: Figure 2(a) shows a VOFET with V₂O5 as the carrier injection layer (Transistor 1) resulting in P-type operation. The gate voltage is varied from 0 to -5 V with 1 V steps. The inset figure depicts the same data on a logarithm scale and was used to extract the ON/OFF ratio.

Figure 2(b) shows Transistor 1 during N-type operation. Figure 2(c) shows a VOFET with a single Al layer electrode (Transistor 2) during P-type operation. Figure 2(d) shows Transistor 2 during N-type operation.

Figure 3 (a) shows C Is XPS spectra of pentacene covered by different thicknesses of Al. The core level shift can be treated as the result of band bending, V_b. V_b=0.33eV.

Figure 3(b) shows UPS spectra (He I) with the increasing thicknesses of the metal film. The work function difference, ΔΦ, is 0.34 eV.

Figure 3(c) shows band diagrams of the Al/pentacene interface. The electron injection barrier is 0.76 eV. The hole injection barrier is 1.04 eV.

Figure 3(d) shows an energy level diagram of Al, V₂O5 and pentacene.

Figure 4 is a plot of device output currents versus oxide thickness for P- and N-type behavior: The dashed line shows N-type behavior. The solid line shows the P-type behavior.

Figure 5 shows transfer characteristics of an ambipolar vertical OFET inverter according to an embodiment of the current invention. Respective equivalent circuits and differential curve are shown in the insets. Figure 6 shows a VOFET with Molybdenum oxide (MOO₃) as the carrier injection layer resulting in P-type operation. Figure 7 shows the VOFET with Molybdenum oxide (MOO₃) as the carrier injection layer of Figure 6 resulting in N-type operation.

DETAILED DESCRIPTION

In describing embodiments of the present invention illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. It is to be understood that each specific element includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.

Organic field-effect transistors have been the subject of much recent inquiry due to their unique properties. However, high operating voltages, low current output and unipolar behavior are major limitations for many applications. Here, we disclose an ambipolar vertical organic field-effect transistor. The ambipolar vertical organic field-effect transistor according to an embodiment of this invention comprises a capacitor cell vertically stacked with an organic active cell. (See PCT/US2004/027579 published March 17, 2005 as WO 2005/024907 A2 for some details regarding VOFET structures, the entire contents of which are hereby incorporated herein by reference.) A transition metal oxide layer at the source/organic interface is provided as a charge injection layer. Such a structure can provide organic transistors with ambipolar behavior as well as low working voltage and high current output. A model is also presented to describe the ambipolar mechanism of the devices. The very thin transition-metal-oxide layer performs a role in device operation in an embodiment of this invention by balancing the hole and electron injection barrier height at the source/organic contact. An embodiment of an organic complementary metal-oxide-semiconductor inverter according to the current invention is also disclosed in an example that has a gain of 6.7. It is anticipated that the ambipolar vertical organic transistors according to this invention may push organic transistors into many new applications. .

According to the current invention, an ambipolar VOFET has a transition- metal-oxide layer at the source/organic interface which serves as the charge injection layer and provides high output current for both N-type and P-type operational modes. The transition metal oxide is defined as the oxidation products of transition metal. Transition metals, are a group of elements in the periodic table that belong to "B series" elements that have an ability to use either the penultimate and outermost electron shells to bond with other substances. They can form different states of transition metal oxides with different "d" electrons. For example, Vanadium Oxide can be vanadium(II) monoxide (VO), Vanadium (IV) oxide (VO₂), Vanadium (III) oxide (V₂O3) and Vanadium Pentoxide (V₂O₅). The transition metal oxide buffer layer can be, but is not limited to, for example:

a. composed of a single transition metal oxide such as V₂O₅, tungsten oxide

(WO₃), Molybdenum oxide (MOO3);

b. a mixture of different transition metal oxides such as, V₂Os with VO, WO₃ and V₂O₅ and MOO₃ with V₂O₅; and/or

c. a transition metal oxide and metal mixture layer, such as Vanadium Pentoxide with aluminum.

The device structure illustrated schematically in (Figure l(a)) according to an embodiment of the current invention has an active cell stacked with a capacitor cell. The middle electrode is defined as the common-source electrode, which is very thin and rough. The top electrode and the bottom electrode are defined as the drain and gate electrode, respectively. A charge injection layer is inserted between the contact of the organic layer and the electrodes. The device area is 2.4 mm², defined by the crossover area between the drain and source electrodes. This device can be operated by controlling the charge injection from the source electrode into the organic layer. Figure l(b) is a schematic illustration of operating modes of the device illustrated in Figure l(a). The source electrode is connected to ground. By applying different polarities of voltage bias to the gate and the drain electrodes, the ambipolar behavior can be obtained. When a negative bias is applied to the drain and gate electrodes, P- type conduction can be obtained. (See Figure l(b), left side.) For a positive bias applied to the same electrodes, N-type conduction can be obtained. (See Figure l(b), right side.)

An example of an ambipolar transistor according to the structure of the embodiment of Figure l(a) is labeled herein as Transistor 1. This transistor in this example consists essentially of (from the bottom up) glass/Aluminum (Al) (35 nm) /Lithium fluoride (LiF) (350 nm)/Al (18 nm)/ Vanadium Pentoxide (V₂O₅)(7.5 nm)/Pentacene (230 nm)/V₂C>5(7.5 nm)/Al(35 nm). One should note however, that the general concepts of this invention are not limited to these particular materials and the particular dimensions. In Figures 2(a) and (b), source-drain currents (I_SD) of Transistor 1 versus bias voltages (V_SD) are plotted for P and N-type behavior under different operating conditions. When operated as a P-type device, the output current was 1.8 mA, with V_SD and gate voltage (V_G) both at negative 5 volts. The ON/OFF ratio was approaching 10⁴. When operated as an N-type device, Transistor 1 showed an output current around 300 μA, when V_SD and V_G were both 5 volts. The ON/OFF ratio was near 10². The leakage currents, the current from the gate electrode to the source electrode, were under 5 μA. This is an example corresponding to an embodiment of the invention. The broad concepts of this invention are not limited to this particular example. In addition, although the specific examples provided herein have organic semiconductors for the active cell, the general concepts are intended to include all types of semiconductors, including inorganic semiconductors.

We also fabricated devices without the V₂O₅ charge injection layer (Transistor 2). Figures 21 and 2(d) show the output characteristics for Transistor 2 under different operating conditions. Obviously, the P-type behavior could not be observed for Transistor 2. For N-type applications, the output current was 271 μA and ON/OFF ratio was near 18. Both devices, Transistors 1 and 2, have similar N-type output characteristics except for Transistor 2's low ON/OFF ratio. When we compare device P-type behaviors, we should expect that adding a V₂O₅ layer plays a very important role in the P-type operating mechanism. The VOFET working mechanism is charge injection from the source electrode into the organic layer through barrier modification by the gate electric field. To understand why the device devoid of the V₂O₅ film only has an N-type mode, we used X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) to study the interface formation between pentacene and Al, the organic layer and source electrode. XPS measurements, in association with UPS measurements, allowed us to determine the alignment of energy levels at metal/semiconductor or semiconductor/semiconductor interfaces. Details of the procedure for determining the energy level alignment of such interfaces using XPS and UPS are described elsewhere (Li Yan, M. G. Mason, C. W. Tang, and Yongli Gao, Appl. Surf Sci 175-176, 412- 418 (2001); P. G. Schroeder, C. B. France, J. B. Park, and B. A. Parkinsona, J. Appl Ftiys. 91, 3010-3014 (2002)). Figure 3(a) shows the development of the XPS C Is core level of pentacene with varying Al coverage on top of the pentacene film. These C Is peaks display only shifting, not broadening, to the higher binding energy. The core level binding energy change can be seen as the result of band bending, Vb ( Li Yan, M. G. Mason, C. W. Tang, and Yongli Gao, Appl. Surf. ScL 175-176, 412-418 (2001)). The UPS results reveal a strong shift of 0.33 eV, shown in Figure 3(b), in the secondary cut-off edge between the bare pentacene film and the 1.4 nm thick Al film, indicating the presence of a significant surface dipole. The UPS secondary cut-off difference can be defined as the work function change, ΔΦ. The lowest unoccupied molecular orbital (LUMO) position cannot be measured by XPS or LTPS. Here, assuming the optical band gap equals the difference between LUMO and the highest occupied molecular orbital (HOMO), we can estimate the LUlSlO position (P. G. Schroeder, C. B. France, J. B. Park, and B. A. Parkinsona, J. Appl. Phys. 91, 3010- 3014 (2002)). Once we know V_b, ΔΦ and the inferred HOMO and LUMO position, we can plot the energy diagram shown in Figure 31. The magnitudes of the electron and hole injection barriers as well as of C Is core-level energy shift, as measured with XPS, are determined. The electron injection barrier (E_b) at the interface can be calculated using the equation, Eb=EmMo-Vb, where ELII_MO ^1S the energy level of the LUMO of pentacene referring to the Fermi level of the electrode. The hole injection barrier can be calculated similarly, but using the HOMO energy level instead of the LUMO energy level of the organic material. We found that the charge injection energy barrier at pentacene/Al interface is 0.76 eV and 1.04 eV for electron and hole, respectively. For the VOFET, using LiF as a dielectric layer, the maximum barrier height which the gate can modify is less than 1.0 eV. However, the hole injection barrier is 1.04 eV, so we cannot observe P-type behavior in the Al electrode device due to the large injection barrier. The energy level diagram for Al, V₂Os, and pentacene is illustrated in Figure 3(d). The valance band Of V₂Os was estimated to be 4.7 eV by UPS. From a reference, the band gap Of V₂O₅ is 2.2 eV (Transition Metal Oxide; An Introduction to their Electronic structure and Properties, P. A. Cox, (Clarendon Press, Oxford, 1992)). The conduction band can be calculated to be 2.5 eV from the equation E_C=E_V-E_g,, where E₀ and E_v are the conductance band and valence band of V₂Os, respectively, and E_g is the band gap. The hole injection barrier for Transistor 1 should exist at the transition metal oxide and organic interface. By applying the simple band model, the hole injection barrier is 0.3 eV for Transistor 1 when a charge injection layer is used to modify the source/organic interface. This barrier height can be tuned by applying gate bias. With N-type operation, electrons at the source electrode can tunnel through the thin transition-metal-oxide layer into the organic film, so the injection barrier is still controlled by the Al electrode. The electron injection barrier heights for Al/ pentacene and Al/ V₂O₅ /pentacene should be almost identical due to the tunneling effect.

When V₂O₅ is included in the film stack, the device operation mechanism can be described in the following manner. The I_SD should be controlled by carrier injection from the source electrode into the organic film. At zero gate bias, the injection barrier is high enough to confine the carriers injected from source to organic, so I_SD is quite low at zero gate bias. However, when the gate is under positive or negative bias, the capacitor is fully charged. These charges build up within the source electrode due to the rough and partially oxidized interface (L. P. Ma and Y.Yang, Appl. Phys. Lett. 87, 123503 (2005)). The charge build-up is modulated by the gate bias. As a result, the injection barrier height is likewise modified and decreases with increasing gate bias. Therefore, the effective injection barrier height is decreased by applying gate bias and the I_SD is increased. For Transistor 2, the hole injection barrier height, 1.04 eV, is large enough to stop the current flow, even under negative gate bias. This is the reason why only N-type behavior is seen. By introducing a very thin V₂O₅ charge injection layer, the hole injection barrier height can be reduced to 0.3 eV while the electron injection property is scarcely affected. Therefore, the transistors work under both N-type and P-type modes. In order to investigate the influence of the V₂O₅ layer, the source transition-metal-oxide thickness versus maximum I_SD (V_SD and V_G both equal to 5 volts) is also plotted, see Figure 4. When the transistor is operated as a P-type device, hole injection from the source electrode into the organic layer dominates the source-drain current. As shown in Figure 4, the solid curve, I_SD increases from 50 nA to 2 mA with increasing the thickness of the V₂O₅ charge injection layer. This result also shows that the charge injection layer dominates the device performance. Without the V₂O5 layer, the injection barrier is too high for the capacitor to modify. When the V2O5 layer is only 2.5 nm thick, the source electrode surface is not fully covered by V₂O5, and as a result, the effective area (the area covered by V₂O₅) for hole injection is small. As the V₂O₅ layer thickness increases, the effective area for hole injection also increases until it reaches the geometric area of the device, where I_SD is governed by the injection barrier between V₂O5 and pentacene because the AI/V₂O5 contact is believed to be υnmic tor holes (C. W. Chu, S. H. Li₅ C. W. Chen, V. Shrotriya and Y.Yang, Appl Phys, Lett. (Accepted)). It can be seen from the solid curve of Figure 4 that after the V2O5 thickness is over 5 nm, the ISD shows a saturation tendency. The slight decrease of the I_Sd for V₂Os above 10 nm can be understood as the increasing resistance of the V₂Os layer.

For N-type operation of the transistor, electron injection from the source electrode into the organic layer dominates the source-drain current. As shown by the dashed curve of Figure 4, we notice a totally different trend between the N-type mode and the P-type mode. The ISD is reduced with increasing V2O5 layer thickness. As can be seen from Figure 3(d), there is a big energy barrier for electron injection from Al into the conductance band of the V₂O5. Therefore, during N-type operation, electrons go through the V₂O₅ layer by tunneling. When the oxide layer is very thin or discontinuous, the electron injection barrier height is dominated by the Al/pentacene contact. When the oxide thickness increases, the tunneling currents and Al/pentacene contact area should also diminish, so the total output current will become smaller. Once the V₂O₅ layer thickness is over 20 nm, the N-type behavior is not observed. For a transition-metal-oxide VOFET₅ the determining factor for device fabrication and operation is how to control the interface between the source electrode and organic layer. In OFET circuit design, CMOS technology is recommended due to its very low-power consumption and compensating effects in device tolerances. In order to get the highest performance, integrating both N- and P-type OFETs using the same semiconductor layer is an important requirement. Our novel ambipolar VOFETs provide an excellent building block to fulfill this basic demand. Figure 5 shows the transfer characteristic of the complementary inverter. The equivalent circuit is also shown as the inset of Figure 5. The inverter operating voltage is clearly below 5 V.

In summary, we have studied the ambipolar properties of vertical organic field-effect transistors that use V₂O5 as a charge injection material in conjunction with pentacene. A particular significance of adding the transition-metal-oxide is that it lowers the hole injection barrier at the organic interface. The electron injection barrier is still controlled by the Al and pentacene contact or the tunneling current from source to organic layer. We also examined the effect of varying factors such as oxide thickness and structure of the device, and showed an optimum V₂Os thickness to achieve the maximum current output according to an embodiment of this invention. We fabricated a vertical organic CMOS inverter using a single semiconductor layer with symmetric source and drain electrodes to verify device performance. With this ambipolar behavior, the illustrated device may open a new application direction for organic field-effect transistors.

Examples

Anibiploar vertical organic transistor with V₂Os as charge injection layer

Vacuum thermal evaporation methods were used for the device fabrication of particular embodiments of this invention. The pressure for deposition processes was held below 4*10^"6 torr. Pentacene and V₂O₅ were purchased from ALDRICH and were used as received. Aluminum, as the bottom gate electrode, was first deposited onto a pre-cleaned glass substrate. A lithium fluoride layer, 350 nm thick, followed as the gate dielectric layer, and then the source aluminum electrode deposition was performed. A charge injection layer, vanadium pentoxide, was deposited on top of the source electrode surface, followed by the pentacene layer deposition. Finally, the top electrode, the drain electrode, was deposited to complete fabrication of the device. In order to get high capacitance for the capacitor, devices should be tested in a moist environment. All example devices mentioned herein were examined with an ambient relative humidity of 45% using an AGILENT 4156C precision semiconductor parameter analyzer.

XPS/UPS spectrometry analysis of band bending at interface

The surface state was measured by an OMICRON NanoTechnology system with Al K excitation (1486.6 eV) for the XPS spectra and He I excitation (21.2 eV) for UPS spectra. Pentacene films were deposited onto 100 nm silver-coated silicon substrates. Samples were then transferred into a preparation chamber in which a variety of thin layers of Al could be deposited in ultrahigh vacuum. Base pressures in the preparation and analyzer chamber were 6^10^"'° and 2^χlO^"10 mbar, respectively. LIPS spectra were recorded with a sample bias of -5 V.

Another example uses Molybdenum oxide as a buffer layer for an ambipolar vertical organic field-effect transistor according to an embodiment of the current invention. In the case the data are illustrated in Figures 6 and 7. The output performance can be summarized as follows: P-type: 0.12 A/cm² (at V_G=-5V) N-type: 0.07 A/cm² (at V_G=5V)

Compared with Vanadium Pentoxide examples, such as discussed above, this device has higher performance. It has a high current output, high on/off ratio and saturation region.

Furthermore, the charge injection layer may be selected from inorganic semiconductor materials, organic semiconductor materials, insulating materials, conducting materials, and combinations thereof.

The current invention is not limited to the specific embodiments of the invention illustrated herein by way of example, but is defined by the claims. One of ordinary skill in the art would recognize that various modifications and alternatives to the examples discussed herein are possible without departing from the scope and spirit of this invention.

Claims

WE CLAIM;

1. An ambipolar vertical organic field effect transistor, comprising: a drain electrode; a gate electrode spaced apart from said drain electrode; a source electrode disposed between said drain electrode and said gate electrode; a dielectric layer disposed between said gate electrode and said source electrode; an organic semiconductor layer disposed between said drain electrode and said source electrode; and a charge injection layer arranged between and in contact with said organic semiconductor layer and said source electrode, wherein, in operation, said charge injection layer balances hole and electron injection barrier heights to provide both N-type and P-type transistor operation modes.

2. An ambipolar vertical organic field effect transistor according to claim 1, wherein said charge injection layer comprises a transition-metal-oxide material.

3. An ambipolar vertical organic field effect transistor according to claim 1, wherein said charge injection layer consists essentially Of V₂Os.

4. An ambipolar vertical organic field effect transistor according to claim 1, wherein said charge injection layer consists essentially of MOO3.

5. An ambipolar vertical organic field effect transistor according to claim 1, wherein said charge injection layer consists essentially of WO3.

6. An ambipolar vertical organic field effect transistor according to claim 1, wherein said charge injection layer consists essentially of a mixture of at least one Of V₂O₅ with VO, WO₃ and V₂O₅ and MoO₃ with V₂O₅.

7. An ambipolar vertical organic field effect transistor according to claim 1, wherein said charge injection layer comprises a mixture of a transition-metal- oxide material and a metal.

8. An ambipolar vertical organic field effect transistor according to claim 7, wherein said metal is Aluminum.

9. An ambipolar vertical organic field effect transistor according to claim 1, wherein said organic semiconductor layer is selected from the group of materials consisting of organic transistor materials, organic light-emitting materials, organic photovoltaic materials and combinations thereof.

10. An ambipolar vertical organic field effect transistor according to claim 1, wherein said organic semiconductor layer consists essentially of pentacene.

1 1. An ambipolar vertical organic field effect transistor according to claim 1, further comprising a charge injection layer disposed between and in contact with said organic semiconductor layer and said drain electrode.

12. An electronic or electro-optic device, comprising: an ambipolar vertical field effect transistor, comprising: a drain electrode; a gate electrode spaced apart from said drain electrode; a source electrode disposed between said drain electrode and said gate electrode; a dielectric layer disposed between said gate electrode and said source electrode; an organic semiconductor layer disposed between said drain electrode and said source electrode; and a charge injection layer arranged between and in contact with said organic semiconductor layer and said source electrode, a power source in electrical connection with said drain electrode to provide a bias voltage between said drain electrode and said source electrode; and a power source in electrical connection with said gate electrode to provide a bias voltage between said gate electrode and said source electrode, wherein, in operation, said charge injection layer balances hole and electron injection barrier heights to provide both N-type and P-type transistor operation modes.

13. An ambipolar vertical field effect transistor, comprising: a drain electrode; a gate electrode spaced apart from said drain electrode; a charge injection layer disposed between said drain electrode and said gate electrode; a dielectric layer disposed between said gate electrode and said charge injection layer; and a semiconductor layer disposed between said drain electrode and said charge injection layer, wherein, in operation, said charge injection layer balances hole and electron injection barrier heights to provide both N-type and P-type transistor operation modes.

14. An ambipolar vertical organic field effect transistor according to claim 13, wherein said charge injection layer comprises a transition-metal-oxide material.

15. An ambipolar vertical organic field effect transistor according to claim 13, wherein said charge injection layer consists essentially OfV₂Os.

16. An ambipolar vertical organic field effect transistor according to claim 13, wherein said charge injection layer consists essentially of MOO₃.

17. An ambipolar vertical organic field effect transistor according to claim 13, wherein said charge injection layer consists essentially of WO₃.

18. An ambipolar vertical organic field effect transistor according to claim 13, wherein said charge injection layer consists essentially of a mixture of at least one Of V₂O₅ with VO, WO₃ and V₂O₅ and MoO₃ with V₂O_S.

19. An ambipolar vertical organic field effect transistor according to claim 13, wherein said charge injection layer comprises a mixture of a transition-metal- oxide material and a metal.

20. An ambipolar vertical organic field effect transistor according to claim 19, wherein said metal is Aluminum.

21. An ambipolar vertical organic field effect transistor according to claim 13, wherein said semiconductor layer is selected from the group of materials consisting of organic semiconductor materials and inorganic semiconductor materials.

22. An ambipolar vertical organic field effect transistor according to claim 13, wherein said semiconductor layer^' is selected from the group of materials consisting of organic transistor materials, organic light-emitting materials, and organic photovoltaic materials

23. An ambipolar vertical organic field effect transistor according to claim 13, wherein said semiconductor layer consists essentially of pentacene.

24. An ambipolar vertical organic field effect transistor according to claim 13, further comprising a charge injection layer disposed between and in contact with said organic semiconductor layer and said drain electrode.

25. An ambipolar vertical organic field effect transistor according to claim 13, further comprising a source electrode a source electrode disposed between said drain electrode and said gate electrode.

26. An ambipolar vertical organic field effect transistor according to claim 13, wherein said charge injection layer is selected from the group materials consisting of inorganic semiconductor materials, organic semiconductor materials, insulating materials, conducting materials, and combinations thereof.