Abstract:
A transistor includes a first semiconductor layer associated with a first electrode; a second semiconductor layer associated with a second electrode; and a discontinuous layer between the first and second semiconductor layer. The discontinuous layer has a plurality of openings being formed on a nonuniform organic surface. Applications of the transistor include an inverter that operates at low supply voltage and high frequency.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 61/099,268, filed Sep. 23, 2008, and entitled “Organic Transistor Inverter,” the contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    This invention relates to organic transistors and inverters. 
         [0003]    Organic thin-film transistors (OTFTs) have several advantages over traditional transistors, including, for example, low cost, low processing temperature, mechanical flexibility, and large area coverage. Applications of OTFTs in consumer electronics and optoelectronics include flat-panel display drivers, radio-frequency identification (RFID) tags, smart cards, and sensors. 
         [0004]    Conventional OTFTs are designed using the architecture of a typical inorganic metal-oxide-semiconductor field-effect transistor, in which a source terminal and a drain terminal are usually formed on the same plane above or below the transistor&#39;s semiconductor layer. The performance of conventional OTFTs is generally influenced by two design parameters: channel length (L) and field-effect mobility (μ FE ). One characterization of the performance is the source-drain response time t SD , which can be determined by the following equation: t SD =L 2 /(μ FE ×V DD ), where V DD  is the supply voltage. 
         [0005]    With conventional OTFTs, it is difficult to reduce this source-drain response time t SD  to achieve high operation frequency. One difficulty, for example, is the low field-effect mobility μ FE  of organic semiconductors used in OTFTs. Another difficulty relates to the channel length of the OTFT, which cannot be easily shortened using nano-lithographic techniques. In cases where the channel length of an OTFT can be reduced to the order of sub-micrometers, the contact resistance between its semiconductor layer and electrodes becomes another factor that can affect the performance of this organic device. 
       SUMMARY 
       [0006]    One aspect of the invention relates to a transistor that includes a first semiconductor layer; a second semiconductor layer; and a discontinuous layer between the first and second semiconductor layer. The discontinuous layer has a plurality of openings being formed on a nonuniform organic surface. 
         [0007]    Embodiments of this transistor may include one or more of the following features. 
         [0008]    The first semiconductor layer represents an emitter layer, the second semiconductor layer represents a collector layer, and the discontinuous layer represents a base layer. 
         [0009]    The first semiconductor layer may include a pentacene layer, and may further include an enhancement layer in contact with the pentacene layer. The enhancement layer may include a LiF layer and a NPB layer. The enhancement layer includes at least one of tungsten oxide (WO 3 ), vanadium oxide (V 2 O 5 ), molybdenum oxide (MoO 3 ), and cesium carbonate (Cs 2 CO 3 ). The second semiconductor layer may include a pentacene layer. The discontinuous layer may be made of Al and may have an average thickness of less than 10 nm. 
         [0010]    The transistor may further include first and second electrodes electrically connected to the first and second semiconductor layers, respectively. Each of the first and the second electrodes can be made of various materials, including metals such as gold (Au), titanium (Ti), copper (Cu), silver (Ag), and aluminum (Al). The transistor may also include a third electrode electrically connected to the discontinuous layer. The third electrode may be made of various materials that have relatively low work functions, including metals such as aluminum (Al), silver (Ag), and magnesium (Mg). 
         [0011]    Another aspect of the invention relates to an inverter that includes a transistor for receiving an input signal and for generating an output signal, and a resistor coupled in series with the transistor. The transistor includes a first semiconductor layer associated with a first electrode; a second semiconductor layer associated with a second electrode; and a discontinuous layer between the first and second semiconductor layer. The output signal satisfies a predetermined criterion in relation to the input signal. 
         [0012]    Embodiments of this inventor may include one or more of the following features. 
         [0013]    The input signal includes an input voltage signal, and the output signal includes an output voltage signal. The predetermine criterion includes, when a magnitude of the input voltage signal is within a predetermined range, the output voltage signal is in reverse polarity with the input voltage signal. 
         [0014]    The discontinuous layer is associated with a third electrode. The input signal is received at the third electrode and the output signal is generated at the second electrode. 
         [0015]    The inverter may be further configured to be able to operate at a frequency of at least 2000 Hz. 
         [0016]    The first semiconductor layer may include a pentacene layer or a PTCDI layer. The second semiconductor layer may include a pentacene layer or a C 60  layer. The discontinuous layer may be made of Al. The plurality of openings are formed on a nonuniform organic surface. 
         [0017]    Another aspect of the invention relates to an inverter that includes a first transistor having a first base electrode for receiving an input signal and a first collector electrode for providing an output signal that satisfies a predetermined criterion in relation to the input signal; and a second transistor having a second base electrode and emitter electrode each connected to the first collector electrode of the first transistor. At least one of the first and second transistors has a first and a second semiconductor layer and a discontinuous layer between the first and second semiconductor layer. Each of the first and second transistors may be a p-type transistor, or alternatively, an n-type transistor. 
         [0018]    Another aspect of the invention relates to an inverter that has a first transistor having a first base electrode for receiving an input signal and a first collector electrode for providing an output signal that satisfies a predetermined criterion in relation to the input signal; and a second transistor having a second base electrode connected to the first base electrode and a second collector electrode connected to the first collector electrode. At least one of the first and second transistors has a first and a second semiconductor layer and a discontinuous layer between the first and second semiconductor layer. The first and second transistors may be respectively a p-type and n-type transistor, or alternatively, an n-type and p-type transistor, or both be ambipolar transistors. 
         [0019]    Among many advantages and features, an organic transistor has a vertical layout in which the conductive channel is perpendicular to the substrate. With this arrangement, the channel length L can be directly controlled by the combined thickness of the transistor&#39;s organic films, which can be reduced, for example, to sub-micrometers. As a result, higher operating frequency and lower operating voltage can be achieved. 
         [0020]    One embodiment of the organic transistor is in the form of an organic-based modulation triode (OBMT), which can be fabricated by employing two back-to-back diodes on a flexible substrate. When various supply voltages are applied to the base electrode, the OBMT exhibits current modulations with apparent saturation. By integrating an OBMT with a suitable load resistor or with another OBMT of similar configuration, an organic inverter that operates at low supply voltage and high frequency can be achieved. 
         [0021]    Other features and advantages of the invention are apparent from the following description, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0022]      FIG. 1A  is a schematic diagram of an exemplary OBMT. 
           [0023]      FIG. 1B  is a diagrammatic view of the OBMT of  FIG. 1A . 
           [0024]      FIG. 2  is a graph showing current-voltage characteristics for the OBMT of  FIG. 1A . 
           [0025]      FIG. 3  is a graph showing current density as a function of voltage for the collector-base diode and emitter-base diode in the OBMT of  FIG. 1A . 
           [0026]      FIG. 4  is a schematic diagram of a test circuit for measuring the current dynamics of the OBMT of  FIG. 1A   
           [0027]      FIGS. 5A-5C  are graphs showing the current density dynamics of the OBMT under input voltage of varying frequencies. 
           [0028]      FIG. 6  is a schematic diagram of one embodiment of an organic inverter that includes the OBMT of  FIG. 1A . 
           [0029]      FIGS. 7A and 7B  are graphs showing the output voltage and gain, respectively, of the organic inverter as a function of input voltage. 
           [0030]      FIG. 8  is a schematic diagram of a second embodiment of an organic inverter that includes two n-type OBMTs. 
           [0031]      FIG. 9  is a schematic diagram of a third embodiment of an organic inverter that includes a pair of n-type and p-type OBMTs. 
           [0032]      FIG. 10  is an atomic force microscopy (AFM) image of the surface morphology of an exemplary non-uniform organic surface over which the base layer of  FIG. 1B  can be formed. 
           [0033]      FIG. 11  is a graph of the various energy levels of the metal and semiconductor layers of the OBMT of  FIG. 1B . 
           [0034]      FIG. 12  is a diagrammatic view of an organic complementary inverter. 
           [0035]      FIGS. 13A and 13B  are graphs showing the current-voltage characteristic for a p-type OBMT and an n-type OBMT, respectively. 
           [0036]      FIGS. 14A and 14B  are graphs showing the output voltage and gain, respectively, as a function of input voltage for the organic complementary inverter of  FIG. 12A . 
           [0037]      FIG. 15  is a graph showing the output voltage of the inverter of  FIG. 12A  as a function of input voltage in the forward and reverse directions. 
       
    
    
     DETAILED DESCRIPTION 
     1 Organic Transistor 
       [0038]    Referring to  FIG. 1A , an organic-based modulation triode (OBMT)  100  includes two diodes  102  and  104  in a back-to-back configuration. Here, diode  102  is referred to as an emitter-base (EB) diode and diode  104  is referred to as a collector-base (CB) diode. OBMT  100  also includes three terminals (electrodes), including emitter  101 , base  103 , and collector  105 . These three terminals provide OBMT  100  with electrical connection to external devices and/or circuits. By controlling an input voltage at base  103 , current flowing from emitter  101  to collector  105  can be modulated. 
         [0039]    Referring to  FIG. 1B , one example of OBMT  100  is formed over a glass or plastic substrate  140 . In this example, CB diode  104  includes a collector layer  130  and a base layer  120 . EB diode  102  includes base layer  120  and an emitter layer  110 . 
         [0040]    Collector layer  130  is made of semiconductor materials, including for example, a phthalocyanine (CuPc) layer  134  and a pentacene layer  132 . 
         [0041]    Base layer  120  is formed over collector layer  130  using a thin aluminum (Al) layer that has multiple voids  122 . Voids  122  are formed, for example, by thermally evaporating a thin Al layer on a non-uniform organic surface  150  (for example, a non-uniform pentacene surface). Base layer  120  provides a path for current to flow from emitter layer  110  to collector layer  130  through voids  122 . 
         [0042]      FIG. 10  illustrates an exemplary non-uniform surface upon which the Al layer  120  can be formed. The surface roughness is measured using an atomic force microscopy (AFM) technique. In this example, the root-mean-square (RMS) of the surface morphology is 13.3 nm. In general, the RMS roughness is greater than about 10 nm. 
         [0043]    Emitter layer  110  is made of semiconductor materials, including for example, a pentacene layer  112 , a N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB) layer  114 , and a LiF layer  116 . Generally, LiF layer  116  serves as a carrier enhancement layer, and NPB layer  114  serves as a carrier energy enhancement layer (also referred to herein as an “energizer”), as described below. In other embodiments, layer  116  is formed of a transition metal oxide such as tungsten oxide (WO 3 ), vanadium oxide (V 2 O 5 ), molybdenum oxide (MoO 3 ), cesium carbonate (Cs 2 CO 3 ), or another material capable of enhancing the performance of OBMT  100 . 
         [0044]      FIG. 11  shows an example of the work functions of different metals and the highest occupied molecular orbital (HOMO) below vacuum levels of the different organic materials in OBMT  100 . Because the HOMO energy of NPB layer  114  is greater than pentacene layer  112 , an energy gap exists between NPB layer  114  and pentacene layer  112 . This energy gap can enhance carrier energy when carriers travel from pentacene layer  112  into NPB layer  114 . LiF layer  116  can be considered as an insulator, serving as a barrier between NPB layer  114  and base layer  120 . As the carriers tunnel through the barrier into collector layer  130 , a tunnel current is formed. The tunnel current can be raised exponentially and the LiF layer  116  serves as a carrier enhancement layer. 
         [0045]    The three electrodes of OBMT  100 , which are collector  105 , base  103 , and emitter  101 , are made of thin strips of gold (Au), aluminum (Al), and gold, respectively. Generally, in a p-type configuration, collector  105  and emitter  101  can be made of various materials that have relatively high work functions, such as gold (Au), titanium (Ti) and copper (Cu). Base  103  can be made of various materials that have relatively low work functions, such as aluminum (Al), silver (Ag), and magnesium (Mg). 
         [0046]    In this example, OBMT  100  is essentially a p-type transistor because of the nature of the selected semiconductor materials. However, in other examples, n-type OBMTs can be conveniently configured using alternative semiconductor materials. Accordingly, the emitter and collector electrodes of an n-type OBMT can be made of low-work-function materials, while the base electrode can be made of high-work-function materials. 
         [0047]    Referring to  FIG. 2 , the collector-to-emitter current (I CE ) of OBMT  100  is shown as a function of collector-to-emitter voltage (V CE ). When supplied with a low base voltage V B , OBMT  100  is able to provide a relatively large output current I CE  with an apparent saturation region. For example, with V B  applied at −3 V, I CE  can reach up to −9.33 μA and saturate near V CE  of −3 V. The on/off current ratio, defined as J CE  (V B =−3 V)/J CE  (V B −0 V) at V CE =−3 V, is about 18 in this example. As a characteristic of the switching performance of OBMT  100 , this on/off ratio can be further increased by 1) reducing the off-current J CE  (V B =0 V), for example, by increasing the thickness of the CB diode  104 , or 2) enhancing the on-current J CE  (V B =−3 V), for example, by selecting electrodes with suitable work-functions as well as by reducing the thickness of EB diode  102 . 
         [0048]    Although OBMT  100  exhibits typical p-channel characteristics similar to conventional OTFTs, it works under different scenarios. As shown in  FIG. 2 , when CB diode  104  is under reverse bias (e.g., V B =−2.5 V), increasing the magnitude of V CE  can cause the diode to saturate. When saturation occurs, I CE  is dominated by the emitter-to-base current I EB . As the magnitude of bias V B  increases, the positive current flow (i.e., the emitter-to-collector current I EC ) also increases. Because base layer  120  is a thin Al layer with multiple voids, the base recombination current can be effectively reduced, and part of the emitter current can flow through base  103  to form a collector current. Consequently, I CE  can be modulated with apparent saturation by varying base voltage V B . This process requires that the thickness of base layer  120  be less than the mean free path of the carriers. When base layer  120  is made of a thick Al film that has few or no voids, OBMT  100  shows either lower current modulation without an apparent saturation region, or no current modulation. The surface morphology (e.g., roughness and nonuniformity) of organic surface  150  on which thin Al layer  120  is formed and the thickness of the base electrode  103  can also affect the current modulation of OBMT  100 . 
         [0049]    The field-effect mobility of OBMT  100  can be determined as follows. Since OBMT  100  can be viewed as essentially being composed of two diodes  102  and  104 , the effective field-effect mobility (μ*) is given by: 
         [0000]    
       
         
           
             
               
                 
                   
                     1 
                     
                       μ 
                       * 
                     
                   
                   = 
                   
                     
                       1 
                       
                         μ 
                         EB 
                       
                     
                     + 
                     
                       1 
                       
                         μ 
                         CB 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where μ EB  is the field-effect mobility of EB diode  102 , and μ CB  is the field-effect mobility of CB diode  104 . To calculate μ*, μ EB  and μ CB  can be obtained as discussed below. 
         [0050]    Referring to  FIG. 3 , the current density (J) versus voltage (V) characteristics of EB diode  102  and CB diode  104  are used for estimating values for μ EB  and μ CB . The x-axis in  FIG. 3  refers to a corrected voltage V, obtained by subtracting a built-in potential V bi  from an actual input voltage V applied  across a diode. Here, the built-in potential V bi  corresponds to the work function differential between EB and CB diode  102  and  104 . When V applied  overcomes V bi , indicating that a flat band condition is reached, the current density J increases quadratically with corrected voltage V, which is common for low-mobility and disordered semiconductors. Using a space charge limited conduction (SCLC) model, the field-effect mobility of each of the EB and CB diode  102  and  104  can be determined by: 
         [0000]    
       
         
           
             
               
                 
                   J 
                   = 
                   
                     
                       8 
                       9 
                     
                      
                     
                       ɛ 
                       0 
                     
                      
                     
                       ɛ 
                       r 
                     
                      
                     μ 
                      
                     
                       
                         V 
                         2 
                       
                       
                         L 
                         3 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where J is the current density, ∈ 0  is the vacuum permittivity, ∈ r  is the dielectric constant, V is the corrected voltage drop across each diode, and L is the combined thickness of all of the semiconductor layers in each diode. From equation (2), the field-effect mobility of EB diode  102  and CB diode  104  are 9.75×10 −5  and 1.05×10 −5  cm 2 /V−s, respectively. Therefore, the effective field-effect mobility μ* of OBMT  100  is 9.48×10 −6  cm 2 /V−s, using equation (1) above. Although μ* of OBMT  100  is much smaller than that of conventional OTFTs, which typically is about 5.5×10 −2  cm 2 /V-s, the short channel length of OBMT  100  (i.e., between 400˜500 nm in this example) still allows it to be operated at relatively low voltages with a high output current. 
         [0051]    Referring to  FIG. 4 , the dynamic characteristics of OBMT  100  can be further analyzed in a test circuit  400 . Here, collector  105 , base  103 , and emitter  101  (also shown as C, B, and E) are respectively connected to a DC source  401 , a function generator  402 , and a resistor  403  of 300 kΩ. Function generator  402  provides an input voltage V B  at base  103  in an alternating rectangular waveform. An oscilloscope  404  measures the density J BE  of emitter current that flows through resistor  403 . 
         [0052]    Referring to  FIGS. 5A-5C , the emitter current density J BE  is shown as a function of input voltage V B  at varying frequencies of 100 Hz, 2 kHz, and 5 kHz, respectively. In  FIG. 5A , the waveform of J BE  follows the alternating square waveform of input V B  at 100 Hz. As the input frequency increases from 100 Hz to 2 kHz and further to 5 kHz, partial and full distortions in output waveforms are observed (in  FIGS. 5B and 5C ). Here, the maximum operating frequency of OBMT  100  is 5 kHz. In comparison, the operating frequency of some conventional pentacene-based OTFTs&#39; is less than 100 Hz. With a shorter channel length, OBMT  100  exhibits not only lower power consumption, but also a higher operating frequency than some conventional OTFTs. The comparison of the performance of these two types of transistors is further shown in Table 1. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Electrical properties of OBMTs and 
               
               
                 conventional pentacene-based OTFTs. 
               
             
          
           
               
                   
                 OBMT 
                 Planar-type OTFT 
               
               
                   
                   
               
             
          
           
               
                   
                 Field-effect mobility 
                 9.48 × 10 −6   
                 5.5 × 10 −2   
               
               
                   
                 (cm 2/Vs) 
               
               
                   
                 Output current (μA) 
                 9.33 (V B  = −3, 
                 1.67 (V G  = −40, 
               
               
                   
                   
                 V CE  = −3) 
                 V SD  = −40) 
               
               
                   
                 Current on/off ratio 
                  18 
                   4 × 10 4   
               
               
                   
                 Operating frequency (Hz) 
                 2000 
                 70 
               
               
                   
                   
               
             
          
         
       
     
       2 Organic Inverters 
       [0053]    OBMTs are useful in many applications. One application, for example, is an organic inverter made by an OBMT coupled in series with a resistor. 
         [0054]    Referring to  FIG. 6 , one embodiment of a load-resistance inverter  500  includes a variable resistor  510  connected to collector  505  of an OBMT  520 . Variable resistor  510  is controlled to provide varying resistances ranging from 100 to 500 kΩ at steps of 100 kΩ. A supply DC voltage V DD  of −5 V is provided at terminal  512 . An input voltage V in  is provided at base  503 , for example, by a function generator (not shown). Output voltage V out  is measured at collector  505 . 
         [0055]    Referring to  FIGS. 7A and 7B , the voltage transfer characteristics and corresponding gain (defined as −dV out /dV in ) of inverter  500  are shown as a function of input voltage V in . When the input voltage V in  is “low” (e.g., at −1 V), OBMT  520  is turned off, and the output voltage V out  corresponds to “high” voltage (e.g., near −5.0 V). When the input voltage is “high” (e.g., at −5 V), OBMT  520  is turned on, and the output voltage becomes relatively “low” (e.g., near −2.5 V). Ideally, when OBMT  520  is off at “low” input voltages, no current passes through resistor  510  and V out  should be equal to V DD . In practice, however, the magnitude of V out  is slightly lower than the magnitude of V DD  due to the presence of a leakage current through OBMT  520 . In this example, the highest voltage gain (about 1.5) occurs when OBMT  520  is connected to a resistor of 400 kΩ and the input voltage V in  is near −4 V. The voltage gain of inverter  500  can be further improved, for example, by lowering the off-current of OBMT  520 . 
         [0056]    Referring to  FIG. 8 , a second application of OBMTs is an organic inverter  600  that includes two n-type (or alternatively, p-type) OBMTs  610  and  620  connected in series. Both of the base and emitter of OBMT  610  are connected to the collector of OBMT  620 . A supply voltage V DD  is provided at the collector of OBMT  610 . The base of OBMT  620  serves as an input terminal for receiving input voltage V in , whereas the collector of OBMT  620  serves as an output terminal for providing output voltage V out . 
         [0057]    Referring to  FIG. 9 , a third application of OBMTs is an organic inverter  700  that includes a pair of p-type OBMT  710  and n-type OBMT  720 . A supply voltage V DD  is provided at the emitter of OBMT  710 . The bases of OBMTs  710  and  720  are connected and together serve as an input terminal for receiving input voltage V in . The collectors of OBMTs  710  and  720  are connected and together serve as an output terminal for providing output voltage V out . 
         [0058]    Referring to  FIG. 12A , complementary organic inverter  700  includes p-type OBMT  710  and n-type OBMT  720  formed on a glass or plastic substrate  1200 . P-type OBMT  710  includes a collector layer  1230 , an Al base layer  1220 , and an emitter layer  1210 . Collector layer  1230  and emitter layer  1210  are formed of layers of organic semiconductors. Collector layer  1230  includes a phthalocyanine (CuPC) layer  1234  and a pentacene layer  1232  and is formed in contact with a collector electrode  1205 . Above collector layer  1230 , Al base layer  1220  connects to an Al base electrode  1203 . In some embodiments, base layer  1220  contains voids (not shown) that provide a path for current to flow from emitter layer  1210  to collector layer  1230 . Emitter layer  1210  includes a thin LiF layer, which functions as a hole injection enhancement layer; an NPB layer  1214 , and a pentacene layer  1212 . In some embodiments, the hole injection enhancement layer is formed of a transition metal oxide such as tungsten oxide (WO 3 ), vanadium oxide (V 2 O 5 ), molybdenum oxide (MoO 3 ), cesium carbonate (Cs 2 CO 3 ), or another material capable of enhancing the performance of OBMT  710 . Above emitter layer  1210 , an emitter electrode  1201  is formed of a layer of tungsten (VI) oxide (WO 3 ) and a layer of Al. P-type OBMT  710  is similar to the p-type OBMT  100  shown in  FIG. 1B , with the exception that emitter electrode  1201  of OBMT  710  is formed of WO 3  and Al, while emitter electrode  101  of OBMT  100  is formed of Au. 
         [0059]    N-type OBMT  720  is also formed of a collector layer  1250 , a base layer  1252 , and an emitter layer  1254  deposited over collector electrode  1205 . Collector layer  1250  is formed of N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI). Al base layer  1252  connects to base electrode  1203 . In some embodiments, base layer  1252  contains voids that provide a path for current to flow from emitter layer  1254  to collector layer  1250 . Emitter layer  1254  is formed of C 60 . In contact with emitter layer  1254 , an emitter electrode  1256  is formed of silver (Ag). 
         [0060]    Referring to  FIGS. 13A and 13B , the collector-to-emitter current (I CE ) is shown as a function of the collector-to-emitter voltage (V CE ) for p-type OBMT  710  and n-type OBMT  720 , respectively. Each curve corresponds to a different base voltage Base voltages ranging from 0 V to −3 V, with a step size of 0.5 V, were used for measurements on p-type OBMT  710 ; base voltages ranging from 0 V to 3 V, with a step size of 0.5 V, were used for measurements on n-type OBMT  720 . The ON current, OFF current, and turn-on voltage (i.e., the voltage when I CE =0) are shown for p-type OBMT  710  and n-type OBMT  720  are given in Table 2. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Performance of p-type and n-type OBMTs 
               
             
          
           
               
                   
                 ON current 
                 OFF current 
                 Turn-on voltage 
               
               
                   
                   
               
             
          
           
               
                 p-type OBMT 
                 −229 mA 
                 −67.6 nA 
                 −0.8 V 
               
               
                   
                 (V B  = −3 V) 
                 (V B  = 0 V) 
               
               
                 n-type OBMT 
                   377 μA 
                   86.9 nA 
                   0.4 V 
               
               
                   
                 (V B  = 3 V) 
                 (V B  = 0 V) 
               
               
                   
               
             
          
         
       
     
         [0061]    For comparison, the ON current for the p-type OBMT  100  shown in  FIGS. 1B and 2  is −9.33 μA at V B =−5 V, which is about an order of magnitude less than the ON current of OBMT  710 . The ON current of OBMT  100  is also significantly less than the ON current of a comparable n-type OBMT due to the fact that electrons have a longer mean free path in metal films than do holes. As a result, electrons have a lower probability of recombining at the base layer of an n-type OBMT than do holes at the base layer of a p-type OBMT. Thus, a greater number of electrons than holes diffuse through the base layer and into the collector layer of their respective OBMT. 
         [0062]    For efficient operation of a complementary organic inverter (e.g., inverter  700 ), current matching between the p-type OBMT and the n-type OBMT is important. Increasing the ON current of the p-type OBMT to approach the level of the ON current of the n-type OBMT will contribute to improved performance of the inverter. In inverter  700 , the emitter electrode  1201  of p-type OBMT  710  is formed of WO 3  and Al instead of Au, as it is in OBMT  100 . The WO 3  lowers the barrier between the Al layer of emitter electrode  1201  and pentacene layer  1212 , thus preventing the reaction and/or diffusion of the Al into the active emitter layer  1210 . The use of WO 3  and Al rather than Au in the emitter electrode increases the ON current of the OBMT by about an order of magnitude. However, the ON current of p-type OBMT  710  remains slightly less than the ON current of n-type OBMT  720 . To further improve the performance of OBMT  710 , a material having a higher energy level may be inserted between NPB layer  1214  and the thin Al layer  1220  in order to raise the energy of the carriers in the device. 
         [0063]    Referring to  FIGS. 14A and 14B , the voltage transfer characteristics and the corresponding gain in complementary organic inverter  700  are shown as a function of input voltage for supply voltages V DD =2 V, 3 V, and 4 V. When the input voltage V IN  is low (e.g., −1 V), a high voltage is output; when the input voltage is high (e.g., −3 V), the output voltage becomes relatively low. The maximum gain of the inverter increases with increasing V DD , reaching a peak value of −8.75 for V DD =4 V. In comparison to the resistance-load inverter  500  whose behavior is shown in  FIGS. 7A and 7B , the complementary organic inverter  700  exhibits enhanced gain at a lower driving voltage. 
         [0064]    Referring to  FIG. 15 , the voltage transfer characteristics for the complementary organic inverter  700  of  FIG. 12A  for V DD =−4 V reveal a slight hysteresis. An ideal OBMT does not include the LiF insulator layer; a complementary organic inverter applying an ideal OBMT does not exhibit hysteresis. The minor hysteresis of inverter  700  can be attributed to carriers trapped by minor impurities in the active organic layers of OBMTs  710  and  720 . In addition to hysteresis, the noise margin is an important performance parameter of an inverter.  FIG. 15  shows a noise margin low (NM L ) value (i.e., a voltage range within which the inverter output is interpreted as a “0”) of 0.97 V and a noise margin high (NM H ) value (i.e., a voltage range within which the inverter output is interpreted as a “1”) of 0.82 V. These measured noise margin values are less than the theoretical values, predicted to be NM H =NM L =V DD /2 (i.e., 2 V). This discrepancy is due to (1) the current mismatch between p-type OBMT  710  and n-type OBMT  720 ; (2) the non-zero OFF current in the OBMTs; and (3) the gradual shift in the turn-on voltage upon increasing the base voltage. 
         [0065]    P-type OBMT  710  in inverter  700  is operated in a linear regime for low values of V IN . A shift in the turn-on voltage of OBMT  710  would result in a change in the value of V OUT  upon increasing V IN  and a corresponding decrease in the values of NM H  and NM L . The electrical properties of complementary organic inverter  700  can be further improved by lowering the OFF currents and turn-on voltages of both p-type OBMT  710  and n-type OBMT  720 . 
       3 Fabrication of OBMTs 
       [0066]    The following section provides examples of fabricating OBMTs. 
         [0067]    Referring again to  FIG. 1B , substrate  108  (glass/plastic substrate) was first pre-cleaned by detergent, acetone, and isopropyl alcohol, and was then treated with an ultraviolet (UV) ozone cleaner for 15 min. Collector  105  is a 30 nm-thick Au layer deposited on substrate  108 . Collector layer  130  includes a 40 nm-thick CuPc layer  134  (Luminescence Technology Corp.) thermally evaporated on collector  105  for smoothing surface morphology, and a 280 nm-thick pentacene layer  132  (Sigma-Aldrich, ˜98% purity) thermally evaporated at a rate of 0.10-0.13 nm/sec from a crucible. 
         [0068]    Base  103  is a 31 nm-thick Al layer thermally evaporated on pentacene layer  132 . A 5 nm-thick Al layer is formed as base layer  120  over base  103 . A 0.4 nm-thick LiF layer  116  was then thermally evaporated onto base layer  120  as a carrier injection enhancement layer. A 20 nm-thick NPB layer  114  was thermally evaporated onto LiF layer  116  to enhance carrier energy. Further, a 140 nm-thick pentacene layer  112  was thermally evaporated at a rate of 0.10-0.13 nm/sec onto NPB layer  114 . Finally, a 30 nm-thick Au layer was thermally evaporated onto pentacene layer  112  to be emitter  101 . 
         [0069]    The above process was patterned by a metal mask. All organic and semiconductor materials and metal electrodes were deposited in a thermal evaporation chamber at a base pressure of 10 −6  Torr. The active area of OBMT  100  is 4.2×10 −3  cm 2 , which is defined by the intersectional area of emitter  101  and collector  105 . 
         [0070]    Referring again to  FIG. 12 , in another example, glass substrate  1200  was cleaned as described above. A 30 nm thick Au layer was deposited onto cleaned substrate  1200  and patterned to form collector electrode  1205 . To form n-type OBMT  720 , a layer of N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI, Sigma-Aldrich, ˜98% purity) was deposited above a portion of collector electrode  1205  to form collector layer  1250 . A 30 nm thick Al strip was then formed on collector layer  1250  to serve as base electrode  1203  for both p-type OBMT  710  and n-type OBMT  720 . A 15 nm thick Al layer was deposited over base electrode  1203 , forming the base layer  1252  of n-type OBMT  720 . To decrease the OFF current of inverter  700 , the partially formed device was then annealed at 150° C. for 2 hours. 
         [0071]    After the anneal, a 100 nm thick layer of C 60  was thermally evaporated onto base layer  1252  to function as the emitter layer  1254 . The emitter electrode  1256  was formed by depositing and patterning a 30 nm thick silver (Ag) layer over emitter layer  1254  of n-type OBMT  720 . 
         [0072]    For p-type OBMT  710 , a 50 nm thick layer  1234  of copper phthalocyanine (CuPc, Luminescence Technology Corp.) was thermally evaporated onto a portion of collector electrode  1205  to smooth the surface morphology, and a 270 nm thick layer  1232  of pentacene (Sigma Aldrich, ˜98% purity) was thermally evaporated to form collector layer  1230 . Base layer  1220  was formed by deposition of a 10 nm thick Al film. Thermal evaporation of a 0.4 nm thick LiF layer formed hole injection enhancement layer. A 20 nm thick layer  1214  of NPB was then thermally evaporated onto LiF layer  1216  as a carrier energy enhancement layer. To complete emitter layer  1210 , a 100 nm thick layer  1212  of pentacene was thermally evaporated onto NPB layer  1214 . Finally, a film containing a 10 nm layer of WO 3  and 30 nm of Al was deposited onto emitter layer  1210  of p-type OBMT  710  to form the emitter electrode  1201 . 
         [0073]    All organic materials were used as received and thermally evaporated in a thermal evaporation chamber at a base pressure of 10 −6  Ton. Patterning was performed using a metal mask; the active area of each OBMT  710 ,  720  was 0.04 cm 2 . 
       4 Other Experimental Conditions 
       [0074]    The pentacene-based planar-type OTFT listed in Table 1 was fabricated on heavily doped p-type silicon wafer with a 300 nm-thick layer of thermally oxidized silicon dioxide (SiO 2 ). A 50 nm pentacene film was then deposited onto SiO 2  through a thermal evaporation process. The width (W) and length (L) of the planar-type OTFT is 2000 μm and 100 μm, respectively. The field-effect mobility of the planar-type OTFT in saturation region is estimated by using the following equation: 
         [0000]    
       
         
           
             
               
                 
                   I 
                   = 
                   
                     
                       1 
                       2 
                     
                      
                     
                       W 
                       L 
                     
                      
                     
                       C 
                       i 
                     
                      
                     
                       
                         μ 
                          
                         
                           ( 
                           
                             
                               V 
                               GS 
                             
                             - 
                             
                               V 
                               T 
                             
                           
                           ) 
                         
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where C i  is the capacitance per unit area of SiO 2 , V T  is the threshold voltage, V GS  is the gate-source voltage, and μ is the field-effect mobility. 
         [0075]    The on- and off-currents of the conventional OTFT are defined as the source-drain current (I DS ) at gate voltage (V G ) of −40 and 0 V, respectively, when a source-drain voltage (V DS ) of −40 V is applied. The current-voltage (I-V) characteristics of the planar-type OTFT were measured by a HP 4145B semiconductor parameter analyzer or a Keithley 4200-SCS semiconductor parameter analyzer. The capacitance-voltage (C-V) measurements were performed with a HP 4980A Precision LCR meter. 
         [0076]    The operating frequencies of OBMT  100  and the conventional OTFT were measured using test circuit  400  shown in  FIG. 1A . As described earlier, test circuit  400  includes function generator  402  (e.g., Tektronix AFG 3022), oscilloscope  404  (e.g., Instek GDS-8065), and resistor  403  of 300 kΩ. All the electrical properties of the OBMT and OTFT were measured in dark under ambient environment or under a nitrogen (N 2 ) atmosphere. 
       5 Other Embodiments 
       [0077]    All of the features disclosed in this specification may be combined in any combination. Each feature disclosed in this specification may be replaced by an alternative feature serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features. 
         [0078]    From the above description, one skilled in the art can easily ascertain the essential characteristics of the present invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, other embodiments are also within the claims.