Organic Complementary Inverters Based on Vertical-Channel Dual-Base Thin-Film Transistors with Time Constants < 10ns

Lateral-channel dual-gate organic thin-lm transistors (OTFTs) are utilized in organic pseudo-CMOS inverters to realize switching voltage control. However, the relatively long channel length will slow the inverter operation. Vertical-channel dual-gate OTFTs are an attractive alternative due to the short channel length. In this work, controllable and reliable complementary inverters are presented using vertical n-channel organic permeable dual-base transistors (OPDBTs) and vertical p-channel organic permeable base transistors (OPBTs). With operating voltages < 2.0 V, the threshold voltages of the n-type OPDBTs are changed across a wide range from 0.12 to 0.82 V by varying the voltage of the additional base. The fabricated tunable organic complementary inverter features switching voltage shift and gain enhancement. In addition, the inverters show very small switching time constants (< 10 ns) at 10 MHz. Our work represents a signicant step towards the application of vertical dual-gate/base transistors in power-ecient organic complementary inverters, offering the capability to easily tune the switching voltage of organic complementary inverters. This facilitates the development of high-performance and complex organic digital integrated circuits.


Introduction
Organic thin-lm transistors (OTFTs) have attracted considerable attention for realizing numerous applications in large-area electronics, such as electronic-paper displays, microprocessors, radio-frequency identi cation circuits, exible sensors, and electronic skins [1][2][3][4][5] . An essential electronic build blocking for practically any kind of digital circuitry are logic gates and in particular inverters circuits. Ideally, they are characterized by a low driving voltage, low static and dynamic power uptake, high gain, and fast response time in order to enable complex and power-e cient devices. In general, low driving voltage of OTFTs may be achieved using high capacitance dielectric layers and reducing the density of the sub-gap density-of-states in the channel. However, a low static and dynamic power uptake, high signal integrity, and high gain of an organic semiconductor based inverter is a far more di cult target to reach. The best strategy to achieve the above-described goals is to use a complementary circuit technology composed of p-type and n-type transistors. Compared to unipolar inverters where a power is constantly dissipated through the load resistance, complementary circuits offer a signi cantly reduced static and dynamic power uptake because one transistor operates in its off-state. However, in order to obtain complementary inverters with a high gain, large noise margin and good signal integrity, it is essential to have balanced charge carrier transport in the n-type and p-type transistors. In this regard, the threshold voltage (V TH ) is of particular importance because it determines the trip point of the inverter, which is the input bias at which the gate inverts the output signal. For standard silicon complementary metal-oxide-semiconductor transistors (CMOS), the threshold voltage at the onset of inversion can be accurately set by the amount of doping applied by ion implantation 6 .
Although channel doping has been successfully demonstrated in organic transistors [7][8][9] , it may not be the best strategy for threshold voltage tuning in OTFTs due to deterioration of charge carrier mobility, reduction of on/off-ratio, or insu cient controllability. An alternative method for threshold voltage tuning is the use of dual-gate transistors. In this case, a second gate electrode is used to precisely set the threshold voltage. The practical implantation of dual-gate transistors and control of V TH  All the inverters discussed above are based on lateral-channel dual-gate OTFTs. However, due to the micrometer-long channel length, the limits of the contact resistance, and the large capacitance in the lateral-channel dual-gate devices, it is a substantial challenge to further increase the operation frequency without employing costly high-resolution patterning techniques 17,18 . Vertical organic transistor architecture though, offers sub-micrometer channel devices enabling low-voltage and hightransconductance operation 19 . In particular, the performance and stability of vertical organic transistors has been improved signi cantly in recent years due to advancements in device manufacturing as well as improvements of the layer structure 20-28 . One vertical organic transistor structure that stands out of all proposed approaches is the organic permeable base transistor (OPBT) which is the organic transistor with the highest transition frequency reported 29 . Furthermore, OPBTs show excellent stability comparable to the best lateral OTFTs 30 . However, complementary inverter circuits based on vertical organic transistors have not been reported so far which is mainly caused by the lack of proper p-or n-type transistors. Thus, so far vertical organic transistors have never proven their advantages in terms of highfrequency operation in functional circuits.
In this study, we report, for the rst time, the fabrication and measurements of organic complementary inverters with vertical-channel dual-gate TFTs. We demonstrate a complementary inverter using n-type organic permeable dual-base transistors (OPDBTs) and p-type OPBTs. The threshold voltage of n-type OPDBTs can be adjusted by applying a bias on the second base. Finally, the proposed complementary inverters show reliable output curves and switching voltage tunability by independently adjusting the applied base bias of the n-type OPDBTs. In addition, the organic inverters can maintain the switching states above 10 MHz with time constants for the fall and rise time of less than 10 ns. With these performance measures, the complementary inverters proposed in this work set a new record for the switching rate of low-voltage organic logic circuits. Moreover, this work proves that the vertical organic transistor concept is a suitable strategy to further improve the performance of organic electronic circuits.

Results
Transistor design. Figure 1 shows a schematic illustration of the organic complementary inverter consisting of an n-type OPDBT and a p-type OPBT. OPBTs resemble a triode-like structure and hence instead of the gate, the control electrode is named as the base. In an OPDBT or an OPBT, organic semiconductor layers are embedded between the collector electrode (C), emitter electrode (E), and base electrode (B). Typically, the layer thicknesses of the collector and emitter are 100 nm aluminum (Al), with an additional layer of 20 nm chromium (Cr) for n-type OPDBTs, or 20 nm gold (Au) for p-type OPBTs. As organic semiconductor materials, C 60 and pentacene are used for the n-type OPDBT and the p-type OPBT, respectively. A 20-nm-thick layer of C 60 doped with the strong electron donor W 2 (hpp) 4 (n-C 60 , dopant concentration of 1.0 wt.%) inserted underneath the emitter is used to reduce the contact resistance and enable the Ohmic-like injection of electrons from the metal electrode in n-type OPDBTs. 31 A 50-nm-thick layer of pentacene doped with the p-dopant F6TCNNQ is also used to reduce the contact resistance between the metal and organic semiconductors, and thus to enhance the injection of holes from the emitter of p-type OPBTs. 31 The base electrode in an OPBT consists of a 15-nm-thick aluminum lm which, due to intentional oxidation, is covered with a native 2-3 nm thin Al 2 O 3 layer. 20 Furthermore, due to the mechanical strain during the oxidation process, nanometer-size pinholes open up in the base and enable charge carrier transport from the emitter to the collector. Details on the device fabrication are given in the Experimental Section. The properties of the base layer are investigated by transmission electron microscopy (TEM) and the detailed explanation can be found in Supplementary Fig. S1.
In an OPBT, charge carriers are injected from the emitter electrode into the organic semiconductor material by applying an emitter-collector voltage. When a positive base bias (negative bias for p-type OPBTs) is applied, the OPBT is turned on, and the charge carriers from the emitter are accumulated at the Al 2 O 3 of the base. Once accumulated, electrons (or holes) will laterally diffuse to the pinholes in the base, and then be pulled from the pinholes to the collector. If the base-emitter voltage is not applied, electrons (or holes) cannot pass through the base and therefore cannot reach the collector. Consequently, the OPBT is in its off-state. In an OPDBT, the second base will also control the transport of charge carriers. Charge carriers can go through the pinholes in both base electrodes and reach the collector only when the potential of base1 is non-zero and the potential of base2 is larger than that of base1 32 .
Electrical characterization of n-and p-type transistors. All electrical measurements of the transistors are carried out in ambient air with encapsulation at room temperature. Figures 2a and b Table S1. When V B1 = 0 V, the OPDBT stays in its offstate. As V B1 increases from 0 to 0.5 V, it can be turned on. The on-current increases from 0.96 to 4.06 mA as V B1 increases from 0.5 to 2.0 V. Furthermore, a transmission value of 99.999%, an on/off ratio of 4.7 × 2.0 V. Importantly, the threshold voltages of the OPDBT shift from 0.12 to 0.82 V when V B1 increases from 0.5 to 2.0 V (see Supplementary Fig. S2) 30 . Thus, by changing the applied base1 voltages, the threshold voltage of the OPDBTs can be set at a desired value. Figure 2b shows the output characteristics of the ntype OPDBTs. A small undesirable nonlinearity of the collector current at small collector-emitter voltages can be observed, which is caused by the space-charge-limited current in the organic semiconductor layer 33 . Overall, the performance of OPDBTs is comparable to single-base devices showing a transition frequency of 40 MHz 29 . The larger overall layer thickness for dual-base devices though caused the current to be slightly lower. The electrical characteristics of a p-type OPBT are shown in Figs. 2c, d, and the corresponding performance parameters are summarized in Supplementary Table S2. An on-current of 5.28 mA, a transmission value of 99.87%, an on/off ratio of 5 × 10 3 , a subthreshold swing of 258 mV dec -1 , and a current gain of 9 × 10 3 are achieved as shown in Fig. 2c. The output characteristics of p-type OPBTs are measured and presented in Fig. 2d. According to the output curves in the linear region, the semiconductor/electrode contact is Ohmic rather than Schottky-like. I C shows good saturation in the output curves. Overall, the performance of the n-and p-type transistors is quite nicely balanced.
Application in complementary inverters. Using an n-type OPDBT and a p-type OPBT, we realize an organic complementary inverter. The complementary inverter circuit is assembled as shown in Fig. 1 and Fig. 3a. The p-type OPBT is connected through its emitter electrode to the positive supply voltage (V CC ) and by the collector electrode to the output terminal. The n-type OPDBT is also connected by the collector electrode to the output terminal and by the emitter to the ground. The base2 electrode of the n-type OPDBT is connected to the base electrode of the p-type OPBT and the input signal. The static voltage transfer curves of the inverter for different supply voltages at constant control voltage (V Control ) of 2.0 V are presented in Figs. 3b and c. With different supply voltages, remarkable inverter characteristics and excellent gains of 16.4-28 are achieved, which is comparable to the organic inverters with lateral-channel transistors [34][35][36] . The tripping point of the inverter is not at its ideal position V CC /2 due to the difference in the threshold voltage of the n-and p-type transistor. However, as we will show later, this effect can be reduced using the function of the OPDBT.  Fig. 3 f. τ rise and τ fall are de ned as a time from 10 to 90% of the change between the two steady-state values (see Supplementary Fig. S3). The smallest time constants of 5 ns and 6 ns are observed at a supply voltage of 4.0 V. To date, organic unipolar inverters reported by Borchert et al.
show the smallest time constants of 19 ns and 56 ns, which are achieved at a frequency of 2 MHz and a supply voltage of 2.5 V 37 . However, the gain of these inverter structures is smaller than 10. Therefore, complementary inverters based on OPBTs allow the dynamic performance of the organic inverters to reach a higher frequency, and respond a smaller time constants, and operate with a higher gain.
Finally, the effect of the additional base electrode in an OPDBT on the transfer characteristics of the complementary inverter is investigated (see Fig. 4). A supply voltage of 3.5 V is applied and V Control is ranged from 0.5 to 2.0 V. As can be seen, varying V Control from 0.5 to 2.0 V causes the threshold voltages of the n-type OPDBTs and hence the switching voltage of the inverter to shift systematically toward more positive voltages. In this way, dual-base transistors enable a wide-range of switching voltage controllability of a complementary inverter over 0.8 V at an input voltage < 2.0 V in a deterministic manner. The highest gain of 28.2 is achieved when V Control = 0.5 V, which can be attributed to the highest threshold swing value of 113 mV dec -1 . Hence, a switching voltage tunable inverter circuit is realized by using a V TH tunable n-type OPDBT and a p-type OPBT.

Conclusions
In summary, this work constitutes the rst application of vertical dual-base transistors to form lowvoltage organic complementary inverters. The threshold voltage control of an n-type OPDBT is shown, and this technique is applied to tune the switching voltage of organic complementary inverters.  Supplementary Fig. S1).

Data Availability
The data that support the plots within this paper and other ndings of this study are available from the corresponding author upon reasonable request.