Integrated complementary inverters and ring oscillators based on vertical-channel dual-base organic thin-film transistors

Lateral-channel dual-gate organic thin-film transistors have been used in pseudo complementary metal–oxide–semiconductor (CMOS) inverters to control switching voltage. However, their relatively long channel lengths, combined with the low charge carrier mobility of organic semiconductors, typically leads to slow inverter operation. Vertical-channel dual-gate organic thin-film transistors are a promising alternative because of their short channel lengths, but the lack of appropriate p- and n-type devices has limited the development of complementary inverter circuits. Here, we show that organic vertical n-channel permeable single- and dual-base transistors, and vertical p-channel permeable base transistors can be used to create integrated complementary inverters and ring oscillators. The vertical dual-base transistors enable switching voltage shift and gain enhancement. The inverters exhibit small switching time constants at 10 MHz, and the seven-stage complementary ring oscillators exhibit short signal propagation delays of 11 ns per stage at a supply voltage of 4 V. Organic n- and p-type vertical transistors, with considerably shorter channel lengths than their planar counterparts, can be used to create complementary metal–oxide–semiconductor (CMOS)-like inverters and ring oscillators that operate in the megahertz frequency range.

O rganic thin-film transistors (OTFTs) are of potential use in large-area electronic applications, including electronic-paper displays, microprocessors, radiofrequency identification circuits, flexible sensors and electronic skins [1][2][3][4][5] . Essential building blocks in any kind of digital circuit are logic gates and, in particular, inverter circuits. To deliver complex and power-efficient systems, such transistors and circuits should ideally have a low driving voltage, low static and dynamic power consumption, high gain and fast response times. Low-drive-voltage OTFTs can be created using high-capacitance dielectric layers and by reducing the density of the sub-gap density of states in the channel. However, inverters with low static and dynamic power uptake, high signal integrity and high gain are more difficult to achieve. The preferred way to overcome these issues is to use complementary circuit technology composed of p-type and n-type transistors.
Compared to unipolar inverters, where power is constantly dissipated through a load resistance, complementary circuits offer significantly reduced static and dynamic power consumption because one transistor operates in its off state. However, 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. The threshold voltage (V TH ) is of particular importance because it determines the trip point of the inverter-the input bias at which the gate inverts the output signal. For standard silicon complementary metal-oxide-semiconductor (CMOS) transistors, the threshold voltage at the onset of inversion can be accurately set by the amount of doping applied by ion implantation 6 .
Channel doping has been successfully demonstrated in organic transistors [7][8][9] , but it is potentially not the best strategy for threshold voltage tuning in OTFTs because of deterioration of charge carrier mobility, reduction of on/off ratios and insufficient controllability. An alternative approach is to use dual-gate transistors, where a second gate electrode is used to precisely set the threshold voltage. Dual-gate control of V TH in OTFTs has been demonstrated [10][11][12][13][14] and, when used in inverter structures, can provide control over inputoutput characteristics 14 . Dual-gate inverters with back-gates on both the load and driver transistor have also been developed 15 , and a comprehensive study of the use of dual-gate transistors to control the threshold voltage of the load transistor and drive transistor of inverters and digital circuits has been reported 16 .
These inverters are, however, based on lateral-channel dual-gate OTFTs, and their long channel lengths, contact resistance and high parasitic capacitance make it difficult to increase operational frequencies without using costly high-resolution patterning techniques 17,18 . Vertical organic transistor architectures can, on the other hand, offer submicrometre channel devices with low-voltage operation and high transconductance 19 , and the performance and stability of vertical organic transistors has improved significantly in recent years due to advancements in device manufacturing and improvements in layer structures [20][21][22][23][24][25][26][27][28] . Organic permeable base transistors (OPBTs) can, in particular, provide very high transition frequencies with excellent stability, comparable to the best lateral OTFTs 29,30 . However, the lack of proper p-or n-type devices has made the development of complementary inverter circuits using vertical organic transistors challenging.
In this Article, we report integrated organic complementary inverters and complementary organic ring oscillators using n-type organic permeable single-and dual-base transistors and p-type OPBTs. The complementary inverters exhibit reliable output curves and switching voltage tunability by independent adjustment of the applied base bias on the n-type dual-base transistors. We evaluate the switching frequencies of the integrated complementary organic ring oscillators based on vertical organic transistors. At operating voltages of less than 4.0 V, switching speeds in the megahertz regime are possible. A signal propagation delay per stage of 11 ns was measured for the integrated complementary organic ring oscillators based on p-and n-type vertical organic transistors. Our results show that the vertical organic transistor concept is a suitable strategy to improve the performance of organic electronic circuits.

Vertical organic transistor design
A schematic cross-section of an n-type organic permeable dual-base transistor (OPDBT) and a p-type OPBT is shown in Fig. 1, along with the chemical structures of four organic semiconductors investigated in this study (C 60 , W 2 (hpp) 4 , pentacene, F6TCNNQ). 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 layers of the collector and emitter are 100-nm aluminium (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 aluminium film, which, due to intentional oxidation, is covered with a native 2-3-nm thin Al 2 O 3 layer 20 . Owing to mechanical strain during the oxidation process, nanometre-size pinholes open up in the base and enable charge carrier transport from the emitter to the collector. Details on device fabrication are provided in the Methods. The properties of the base layer were investigated by transmission electron microscopy (TEM) and a detailed explanation is provided in Supplementary Fig. 1.
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 AlO x 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 pass through the pinholes in both base electrodes and reach the collector only when the potential of base 1 is non-zero and the potential of base 2 is larger than that of base 1 32 .

Electrical characterization of n-and p-type transistors
All electrical measurements of the transistors were carried out in ambient air with encapsulation, at room temperature. Figure 2a,b presents the electrical characteristics of an n-type OPDBT. The transfer curves were measured as a function of the base 2-emitter voltage (V B2 ) at different base 1-emitter voltages (V B1 ) of 0, 0.5, 1.0, 1.5 and 2.0 V, respectively. The corresponding performance parameters of the transfer curves-for example, transmission values (I C /(I C + I B )), on currents, on/off ratios, threshold voltages (V TH ), subthreshold swing (SS) and current gain (β = I C /I B )-were extracted and are summarized in Supplementary Table 1. When V B1 = 0 V, the OPDBT stays in its off state. 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 × 10 4 , a subthreshold swing of 123 mV dec −1 and a current gain of 2 × 10 7 are achieved when V B1 reaches 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 ( Supplementary Fig. 2) 30 . Thus, by changing the applied base 1 voltages, the threshold voltage of the OPDBTs can be set at a desired value. The base 1 current is shown in Supplementary Fig. 3, and it is observed that OPDBTs show larger base 1 current than base 2 current, which may be attributed to the oxidation quality of the base 1 electrode. Figure 2b shows the output characteristics of the n-type OPDBT. A small undesirable offset of the collector current at small collector-emitter voltage can be observed, which may be caused by the large base 1 current at low operating voltages. This increased base current level for base 1 is attributed to the lower oxidation quality of base 1 compared to base 2. In addition, the output curve of the n-type OPDBT shows a distinct nonlinearity for V CE < 1.0 V, which is explained by the space-charge-limited current (SCLC) in the organic semiconductor layer, as also observed for single-base OPBTs 33 . The SCLC is not observed in p-type OPBTs due to the background charge carrier concentration in pentacene and the additional built-in field in the base-emitter diode 28 .
The device-to-device reproducibility of OPDBTs is characterized in Supplementary Fig. 4, where the maximum on-current distribution of 50 OPDBT devices is summarized. As indicated by the Gaussian fitting curve, OPDBTs can exhibit well-reproducible device characteristics. Overall, the performance of OPDBTs is comparable to single-base devices, showing a transition frequency of 40 MHz (ref. 29 ). However, the larger overall layer thickness for dual-base devices caused the current to be slightly lower. The electrical characteristics of a p-type OPBT are shown in Fig. 2c,d, and the corresponding performance parameters are summarized in Supplementary  Table 2. An on current of 5.28 mA, transmission value of 99.87%, on/ off ratio of 5 × 10 3 , subthreshold swing of 258 mV dec −1 and 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 performances of the n-and p-type transistors are quite nicely balanced.

Static and dynamic circuit characteristics
By connecting an n-type OPDBT and a p-type OPBT, an organic complementary inverter is realized. The effect of the additional base electrode in an OPDBT on the transfer characteristics of the complementary inverter was also investigated (Fig. 3). The integrated complementary inverter was assembled as shown in 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 base 2 electrode of the n-type OPDBT is connected to the base electrode of the p-type OPBT and the input signal. The base 1 electrode of the OPDBT is connected as the control voltage (V Control ). A supply voltage of 3.5 V was applied and V Control was ranged from 0.5 to 2.0 V. As can be seen in Fig. 3b, varying V Control from 0.5 to 2.0 V caused the threshold voltage of the n-type OPDBTs and hence the switching voltage of the inverter to shift systematically towards 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 of <2.0 V, in a deterministic manner. The highest gain of 28.2 was 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 was realized by using a V TH -tunable n-type OPDBT and a p-type OPBT.
The complementary inverter was next investigated by varying the supply voltage. An image of an integrated complementary inverter is shown in Fig. 4a. The static voltage transfer curves of the inverter for different supply voltages at a constant control voltage of 2.0 V are presented in Fig. 4b,c. With different supply voltages, remarkable inverter characteristics and excellent gains of 16.4 to 28 were achieved, comparable to 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 voltages of the n-and p-type transistors. However, as we will show later, this effect can be reduced using the function of the OPDBT. Fine-tuning of the device parameters (for example, semiconductor thickness) could help to further increase the inverter gain. However, given that OPBTs do not obey the standard models developed for organic thin-film transistors, a physics-based compact model for OPBTs would be needed to further increase the performance (in particular the gain) of the inverter circuit.
The dynamic response characteristics of the present inverter were evaluated by applying a square-wave input signal with an amplitude (V IN ) of 2.0 V (adjusting the supply and control voltage so that V CC = V Control = 2.0 V) and frequencies (f) of 1 and 10 MHz, respectively. The results are shown in Fig. 4d,e, where the black and red curves indicate the input voltage V IN and output voltage V OUT , respectively. The output voltage exhibits an inverter response to the input voltage with a transient behaviour just after the changes in V IN . The high output almost reaches 2.0 V (V CC ) and the low output is close to reaching 0 V at a frequency of 1 MHz. On increasing the frequency to 10 MHz, the output voltage drops slightly, but it is still close to 2.0 V and the state of the inverter can be maintained. The rise (τ rise ) and fall (τ fall ) time constants of the switching events at different supply voltages are plotted in Fig. 4f. τ rise and τ fall are defined as the time from 10 to 90% of the change between   37 ). However, the gain of these inverter structures was smaller than 10. Therefore, our complementary inverters based on OPBTs allow the dynamic performance of the organic inverters to reach a higher frequency, respond with a smaller time constant and operate with higher gain. The static power consumption of the complementary single-base inverter was determined to be 70 nW at V CC = 2.0 V; this is mainly governed by the off-state current of the p-type OPBT. The dynamic power uptake at 10 MHz (V CC = 4.0 V) is ~350 mW and is determined by the large capacitance of the p-type OPBT. In future, static and dynamic power consumption could be further decreased, for example by reducing the active area of the device (hence reducing the capacitance) and optimizing the processing conditions (for example, wet-chemical anodization of the base layer 21 ). In addition to the organic complementary inverter, to demonstrate the benefit of vertical organic transistors for dynamic performance, seven-stage complementary organic ring oscillators were fabricated. All the vertical organic transistors in the ring oscillators had the same structural geometry as discussed above. Figure 5a presents the circuit diagram and images of a seven-stage ring oscillator based on complementary inverters fabricated from n-and p-type OPBTs. Figure 5b and Supplementary Fig. 6 present the measured output signal of the ring oscillator. The oscillation frequencies (f OSC ) of the ring oscillators operating at different supply voltages were determined by fitting the measurement output signal to a sine wave. At a supply voltage of 4.0 V, the fitted oscillation frequency is 6.49 MHz. Even at a low V CC of 2.0 V, the oscillation frequency of the seven-stage complementary organic ring oscillator is still higher than 0.5 MHz. The propagation delay per stage (τ) was calculated from the oscillation frequency and the number of stages (N) in the loop by τ = 1/(2Nf OSC ). The operation frequency and the propagation delay per stage of the ring oscillator were studied as a function of supply voltage. As shown in Fig. 5c, the oscillation frequency increases as a function of supply voltage, and the propagation delay time per stage decreases. At a supply voltage of 4.0 V, the measured signal propagation delay is 11 ns per stage for the ring oscillator based on vertical organic transistors, which is in a similar range as the rise and fall time of the single inverter in Fig. 4. These signal delays are short in comparison to those reported so far for organic ring oscillators on any substrate at supply voltages of less than 10 V (refs. [37][38][39][40][41][42][43].

conclusions
We have shown that vertical dual-base transistors can be used to create low-voltage organic complementary inverters and complementary ring oscillators. We have demonstrated threshold voltage control of an n-type OPDBT, and the technique was used to tune the switching voltage of organic complementary inverters. We have also shown that dual-base transistors can enable a wide range of switching voltage control of over 0.8 V for a complementary inverter with an input voltage of less than 2.0 V. The complementary inverters can maintain the switching states and operate with small time constants of less than 10 ns at 10 MHz. The p-and n-type-based vertical organic transistors also provide signal propagation delays of 11 ns per stage at a supply voltage of 4.0 V in seven-stage complementary ring oscillators. Our work highlights the potential of vertical organic transistors for high-frequency logic circuit applications.

Methods
Device fabrication. Both the n-type OPDBTs and p-type single-base OPBTs presented were fabricated in a single-chamber UHV device. The glass substrate was previously cleaned with N-methylpyrrolidone, deionized water, ethanol and an ultraviolet-ozone cleaning system. By using thermal vapour deposition under ultrahigh-vacuum conditions, the layer stack ( Fig. 1)  SiO 100 nm with a free stripe of 250 µm (1 Å s −1 )/Cr 20 nm (0.1 Å s −1 )/Al 100 nm (2 Å s −1 ). The layer stack, evaporation rates and treatments of the p-type OPBTs were Al 100 nm (2 Å s −1 )/Au 20 nm (0.3 Å s −1 )/pentacene:F6TCNNQ 50 nm (0.6 Å s −1 )/pentacene 200 nm (2 Å s −1 )/Al 15 nm (1 Å s −1 )/15-min oxidation in ambient air/pentacene 200 nm (2 Å s −1 )/pentacene:F6TCNNQ 50 nm (0.6 Å s −1 )/ Au 20 nm (0.3 Å s −1 )/Al 100 nm (2 Å s −1 ). Finally, both n-type and p-type devices were encapsulated under a nitrogen atmosphere (<1 ppm O 2 and H 2 O) using UV-cured epoxy glue and cavity glasses, without UV exposure of the active area. Annealing for 2 h at 150 °C on a hotplate in a nitrogen glovebox was performed for all n-type OPDBTs. The active areas of the n-type OPDBTs and p-type OPBTs were 0.0625 mm 2 and 6.25 mm 2 , respectively. The integrated complementary inverters and seven-stage ring oscillators were also fabricated by subsequent deposition of thin films through a set of shadow masks on glass substrates, with the same procedure used for the n-type OPDBTs and p-type OPBTs described above.
Device characterization. All electrical measurements were performed at room temperature and in ambient air. Static transfer and output characteristics of the transistors and complementary inverters were measured using a five-probe system with a semiconductor parameter analyser (Keithley 4200-SCS). For all the electrical characterizations, the measurement software SweepMe! (sweep-me.net) was used. The dynamic performance of the organic complementary inverters and complementary seven-stage ring oscillators was measured using a Hewlett Packard 8114A 100-V/2-A pulse generator, a Rohde & Schwarz HMO3004 4 GSa s −1 /8 MB oscilloscope and a five-probe system with the Keithley 4200-SCS parameter analyser. High-angle annular dark-field scanning transmission electron microscopy and spectrum imaging analysis based on energy-dispersive X-ray spectroscopy were conducted with a Talos F200X microscope (Thermo Fisher Scientific/FEI) operated at 200 kV and equipped with a Super-X energy-dispersive X-ray detector, thus providing detailed elemental maps (O, Al) of the base layer ( Supplementary  Fig. 1). The photographs of the complementary inverters and ring oscillators were taken with an optical microscope (VHX5000 series, Keyence).

Data availability
All the data that support this study are included in this article and its Supplementary Information files. Source data are provided with this paper.