Organic Permeable Base Transistors Research Articles

High‐Frequency fT and fmax in Organic Transistors: Performance and Perspective

AbstractIn state‐of‐the‐art organic transistors, the transit frequency fT is reported to be fT = 40 MHz for vertical organic transistors, where a voltage‐normalized fT corresponds to 0.36 MHz V−2. The reported highest fT for conventional organic transistors is fT = 160 MHz, where the voltage‐normalized fT is 0.1 MHz V−2. While the reported transit frequency fT of n‐MOSFETs for silicon technology when normalized by the applied biases exceeds 300 GHz V−2. The reported carrier mobility of organic semiconductors is over 100 cm2 V−1 s−1, which is almost an order of magnitude lower than that of silicon. However, the transit frequency fT of organic transistors lags far behind silicon technology by about six orders of magnitude at a comparable device length below 300 nm. In this work, a technology computer‐aided design (TCAD) simulator is adjusted to the experimental DC and AC characteristics of the 200 nm device length vertical organic permeable‐base transistors (OPBTs) based on C60 as semiconductor. The AC performance of the devices is investigated and quantitatively analyzed the influence and contribution of key design, structure and material parameters that determine fT and fmax. The feasible ways to enhance and optimize the highest achievable fT and fmax are identified to reach the and overcome the limitations to high‐frequency operation in organic transistors.

Analytical modeling of organic permeable-base transistors based on geometrical parametrization

Analytical modeling of organic permeable-base transistors based on geometrical parametrization

Reliable p-type organic permeable base transistors—the missing component for integrated circuits

Due to their nm-range channel length, vertical organic transistors are gaining scientific interest to overcome the frequency limitations of lateral organic transistor. Especially, vertical organic permeable base transistors (OPBTs) with a structure resembling a vacuum-tube-triode stand out due to their excellent electrical performance. Recently, n-type OPBTs have been demonstrated with excellent performance and great potential to be applied in high-frequency logic circuits, display driving circuits, and light-emitting devices. However, achieving adequate p-type OPBTs to catch up with the performance of n-type devices is still challenging. Here, we report on the peculiar finding that the extended exposure of p-type OPBTs to ambient air during the fabrication has a positive effect on the electrical performance and tremendously improves the yield of devices to . Upon exposing the fresh device to air, oxygen diffuses into the semiconductor layer interacting with the base electrode, which facilitates the formation of pinholes in the base and creates an insulator layer around the base. Moreover, as oxygen acts as a p-dopants to the p-type semiconductors used in this work, the device performance is further improved, manifesting in an increased on-state current. However, as the exposure time increases, different p-type organic semiconductors show different behavior depending on their ionization potential. Comparing two semiconductor materials with a difference in their ionization potential of 0.4 eV, we show that long-term degradation could be effectively suppressed. Therefore, the positive effect of air exposure could be utilized to optimize p-type OPBTs, which offers a simple and universal way to construct high performance p-type OPBTs and possibly also complementary circuits.

Field and Thermal Emission Limited Charge Injection in Au-C60-Graphene van der Waals Vertical Heterostructures for Organic Electronics.

Among the family of 2D materials, graphene is the ideal candidate as top or interlayer electrode for hybrid van der Waals heterostructures made of organic thin films and 2D materials due to its high conductivity and mobility and its inherent ability of forming neat interfaces without diffusing in the adjacent organic layer. Understanding the charge injection mechanism at graphene/organic semiconductor interfaces is therefore crucial to develop organic electronic devices. In particular, Gr/C60 interfaces are promising building blocks for future n-type vertical organic transistors exploiting graphene as tunneling base electrode in a two back-to-back Gr/C60 Schottky diode configuration. This work delves into the charge transport mechanism across Au/C60/Gr vertical heterostructures fabricated on Si/SiO2 using a combination of techniques commonly used in the semiconductor industry, where a resist-free CVD graphene layer functions as a top electrode. Temperature-dependent electrical measurements show that the transport mechanism is injection limited and occurs via Fowler-Nordheim tunneling at low temperature, while it is dominated by a nonideal thermionic emission at room and high temperatures, with energy barriers at room temperature of ca. 0.58 and 0.65 eV at the Gr/C60 and Au/C60 interfaces, respectively. Impedance spectroscopy confirms that the organic semiconductor is depleted, and the energy band diagram results in two electron blocking interfaces. The resulting rectifying nature of the Gr/C60 interface could be exploited in organic hot electron transistors and vertical organic permeable-base transistors.

Vertical organic transistors with a permeable base: from fundamentals to performance prediction

A combined experimental and theoretical investigation establishes rational design rules for high-performance vertical organic permeable-base transistors (OPBTs).

전기화학적으로 산화된 투과성 베이스 전극을 갖는 수직구조 유기 트랜지스터

Vertical organic thin film transistors (VOTFTs) are emerging electronic devices that can overcome the limitations of conventional organic thin film transistors (OTFTs) based on lateral architecture. Here, organic permeable base transistors (OPBTs) are formed with an oxide layer on the base electrode through an electrochemical process called anodization, and this oxide layer acts as a dielectric layer. Through this electrochemical method, the manufacturing process is simplified and the charge carrier transport of the VOTFT is improved. In this study, the thickness of AlOx formed by anodizing the Al electrode was calculated through the capacitance-voltage measurement of the metal-insulator-metal device, and OPBT operations were observed according to the anodizing potential and interfacial layer, respectively. In addition, we observed the specific behaviors of vertical OPBT depending on the applied potential correlations between middle base electrode and bottom collector electrode.

Organic Permeable Base Transistors – Insights and Perspectives

AbstractVertical organic transistors have emerged as a new device concept holding great promise to overcome the limitations of lateral organic transistors. In this regard, the organic permeable base transistor (OPBT), a special kind of vertical transistor, stands out due to excellent performance figures such as low‐voltage operation and high transition frequency measured in a pulse‐biasing mode. On the occasion of Prof. Karl Leo's 60th birthday, his contributions to the development of high‐performance OPBTs are honored and a perspective for the future of OPBT development is provided. The current state of the art of OPBTs is reviewed and the principles of the device operation are summarized. New insights into the formation of the permeable base layer, which is vital to the function of OPBTs, are discussed. Additionally, a full yield analysis on a batch of 144 equivalent OPBTs is reported and the device yield and temporal evolution of device parameters are statistically analyzed. This analysis proves a device yield of >90% for these short‐channel transistors, rendering the possibility for circuit integration. Finally, the scaling laws of OPBTs are derived and strategies for new materials and device layouts to push the performance to the gigahertz regime are concluded.

Unraveling Structure and Device Operation of Organic Permeable Base Transistors

AbstractOrganic permeable base transistors (OPBTs) are of great interest for flexible electronic circuits, as they offer very large on‐current density and a record‐high transition frequency. They rely on a vertical device architecture with current transport through native pinholes in a central base electrode. This study investigates the impact of pinhole density and pinhole diameter on the DC device performance in OPBTs based on experimental data and TCAD simulation results. A pinhole density of NPin = 54 µm−2 and pinhole diameters around LPin = 15 nm are found in the devices. Simulations show that a variation of pinhole diameter and density around these numbers has only a minor impact on the DC device characteristics. A variation of the pinhole diameter and density by up to 100% lead to a deviation of less than 4% in threshold voltage, on/off current ratio, and sub‐threshold slope. Hence, the fabrication of OPBTs with reliable device characteristics is possible regardless of statistical deviations in thin film formation.

Stability of organic permeable base transistors

Organic Permeable Base Transistors (OPBTs) reach a very high transit frequency and large on-state currents. However, for a later commercial application of this technology, a high operational stability is essential as well. Here, the stability of OPBTs during continuous cycling and during base bias stress is discussed. It is observed that the threshold voltage of these transistors shifts toward more positive base voltages if stressed by applying a constant potential to the base electrode for prolonged times. With the help of a 2D device simulation, it is proposed that the observed instabilities are due to charges that are trapped on top of an oxide layer formed around the base electrode. These charges are thermally released after removing the stress, and the device reaches its initial performance after around 24–48 h.

Electrically Stable Organic Permeable Base Transistors for Display Applications

AbstractVertical organic permeable base transistors (OPBT) can drive large current densities of kA cm−2 and achieve record‐high transition frequencies of up to 40 MHz. They are therefore an interesting candidate for numerous applications, including the use in active‐matrix organic light emitting display (AMOLED) backplanes. However, the transistor characteristics are required to be stable against electrical stress. Here, the threshold voltage shifts under current‐ and voltage‐stress conditions over various temperatures and illumination conditions are investigated. OPBTs show excellent stability under the probed conditions, even at extended exposure to large on‐state currents of 3 A cm−2. The strongest shifts are observed at elevated temperatures, suggesting a temperature activation of the stress effect. On‐state current stress scales linearly with the current density, while off‐state stress is induced by the applied voltage. The effects of both kinds of stress are complementary and can compensate each other. It is found that the OPBT is perfectly suitable as a switching transistor in AMOLED displays.

Suppressing Base Currents in Organic Permeable-Base Transistors by Anodization of the Base Electrode

Organic permeable base transistors (OPBTs) have long been investigated as an alternative to conventional OFETs. Their vertical architecture allows for simple low-resolution fabrication techniques, ...

Modeling tunnel currents in organic permeable-base transistors

Research in organic permeable base transistors (OPBTs) has led to a significant increase in their performance. However, despite this progress, understanding of the working mechanism of OPBTs is still limited. Although first numerical models of OPBTs are able to describe the switching mechanism of OPBTs correctly, they neglect currents injected at the base electrode, which leads to unrealistically low off-currents and high ON/OFF ratios.Here, a tunneling model is developed that is capable of describing injection of charges through a thin oxide layer formed around the base electrode of OPBTs. With the help of this injection model, the performance of the base–collector diode of OPBTs is discussed. In particular, the model is used to explain the reduction in backward currents due to an exposure to ambient air by an increase in the thickness of the oxide layer. Furthermore, the tunnel model is used to show that the reduction in backward currents of the base–collector diode leads to a decrease in off-currents of complete OPBTs, which in turn leads to an increase in their ON/OFF ratio.

Vertical Organic Thin-Film Transistors with an Anodized Permeable Base for Very Low Leakage Current.

The organic permeable base transistor (OPBT) is currently the fastest organic transistor with a transition frequency of 40 MHz. It relies on a thin aluminum base electrode to control the transistor current. This electrode is surrounded by a native oxide layer for passivation, currently created by oxidation in air. However, this process is not reliable and leads to large performance variations between samples, slow production, and relatively high leakage currents. Here, for the first time it is demonstrated that electrochemical anodization can be conveniently employed for the fabrication of high-performance OPBTs with vastly reduced leakage currents and more controlled process parameters. Very large transmission factors of 99.9996% are achieved, while excellent on/off ratios of 5 × 105 and high on-currents greater than 300 mA cm-2 show that the C60 semiconductor layer can withstand the electrochemical anodization. These results make anodization an intriguing option for innovative organic transistor design.

Scaling of High‐Performance Organic Permeable Base Transistors

AbstractOrganic permeable‐base transistors (OPBTs) show potential for high‐speed, flexible electronics. Scaling laws of OPBTs are discussed and it is shown that OPBT performance can be increased by reducing their effective device area. Comparing the performance of optimized OPBTs with state‐of‐the‐art organic field‐effect transistors (OFETs), it is shown that OPBTs have a higher potential for an increased transit frequency. Not only do OPBTs reach higher transconductance values without the need for sophisticated structuring techniques, but they are also less sensitive to parasitic contact resistances. With the help of a 2D numerical model, the reduced contact resistances of OPBTs are explained by a homogeneous injection of current across the entire emitter electrode, compared to injection in a small area along the edge of the source of OFETs.

The Effect of Base–Emitter Interfacial Layer on the Organic Permeable Base Transistor’s Characteristics

ABSTRACTA thin interfacial layer of 20 nm has been introduced to increase the barrier from 0.9 to 1.1 eV or 1.2 eV or 1.4 eV between the base and emitter of an organic permeable base transistor (OPBT). The effect of this barrier height has been studied through the OPBTs fabricated with 5,5′′′′′-dihexyl-2,2′:5′,2′′:5′′,2′′′:5′′′,2′′′′:5′′′′,2′′′′′-sexithiophene as an active material for forming emitter and collector regions. First time, it has been observed that output characteristics of OPBT’s mimics the near-ideal characteristics of the metal oxide semiconductor field effect transistor and experiences significant change as the barrier height increases and the device enters into the velocity saturation region at lower applied voltage. The gain factor (β) has been found to vary from 0.5 to 2.7 µAV–2 with the increase in the barrier from 0.9 to 1.2 eV while at the high barrier of 1.4 eV, it is almost negligible.

Non-Linear Self-Heating in Organic Transistors Reaching High Power Densities

The improvement of the performance of organic thin-film transistors is driven by novel materials and improved device engineering. Key developments are a continuous increase of the charge carrier mobility, a scale-down of transistor dimensions, and the reduction of contact resistance. Furthermore, new transistor designs such as vertical devices are introduced to benefit from drastically reduced channel length while keeping the effort for structuring moderate. Here, we show that a strong electrothermal feedback occurs in organic transistors, ultimately leading to output characteristics with regions of S-shaped negative differential resistance. For that purpose, we use an organic permeable-base transistor (OPBT) with outstanding current densities, where a strong and reproducible, non-linear electrothermal feedback is revealed. We derive an analytical description of the temperature dependent current-voltage behavior and offer a rapid investigation method for material systems, where a temperature-activated conductivity can be observed.

Modeling of Organic Permeable Base Transistor Based on Inverse of Transistor Efficiency ( ${I}_{C}$ / ${g}_{m}$ )

For the first time, we are proposing characteristic equation, which describes the behavior of an organic permeable base transistor (OPBT). For this, an OPBT has been fabricated and studied through the experimentally obtained ${I}_{C}$ / ${g}_{m}$ versus ${V}_{\mathsf {BE}}$ (base-emitter voltage) characteristics. The characteristic equation, i.e., collector current, ${I}_{C} = {k}$ ( ${V}_{\mathsf {BE}} - {V}_{\mathsf {th}})^{n}$ has been obtained through curve fitting and mathematical formulations. Six OPBTs (modified by inserting a layer of barrier material of either N, $\text{N}^{\prime }$ [(-di-[( $\alpha$ -naphthyl)-N, $\text{N}^{\prime }$ -diphenyl]-1, $1^{\prime }$ -biphenyl)-4, $4^{\prime }$ -diamine ( $\alpha$ -NPD) or N, $\text{N}^{\prime }$ -Bis(naphthalen-1-yl)-N, $\text{N}^{\prime }$ -bis(phenyl)-2,7-diamino-9, 9-(Spiro-NPB) between emitter and base layers) confirm that their transfer characteristics follow this equation with n varying between 1.1 and 2.2. The n has been found to be dependent on barrier material. The gain of the device is ${A}_{O} = \textit {nV}_{E}$ / ${V}_{\mathsf {BE}}$ , ${V}_{E}$ being the early voltage.

Organic Power Electronics: Transistor Operation in the kA/cm2 Regime

In spite of interesting features as flexibility, organic thin-film transistors have commercially lagged behind due to the low mobilities of organic semiconductors associated with hopping transport. Furthermore, organic transistors usually have much larger channel lengths than their inorganic counterparts since high-resolution structuring is not available in low-cost production schemes. Here, we present an organic permeable-base transistor (OPBT) which, despite extremely simple processing without any high-resolution structuring, achieve a performance beyond what has so far been possible using organic semiconductors. With current densities above 1 kA cm−2 and switching speeds towards 100 MHz, they open the field of organic power electronics. Finding the physical limits and an effective mobility of only 0.06 cm2 V−1 s−1, this OPBT device architecture has much more potential if new materials optimized for its geometry will be developed.

Operation mechanism of high performance organic permeable base transistors with an insulated and perforated base electrode

The organic permeable base transistor is a vertical transistor architecture that enables high performance while maintaining a simple low-resolution fabrication. It has been argued that the charge transport through the nano-sized openings of the central base electrode limits the performance. Here, we demonstrate by using 3D drift-diffusion simulations that this is not the case in the relevant operation range. At low current densities, the applied base potential controls the number of charges that can pass through an opening and the opening is the current limiting factor. However, at higher current densities, charges accumulate within the openings and in front of the base insulation, allowing for an efficient lateral transport of charges towards the next opening. The on-state in the current-voltage characteristics reaches the maximum possible current given by space charge limited current transport through the intrinsic semiconductor layers. Thus, even a small effective area of the openings can drive huge current densities, and further device optimization has to focus on reducing the intrinsic layer thickness to a minimum.

Advanced Organic Permeable-Base Transistor with Superior Performance.

An optimized vertical organic permeable-base transistor (OPBT) competing with the best organic field-effect transistors in performance, while employing low-cost fabrication techniques, is presented. The OPBT stands out by its excellent power efficiency at the highest frequencies.