Metal-base Organic Transistors Research Articles

Interface-Induced Face-on Orientation of Organic Semiconductors with a Template Layer and Its Application to Vertical-Type Organic Transistors

Molecular orientation control with a template material, perylene 3,4,8,9-tetracarboxylic dianhydride (PTCDA), was investigated for various π-conjugated crystalline molecules, namely copper phthalocyanine (CuPc), N,N’-Dimethyl-3,4,9,10-perylenedicarboximide (MePTC), pentacene and dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (DNTT). The molecular orientation of CuPc and MePTC clearly changed from edge-on to face-on orientation by depositing on PTCDA. The electron mobility of MePTC, which is an n-type organic semiconductor suitable for vertical carrier transport, improved approximately three times owing to face-on molecular orientation. Thin-film X-ray diffraction studies revealed that the molecular packing mode (stacking or herringbone) affects the molecular orientation control. The MePTC film oriented with PTCDA was applied to a vertical-type organic transistor, a metal base organic transistor (MBOT), resulting in higher output current and current gain.

Investigation on the Conduction Mechanisms in Metal-Base Vertical Organic Transistors by DC and LF-Noise Measurements

Vertical organic transistors are a promising candidate to overcome the scaling limits of conventional horizontal organic field-effect devices. In this paper, based on the results of direct current (dc) and low-frequency noise (LFN) measurements, we propose a picture for the carrier transport in metal-base organic transistors in which transmission across the base is due to the combined action of both hot-carriers (HCs) and high-conduction paths (pores) in the metal base. Investigated devices employ pentacene and copper phthalocyanine as active layers for the emitter and collector regions, and a bilayer consisting of Au/MoO3 as injecting electrode. The dc analysis highlights that the charge transport in the investigated devices is due to drift diffusion of the emitter-injected carriers in the highest occupied molecular orbital (HOMO) band and is dominated by HC injection, although conduction via pores increases and becomes not negligible at high base–emitter fields. The LFN analysis highlights the presence of two dominant and uncorrelated 1/f current noise sources, one located between base and emitter, due to the fluctuations of the not-transmitted HC current, and a second source located between collector and emitter, due to the fluctuations of the transmitted HC current and/or to the fluctuations of the current through the pores in the metallic base.

Resistor-transistor logic circuits using vertical-type organic transistors

ABSTRACTNOR gate operation using organic devices was demonstrated based on resistor-transistor logic (RTL). The RTL was composed of an OR gate using two single layered devices as input resistors and a NOT gate (inverter) using a vertical-type metal base organic transistor (MBOT). The input resistors were connected to the base electrode of the MBOT. When either input was turned from low to high voltage, the output voltage changed from high to low, indicating NOR operation. Dynamic characteristics of the RTL circuit was also evaluated, and operation frequency was estimated to be 300 Hz.

Double-layered thin collector in n-type metal-base organic transistors

A very short channel length was achieved in vertical-type metal-base organic transistors using a double-layered collector composed of N,N′-dimethyl-3,4,9,10-perylene tetracarboxylic diimide (Me-PTC) and N,N′-distearyl-3,4,9,10-perylene tetracarboxylic diimide (St-PTC). St-PTC showed low electric conductivities, but inserting it at the interface with the collector electrode improved the on/off ratio to 4 × 104 for a total collector thickness of 105 nm. The decrease in the off current was attributed to the low electric conductivity of St-PTC, and the increase in the on current was discussed from the viewpoint of the morphological change of the collector layer.

Polymer Material Dependence in the Polymer/Small Molecule Metal-Base Organic Transistors

A hybrid metal-base organic transistor composed of a vacuum-deposited small-molecule emitter layer and a solution-processed polymer collector layer was fabricated using three types of conductive polymer, poly(3-hexylthiophene) (P3HT), poly(2-methoxy-5(2′-ethyl)hexoxy-phenylenevinylene (MEH-PPV) and poly(9, 9-dioctylfluorene-co-benzothiadiazole) (F8BT). Clear current amplification was observed only in the device with P3HT, and output current density of 12.6 mA/cm2, on/off ratio of 105, and current amplification factor of 76 were achieved at a collector voltage of 5 V and base voltage of 3 V. The output current decreased in order of P3HT > MEH-PPV > F8BT. The relationship between energy levels of polymer materials and carrier transmission process through the base electrode were discussed.

Surface‐light‐emitting transistors based on vertical‐type metal‐base organic transistors

Abstract— Light‐emitting transistors having a metal‐base organic transistor (MBOT) structure demonstrate both the function of an organic thin‐film transistor (OTFT) and organic light‐emitting diode (OLED). The MBOT is a vertical‐type organic transistor having a simple structure composed of organic/metal/organic layers demonstrating high‐current and low‐voltage operation. The light‐emitting MBOT was fabricated simply by inserting additional layers of hole‐transporting and emissive materials used in the OLED into the col lector layer. The device showed perfect surface emission similar to an OLED. A luminance modulation of 370 cd/m2 was observed at a collector voltage of 20 V and a base voltage of 3 V. This device can be applied to an OLED display device to increase the numerical aperture or reduce the required current of the TFT backplane.

Influence of charge carrier injection at emitter electrode/emitter interface on the performance of metal-base organic transistors

Pentacene-based metal-base organic transistors (MBOTs) are fabricated. The influence of the charge carrier injection efficiency at the emitter electrode/emitter interface on the device performance is investigated. It is found that the current modulation and the on/off ratio increase with the injection efficiency. By inserting poly(3,4-ethylenedioxythiophene) (PEDOT): PSS/m-MTDATA layers at the emitter electrode/emitter interface, the current modulation and the on/off ratio reach 6.7 mAcm−2 and 23, respectively. Meanwhile, the current gain is 95–96 in our experiment, which is almost independent on the injection efficiency.

LiF/Al Base Electrodes in Vertical Metal-Base Organic Transistors for Heat-Treatment-Free Process

Clear current modulation and current amplification were observed in vertical metal-base organic transistors (MBOTs) without heat treatment in air that was previously essential. The devices, which use a LiF/Al base electrode and a C60 layer under it, were prepared without heat treatment and showed a large current modulation exceeding 114.5 mA cm-2 and a current amplification factor of 51.7 at a collector voltage of 5 V and a base voltage of 3 V. The LiF/Al base electrode and the C60 layer under it were found to contribute to the increase in on current and to the decrease in off current, respectively.

Current Enhancement in the Vertical-Type Metal-Base Organic Transistors

The vertical-type metal-base organic transistors (MBOT) are high-performance transistors which have a simple layered structure of organic/metal/organic layers. In this study, we investigated various structures for the collector layer to enhance current modulation and reduce operation voltage. The device having collector layers composed of perylene derivatives/bathocuproine(BCP)/C60 and base electrode of LiF/Al showed very high current modulation of 165.1 mA/cm2 at 2 V of collector voltage and 1 V of base voltage. We found that the LiF layer and BCP layer contribute to current enhancement, and the C60 layer contributes to suppression of leakage current, respectively.

Improvement in the on/off ratio of a vertical-type metal-base organic transistor by heat treatment in air

The on/off ratio of a vertical-type metal-base organic transistor was significantly improved by subjecting it to heat treatment in air. The heat treatment of the collector layer and the base electrode reduced the off current that is mainly due to leakage current between the base and the collector, resulting in a considerable decrease in the off current. As a result, a high on/off ratio greater than 10 5 was achieved, in addition to the low-voltage and high current operation of the vertical-type organic transistor.

Ambipolar permeable metal-base transistor based on NPB/C 60 heterojunction

In this paper, we report the fabrication of permeable metal-base organic transistors based on N, N′-diphenyl- N, N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine (NPB)/C 60 heterojunction as both emitter and collector. By applying different polarities of voltage bias to the collector and the base, and input current to the emitter, the ambipolar behavior can be observed. The device demonstrates excellent common-base characteristics both in P-type and N-type modes with common-base current gains of 0.998 and 0.999, respectively.

Vertical structure p-type permeable metal-base organic transistors based on N,N′-diphentyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine

In this article, vertical structure p-type permeable-base organic transistors were proposed and demonstrated. A hole-type organic semiconductor N,N′-diphentyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine was used as emitter and collector. In the permeable-base transistors, the metal base was formed by firstly coevaporating Al and Ca in vacuum and then annealing at 120°C for 5min in air, followed by a thin Al deposition. These devices show a common-base current gain of near 1.0 and a common-emitter current gain of ∼270.

Large current gain and low operational voltage permeable metal-base organic transistors based on Au/Al double layer metal base

In order to realize the common-emitter characteristics of the tris(8-hydroxyquinoline) aluminium (Alq3)-based organic transistors, we used Au/Al double metal layer as the base, thus the vertical metal-base transistors with structure of Al/n-Si/Au/Al/Alq3/LiF/Al were constructed. It was found that the contact properties between the base and the organic semiconductors play an important role in the device performance. The utilization of Au/Al double layer metal base allows the devices to operate at high gain in the common-emitter and common-base mode at low operational voltage.

Stacked Device of Polymer Light-Emitting Diode Driven by Metal-Base Organic Transistor

We fabricated a new light-emitting device that combined a polymer light-emitting diode (PLED) and a vertical-type metal-base organic transistor (MBOT) through a floating electrode. By employing a layered floating electrode of Mg:Ag/Au, the MBOT on the PLED was operated successfully and a current amplification factor of approximately 20 was observed. The PLED luminescence exceeding 100 cd/m2 can be modulated using the MBOT with a low base voltage (2.8 V) and VCC (8 V). The emission contrast (on/off ratio) was improved with insertion of an insulating layer under the base, and the cut-off frequency was estimated to be 8 kHz. This device is expected to be a promising driving system of organic light-emitting diode (OLED), realizing low voltage and high numerical aperture.

Role of Oxidation Layer of Aluminum Base Electrode in Metal-Base Organic Transistors

Metal base organic transistor (MBOT) is a new type of vertical organic transistor that has simple organic/metal/organic layered structure. MBOT showed a high output current modulation for very low operation voltage, but large off-current resulted in poor ON/OFF ratio around several tens. We found that leaving the device under atmospheric condition caused decrease of the OFF current and the ON/OFF ratio reaching 330 was attained. The effect of exposure to air was investigated from the viewpoint of oxidation layer of electrode surface.

High-current and low-voltage operation of metal-base organic transistors with LiF∕Al emitter

In order to improve the performance and clarify the operation mechanism of the vertical-type metal-base organic transistor with a simple organic/metal/organic layered structure, the influence of the electron injection at the emitter electrode interface on modulation current has been investigated using the several emitter metal electrodes. The low injection barrier resulted in large modulation current. When a LiF∕Al emitter electrode was used for C60 organic semiconductor, this device achieved a high current modulation—exceeding 250mA∕cm2 for a low collector voltage of 3.0V and a base voltage of 1.25V.

Very High On/Off Ratio in Vertical-Type Metal-Base Organic Transistors

ABSTRACTThe on/off ratio of the vertical-type metal-base organic transistors was drastically improved by heat treatment in air. The heat treatment after deposition of the collector layer and base electrode reduced the leakage current between the base and collector, resulting in remarkable suppression of the off current. As a result, in addition to the advantage of low voltage and high current operation based on the vertical structure, very high on/off ratio exceeding 105 was achieved.