Hiroyuki Iechi

Inverter Circuits using Pentacene and ZnO Transistors

We report two types of integrated circuits based on a pentacene static-induction transistor (SIT), a pentacene thin-film transistor (TFT) and a zinc oxide (ZnO) TFT. The operating characteristics of a p-p inverter using pentacene SITs and a complementary inverter using a p-channel pentacene TFT and an n-channel ZnO TFT are described. The basic operation of logic circuits at a low voltage was achieved for the first time using the pentacene SIT inverter and complementary circuits with hybrid inorganic and organic materials. Furthermore, we describe the electrical properties of the ZnO films depending on sputtering conditions, and the complementary circuits using ZnO and pentacene TFTs.

Electrical characteristics of flexible organic static induction transistors under bending conditions

Organic static induction transistors (OSITs) with vertical channel structures that operate at low voltages have potential as components of portable or ubiquitous devices. In this study, the static characteristics of pentacene OSITs on flexible substrates were examined under bending conditions. Bending tests of the flexible substrate were carried out under various compressive and tensile strains and at various bending radii. The results demonstrate that OSITs exhibit stable electrical characteristics at compressive and tensile strains up to a bending radius of 5mm, suggesting a possible application as transistors in the field of flexible electronics.

Selectively Doped n-GaInP/GaAs Heterostructures Grown by MOCVD

We report the first growth of selectively doped n-GaInP/GaAs heterostructures by low pressure metalorganic chemical vapor deposition (MOCVD) using trimethylindium, triethylgallium, phosphine and arsine as source materials. The Shubnikov-de Haas oscillation and the temperature dependence of electron mobility indicate that a two-dimensional electron gas is present at the GaInP/GaAs interface. It is noted that the persistent photoconductivity observed in these heterostructures was very weak. These results suggest potential application of this material system to high electron mobility transistors (HEMT).

Improvement in On/Off Ratio of Pentacene Static Induction Transistors with Ultrathin CuPc Layer

Conventional organic static induction transistors (OSITs) have been directly fabricated on a source electrode formed on a substrate without the investigation of the effect of a surface treatment on device characteristics. We have fabricated the OSITs based on pentacene thin film with an ultrathin copper phthalocyanine (CuPc) layer. The influence of depositing this ultrathin CuPc layer on the source electrode on static characteristics was investigated. The obtained results provided an important fact indicating that a high on/off ratio of 230 and a high current of 3 µA in an organic SIT are achieved by the formation of a hole injection barrier that works as a tunneling layer on a source electrode. The barrier effectively controls the tunnel current from the source electrode by applying a gate voltage.

Organic Inverter Using Monolithically Stacked Static Induction Transistors

We fabricated monolithic organic inverters using a stacked structure of two organic static induction transistors (SITs). The operating characteristics of a p-channel transistor load and p-channel transistor drive-type inverter in which pentacene films are employed as an organic semiconductor material are described. The gain transfer characteristics of a pentacene SIT inverter based on an enhancement-load/enhancement-drive layout showed a threshold voltage of approximately -1 V and a relatively large voltage gain of approximately 1.5. The advantages of this novel device structure are the controllability of the operational characteristics of each SIT and the simple device fabrication process.

Improvement in On/Off Ratio of Pentacene Static Induction Transistors by Controlling Hole Injection Barrier

For the realization of high-performance organic static induction transistors (OSITs), it is important to investigate the effect of a hole injection barrier at the interface between a pentacene films and a source electrode in OSITs. In this study, the OSITs based on pentacene films were fabricated on various metallic source electrodes with different work functions and on copper phthalocyanine (CuPc)/indium tin oxide (ITO) electrodes with different CuPc thicknesses. The hole injection barrier was affected by the work function of metallic source electrodes (ITO, Au, and Pt) and the thickness of CuPc (0, 3, and 5 nm). The obtained results demonstrate that a high on/off ratio is achieved when a hole injection barrier with a moderate height is formed at the interface. It was found that controlling the hole injection barrier is effective for improving the characteristics of OSITs.

Characterization of Zinc Oxide and Pentacene Thin Film Transistors for CMOS Inverters

We fabricated both thin film transistors (TFTs) and diodes using zinc oxide (ZnO) and pentacene, and investigated their basic characteristics. We found that field-effect mobility is influenced by the interface state between the semiconductor and dielectric layers. Furthermore, the complementary metal oxide semiconductor (CMOS) inverter using a p-channel pentacene field-effect transistor (FET) and an n-channel ZnO FET showed a relatively high voltage gain (8 - 12) by optimizing the device structure. The hybrid complementary inverters described here are expected for application in flexible displays, radio frequency identification cards (RFID) tags, and others.

Interrelation of Si Internal Stress and Si/SiO2 Interface Stress

Both Si/SiO2 interface stress (σint) and Si internal stress (σsi) of identical CZ-Si wafers with oxidized SiO2 film are estimated and analyzed. In the oxidation temperature from 1000 to 1100°C, temperature dependence of σint and σsi is similar in the small warpage region, but quite different in the large warpage region. Furthermore, it is observed that the values of σint are one order of magnitude larger than those of σsi. These experiments suggest that the relaxation phenomenon occurs near Si/SiO2 interface, and the relaxation mechanism in the small warpage region differs from that in the large warpage region.

Organic Complementary Inverters Based on Step-Edge Vertical Channel Organic Field-Effect Transistors

An organic complementary inverter which consists of a p-channel pentacene step-edge vertical channel organic field-effect transistor (SVC-OFET) and an n-channel hexadecafluorophthalocyanine (F16CuPc) SVC-OFET has been demonstrated. The two short-channel SVC-OFETs have been fabricated on the two steep side walls of a single gate line using a self-aligning technique. Such a structure significantly simplifies the layout and wiring of the inverter circuit. The inverter exhibited the same input and output swing ranges as the supply voltage, a logic threshold voltage very close to half of the supply voltage and a gain larger than 3.

Study of vertical type organic light emitting transistor using ZnO

We propose a new type organic light emitting transistor (OLET) combining static induction transistor (SIT) with double hetero junction type organic light emitting diodes (OLED) using n-type zinc oxide (ZnO) films which works as a transparent and electron injection layer. The device characteristics of newly developed OLED and ZnO-SIT showed relatively high luminance of about 500 cd/m2 at 7.6 mA/cm2 and is able to control by gate voltage as low as a few volts, respectively. The crystal structures of the ZnO films as a function of Ar/O2 flow ratio and the basic characteristics of the thin film transistor (TFT) and SIT depending on the ZnO sputtering conditions are investigated. The results obtained here show that the OLET using ZnO film is a suitable element for flexible sheet displays.