Electronic structure of diluted magnetic semiconductor superlattices: In-plane magnetic field effect

The electronic structure of diluted magnetic semiconductor (DMS) superlattices under an in-plane magnetic field is studied within the framework of the effective-mass theory; the strain effect is also included in the calculation. The numerical results show that an increase of the in-plane magnetic field renders the DMS superlattice from the direct band-gap system to the indirect band-gap system, and spatially separates the electron and the hole by changing the type-I band alignment to a type-II band alignment. The optical transition probability changes from type I to type II and back to type I like at large magnetic field. This phenomenon arises from the interplay among the superlattice potential profile, the external magnetic field, and the sp-d exchange interaction between the carriers and the magnetic ions. The shear strain induces a strong coupling of the light- and heavy-hole states and a transition of the hole ground states from "light"-hole to "heavy"-hole-like states.

Comparison of field effect transistor characteristics between space-grown and earth-grown gallium arsenide single crystal substrates

Semi-insulating gallium arsenide single crystal grown in space has been used in fabricating low noise field effect transistors and analog switch integrated circuits by the direct ion-implantation technique. All key electrical properties of these transistors and integrated circuits have surpassed those made from conventional earth-grown gallium arsenide. This result shows that device-grade space-grown semiconducting single crystal has surpassed the best terrestrial counterparts. (C) 2001 American Institute of Physics.

Optical and electrical investigation of low dimensional self-assembled InAs quantum dot field effect transistors

Self-assembled InAs QD dot-in-a-well (DWELL) structures were grown on GaAs substrate by MBE system, and heterojunction modulation-doped field effect transistor (MODFET) was fabricated. The optical properties of the samples show that the photoluminescence of InAs/GaAs self-assembled quantum dot (SAQD) is at 1.265 mu m at 300 K. The temperature-dependence of the abnormal redshift of InAs SAQD wavelength with the increasing temperature was observed, which is closely related with the inhomogeneous size distribution of the InAs quantum dot. According to the electrical measurement, high electric field current-voltage characteristic of the MODFET device were obtained. The embedded InAs QD of the samples can be regard as scattering centers to the vicinity of the channel electrons. The transport property of the electrons in GaAs channel will be modulated by the QD due to the Coulomb interaction. It has been proposed that a MODFET embedded with InAs QDs presents a novel type of field effect photon detector.

IMPROVEMENT OF STRUCTURAL INSTABILITY OF THE ION-SENSITIVE FIELD-EFFECT TRANSISTOR (ISFET)

Three causes involved in the instability of the ISFET are proposed in this study. First, it is ascertained that hydroxyl group resident at the surface of the Si3N4 film or in the electrolyte solution is most active and subject to gain or loss of electrons. This is one of the main causes for ISFET structural instability. Secondly, the stability of the pH-sensitive FET varies with deposition conditions in the fabrication process of the ISFET. This proves to be another cause of ISFET instability. Thirdly, the pH of the measured solution varies with the measuring process and time, contributing to the instability, but is not a cause of the instability of the pH-ISFET itself. We utilized the technique of readjusting and controlling the ratio of hydroxyl groups to amine groups to enhance the stability of the ISFET. Our techniques to improve stability characteristics proved to be effective in practice.

FIELD-EFFECT ON THERMAL EMISSION FROM THE 0.40 EV ELECTRON LEVEL IN INGAP

Results are reported of electric-field dependence on thermal emission of electrons from the 0.40 eV level at various temperatures in InGaP by means of deep-level transient spectroscopy. The data are analyzed according to the Poole-Frankel emission from the potentials which are assumed to be Coulombic, square well, and Gaussian, respectively. The emission mte from this level is strongly field dependent. It is found that the Gaussian potential model is more reasonable to describe the phosphorus-vacancy-induced potential in InGaP than the Coulombic and square-well ones.

Threshold behavior in backgating in GaAs metal-semiconductor field-effect transistors: Induced by limitation of channel-substrate junction to leakage current

An analytical model is proposed to understand backgating in GaAs metal-semiconductor field-effect transistors (MESFETs), in which the effect of channel-substrate (CS) junction is included. We have found that the limitation of CS junction to leakage current will cause backgate voltage to apply directly to CS junction and result in a threshold behavior in backgating effect. A new and valuable expression for the threshold voltage has been obtained. The corresponding threshold electric field is estimated to be in the range of 1000-4000 V/cm and for the first time is in good agreement with reported experimental data. More, the eliminated backgating effect in MESFETs that are fabricated on the GaAs epitaxial layer grown at low temperature is well explained by our theory. (C) 1997 American Institute of Physics.

BACKGATING AND LIGHT SENSITIVITY IN GAAS METAL-SEMICONDUCTOR FIELD-EFFECT TRANSISTORS

Three different types of GaAs metal-semiconductor field effect transistors (MESFET) by employing ion implantation, molecular beam epitaxy (MBE) and low-temperature MBE (LT MBE) techniques respectively were fabricated and studied in detail. The backgating (sidegating) measurement in the dark and in the light were carried out. For the LT MBE-GaAs buffered MESFETs, the output resistance R(d) and the peak transconductance g(m) were measured to be above 50 k Omega and 140 mS/mm, respectively, and the backgating and light sensitivity were eliminated. A theoretical model describing the light sensitivity in these kinds of devices is given. and good agreement with experimental data is reached.

p-p isotype organic heterojunction and ambipolar field-effect transistors

We realized ambipolar transport behavior in field-effect transistors by using p-p isotype heterojunction films as active layers, which consisted of two p-type semiconductor materials, 2, 2'; 7', 2 ''-terphenanthrenyl (Ph3) and vanadyl-phthalocyanine (VOPc). The ambipolar charge transport was attributed to the interfacial electronic structure of Ph3-VOPc isotype heterojunction, and electrons and holes were accumulated at both sides of the narrow band-gap VOPc and the wide band-gap Ph3, respectively, which were confirmed by the capacitance-voltage relationship of metal-oxide-semiconductor diodes. The accumulation thickness of carriers was also obtained by changing the heterojunction active layer thickness. Furthermore, the results indicate that the device performance is relative to interfacial electronic structures.

Tin (IV) phthalocyanine oxide: An air-stable semiconductor with high electron mobility

Air-stable n-type field effect transistors were fabricated with an axially oxygen substituted metal phthalocyanine, tin (IV) phthalocyanine oxide (SnOPc), as active layers. The SnOPc thin films showed highly crystallinity on modified dielectric layer, and the electron field-effect mobility reached 0.44 cm(2) V-1 s(-1). After storage in air for 32 days, the mobility and on/off ratio did not obviously change. The above results also indicated that it is an effective approach of seeking n-type semiconductor by incorporating the appropriate metal connected with electron-withdrawing group into pi-pi conjugated system.

High-mobility pentacene thin-film transistors with copolymer-gate dielectric

Pentacene thin-film transistors have been obtained using polymethyl-methacrylate-co-glyciclyl-methacrylate (PNIMA-GMA) as the gate dielectric. The optimum active layer thickness in thin-film transistors (OTFTs) was investigated. The present devices show a wide operation voltage range. The on/off current ratio is as high as 10(5). In linear region (V-DS = -2V), the field-effect mobility of device increases with the increase in gate field at low-voltage region (V-G < - 20 V), and a mobility of 0.33 cm(2)/Vs can be obtained when V-G > 20 V. In saturation region, the mobility increases linearly with the gate field, and a high mobility of 1.14 cm(2)/Vs can be obtained at V-G = -95V. The influence of voltage on mobility of device was investigated.

Micropatterning of metal films coated on polymer surfaces with epoxy mold and its application to organic field effect transistor fabrication

In this letter, a simple and versatile approach to micropatterning a metal film, which is evaporated on a Si substrate coated with polymer, is demonstrated by the use of a prepatterned epoxy mold. The polymer interlayer between the metal and the Si substrate is found important for the high quality pattern. When the metal-polymer-Si sandwich structure is heated with the temperature below T-m but above T-g of the polymer, the plastic deformation of the polymer film occurs under sufficiently high pressure applied. It causes the metal to crack locally or weaken along the pattern edges. Further heating while applying a lower pressure results in the formation of an intimate junction between the epoxy stamp and the metal film. Under these conditions the epoxy cures further, ensuring adhesion between the stamp and the film. The lift-off process works because the adhesion between the epoxy and the metal film is stronger than that between the metal film and the polymer. A polymer field effect transistor is fabricated in order to demonstrate potential applications of this micropatterning approach.

Metal transfer printing and its application in organic field-effect transisitor fabrication

The fabrication of multilayer microstructures, for example for organic field-effect transistors, using metal transfer printing (MTP) is demonstrated. The Figure shows a two-layer gold structure produced by MTP. Since MTP is a purely additive technique, in which mechanical adhesion acts as the patterning driving force, it is considered an attractive approach to reel-to-reel processing.

Morphology and crystalline transition of poly (3-butylthiophene) associated with its polymorphic modifications

Poly (3-butylthiophene) (P3BT) is a much less studied conjugated polymer despite its high crystallizability and thus excellent electrical property. In this work, morphology of P3BT at different crystalline polymorphs and solvent/thermal induced phase transition between form I and U modifications have been intensively investigated by using optical microscopy, electron microscopy, differential scanning calorimetry, and X-ray diffraction. It is shown that a direct deposition from carbon disulfide (CS2) at fast evaporation results in P3BT crystals in form I modification, giving typical whiskerlike morphology. In contrast, low evaporation rate from CS, leads to formation of form II crystals with spherulitic morphology, which is so far scarcely observed in polythiophene.