Effects of post-annealed floating gate on the performance of AlGaN/GaN heterostructure field-effect transistors

AlGaN/GaN heterostructure field-effect transistors(HFETs) with different floating gate lengths and floating gates annealed at different temperatures, are fabricated. Using the measured capacitance–voltage curves of the gate Shottky contacts for the AlGaN/GaN HFETs, we find that after floating gate experiences 600?C rapid thermal annealing, the larger the floating gate length, the larger the two-dimensional electron gas electron density under the gate region is. Based on the measured capacitance–voltage and current–voltage curves, the strain of the AlGaN barrier layer in the gate region is calculated, which proves that the increased electron density originates from the increased strain of the AlGaN barrier layer.     

Synthesis of thermally stable HfO_xN_y as gate dielectric for AlGaN/GaN heterostructure field-effect transistors

In this paper, we adopted thermally stable HfO_xN_y as gate dielectric for TiN/HfO_xN_y/AlGaN/GaN heterostructure field-effect transistors(HFETs) application. It demonstrated that the surface morphologies, composition, and optical properties of the HfO_xN_y films were dependent on oxygen flow rate in the O_2/N_2/Ar mixture sputtering ambient. The obtained metal–oxide–semiconductor heterostructure field-effect transistors by depositing HfO_2 and HfO_xN_y dielectric at different oxygen flow rates possessed a small hysteresis and a low leakage current. After post deposition annealing at 900℃, the device using HfO_xN_y dielectric operated normally with good pinch-off characteristics, while obvious degradation are observed for the HfO_2 gated one at 600℃. This result shows that the HfO_xN_y dielectric is a promising candidate for the self-aligned gate process.     

Back doping design in delta-doped AlGaN/GaN heterostructure field-effect transistors

In this paper, we investigate theoretically the electron transport in AlGaN/GaN single-barrier and in AlGaN/GaN/AlGaN double-barrier heterostructures, aimed to operate as high-power and high-temperature field-effect transistors. The presence of spontaneous and piezoelectric polarizations as well as the heterointerface polarity are evoked and taken into account in the modelling part. Delta-doping is used as a source of electrons for the channel quantum well. Calculations of the electron-band parameters are made by using self-consistent solutions of coupled Schrodinger-Poisson equations. It is found that the polarization fields act to significantly increase the two-dimensional sheet charge concentration. Moreover, the AlGaN/GaN heterostructures with higher Al compositions are found to be favourable for higher electron densities. On the other hand, the employment of a back doping with delta-shaped profiles is shown to improve further the electrical behaviour of the field-effect transistors studied.     

Self-aligned-gate AlGaN/GaN heterostructure field-effect transistor with titanium nitride gate

Self-aligned-gate heterostructure field-effect transistor(HFET) is fabricated using a wet-etching method.Titanium nitride(TiN) is one kind of thermal stable material which can be used as the gate electrode.A Ti/Au cap layer is fixed on the gate and acts as an etching mask.Then the T-shaped gate is automatically formed through over-etching the TiN layer in 30% H_2O_2 solution at 95 ℃.After treating the ohmic region with an inductively coupled plasma(ICP) method,an Al layer is sputtered as an ohmic electrode.The ohmic contact resistance is approximately 0.3 Ω·mm after annealing at a low-temperature of 575 ℃ in N_2 ambient for 1 min.The TiN gate leakage current is only 10~(-8) A after the low-temperature ohmic process.The access region length of the self-aligned-gate(SAG) HFET was reduced from 2 μm to 0.3 μm compared with that of the gate-first HFET.The output current density and transconductance of the device which has the same gate length and width are also increased.     

Normally-off AlGaN/GaN heterojunction field-effect transistors with in-situ AlN gate insulator

AlGaN/GaN heterojunction field-effect transistors (HFETs) with p-GaN cap layer are developed for normally-off operation, in which an in-situ grown AlN layer is utilized as the gate insulator. Compared with the SiN_x gate insulator, the AlN/p-GaN interface presents a more obvious energy band bending and a wider depletion region, which helps to positively shift the threshold voltage. In addition, the relatively large conduction band offset of AlN/p-GaN is beneficial to suppress the gate leakage current and enhance the gate breakdown voltage. Owing to the introduction of AlN layer, normally-off p-GaN capped AlGaN/GaN HFET with a threshold voltage of 4 V and a gate swing of 13 V is realized. Furthermore, the field-effect mobility is approximately 1500 cm²·V^-1·s^-1 in the 2DEG channel, implying a good device performance.     

The current-voltage characteristics of field-effect transistors with short channels

The current-voltage characteristics of MOS field-effect transistors is investigated theoretically and experimentally in the region of extremely high drain electric fields E_D using the electron temperature concept in the classical three-dimensional theory. It is found that the drain field at which the drain current becomes non-ohmic on the basis of hot electron effects is related to the surface field E_zs by E_D～√E_zs. Other expressions for the field dependent mobility are also given which allow the construction of the current-voltage characteristics.     

Organic field-effect transistors

The paper reviews the recent year publications concerning organic field-effect transistors (OFETs). A lot of works have been performed to help understanding the structural and electrical properties of materials used to construct OFETs. It has been established that in partially ordered systems, the charge transport mechanism is thermally activated and field-assisted hopping transport and the hopping transport between disorder-induced localized states dominate over intrinsic polaronic hopping transport seen in organic single crystals. Many research attempts have been carried out on the design of air-stable organic semiconductors with a solution process which is capable of producing OFETs with excellent properties and good stability when subjected to multiple testing cycles and under continuous electrical bias. Recent experiments have demonstrated ambipolar channel conduction and light emission in conjugated polymer FETs. These achievements are the basis for construction of OLED based displays driven by active matrix consisting of OFETs.     

Parasitic source resistance at different temperatures for AlGaN/AlN/GaN heterostructure field-effect transistors

The parasitic source resistance(RS) of Al Ga N/Al N/Ga N heterostructure field-effect transistors(HFETs) is studied in the temperature range 300–500 K. By using the measured RSand both capacitance–voltage(C–V) and current–voltage(I–V) characteristics for the fabricated device at 300, 350, 400, 450, and 500 K, it is found that the polarization Coulomb field(PCF) scattering exhibits a significant impact on RSat the above-mentioned different temperatures. Furthermore, in the Al Ga N/Al N/Ga N HFETs, the interaction between the additional positive polarization charges underneath the gate contact and the additional negative polarization charges near the source Ohmic contact, which is related to the PCF scattering, is verified during the variable-temperature study of RS.     

Plasma-assisted surface treatment for low-temperature annealed ohmic contact on AlGaN/GaN heterostructure field-effect transistors

In this study, a low-temperature annealed ohmic contact process was proposed on Al Ga N/Ga N heterostructure field effect transistors(HFETs) with the assistance of inductively coupled plasma(ICP) surface treatment. The effect of ICP treatment process on the 2DEG channel as well as the formation mechanism of the low annealing temperature ohmic contact was investigated. An appropriate residual Al Ga N thickness and a plasma-induced damage are considered to contribute to the low-temperature annealed ohmic contact. By using a single Al layer to replace the conventional Ti/Al stacks, ohmic contact with a contact resistance of 0.35 ?·mm was obtained when annealed at 575?C for 3 min. Good ohmic contact was also obtained by annealing at 500?C for 20 min.     

Influence of temperature on strain-induced polarization Coulomb field scattering in AIN/GaN heterostructure field-effect transistors

Electron mobility scattering mechanism in AlN/GaN heterostuctures is investigated by temperature-dependent Hall measurement, and it is found that longitudinal optical phonon scattering dominates electron mobility near room temperature while the interface roughness scattering becomes the dominant carrier scattering mechanism at low temperatures （~ 100 K）. Based on measured current-voltage characteristics of prepared rectangular AlN/GaN heterostructure field-effect transistor under different temperatures, the temperature-dependent variation of electron mobility under different gate biases is inves- tigated. The polarization Coulomb field （PCF） scattering is found to become an important carrier scattering mechanism after device processing under different temperatures. Moreover, it is found that the PCF scattering is not generated from the thermal stresses, but from the piezoelectric contribution induced by the electrical field in the thin A1N barrier layer. This is attributed to the large lattice mismatch between the extreme thinner AlN barrier layer and GaN, giving rise to a stronger converse piezoelectric effect.     

Influence of temperature on strain-induced polarization Coulomb field scattering in AlN/GaN heterostructure field-effect transistors

Electron mobility scattering mechanism in AlN/GaN heterostuctures is investigated by temperature-dependent Hall measurement, and it is found that longitudinal optical phonon scattering dominates electron mobility near room temperature while the interface roughness scattering becomes the dominant carrier scattering mechanism at low temperatures(～100 K).Based on measured current–voltage characteristics of prepared rectangular AlN/GaN heterostructure field-effect transistor under different temperatures, the temperature-dependent variation of electron mobility under different gate biases is investigated. The polarization Coulomb field(PCF) scattering is found to become an important carrier scattering mechanism after device processing under different temperatures. Moreover, it is found that the PCF scattering is not generated from the thermal stresses, but from the piezoelectric contribution induced by the electrical field in the thin AlN barrier layer. This is attributed to the large lattice mismatch between the extreme thinner AlN barrier layer and GaN, giving rise to a stronger converse piezoelectric effect.     

Strong quantum confinement and high carrier concentration in AlGaN/InGaN/GaN heterostructure field-effect transistors

In this letter, we investigate the carrier features in AlGaN/InGaN/GaN heterostructure field-effect transistors. A study of charge control in the AlGaN/InGaN/GaN structure is performed by self-consistently solving Schrödingers equation in conjunction with Poissons equation. The results indicate that the concentration of two-dimensional electron gas can be largely increased with the incorporation of an InGaN layer. Both carrier density and quantum confinement are very sensitive to the strain in the InGaN channel layer. These novel features are attributed to the strong polarization effect in the AlGaN/InGaN and InGaN/GaN interfaces. PACS 85.30.De; 85.30.Tv; 77.65.Ly; 85.35.Be; 02.60.Cb     

Influence of drain bias on the electron mobility in AIGaN/AIN/GaN heterostructure field-effect transistors

Using measured capacitance-voltage curves and current-voltage characteristics for the AlGaN/AlN/GaN heterostructure field-effect transistors with different gate lengths and drain-to-source distances, the influence of drain bias on the electron mobility is investigated. It is found that below the knee voltage the longitudinal optical （LO） phonon scattering and interface roughness scattering are dominant for the sample with a large ratio of gate length to drain-to-source distance （here 4/5）, and the polarization Coulomb field scattering is dominant for the sample with a small ratio （here 1/5）. However, the above polarization Coulomb field scattering is weakened in the sample with a small drain-to-source distance （here 20 μm） compared with the one with a large distance （here 100 μm）. This is due to the induced strain in the AlGaN layer caused by the drain bias.     

Influence of drain bias on the electron mobility in AlGaN/AlN/GaN heterostructure field-effect transistors

Using measured capacitance-voltage curves and current-voltage characteristics for the AlGaN/AlN/GaN heterostructure field-effect transistors with different gate lengths and drain-to-source distances,the influence of drain bias on the electron mobility is investigated.It is found that below the knee voltage the longitudinal optical(LO) phonon scattering and interface roughness scattering are dominant for the sample with a large ratio of gate length to drain-to-source distance(here 4/5),and the polarization Coulomb field scattering is dominant for the sample with a small ratio(here 1/5).However,the above polarization Coulomb field scattering is weakened in the sample with a small drain-to-source distance(here 20 μm) compared with the one with a large distance(here 100 μm).This is due to the induced strain in the AlGaN layer caused by the drain bias.     

High temperature characteristics of bilayer epitaxial graphene field-effect transistors on SiC Substrate

In this paper,high temperature direct current(DC) performance of bilayer epitaxial graphene device on SiC substrate is studied in a temperature range from 25℃ to 200℃.At a gate voltage of-8 V(far from Dirac point),the drainsource current decreases obviously with increasing temperature,but it has little change at a gate bias of +8 V(near Dirac point).The competing interactions between scattering and thermal activation are responsible for the different reduction tendencies.Four different kinds of scatterings are taken into account to qualitatively analyze the carrier mobility under different temperatures.The devices exhibit almost unchanged DC performances after high temperature measurements at 200℃ for 5 hours in air ambience,demonstrating the high thermal stabilities of the bilayer epitaxial graphene devices.     

Evaluation of a gate-first process for AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors with low ohmic annealing temperature

In this paper, TiN/AlO_x gated Al Ga N/Ga N metal–oxide–semiconductor heterostructure field-effect transistors(MOSHFETs) were fabricated for gate-first process evaluation. By employing a low temperature ohmic process, ohmic contact can be obtained by annealing at 600℃ with the contact resistance approximately 1.6 ?·mm. The ohmic annealing process also acts as a post-deposition annealing on the oxide film, resulting in good device performance. Those results demonstrated that the TiN/AlO_x gated MOS-HFETs with low temperature ohmic process can be applied for self-aligned gate Al Ga N/Ga N MOS-HFETs.     

Impact of dopants in GaN on the formation of two-dimensional electron gas in AlGaN/GaN heterostructure field-effect transistors

The two-dimensional electron gas distribution in AlGaN/GaN high electron mobility transistors is determined from the solution of the coupled Schr?dinger’s and Poisson’s equations. Considering the piezoelectric effect, the two-dimensional electron gas concentration is calculated to be as high as 7.7×10¹⁹ cm^-3. In order to obtain an understanding of how the two-dimensional electron gas distribution is influenced by dopants in GaN, we observed the two-dimensional electron gas concentration and occupation of sub-bands versus dopant concentration in the GaN layer of an AlGaN/GaN heterostructure. Our results show that the two-dimensional electron gas concentration is slightly increased at higher doping levels in GaN, while the quantum confinement in the AlGaN/GaN heterostructure is weakened with the increase of donor concentration in the GaN layer. Received: 26 May 2001 / Accepted: 23 July 2001 / Published online: 23 January 2002