Characteristics of submicron gate GaAs FET's with Al0.3Ga0.7As buffers: Effects of interface quality

GaAs field effect transistors (FET's) having submicron gate lengths (0.7 µm) and Al0.3Ga0.7As buffer layers were fabricated. The saturation, and particularly the pinch-off characteristics, showed a considerable dependence on the growth conditions used during preparation by molecular beam epitaxy (MBE). The structures grown at high substrate temperatures exhibited an excellent pinch-off characteristic, while those grown at low temperatures showed an inferior pinch-off characteristic. A transconductance of 160 mS/mm was obtained in all structures, regardless of the growth temperature.     

Temperature dependence of Al/undoped Al0.5Ga0.5As/GaAs capacitors

Undoped Al0.5Ga0.5As is used in place of the insulator layer in the fabrication of MIS-type capacitors with Schottky gates. The current-voltage and capacitance-voltage characteristics of the capacitors were measured as a function of temperature in the range 300-77 K. At high temperatures current is by thermionic emission over the barrier determined by the Schottky contact and the Al0.5Ga0.5As/ GaAs conduction band discontinuity. As the temperature is lowered, Fowler-Nordheim tunneling is observed at sufficiently large gate biases and at 77 K conduction is ohmic. Based on I-V and C-V data the electron accumulation layer density is estimated to be about 1 × 10¹²cm^-2at 77 K when the capacitor is positively biased. The results obtained indicate that for an appropriate choice of parameters it should be possible to fabricate MIS-like transistors suitable for high-speed operation at 77 K.     

A new Al0.3Ga0.7As/GaAs modulation-doped FET

A new Al0.3Ga0.7As/GaAs modulation-doped FET fabricated like a MESFET but operating like a JFET was successfully fabricated and tested. This new device replaces the Schottky gate of the MESFET with an n+/p+ camel diode structure, thereby allowing problems associated with the former to be overcome. The devices, which were fabricated from structures grown by molecular beam epitaxy (MBE), had a 1µm gate length, a 290µm gate width, and a 4µm channel length. The room temperature transconductance normalized to the gate width was about 95 mS/mm, which is comparable to that obtained in similar modulation-doped Schottky barrier FET's. Unlike modulation-doped Schottky barrier FET's, fabrication of this new device does not require any critical etching steps or formation of a rectifying metal contact to the rapidly oxidizing Al0.3Ga0.7As. Relatively simple fabrication procedures combined with good device performance make this camel gate FET suitable for LSI applications.     

Preparation and properties of molecular beam epitaxy grown (Al0.5Ga0.5)0.48In0.52As

The first reported growth of the quaternary AlGaInAs on an InP substrate by molecular beam epitaxy had an equal aluminum-to-gallium mole fraction ratio, and exhibited a 5 K bandgap energy of 1.237 eV. This is intermediate between the 5 K band gap energy of Ga0.47In0.53As (0.810 eV) and that of Al0.48In0.52As (1.56 eV). A Schottky diode and a MESFET were fabricated on this material.     

Silicon Oxide enhanced Schottky gate In0.53Ga0.47As FET's with a self-aligned recessed gate structure

We present the fabrication and characterization of an In0.53Ga0.47As enhanced Schottky gate FET with a self-aligned recessed gate structure. A thin layer of e-beam evaporated silicon oxide was used to reduce the gate leakage current. For a n-channel doping of 8 × 10¹⁶cm^-3and a gate length of 1.5 µm, these devices showed good pinchoff characteristics with transconductances of 150 mS/mm. The effective velocity of electrons at current saturation is deduced to be 2.4 × 10⁷cm/s at the drain end of the gate. At 3 GHz these devices have a maximum available gain of 10 dB, decreasing to 6 dB at 6 GHz.     

Submicron gate GaAs/Al0.3Ga0.7AS MESFET's with extremely sharp interfaces (40 Å)

GaAs field-effect transistors (FET's) having a gatelength of 0.7 µm and an Al0.3Ga0.7As buffer layer were fabricated. The structures were grown by molecular beam epitaxy (MBE) in a substrate surface temperature range of 580-700°C. Samples grown at 700°C showed excellent pinchoff characteristics, while those grown at the lower end of the temperature spectrum exhibited degraded pinchoff characteristics. Compared to GaAs/GaAs, all of the structures, particularly those grown at 700°C, showed flatter saturation characteristics, especially for large drain voltages. The transconductance near the surface was about 160 mS/mm, regardless of the growth temperature. The saturation velocity of electrons in the channel layer was deduced to be about 1.6 × 10⁷cm/s, again, regardless of the growth temperature. The sharpness of the interface was very dependent on the growth temperature. Sharpnesses of 40, 100, and 550 Å were obtained in structures g own at 700, 640, and 580° C, respectwely. These figures compare with 300 Å obtained in channel layers with a GaAs buffer layer.     

Investigation of In0.53Ga0.47As for high-frequency microwave power FET's

Submicrometer In0.53Ga0.47As junction field-effect transistors (JFET's) were fabricated using a chemical etching technique. In spite of the well-known low bulk breakdown fields of InGaAs, the source-drain breakdown voltages of the FET's were close to 20 V under pinchoff conditions, indicating a potentially high power-handling capability. At 11 GHz, a 250-µm-wide FET showed a linear gain of 5.2 dB and 17.2-dB . m (53 mW) output power at 1-dB compression point, with a power-added efficiency of 14 percent. Problems of an unexpectedly low electron mobility in the channel, annealing of implanted Be, and oscillations in the drain current-voltage characteristic are discussed.     

A proposal for a high-speed In0.52Al0.48As/In0.53Ga0.47As MODFET with an (InAs)m(GaAs)msuperlattice channel

In this letter we examine theoretically the potential of an In0.52Al0.48As/In0.53Ga0.47As modulation-doped field-effect transistor in which the usual InGaAs channel is replaced by an (InAs)m(GaAs)msuperlattice with m ≲ 4, extending over 100 ∼ 200 Å from the InAlAs interface. For small m the superlattice bandstructure is essentially the same as that of the alloy and the effects of the very small lattice mismatch are negligible. More importantly, the electrons in the active channel are not expected to suffer any alloy scattering since the channel now has perfect long-range order with no random potential fluctuations. We show that this MODFET has extremely high mobility and its low-temperature mobility can be an order of magnitude higher than that of the conventional InAlAs/InGaAs MODFET. Comparison is also made with the AlGaAs/GaAs MODFET and results indicate that the proposed structure has superior potential performance.     

Doping effects and compositional grading in AlxGa1-xAs/GaAs heterojunction bipolar transistors

First-order analytical calculations were made for the energy-band diagrams for n-AlxGa1-xAs/p-GaAs heterojunctions for x = 0.15, 0.3, and 0.5 employing different compositional gradings and doping densities specifically for heterojunction-bipolar-transistor (HBT)applications. In the calculations most recently determined, conduction-band discontinuity ΔECof 65 percent of the bandgap difference ΔEgbetween the AlxGa1-xAs and GaAs, and the donor activation energies in n-AlxGa1-xAs of 60 and 160 meV for x = 0.3 and 0.5, respectively, were used. The results show that the position of the heterojunction spike barrier, and the depth and width of the notch in the conduction-band edge for a compositionally abrupt heterointerface depend on the respective doping densities on the p and n sides of the heterojunction. Also, for an abrupt heterointerface the difference in barrier heights for electron and hole injections varies between ΔEgand ΔEV(the valence-band discontinuity), depending on the doping densities and the applied bias, and is not necessarily the generally accepted value of ΔEV. Analytical expressions and curves were obtained to estimate the minimum compositional grading L for eliminating the spike barrier and the notch as a function of the doping densities and the applied bias.     

A new Ga0.47In0.53As field-effect transistor with a lattice-mismatched GaAs gate for high-speed circuits

A novel thin GaAs lattice-mismatched gate Ga0.47In0.53As field-effect transistor (LMG-FET) is reported. The device shows an extrinsic dc transconductance of 108 mS/mm for a 1.4-µm gate length and 240-µm gate width. Despite a 3.7-percent, lattice mismatch between GaAs and Ga0.47In0.53As, the LMG-FET shows stable operation even at 80°C (with a 13-percent increase in transconductances), exhibits negligible current drift, and suffers very little change in threshold voltages (<0.05 V) under illumination, This technology may find applications in high-speed lightwave transmission as well as high-speed digital circuits.     

Double heterojunction AlxGa1-xAs/GaAs bipolar transistors (DHBJT's) by MBE with a current gain of 1650

Double heterojunction AlGaAs/GaAs bipolar junction transistors (DHBJT's) grown by molecular beam epitaxy (MBE) were fabricated and tested. Devices with 0.2-µm and 0.1-µm base thicknesses exhibited common emitter current gains of up to 325 and 1650, respectively, in a wide range of collector currents. To obtain such high current gains, growth conditions had to be optimized and controlled. These high current gains, compared with the previous best value of 120 obtained in a MBE-grown transistor, make the HBJT's very promising for low-power high-speed logic application.     

Anomalous effects of lamp annealing in modulation-doped In0.53Ga0.47As/In0.52Al0.48As and Si-implanted In0.53Ga0.47As

The effects of pulsed halogen-lamp annealing on modulation-doped In0.53Ga0.47As/In0.52Al0.48As heterostructures and Si-implanted In0.53Ga0.47As have been studied to determine the suitabiiity of this process in the fabrication of high-performance field-effect transistors. Implantation and annealing of these materials are necessary for contact and self-aligned gate formation. Mobilities as high as 7400 cm²/ V . s are measured at 300 K in undoped molecular-beam epitaxy In-GaAs implanted with 8 × 10¹²cm^{-2 29}Si⁺and lamp annealed at 700°C for 5 s. Anomalous overactivations (up to 120 percent) are observed in these layers When silox encapsulation is used during annealing, but the effect is absent for GaAs proximity capping. Sharp decreases in sheet-electron concentration and mobility occur in the normal modulation-doped structures for annealing temperatures > 750°C, while this trend is much smaller in the inverted structures. Arsenic loss from the In-AlAs doping layer is attributed as the main mechanism for this behavior, which makes the inverted structure more suitable for device processing. Depth profiling in the modulation-doped structures indicates that there may be serious pinchoff problems in these devices when annealed at higher temperatures due to outdiffusion of impurities from the InP substrate. Values of interdiffusion coefficients at the InGaAs/ InAIAs heterointerfaces, being reported for the first time, are almost three orders higher than those measured in the GaAs/AIAs systems.     

Dispersion of the refractive index of GaAs and AlxGa1-xAs

The real part of the complex refractive index near the fundamental absorption edge is calculated for the ternary compound AlxGa1-xAs,0 leq x leq 0.3, as a function of frequency. An analytical expression fornis given which is derived from a quantum mechanical calculation of the dielectric constant of a semiconductor assuming the band structure of the Kane theory. The expression obtained is a function of bandgap energy, effective electron and heavy hole masses, the spin orbit splitting energy, the lattice constant, and the carrier concentration for n-type or p-type materials. The refractive index at the absorption edge is found as a function of the material parameters above. This enables one to express theoretical results in terms of basic material parameters only, with no adjustable constants. Comparison of theory with available experimental data is given for various reported values of the bandgap energy and effective masses as functions of mole fractionx.     

Design and performance of very high-speed In0.53Ga0.47As/In0.52Al0.48As p-i-n photodiodes grown by molecular beam epitaxy

High-speed In0.53Ga0.47As/In0.52Al0.48As photodiodes have been grown by molecular beam epitaxy (MBE) on semi-insulating InP substrates and fabricated. The measured impulse response characteristics are very close to the analytically calculated ones. The temporal response to pulsed optical excitation is characterized by a rise time of 21 ps and a width (FWHM) of 27 ps. The 25 × 20-µm²diodes have a junction capacitance <0.1 pF, a dark current ∼1 nA, and a peak responsivity of 0.35 A/W. These characteristics are comparable or better than most epitaxial InGaAs photodiodes reported to date and make the devices suitable for a host of high-speed applications and monolithic integration.     

Band offset effect on transport in AlxGa1-xAs/GaAs heterojunction bipolar transistors grown by metalorganic chemical vapor deposition

AlxGa1-xAs/GaAs heterojunction bipolar transistors were grown with x in the emitter from 0 to 0.57 and the band offset effect on electron transport has been studied using a new technique. The data, though preliminary, indicate that the transport is dominated by electrons near the conduction-band minima when x < 0.45, but not when x = 0.57.     

Low-resistance ohmic contacts to AlGaAs/GaAs and In0.52Al0.48As/In0.53Ga0.47As modulation-doped structures obtained by halogen lamp annealing

The successful application of short-term halogen lamp annealing to form ohmic contacts to AlGaAs/GaAs and In0.52Al0.48As/ In0.53Ga0.47As modulation-doped structures is demonstrated. Use of Ti in the electron-beam evaporated metallization scheme and a two-step annealing cycle give contacts with reproducibly good electrical and morphological characteristics. Minimum values of specific contact resistancerho_{c} = 4.0 times 10^{-7}and6.0 times 10^{-7}Ω.cm²for AlGaAs/GaAs and In0.52Al0.48As/In0.53Ga0.47As, respectively, are measured. Corresponding values of the transfer resistance Rcare 0.12 ± 0.02 and 0.18 ± 0.05 Ω.mm. These values are the lowest achieved with lamp annealing and are comparable to the best obtained with transient furnace annealing.     

Characteristics of an In0.53Ga0.47As very shallow junction gate structure grown by molecular beam epitaxy

We demonstrate an In0.53Ga0.47As very shallow (80 ∼ 700 Å) junction gate structure grown by molecular beam epitaxy (MBE). This structure allows the fabrication of submicron devices with simple processing steps. Study of the electrical characteristics suggests that this structure should be useful for both normally-on and normally-off field effect transistor (FET) applications. An FET structure utilizing this gate structure is also proposed.     

InxAl1-xAs/InP heterojunction insulated gate field effect transistors (HIGFET's)

Stability problems in conventional InP metal-insulator-semiconductor field effect transistors (MISFET's) have been overcome in InP heterojunction insulated gate FET's (HIGFET's) by replacing the insulator with InxAl1-xAs. We report on the fabrication and low-frequency operation of the HIGFET with a composition of x = 0.43. Transistor characteristics have been successfully modeled by an analytical MISFET model which indicate a low interfacial state density (≅ 10¹¹/cm²) and near flat-band condition.     

Admittance measurements on In0.53Ga0.47As/InP:Fe-JFET's and GaAs-MESFET's

Admittance measurements as a function of frequency and voltage on field-effect transistor (FET) channels of In0.53Ga0.47As/InP: Fe-JFET's and GaAs-MESFET's are reported. Theoretical and experimental investigations of the conductance and susceptance show that only three independent parameters, like space-charge capacitance, series resistance, and differential resistance, are needed to describe the measured results in the entire voltage and frequency range of investigation.