Compound semiconductors for low-noise microwave MESFET applications

In order to determine the low-noise potential of microwave MESFET's fabricated from materials other than GaAs, a one-dimensional FET model is employed. From material parameters and device geometry the model enables the calculation of a small-signal equivalent circuit from which performance information is acquired. Material parameters, as predicted from Monte Carlo calculations, are used to simulate 1-µm devices fabricated from GaAs as well as InP, Ga0.47In0.53As, InP0.8As0.2, Ga0.27In0.73P0.4As0.6, and Ga0.5In0.5As0.96Sb0.04. Results obtained from simulations comparing a Ga0.5In0.5As0.96- Sb0.04device to an equivalent GaAs device indicate that a decrease in minimum noise figure of almost a factor of two is possible. Considerable improvement in noise performance over a GaAs device is also predicted for devices fabricated from Ga0.47In0.53As and Ga0.27In0.73P0.4As0.6. In addition, the quaternary and ternary devices as well as the InP device should exhibit superior gain and high-frequency performance compared to GaAs devices.     

Improvement of photoluminescence of molecular beam epitaxially grown GaxAlyIn1-x-yAs by using an As2molecular beam

We report the use of an As2(instead of As4) beam during molecular beam epitaxial (MBE) growth of GaxAlyIn1-x-yAs layers, lattice-matched to InP substrate. We also show that use of the As2beam improves the room-temperature photoluminescence intensities of both In0.53Ga0.47As and Ga0.39Al0.08In0.53As epilayers with more significant improvement for the latter. The substrate temperature employed was ∼ 580°C for both.     

Ga0.4In0.6As/Al0.55In0.45As pseudomorphic modulation-doped field-effect transistors

High-performance pseudomorphic Ga0.4In0.6As/ Al0.55In0.45As modulation-doped field-effect transistors (MODFET's) grown by MBE on InP have been fabricated and characterized. DC transconductances as high as 271, 227, and 197 mS/mm were obtained at 300K for 1.6-µm and 2.9-µm gate-length enhancement-mode and 2-µm depletion-mode devices, respectively. An average electron velocity as high as 2.36 × 10⁷cm/s has been inferred for the 1.6-µm devices, which is higher than previously reported values for 1-µm gate-length Ga0.47In0.53As/Al0.48In0.52As MODFET's. The higher bandgap Al0.55In0.45As pseudomorphic barrier also offers the advantages of a larger conduction-band discontinuity and a higher Schottky barrier height.     

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 heterostructure Ga0.47In0.53As MESFETs by MBE

Ga0.47In0.53As MESFETs have been fabricated on InP substrates. The low barrier height of Ga0.47In0.53As (0.20 eV) which makes simple GaInAs MESFETs at this composition impractical, has been overcome by using thin Al0.48In0.52As layers between gate metal and GaInAs active layers. Al0.48In0.52As has also been exploited in the form of buffer layers. The double heterostructure FET wafers with single crystal Al gate metal were grown by molecular beam epitaxy (MBE). The 2.75 µm gate length MESFETs showed d.c. transconductance gm= 57 mS mm^-1in spite of nonoptimized dimensions.     

High-gain Al0.48In0.52As/Ga0.53As vertical n-p-n heterojunction bipolar transistors grown by molecular-beam epitaxy

We report the first vertical n-p-n heterojunction bipolar transistors formed in the (Al,In)As/(Ga,In)As alloy system. The structures grown by molecular-beam epitaxy (MBE) use a wide band-gap (Eg = 1.44 eV) Al0.48In0.52As emitter on a lower band gap (Eg = 0.73 eV) Ga0.47In0.53As base 2500 Å in width. Transistors with both abrupt and graded heterojunction emitters were demonstrated with dc current gains of 140 and 280, respectively, at a collector current of 15 mA. The (Al,In)As/(Ga,In)As heterojunction transistors offer the attractive possibility of optical integration with long wavelength lasers and photodetectors.     

Ga0.47In0.53As vertical photoconductive detectors with high gain and low bias voltage

We demonstrate for the first time a long-wavelength Ga0.47In0.53As vertical photoconductive detector with very high gain, low noise, low-bias voltages, high sensitivity, and high-coupling efficiency. The detector consists of an n+InP, a Fe-doped Ga0.47In0.53As, and an n+Ga0.47In0.53As layer grown successively on a semi-insulating InP substrate. The highly resistive active layer sandwiched between two n+layers creates a uniform electric field perpendicular to the surface, producing a dc gain of 86 at bias voltages as low as 0.5 V. The noise power at 100 MHz is about 11 dB lower than that of a coplanar interdigitated photoconductive detector prepared with undoped GaInAs grown by vapor-phase epitaxy. Preliminary measurements reveal a receiver sensitivity of -28.2 dBm at a bit-error rate of 10^-9at 420 Mbit/s and a wavelength of 1.55 µm.     

A Ga0.47In0.53As/InP heterophotodiode with reduced dark current

We have used a Ga0.47In0.53As/InP heterostructure to produce a photodiode (area =3 times 10^{-4}cm²) which shows a saturated dark current of 100 pA at 23°C and 1.7 nA at 50°C. At this dark current, these photodiodes have near-unity quantum efficiency at 1.6 μm and show good photoresponse over the1.0-1.65 mum region of the optical spectrum.     

Depletion mode modulation doped Al0.48In0.52As-Ga0.47In0.53As heterojunction field effect transistors

We report the first demonstration of a depletion mode modulation doped Ga0.47In0.53As field effect transistor. This transistor combines the advantage of modulation doping and the superior material characteristics of Ga0.47In0.53As. DC transconductances of 31 mmho/ mm at 300 K and 69 mmho/mm at 77 K have been measured for a device with 5.2µm gate length and 340 µm gate width. An enhanced drift mobility is responsible for 88 percent of the improvement in the transconductance at 77 K and the remaining 12 percent is attributed to an improved ohmic contact. A high performance modulation doped Ga0.47In0.53As FET is expected to play an important role in very high speed digital and analog applications.     

Inversion-mode insulated gate Ga0.47In0.53As field-effect transistors

It is demonstrated that inversion-mode n-channel insulated gate field-effect transistors can be made of p-type heteroepitaxially-grown Ga0.47In0.53As layers on semi-insulating InP.     

The p-n junction quantum well APD: A new solid-state photodetector for lightwave communications systems and on-chip detector applications

A novel variation on the doped quantum well avalanche photodiode is presented that provides comparable signal-to-noise performance at more realizable material doping requirements. The device consists of repeated unit cells formed from a p-n Al0.48In0.52As junction immediately followed by near-intrinsic Ga0.47In0.53As and Al0.48In0.52As layers. As in the doped quantum well device, the asymmetric unit cell selectively heats the electron distribution much more than the hole distribution prior to injection into the narrow-gap Ga0.47In0.53As layer in which impact ionization readily occurs. The effects of various device parameters, such as the junction doping, Ga0.47In0.53As and intrinsic Al0.48In0.52As layer widths as well as the overall bias on the electron and hole ionization rates, is analyzed using an ensemble Monte Carlo method. From the determination of the ionization rates and the ionization probabilities per stage, P and Q, an optimal device design can be obtained that provides high gain at low multiplication noise. In addition, a structure that operates at less than 5 V bias is presented that can provide moderate gain at very low noise. It is expected that the device designs presented herein can serve either as high-gain low-noise detectors for lightwave communications systems or as moderate-gain low-noise detectors for on-chip application.     

Ga0.47In0.53As: A ternary semiconductor for photodetector applications

Ga0.47In0.53As has been used to make fast (rt < 1 ns), photodiodes with low dark current (i_{D} < 10^{-8}A) and good quantum efficiency (ηQext > 50 percent over the entire1.0-1.7 mum region of the optical spectrum). The physical properties related to the crystal growth and carrier transport are discussed in this paper in terms of both the design and the operating characteristics of detectors fabricated from this ternary alloy. The results of our work show that Ga0.47In0.53As is a material well-suited to several important semiconductor device applications. A comparison to other semiconductor photodiodes shows that Ga0.47In0.53As is one of the most sensitive detectors available in the1.0-1.7 mum wavelength region. One can expect repeater-free transmission in excess of 150 km at 100 Mbits . s^-1using these detectors in a digital optical fiber link at the 1.55 μm low-loss (alpha < 0.3dB . km^-1) low-dispersion transmission window.     

Short channel Ga0.47In0.53As/Al0.48In0.52As selectively doped field effect transistors

We report a selectively doped Ga0.47In0.53As/Al0.48In0.52As field effect transistor with a 1.2 µm gate length and present a model of two-region operation to analyze its I-V characteristics. This depletion mode transistor shows complete pinch-off and saturation characteristics with a low frequency transconductance of 70 mmho/ mm at 300 K and 125 mmho/mm at 77 K. The theoretical model, which includes the background carriers in the undoped Ga0.47In0.52As layer, agrees with the experimental results.     

Double heterostructure Ga0.47In0.53As MESFETs with submicron gates

MESFETs with GA0.47In0.53As active channel grown by MBE on InP substrates were successfully fabricated. Thin layers of MBE grown Al0.48In0.52As seperated both the single crystal aluminum gate from the active channel and the active channel from the InP substrate so raising the Schottky barrier height of the gate and confining the electrons to the channel. The MESFETs with 0.6µm long gates and gate-to-source separations of 0.8 um exhibited an average gmof 135 mS mm^-1of gate width for Vds= 2V and Vg= 0. This is higher than that reported for GaAs MESFETs with a similar geometry in spite of the intermediate layer between the gate metal and the active layer.     

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.     

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.     

Integrated double heterostructure Ga0.47In0.53As photoreceiver with automatic gain control

The first operation of an integrated differential notch-type photoconductor and dual gate (DG) double heterostructure (DH) MESFET in Ga0.47In0.53As is reported. The starting material was grown by molecular beam epitaxy on a semi-insulating InP substrate. A 2 mW HeNe laser with a spot diameter Of 0.5 mm could modulate the drain current by 300 µA with the upper gate suitably biased.     

Semiconductor-gated InGaAs/InAlAs heterostructure transistors (SISFET's)

We have successfully fabricated FET's with In0.53Ga0.47As channels, lattice-matched In0.52Al0.48As gate barriers, and n+ In0.53- Ga0.47As gates. For a barrier thickness of 600 Å and a gate length of 1.7 µm, the maximum transconductance is 250 mS/mm at T = 300 K. From gate capacitance measurements, the cutoff frequency is inferred to be ft= 15 GHz for this gate length. Self-aligned source and drain implants have been used to permit nonalloyed ohmic contacts with a characteristic resistance of 0.1 Ω.mm. The transconductance remains above 210 mS/mm for forward gate bias up to +1.0 V, confirming the usefulness of this gate structure for enhancement-mode devices.     

The p-n heterojunction quantum well APD: A new high-gain low-noise high speed photodetector suitable for lightwave communications and digital applications

A new Ga0.47In0.53As/Al0.48In0.52As multiquantum well avalanche photodiode, the APD, is presented that provides comparable signal-to-noise performance compared to either the doped quantum well APD or the p-n junction quantum well APD, but without carrier trapping effects even at very low overall applied electric fields. The device is made of repeated unit cells consisting of a p-n junction formed between two dissimilar materials followed by a nearly intrinsic wide-bandgap layer. As in the doped quantum well device, the asymmetric unit cell selectively heats the electron distribution much more than the hole distribution within the narrow-gap Ga0.47In0.53As layer leading to a greatly enhanced electron-to-hole ionization rates ratio. The most significant improvement over the doped and p-n junction quantum well devices is the lack of carrier trapping at the heterojunction without further engineering of the interface (compositional grading). Carrier trapping is avoided, thereby providing very high-speed performance even for low-voltage devices, by doping the narrow-gap layer. The resulting built-in field within the GaInAs layer is sufficiently large of itself that both electrons and holes are heated to energies large enough to overcome the potential barrier at the end of the quantum well. In this way, devices operating at 5 V bias can be built that will provide a gain of about 4 at large bandwidths, ~18 GHz.     

Electron and hole impact ionization rates in InP/Ga0.47In0.53As superlattice

The electron and hole impact ionization rates, α andβ, in an InP/Ga0.47In0.53As superlattice have been experimentally determined from photomultiplication data made on an InP/GaInAs superlattice photodiode. A Monte Carlo simulation of α and β in the superlattice has been developed, and the enhancement of impact ionization due to the effect of the band edge discontinuity has been investigated. The experimental ionization rates have been analyzed by the simulation. The larger β than α in the superlattice has been shown to be explained by the valence band discontinuity about two times larger than the conduction band edge discontinuity in this superlattice structure.