In0.53Ga0.47As FET's with insulator-assisted Schottky gates

Schottky-gate FET's have been fabricated on n-type In0.53Ga0.47As using a thin interfacial silicon nitride layer between the metal and the epitaxial layer to reduce the gate leakage current. In0.53Ga0.47As was grown by molecular beam epitaxy on semi-insulating InP substrates and silicon nitride was grown by plasma-enhanced chemical vapor deposition. Devices with 1.2µm gate length and net donor doping in the mid 10¹⁶cm^-3range show dc transconductance of up to 130mS/mm. Both depletion and enhancement mode operation were observed. The effective saturation velocity of electrons in the channel is deduced to be 2.0 ± 0.5 × 10⁷cm/sec, a value 60 to 70% higher than that in GaAs MESFET's. The insulator-assisted gate technology has many advantages in fabrication flexibility and control compared with other approaches to realizing high-speed microwave and logic in FET's in In0.53Ga0.47As.     

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.     

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.     

An In0.15Ga0.85As/GaAs pseudomorphic single quantum well HEMT

This letter describes high electron mobility transistors (HEMT's) utilizing a conducting channel which is a single In0.15Ga0.85AS quantum well grown pseudomorphically on a GaAs substrate. A Hall mobility of 40 000 cm²/V.s has been observed at 77 K. Shubnikov-de Haas oscillations have been observed at 4.2 K which verify the existence of a two-dimensional electron gas at the In0.15Ga0.85As/GaAs interface. HEMT's fabricated with 2-µm gate lengths show an extrinsic transconductance of 90 and 140 mS/mm at 300 and 77 K, respectively-significantly larger than that previously reported for strained-layer superlattice InxGa1-xAs structures which are nonpseudomorphic to GaAs substrates. HEMT's with 1-µm gate lengths have been fabricated, which show an extrinsic transconductance of 175 mS/mm at 300 K which is higher than previously reported values for both strained and unstrained InxGa1-xAs FET's. The absence of AlxGa1-xAs in these structures has eliminated both the persistent photoconductivity effect and drain current collapse at 77 K.     

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.     

Characterization of InGaAs/AlGaAs pseudomorphic modulation-doped field-effect transistors

High-performance pseudomorphic InyGa1-yAs/Al0.15- Ga0.85As (0.05 le y le 0.2) MODFET's grown by MBE have been characterized at dc (300 and 77 K) and RF frequencies. Transconductances as high as 310 and 380 mS/mm and drain currents as high as 290 and 310 mA/mm were obtained at 300 and 77 K, respectively, for 1-µm gate lengths and 3-µm source-drain spacing devices. Lack of persistent trapping effects,I-Vcollapse, and threshold voltage shifts observed with these devices are attributed to the use of low mole fraction AlxGa1-xAs while still maintaining 2DEG concentrations of about 1.3 × 10¹²cm^-2. Detailed microwave S-parameter measurements indicate a current gain cut-off frequency Of 24.5 GHz Wheny = 0.20, which is as much as 100 percent better than similar GaAs/AlGaAs MODFET structures, and a maximum frequency of oscillation of 40 GHz. These superior results are in part due to the higher electron velocity of InGaAs as compared with GaAs. Velocity field measurement performed up to 3 kV/cm using the magnetoresistance method indicates an electron saturation velocity of greater than 1.7 × 10⁷cm/s at 77 K fory = 0.15, which is 20 percent higher than GaAs/AlGaAs MODFET's of similar structure.     

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.     

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.     

An In0.52Al0.48As/n+-In0.53Ga0.47As MISFET with a heavily doped channel

An In0.52Al0.48As/n⁺-In0.53Ga0.47As MIS-type field-effect transistor (FET) with a channel doped at a 7 × 10¹⁷cm^-3level has been fabricated on an InP substrate. A device with a 2-µm channel length has yielded a maximum transconductance of 152 mS/mm,f_{T} = 12.4GHz, andf_{max} = 50GHz. At 10 GHz, the maximum available gain is 17.4 dB. The performance of this device shows that heavily doped channel FET's are very promising for high-frequency operation.     

Selectively doped heterostructure frequency dividers

The operation of high-speed divide-by-two circuit (binary counter) composed of selectively doped heterostructure logic gates is reported for the first time. These field-effect transistor circuits utilize the enhanced transport properties of high-mobility electrons confined near a heterojunction interface in a selectively doped AlGaAs/GaAs structure. The dividers are based on a Type-D flip-flop composed of six direct-coupled NOR-gates having 1-µm gate lengths and 4-µm source-drain spacings. They are fabricated by conventional optical contact lithography on a four-layer Al.3Ga.7As/GaAs structure grown by molecular-beam epitaxy. Successful operation is demonstrated at 5.9 GHz at 77 K for 1.3-V bias and 30-mW total power dissipation (including output buffers) and 3.7 GHz at 300 K for 1.4-V bias and 19-mW total power dissipation. Total power dissipation values as low as 3.9 mW at 0.65-V bias were also obtained for 2.85-GHz operation at 300 K. These preliminary results illustrate the promise of SDHT logic for ultrahigh-speed low-power applications.     

High transconductance InGaAs/AlGaAs pseudomorphic modulation-doped field-effect transistors

Pseudomorphic In0.15Ga0.85As/Al0.15Ga0.85As modulation-doped field effect transistors (MODFET's) exhibiting extremely good dc characteristics have been successfully fabricated, dc transconductance in these strained-layer structures of 270 mS/mm were measured for 1-µm gate, normally-on devices at 300 K. Maximum drain current levels are 290 mA/mm, with excellent pinch-off and saturation characteristics. The transconductance increased to 360 mS/mm at 77 K while no persistent photoconductivity or drain collapse was observed. Preliminary microwave results indicate a 300-K current gain cutoff frequency of about 20 GHz. These results are equivalent to the best GaAs/AlGaAs MODFET results and are due in part to the improved transport properties and carrier confinement in the InGaAs quantum well.     

High-speed low-voltage ring oscillators based on selectively doped heterojunction transistors

We report ring oscillators which attain high speed at low supply voltage, and thus low power. Selectively doped heterojunction transistors (SDHT's) were used in a direct-coupled circuit. The GaAs/ Al.3Ga.7As heterostructure, grown by MBE, was designed to allow self-limiting etch of the gate recesses in the driver transistors. Devices with 1 µm gates operated at supply voltages as low as 0.23 V at T = 300 K. At 77 K, gate delays as low as 14.7 ps were observed at 1.0-V bias.     

Properties of selectively doped heterostructure transistors incorporating a superlattice donor layer

We report on some salient features of an improved structure of selectively doped heterostructure transistor (SDHT) incorporating a short-period (30 Å) Al0.6Ga0.4As/n-GaAs superlattice donor layer. We show that this superlattice-SDHT (S²DHT) structure is a good candidate for both low-temperature packaged operation and room temperature applications. In addition to eliminating drain I-V distortion at low temperature, the device shows a threshold voltage shift from 300 K to 77 K of only ∼50 mV. The device also has high transconductance (∼250 mS/mm for 1-µm gate lengths at room temperature), larger voltage swing, and higher current driving capability than conventional SDHT's.     

A low-noise microwave HEMT using MOCVD

Low-noise high-electron-mobility Transistors (HEMT's) with AlGaAs/GaAs heterostructures have been successfully fabricated using normal pressure metal-organic chemical vapor deposition (MOCVD). Hall mobilities of the two-dimensional electron gas at the interface are 8030 and 14 8000 cm²/V . s at 300 and 77 K, respectively, with an undoped Al0.3Ga0.7As spacer layer of 100 Å. The HEMT's with 0.65-µm-long and 200-µm-wide gates have exhibited a noise figure of 1.13 dB with 10.8 dB of associated gain at 12 GHz, and a dc transconductance of 280 mS/mm. These values are comparable to other reported HEMT devices using molecular-beam epitaxy (MBE).     

Current—voltage characteristics of an AlGaAs/GaAs heterostructure FET for high gate voltages

Drain-to-source current for high gate voltages in AlGaAs/GaAs heterostructure FET's (HFET's) is found to depend on the electron saturation velocity in the AlGaAs layer. A simple model, which takes into account the current through the undepleted channel in the AlGaAs layer as a function of the electron saturation velocity in AlGaAs, is proposed for describing I-V characteristics of HFET's for high gate voltages. Using the model, effective electron saturation velocity in AlxGa1-xAs for different Al content levels has been obtained from the analysis of the present experimental results; 7 × 10⁶cm/s for x = 0.24 and 3 × 10⁶cm/s for x = 0.3 at a 4 × 10¹⁷cm^-3doping concentration.     

GaAs LSI-directed MESFET's with self-aligned implantation for n+-layer technology (SAINT)

Self-aligned implantation for n⁺-layer technology (SAINT) has been developed for improvement in normally-off GaAs MESFET's to be used in LSI's. This technology has made it possible to arbitrarily control the spacing between the n⁺-implanted layer and gate contact by a dielectric lift-off process utilizing a multilayer resist with an undercut wall shape. SAINT FET's with a 1-µm gate length have above 200 mS/ mm transconductance in the normally-off region. The K value along the square-lawI - Vfitting has been improved by a factor of 3.4, compared to conventional FET's without the n⁺-layer. Thermal emission for carriers from the source n⁺-layer in the subthreshold region has been experimentally formulated. Threshold-voltage shift due to gate shortening for [011] gate FET's is definitely smaller than that for [011] gate FET's. The threshold-voltage standard deviations for [011] gate FET's with 2- and 1-µm gate lengths, obtained from a 6-mm × 9-mm area, are 9 and 34 mV, respectively. An E/D direct-coupled FET logics (DCFL) 15-stage ring oscillator with a 1-µm gate length shows a high switching speed of 45 ps/gate at a low supply voltage of 0.91 V.     

Characterization of deep levels in modulation-doped AlGaAs/GaAs FET's

Deep levels in modulation-doped field-effect transistors (MODFET's) fabricated from MBE-grown AlGaAs/GaAs heterostructures, have been characterized by a modified deep-level transient spectroscopy (DLTS) technique. Assuming donor-like traps in the AlGaAs layer, it is shown that the threshold voltage Vtvaries exponentially with time under pulsed-biased conditions. This result is verified experimentally by observing the transient in the drain current IDin long-gate FET's biased in saturation. The resulting Δ √{I_{D}} DLTS spectrum reveals an electron trap with an activation energy of 0.472 eV in Si-doped Al0.3Ga0.7As.     

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.     

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.     

Low-noise In0.53Ga0.47As:Fe photoconductive detectors for optical communication

Photoconductive detectors were fabricated on In0.53- Ga0.47As/InP made high-resistive by doping with elemental Fe. A mobility of ∼ 6000 cm²/V . s and a net electron concentration of (2-5) × 10¹²cm^-3were measured in layers on which the devices were fabricated. Photoconductive gains of 5-10 were measured with CW and pulsed excitation. The calculation response time to 300-ps pulsed excitation is ∼ 100 ps and can be improved with reduced channel spacings. Typical dark currents in the devices are of the order of 50 µ A at room temperature. The noise power into a 50-Ω load measured at 82°C is -108.9 dBm. This is the lowest value measured in photoconductive detectors made with III-V In0.53Ga0.47As.