A study of high-speed normally off and normally on Al0.5Ga0.5As heterojunction gate GaAs FET's (HJFET)

The dc, small-signal microwave, and large-signal switching performance of normally off and normally on Al0.5Ga0.5As gate heterojunction GaAs field-effect transistors (HJFET) with submicrometer gate lengths are reported. The structure of both types of devices comprises an n-type 10¹⁷-cm^-3Sn-doped active layer on a Cr-doped GaAs substrate, a p-type 10¹⁸-cm^-3Ge-doped Al0.5Ga0.5As gate layer and a p⁺-type 5 × 10¹⁸-cm^-3Ge-doped GaAs "contact and cap" layer on the top of the gate. The gate structure is obtained by selectively etching the p⁺-type GaAs and Al0.5Ga0.5As. Undercutting of the Al0.5Ga0.5As layer results in submicrometer gate lengths, and the resulting p⁺-GaAs overhang is used to self-align the source and the drain with respect to the gate. Normally off GaAs FET's with 0.5- to 0.7-µm long heterojunction gates exhibit maximum available power gains (MAG) of about 9 dB at 2 GHz. Large-signal pulse measurements indicate an intrinsic propagation delay of 40 ps with an arbitrarily chosen 100-Ω drain load resistance in a 50-Ω microstrip circuit. Normally on FET's with submicrometer gate lengths (∼0.6 µm) having a total gate periphery of 300 µm and a corresponding dc transconductance of 20-30 mmhos exhibit a MAG of 9.5 dB at 8 GHz. The internal propagation delay time measured under the same conditions as above is about 20 ps.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Fabrication and characterization of a depletion-mode ZnS0.07Se0.93 MESFET

We report the fabrication and characterization of a depletion-mode n-channel ZnS_0.07Se_0.93 metal-semiconductor field effect transistor (MESFET). A ZnSSe FET could be a key element in opto-electronic integration consisting of light emitters, light receivers and MESFET pre-amplifiers. Mesa isolation, recess etching and self-alignment techniques were adopted to optimize the MESFET performance. Source and drain (S/D) ohmic contacts and gate Schottky contact were formed by Cr/In/Cr and Au deposition, respectively. Depletion mode FET's with varying gate width-to-length ratio of W/L=200 μm/20 μm, 200 μm/4 μm and 200 μm/2 μm were fabricated. A 2 μm FET was characterized as follows: the turn-on voltage, V_on≈1.75 V, the pinch-off voltage, V_p≈-13 V, the unit transconductance, g_m≈8.73 mS/mm, and the breakdown voltage with zero gate-source bias, BV≈28 V     

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.     

Schottky barriers and ohmic contacts on n-type InP based compound semiconductors for microwave FET's

InP, and In0.73Ga0.27As0.6P0.4, and In0.53Ga0.47As lattice matched to InP are of special importance as active FET channel materials because of the high electron velocity and/or high electron mobility they offer. Using a AuGe/Ni/Au metallization system, specific contact resistances of 5 × 10^-7Ω . cm², 8 × 10^-7Ω . cm², and 5.8 × 10^-6Ω cm²were obtained for ohmic contacts on In0.53Ga0.47As, InP, and In0.89Ga0.11As0.24P0.76, respectively. Leakage currents of 10 µA at 7-V reverse bias were observed for 1 × 200-µm gates on InP. and In0.89Ga0.11As0.24P0.76FET's having a SiO2film about 50 Å thick under the gate. A thin SiO2layer underneath the gate improved the Schottky-gate I-V characteristics, but thick oxides severely degraded the microwave performance of the FET's. These excellent ohmic contacts and Schottky barriers resulted in a maximum insertion gain of 15 dB at 8 GHz and a noise figure of 2.5 dB with 8-dB gain at 7 GHz for the InP deviees. For 1.15-eV InxGa1-xAsyP1-yFET's, the resulting gain was 9 dB at 8 GHz.     

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.     

A self-aligned In0.53Ga0.47As junction field-effect transistor grown by molecular beam epitaxy

An In0.53Ga0.47As field-effect transistor has been fabricated on MBE-grown material, using a novel self-alignment technique. This device has a dc transconductance of 60 mS/mm for a 3-µm gate length, one of the highest reported figures for such a length, and a very low gate leakage of 100 nA at -3-V gate bias.     

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.     

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.