GaAs MOSFET with oxide gate dielectric grown by atomic layer deposition

For the first time, a III-V compound semiconductor MOSFET with the gate dielectric grown by atomic layer deposition (ALD) is demonstrated. The novel application of the ALD process on III-V compound semiconductors affords tremendous functionality and opportunity by enabling the formation of high-quality gate oxides and passivation layers on III-V compound semiconductor devices. A 0.65-/spl mu/m gate-length depletion-mode n-channel GaAs MOSFET with an Al/sub 2/O/sub 3/ gate oxide thickness of 160 /spl Aring/ shows a gate leakage current density less than 10/sup -4/ A/cm/sup 2/ and a maximum transconductance of 130 mS/mm, with negligible drain current drift and hysteresis. A short-circuit current-gain cut-off frequency f/sub T/ of 14.0 GHz and a maximum oscillation frequency f/sub max/ of 25.2 GHz have been achieved from a 0.65-/spl mu/m gate-length device.     

Unpassivated AlGaN-GaN HEMTs with minimal RF dispersion grown by plasma-assisted MBE on semi-insulating 6H-SiC substrates

High electron mobility transistors (HEMTs) are fabricated from AlGaN-GaN heterostructures grown by plasma-assisted molecular beam epitaxy (MBE) on semi-insulating 6H-SiC substrates. At a sheet charge density of 1.3 /spl times/ 10/sup 13/ cm/sup -2/, we have repeatedly obtained electron mobilities in excess of 1350 cm/sup 2//Vs. HEMT devices with a gate length of 1/spl mu/m, a gate width of 200 /spl mu/m, and a source-drain spacing of 5 /spl mu/m show a maximum drain current of 1.1 A/mm and a peak transconductance of 125 mS/mm. For unpassivated HEMTs, we measured a saturated power output of 8.2-W/mm continuous wave (cw) at 2 GHz with an associated gain of 11.2 dB and a power-added efficiency of 41%. The achievement of high-power operation without a surface passivation layer suggests that free surface may not be the dominant source of radio-frequency (RF) dispersion in these MBE-grown structures. This data may help discriminate between possible physical mechanisms of RF dispersion in AlGaN-GaN HEMTs grown by different techniques.     

Gate length electric parameter dependences of ultra-submicrometre delta -doped pseudomorphic HEMTs

AlGaAs/InGaAs/GaAs MODFETs having 20% indium in the channel and Si planar doping (5*10/sup 12/ cm/sup -2/) have been fabricated with gate lengths of 0.1-0.7 mu m and a width of 100 mu m. Gates that are longer than 0.2 mu m are T-shaped and the narrower gates (0.1 and 0.15 mu m) are triangular. From DC measurement a maximum G/sub m/ of 1100 mS/mm has been obtained. The current gain cutoff frequency F/sub t/ corrected for the access resistances is 145 GHz, corresponding to an intrinsic transition frequency of 220 GHz.<>     

A novel InGaP/InGaAs/GaAs double /spl delta/-doped pHEMT with camel-like gate structure

The author reports a novel InGaP/InGaAs/GaAs double delta-doped pseudomorphic high-electron mobility transistor (pHEMT) with n/sup +/-GaAs/p/sup +/-InGaP/n-InGaP camel-like gate structure grown by MOCVD. Due to the p-n depletion from the p/sup +/-InGaP gate to the channel region and the presence of /spl Delta/Ec at the InGaP/InGaAs heterostructure, the turn-on voltage of gate is larger than 1.7 V. For a 1/spl times/100-/spl mu/m/sup 2/ device, the experimental results show an extrinsic transconductance of 107 mS/mm and a saturation current density of 850 mA/mm. Significantly, an extremely broad gate voltage swing larger than 6 V with above 80% maximum g/sub m/ is obtained. Furthermore, the unit current cut-off frequency f/sub T/ and maximum oscillation frequency are up to 20 and 32 GHz, respectively. The excellent device performance provides a promise for linear and large signal amplifiers and high-frequency circuit applications.     

Submicron AlGaN/GaN HEMTs with very high drain current density grown by plasma-assisted MBE on 6H-SiC

High electron mobility transistors (HEMTs) were fabricated from AlGaN/-GaN layers grown by plasma-assisted molecular beam epitaxy on semi-insulating 6H-SiC substrates. Room-temperature Hall effect measurements yielded a polarization-induced 2DEG sheet charge of 1.3/spl middot/10/sup 13/ cm/sup -2/ and a low-field mobility of 1300 cm/sup 2//V/spl middot/s. Submicron gates were defined with electron beam lithography using an optimized two-layer resist scheme. HEMT devices repeatedly yielded drain current densities up to 1798 mA/mm, and a maximum transconductance of 193 mS/mm. This is the highest drain current density in any AlGaN-GaN HEMT structure delivering significant microwave power reported thus far. Small-signal testing of 50-/spl mu/m wide devices revealed a current gain cutoff frequency f/sub T/ of 52 GHz, and a maximum frequency of oscillation f/sub max/ of 109 GHz. Output power densities of 5 W/mm at 2 GHz, and 4.9 W/mm at 7 GHz were recorded from 200-/spl mu/m wide unpassivated HEMTs with a load-pull setup under optimum matching conditions in class A device operation.     

Influences of sulfur passivation on temperature-dependent characteristics of an AlGaAs/InGaAs/GaAs PHEMT

The influences of (NH/sub 4/)/sub 2/S/sub x/ treatment on an AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor (PHEMT) are studied and demonstrated. Upon the sulfur passivation, the studied device exhibits better temperature-dependent dc and microwave characteristics. Experimentally, for a 1/spl times/100 /spl mu/m/sup 2/ gate/dimension PHEMT with sulfur passivation, the higher gate/drain breakdown voltage of 36.4 (21.5) V, higher turn-on voltage of 0.994 (0.69) V, lower gate leakage current of 0.6 (571) /spl mu/A/mm at V/sub GD/=-22 V, improved threshold voltage of -1.62 (-1.71) V, higher maximum transconductance of 240 (211) mS/mm with 348 (242) mA/mm broad operating regime (>0.9g/sub m,max/), and lower output conductance of 0.51 (0.53) mS/mm are obtained, respectively, at 300 (510) K. The corresponding unity current gain cutoff frequency f/sub T/ (maximum oscillation frequency f/sub max/) are 22.2 (87.9) and 19.5 (59.3) GHz at 250 and 400 K, respectively, with considerably broad operating regimes (>0.8f/sub T/,f/sub max/) larger than 455 mA/mm. Moreover, the relatively lower variations of device performances over wide temperature range (300/spl sim/510 K) are observed.     

Microwave performance of GaAs MOSFET with wet thermally oxidized InAlP gate dielectric

This paper reports the first demonstration of a microwave-frequency operation of a GaAs MOSFET fabricated using a wet thermal oxidization of InAlP lattice-matched to GaAs to form a native-oxide gate insulator. Devices with 1-/spl mu/m gate lengths exhibit a cutoff frequency (f/sub t/) of 13.7 GHz and a maximum frequency of oscillation (f/sub max/) of 37.6 GHz, as well as a peak extrinsic transconductance of 73.6 mS/mm. A low-leakage current density of 3.8/spl times/10/sup -3/ A/cm/sup 2/ at 1-V bias for an MOS capacitor demonstrates the good insulating properties of the /spl sim/ 11-nm thick native gate oxide.     

GaAs MOSFET using InAlP native oxide as gate dielectric

GaAs metal-oxide-semiconductor field-effect transistors (MOSFETs) using wet thermally oxidized InAlP as the gate insulator are reported for the first time. Leakage current measurements show that the 11-nm-thick native oxide grown from an In/sub 0.49/Al/sub 0.51/P layer lattice-matched to GaAs has good insulating properties, with a measured leakage current density of 1.39/spl times/10/sup -7/ mA//spl mu/m/sup 2/ at 1 V bias. GaAs MOSFETs with InAlP native gate oxide have been fabricated with gate lengths from 7 to 2 /spl mu/m. Devices with 2-/spl mu/m-long gates exhibit a peak extrinsic transconductance of 24.2 mS/mm, an intrinsic transconductance of 63.8 mS/mm, a threshold voltage of 0.15 V, and an off-state gate-drain breakdown voltage of 21.2 V. Numerical Poisson's equation solutions provide close agreement with the measured sheet resistance and threshold voltage.     

High-voltage normally off GaN MOSFETs on sapphire substrates

Gallium nitride self-aligned MOSFETs were fabricated using low-pressure chemical vapor-deposited silicon dioxide as the gate dielectric and polysilicon as the gate material. Silicon was implanted into an unintentionally doped GaN layer using the polysilicon gate to define the source and drain regions, with implant activation at 1100/spl deg/C for 5 min in nitrogen. The GaN MOSFETs have a low gate leakage current of less than 50 pA for circular devices with W/L=800/128 /spl mu/m. Devices are normally off with a threshold voltage of +2.7 V and a field-effect mobility of 45 cm/sup 2//Vs at room temperature. The minimum on-resistance measured is 1.9 m/spl Omega//spl middot/cm/sup 2/ with a gate voltage of 34 V (W/L=800/2 /spl mu/m). High-voltage lateral devices had a breakdown voltage of 700 V with gate-drain spacing of 9 /spl mu/m (80 V//spl mu/m), showing the feasibility of self-aligned GaN MOSFETs for high-voltage integrated circuits.     

Electrical characteristics of 8-/spl Aring/ EOT HfO/sub 2//TaN low thermal-budget n-channel FETs with solid-phase epitaxially regrown junctions

The authors demonstrate high-performing n-channel transistors with a HfO/sub 2//TaN gate stack and a low thermal-budget process using solid-phase epitaxial regrowth of the source and drain junctions. The thinnest devices have an equivalent oxide thickness (EOT) of 8 /spl Aring/, a leakage current of 1.5 A/cm/sup 2/ at V/sub G/=1 V, a peak mobility of 190 cm/sup 2//V/spl middot/s, and a drive-current of 815 /spl mu/A//spl mu/m at an off-state current of 0.1 /spl mu/A//spl mu/m for V/sub DD/=1.2 V. Identical gate stacks processed with a 1000-/spl deg/C spike anneal have a higher peak mobility at 275 cm/sup 2//V/spl middot/s, but a 5-/spl Aring/ higher EOT and a reduced drive current at 610 /spl mu/A//spl mu/m. The observed performance improvement for the low thermal-budget devices is shown to be mostly related to the lower EOT. The time-to-breakdown measurements indicate a maximum operating voltage of 1.6 V (1.2 V at 125 /spl deg/C) for a ten-year lifetime, whereas positive-bias temperature-instability measurements indicate a sufficient lifetime for operating voltages below 0.75 V.     

Optimization of AuGe-Ni-Au ohmic contacts for GaAs MOSFETs

GaAs-based metal-oxide-semiconductor field-effect transistors (MOSFETs) are promising devices for high-speed and high-power applications. One important factor influencing the performance of a GaAs MOSFET is the characteristics of ohmic contacts at the drain and source terminals. In this paper, AuGe-Ni-Au metal contacts fabricated on a thin (930 /spl Aring/) and lightly doped (4/spl times/10/sup 17/ cm/sup -3/) n-type GaAs MOSFET channel layer were studied. The effects of controllable processing factors such as the AuGe thickness, the Ni/AuGe thickness ratio, alloy temperature, and alloy time to the characteristics of the ohmic contacts were analyzed. Contact qualities including specific contact resistance, contact uniformity, and surface morphology were optimized by controlling these processing factors. Using the optimized process conditions, a specific contact resistance of 5.6/spl times/10/sup -6/ /spl Omega//spl middot/cm/sup 2/ was achieved. The deviation of contact resistance and surface roughness were improved to 1.5% and 84 /spl Aring/, respectively. Using the improved ohmic contacts, high-performance GaAs MOSFETs (2 /spl mu/m/spl times/100 /spl mu/m) with a large drain current density (350 mA/mm) and a high transconductance (90 mS/mm) were fabricated.     

Surface passivation of InGaP/InGaAs/GaAs pseudomorphic HEMTs with ultrathin GaS film

We report on the successful surface passivation of wide recess InGaP/InGaAs/GaAs pseudomorphic HEMTs with MBE-grown ultrathin GaS film (2 nm) employing a single precursor, tertiarybutyl-galliumsulfide-cubane ([(t-Bu)GaS]/sub 4/). At the recess length of 1.1 /spl mu/m, a GaS-passivated device with a 0.5-/spl mu/m gate length has the maximum transconductance (g/sub m max/) of 347 mS/mm, which is about 40% higher than that of 240 mS/mm for a device without GaS passivation. We found that one of the causes of an increased g/sub m max/ is the decrease of sheet resistance on the recessed surface because GaS passivation has reduced the depletion layer. Meanwhile, the two-terminal gate-to-drain reverse breakdown voltage (BV/sub gd/) was reduced after GaS passivation. The BV/sub gd/ is independent of the recess length between gate and drain (L/sub gd/) for GaS-passivated devices, unlike that for devices without GaS passivation. According to our calculation of the BV/sub gd/ involving the effects of impact ionization and the interface state, the BV/sub gd/ becomes almost independent of the L/sub gd/, when the interface state density (N/sub int/) is below 1/spl times/10/sup 12/ cm/sup -2/. Then, the calculated surface potential at the recess region is less than 0 eV. This result suggests that GaS passivation can remarkably reduce the N/sub int/ at the recess region.     

High breakdown voltage (Al/sub 0.3/Ga/sub 0.7/)/sub 0.5/In/sub 0.5/P/InGaAs quasi-enhancement-mode pHEMT with field-plate technology

A high breakdown voltage and a high turn-on voltage (Al/sub 0.3/Ga/sub 0.7/)/sub 0.5/In/sub 0.5/P/InGaAs quasi-enhancement-mode (E-mode) pseudomorphic HEMT (pHEMTs) with field-plate (FP) process is reported for the first time. Between gate and drain terminal, the transistor has a FP metal of 1 /spl mu/m, which is connected to a source terminal. The fabricated 0.5/spl times/150 /spl mu/m/sup 2/ device can be operated with gate voltage up to 1.6 V owing to its high Schottky turn-on voltage (V/sub ON/=0.85 V), which corresponds to a high drain-to-source current (I/sub ds/) of 420 mA/mm when drain-to-source voltage (V/sub ds/) is 3.5 V. By adopting the FP technology and large barrier height (Al/sub 0.3/Ga/sub 0.7/)/sub 0.5/In/sub 0.5/P layer design, the device achieved a high breakdown voltage of -47 V. The measured maximum transconductance, current gain cutoff frequency and maximum oscillation frequency are 370 mS/mm, 22 GHz , and 85 GHz, respectively. Under 5.2-GHz operation, a 15.2 dBm (220 mW/mm) and a 17.8 dBm (405 mW/mm) saturated output power can be achieved when drain voltage are 3.5 and 20 V. These characteristics demonstrate that the field-plated (Al/sub 0.3/Ga/sub 0.7/)/sub 0.5/In/sub 0.5/P E-mode pHEMTs have great potential for microwave power device applications.     

Performance of the AlGaN HEMT structure with a gate extension

The microwave performance of AlGaN/GaN HEMTs at large drain bias is reported. The device structures were grown by organometallic vapor phase epitaxy on SiC substrates with a channel sheet resistance less than 280 ohms/square. The breakdown voltage of the HEMT was improved by the composite gate structure consisting of a 0.35 /spl mu/m long silicon nitride window with a 0.18 /spl mu/m long metal overhang on either side. This produced an metal-insulator-semiconductor (MIS) gate extension toward the drain with the insulator, silicon nitride, approximately 40-nm-thick. Transistors with a 150 /spl mu/m total gate width have demonstrated a continuous wave (CW) 10 GHz output power density and power added efficiency of 16.5 W/mm and 47%, respectively when operated at 60 V drain bias. Small-signal measurements yielded an f/sub T/ and f/sub max/ of 25.7 GHz and 48.8 GHz respectively. Maximum drain current was 1.3 A/mm at +4 V on the gate, with a knee voltage of /spl sim/5 V. This brief demonstrates that AlGaN/GaN HEMTs with an optimized gate structure can extend the device operation to higher drain biases yielding higher power levels and efficiencies than have previously been observed.     

30-W/mm GaN HEMTs by field plate optimization

GaN high-electron-mobility-transistors (HEMTs) on SiC were fabricated with field plates of various dimensions for optimum performance. Great enhancement in radio frequency (RF) current-voltage swings was achieved with acceptable compromise in gain, through both reduction in the trapping effect and increase in breakdown voltages. When biased at 120 V, a continuous wave output power density of 32.2 W/mm and power-added efficiency (PAE) of 54.8% at 4 GHz were obtained using devices with dimensions of 0.55/spl times/246 /spl mu/m/sup 2/ and a field-plate length of 1.1 /spl mu/m. Devices with a shorter field plate of 0.9 /spl mu/m also generated 30.6 W/mm with 49.6% PAE at 8 GHz. Such ultrahigh power densities are a dramatic improvement over the 10-12 W/mm values attained by conventional gate GaN-based HEMTs.     

An ultrathin vertical channel MOSFET for sub-100-nm applications

An ultrathin vertical channel (UTVC) MOSFET with an asymmetric gate-overlapped low-doped drain (LDD) is experimentally demonstrated. In the structure, the UTVC (15 nm) was obtained using the cost-effective solid phase epitaxy, and the boron-doped poly-Si/sub 0.5/Ge/sub 0.5/ gate was adopted to adjust the threshold voltage. The fabricated NMOSFET offers high-current drive due to the lightly doped (<1/spl times/10/sup 15/ cm/sup -3/) channel, which suppresses the electron mobility degradation. Moreover, an asymmetric gate-overlapped LDD was used to suppress the offstate leakage current and reduce the source/drain series resistance significantly as compared to the conventional symmetrical LDD. The on-current drive, offstate leakage current, subthreshold slope, and DIBL for the fabricated 50-nm devices are 325 /spl mu/A//spl mu/m, 8/spl times/10/sup -9/ /spl mu/A//spl mu/m, 87 mV/V, and 95 mV/dec, respectively.     

Wideband DHBTs using a graded carbon-doped InGaAs base

We report an InP/InGaAs/InP double heterojunction bipolar transistor (DHBT), fabricated using a mesa structure, exhibiting 282 GHz f/sub /spl tau// and 400 GHz f/sub max/. The DHBT employs a 30 nm InGaAs base with carbon doping graded from 8/spl middot/10/sup 19//cm/sup 3/ to 5/spl middot/10/sup 19//cm/sup 3/, an InP collector, and an InGaAs/InAlAs base-collector superlattice grade, with a total 217 nm collector depletion layer thickness. The low base sheet (580 /spl Omega/) and contact (<10 /spl Omega/-/spl mu/m/sup 2/) resistivities are in part responsible for the high f/sub max/ observed.     

InGaAs/InP DHBTs with 120-nm collector having simultaneously high f/sub /spl tau//, f/sub max//spl ges/450 GHz

InP/In/sub 0.53/Ga/sub 0.47/As/InP double heterojunction bipolar transistors (DHBT) have been designed for increased bandwidth digital and analog circuits, and fabricated using a conventional mesa structure. These devices exhibit a maximum 450 GHz f/sub /spl tau// and 490 GHz f/sub max/, which is the highest simultaneous f/sub /spl tau// and f/sub max/ for any HBT. The devices have been scaled vertically for reduced electron collector transit time and aggressively scaled laterally to minimize the base-collector capacitance associated with thinner collectors. The dc current gain /spl beta/ is /spl ap/ 40 and V/sub BR,CEO/=3.9 V. The devices operate up to 25 mW//spl mu/m/sup 2/ dissipation (failing at J/sub e/=10 mA//spl mu/m/sup 2/, V/sub ce/=2.5 V, /spl Delta/T/sub failure/=301 K) and there is no evidence of current blocking up to J/sub e//spl ges/12 mA//spl mu/m/sup 2/ at V/sub ce/=2.0 V from the base-collector grade. The devices reported here employ a 30-nm highly doped InGaAs base, and a 120-nm collector containing an InGaAs/InAlAs superlattice grade at the base-collector junction.     

A high-speed ITO-InAlAs-InGaAs Schottky-barrier photodetector

A high-speed and high-sensitivity vertical indium-tin-oxide-InAlAs-InGaAs Schottky barrier photodetector has been designed, fabricated, and characterized. The devices had dark current densities as low as 8.87/spl times/10/sup -5/ A/cm/sup -2/ at an applied bias of 5 V. The responsivity for all the devices tested ranged from 0.55-0.60 A/W at a wavelength of 1.31 /spl mu/m, and 0.563-0.583 A/W at 1.55 /spl mu/m. The 15-/spl mu/m diameter devices exhibited a -3 dB bandwidth of 19 and 25 GHz at a wavelength of 1.55 /spl mu/m and an applied bias of 5 and 10 V, respectively. These are the best values of responsivity and bandwidth for a vertical InGaAs-based Schottky-barrier photodetector reported to date.     

An ultra-low power InAs/AlSb HEMT Ka-band low-noise amplifier

The first antimonide-based compound semiconductor (ABCS) MMIC, a Ka-Band low-noise amplifier using 0.25-/spl mu/m gate length InAs/AlSb metamorphic HEMTs, has been fabricated and characterized on a 75 /spl mu/m GaAs substrate. The compact 1.1 mm/sup 2/ three-stage Ka-band LNA demonstrated an average of 2.1 dB noise-figure between 34-36 GHz with an associated gain of 22 dB. The measured dc power dissipation of the ABCS LNA was an ultra-low 1.5 mW per stage, or 4.5 mW total. This is less than one-tenth the dc power dissipation of a typical equivalent InGaAs/AlGaAs/GaAs HEMT LNA. Operation with degraded gain and noise figure at 1.1 mW total dc power dissipation is also verified. These results demonstrate the outstanding potential of ABCS HEMT technology for mobile and space-based millimeter-wave applications.