Abbas Torabi

Monolithic integration of silicon CMOS and GaN transistors in a current mirror circuit

GaN high electron mobility transistors (HEMTs) were monolithically integrated with silicon CMOS to create a functional current mirror circuit. The integrated circuit was fabricated on 100 mm diameter modified silicon-on-insulator (SOI) wafers incorporating a resistive (111) silicon handle substrate and a lightly doped (100) silicon device layer. In a CMOS-first process, the CMOS was fabricated using the (100) device layer. Subsequently GaN was grown by plasma molecular beam epitaxy in windows on the (111) handle substrate surface without wire growth despite using gallium-rich growth conditions. Transmission lines fabricated on the GaN buffer/SOI wafer exhibited a microwave loss of less than 0.2 dB/mm up to 35 GHz. Direct current measurements on GaN HEMTs yielded a current density of 1.0 A/mm and transconductance of 270 mS/mm. At 10 GHz and a drain bias of 28 V, 1.25 mm long transistors demonstrated a small signal gain of 10.7 dB and a maximum power added efficiency of 53% with a concomitant power of 5.6 W...

AlGaN/GaN high electron mobility transistors on 100 mm silicon substrates by plasma molecular beam epitaxy

GaN high electron mobility transistor (HEMT) structures have been grown by plasma molecular beam epitaxy on 100 mm diameter ⟨111⟩ silicon substrates. Crack-free films with thicknesses of up to 1.7 μm were deposited without the use of strain-relaxing buffer layers. X-ray measurements indicate high structural uniformity and the Pendellosung oscillations are observed due to the abruptness of the AlGaN/GaN interface. Capacitance-voltage measurements display a sharp pinch-off with a depleted GaN buffer layer and no measurable charge accumulation at the substrate-epi interface. Transmission line measurements on the GaN HEMT buffer and substrate indicate a loss of less than 0.2 dB/mm up to 20 GHz. An average sheet resistance of 443 Ω/sq with a standard deviation of 0.8% and a mobility of 1600 cm2/V s were obtained for an Al0.25Ga0.75N/GaN HEMT. Transistors were fabricated with a current density of 1.2 A/mm and a transconductance of 290 mS/mm which is quite comparable to GaN HEMTs on SiC.

Rapid silicon outdiffusion from SiC substrates during molecular-beam epitaxial growth of AlGaN∕GaN∕AlN transistor structures

AlGaN∕GaN∕AlN transistor structures were grown onto SiC substrates by molecular-beam epitaxy. Under aluminum-rich growth conditions for the AlN nucleation layer, undesirable n-type conduction is observed near the GaN∕AlN interface for even thick (>1000A) AlN layers. Silicon is identified as the unwanted dopant from secondary-ion mass spectroscopy measurements. Atomic force microscopy surface maps reveal free aluminum metal on AlN surfaces grown under modest aluminum-rich conditions. It is proposed that rapid silicon migration is caused by molten aluminum reacting with the SiC substrate resulting in dissolved silicon that rapidly migrates through the growing AlN layer. This behavior is significantly reduced using a growth flux ratio of aluminum to reactive nitrogen close to unity. The resulting buffer leakage current of the GaN high electron mobility transistor structure is reduced by more than four orders of magnitude.

Improved pseudomorphic high electron mobility transistor structures on InGaAs substrates

Single and double pulse doped pseudomorphic high electron mobility transistor structures with 110-A-thick InGaAs channel layers have been grown on InxGa1−xAs substrates (x=0.04; 0.065) and GaAs substrates. For In0.23Ga0.77As channel layers, higher electron mobilities were obtained on In0.04Ga0.96As substrates due to reduced strain. Transmission electron microscopy micrographs on a GaAs-based sample exhibited a roughened selectively doped heterojunction but no detected misfit dislocations. Pseudomorphic structures with In0.27Ga0.73As channel layers were also grown on In0.065Ga0.935As substrates with good transport and optical properties. The properties of the analogous structure grown on GaAs were severely degraded. Transmission electron microscopy micrographs on the GaAs sample showed a very rough selectively doped heterojunction with misfit dislocations.

Highly uniform AlGaN/GaN HEMT films grown on 200-mm silicon substrates by plasma molecular beam epitaxy

Highly uniform AlGaN/GaN HEMT films with good electron transport properties have been grown on 200-mm silicon substrates by plasma molecular beam epitaxy. X-ray diffraction measurements indicate an AlGaN compositional and thickness variation of ±1% across the wafer, and a 29 point resistance map of a HEMT yielded a sheet resistance of 451 Ω/sq ± 1.1%. The electron mobility for seven measurements taken across the diameter of the wafer was 1555 cm2/Vs ± 1%. The mobility obtained on 200-mm silicon is within 10% of the mobility obtained for GaN HEMTs grown on 100-mm SiC substrates, which have a much smaller lattice mismatch with GaN. The uniform films were obtained at GaN growth rates comparable to 100-mm growth and a chamber pressure well within the free molecular flow regime.

Precise determination of indium composition and channel thickness in pseudomorphic high electron mobility transistors using room temperature photoluminescence

Proper composition and thickness of the InGaAs channel in pseudomorphic high electron mobility transistors (PHEMTs) is critical to assuring good device performance. Typically these characteristics have been measured by high-resolution x-ray diffraction. The results presented in this work show that the subband energy levels obtained from line shape analysis of room temperature photoluminescence spectra on these structures can be correlated very well with thickness and composition obtained from x-ray diffraction. Since the photoluminescence measurement and analysis is quite fast, this technique is suitable for rapid, nondestructive screening of PHEMT epitaxial material.

ALGAAS/INGAAS/ALGAAS DOUBLE PULSE DOPED PSEUDOMORPHIC HIGH ELECTRON MOBILITY TRANSISTOR STRUCTURES ON INGAAS SUBSTRATES

Double pulse doped AlGaAs/InGaAs/AlGaAs pseudomorphic high electron mobility transistor (PHEMT) structures have been grown on InxGa1−xAs (x=0.025–0.07) substrates using molecular beam epitaxy. A strain compensated, AlGaInAs/GaAs superlattice was used for improved resistivity and breakdown. Excellent electrical and optical properties were obtained for 110-A-thick InGaAs channel layers with indium concentrations up to 31%. A room temperature mobility of 6860 cm2/V s with 77 K sheet density of 4.0×1012 cm−2 was achieved. The InGaAs channel photoluminescence intensity was equivalent to an analogous structure on a GaAs substrate. To reduce strain PHEMT structures with a composite InGaP/AlGaAs Schottky layer were also grown. The structures also exhibited excellent electrical and optical properties. Transmission electron micrographs showed planar channel interfaces for highly strained In0.30Ga0.70As channel layers.