Novel Devices ♦ news digest
NIST Tech Beat article, “Extreme Darkness: Carbon Nanotube Forest Covers NIST’s Ultra-dark Detector,” at
www.nist.gov/pml/div686/dark_081710. cfm.
and
NIST Tech Beat article, “Prototype NIST Device Measures Absolute Optical Power in Fiber at Nanowatt Levels,” at
www.nist.gov/pml/div686/ radiometer-122011.cfm.
Microwave GaN-on-Si HEMTs compatible with non-gold metal stack
The devices are believed to exhibit the lowest specific contact resistivity (rc) ever reported for CMOS-compatible non-gold ohmic contacts for conventional gallium nitride HEMTs on a silicon substrate
Researchers from Nanyang Technological University, Singapore have demonstrated 0.15 µm gate-length AlGaN/GaN high-electron-mobility transistors (HEMTs) with direct-current and microwave performances.
The scientists, led by Professor G. I. Ng, say that this is the first microwave performance GaN- on-silicon HEMTs with improved ohmic contact using complementary metal-oxide-semiconductor (CMOS)-compatible non-gold metal stack.
The silicon/tantalum (Si/Ta)-based ohmic contact exhibited the lowest contact resistance (Rc = 0.24 Ω-mm) ever reported thus far with as smooth surface morphology.
This work demonstrates the feasibility of achieving high performance GaN-on-silicon HEMTs using a non-gold metal stack approach which is compatible to the CMOS process in large-volume silicon manufacturing lines.
The fabricated GaN HEMTs exhibited maximum drain current density (IDmax) of 830 mA/ mm, a maximum extrinsic transconductance (gmmax) of 250 mS/mm, and a threshold voltage (Vth) of -3.75 V. The measured current gain
January/February 2013
www.compoundsemiconductor.net 221
cut-off frequencyfTand maximum oscillation frequency fmax are 39 GHz.
The devices also achieved a breakdown voltage of 90 V with a minimum drain current collapse of less than 10 percent for a gate-drain spacing of 1.7 µm. The device Johnson’s figure of merit (J-FOM =fT×BVgd) is in the range between 3.51 THz.V to 3.83 THz.V which is comparable to other reported GaN HEMTs on silicon with a conventional III-V gold-based ohmic contact process.
The GaN HEMT structure was grown by MOCVD with a 2-nm thick GaN cap layer, 18-nm thick Al0.26Ga0.74N barrier, 800-nm thick GaN buffer and 1.4-µm thick transition layer on 4 inch silicon (111) (resistivity > 6000 Ω-cm).
The grown structure exhibited room temperature 2-dimensional electron gas (2-DEG) mobility of 1450 cm2/V.s and sheet carrier density of 1.1x1013 cm-2. An optimised Ta/Si-based ohmic contact metal scheme (Ta/Si/Ti/Al/Ni/Ta) revealed repeated low Rc value of 0.24 Ω-mm (standard deviation of 0.07 Ω-mm) out of 3 separate runs with an average specific contact resistivity (rc) of 1.25x10-6 Ωcm2.
This is believed to be the lowest ever reported for CMOS-compatible non-gold ohmic contacts for conventional GaN HEMTs on silicon and it is also lower than that of recessed ohmic contacts.
With reference to the conventional gold-based ohmic contact [Figure 1(a)], the CMOS-compatible non-gold ohmic metal stack provides a smooth surface morphology with good edge definition [Figure 1 (b)].
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