CURRENT NEWS                                                                              

Microwave GaN-on-Si HEMTs compatible with non-gold metal stack

Jan 14, 2013
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 m 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 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 m. The device Johnson’s figure of merit (J-FOM = fTBVgd) 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-m 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)].

Figure 1 (a) Conventional III-V gold-based Ohmic contact with rough morpholoty, (b) Non-gold Ohmic contact with smooth surface morphology

This simple ohmic scheme also avoids the need to use other complicated techniques such as an ohmic recess or a regrown ohmic contact, which will complicate the manufacturing process. The fabricated devices have also exhibited very low current collapse (less than 10 percent) at gate- and drain-quiescent biases (Vgs0 = -8 V, Vds0 = 10 V) [Figure 1 (c)]. Further device improvement can be realised by optimising the GaN HEMT epi-structure and by reducing the device parasitics.

Figure 1 (c) Pulsed (pulse width = 200 ns; pulse period = 1 ms) IDS-VDS characteristics of fabricated GaN HEMTs on silicon using CMOS compatible non-gold metal stack.

This research work is supported by SERC-A*STAR under the TSRP program grant No.102-169-0126.

Further details of this research are described in the paper, "Demonstration of Submicron-Gate AlGaN/GaN High-Electron-Mobility Transistors on Silicon with Complementary Metal–Oxide–Semiconductor-Compatible Non-Gold Metal Stack," by S. Arulkumaran, G. I. Ng, S. Vicknesh et al, in Applied Physics Express 6, 016501 (2013). DOI:10.7567/APEX.6.016501

Compound Semiconductor. The most respected, authoritative and widely read information source connecting the community since 1995 To view the latest issue of Compound Semiconductor, click here
To register free of charge to receive news via e mail on a weekly basis click here.
Search the Compound Semiconductor web site
Submit your Lab & Fab article
It is imperative that remains a timely resource for this industry, so we are only interested in highlighting very recent work reported in academic papers. Therefore, please only consider writing a short piece highlighting your work if you have a journal paper that has been accepted and about to appear in press, or a paper that has been published within the last month. For further guidelines, click here.
    Email this article to a friend
  Connotea       Cite-U-Like        Digg       



For maximum exposure, become a Corporate Partner.
Contact our sales team.
Buyer's Guide