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First microwave AlGaN/GaN HEMTs on 8 inch silicon

Recent research demonstrates the feasibility of achieving high performance III-nitride HEMTs on 8 inch diameter Silicon (111) for high-frequency and high-power device applications
A Singapore Research Team has demonstrated the direct-current and microwave performances of 0.3-µm-gate-length AlGaN/GaN HEMTs grown on an 8 inch diameter silicon substrate.

The researchers say this is the first demonstration of sub-micron gate GaN HEMTs on 8 inch silicon.

The team was composed of scientists from Nanyang Technological University (NTU), A*STAR Institute of Microelectronics (IME) and A*STAR Institute of Materials Research and Engineering (IMRE).

The High-Electron-Mobility Transistors (HEMTs) were grown using the MOCVD Veeco TurboDisk k465i system.

The fabrication and the characterisation of the submicron devices were performed at NTU. The submicron-gate devices on crack-free AlGaN/GaN HEMT structures exhibited good DC and microwave characteristics.

This work demonstrates the feasibility of achieving good performance AlGaN/GaN HEMTs on 8 inch diameter Silicon (111) for low-cost high-frequency and high-power switching device applications. 

Currently, many major silicon fabs worldwide are also pursuing the growth and process development of GaN devices on 8 inch silicon substrates. They would utilise their 8 inch line to reduce the chip cost and increase the chip functionalities by combining with CMOS circuitries.

Figure 1 shows a photograph of crack-free AlGaN/GaN HEMT structures grown on a full 8 inch diameter Silicon (111) substrate, with a starting substrate resistivity of ~100 Ω-cm.

Figure 1. Photograph of crack-free AlGaN/GaN HEMT structure on 8-inch diameter Si(111)

The device structure is: i-GaN (2nm)/i-Al0.17Ga0.83N (18 nm)/GaN (1050 nm)/AlN (10 nm)/GaN (870 nm)/AlN (10 nm)/GaN/Al1−xGaxN (1000 nm) multiple layers/AlN (100 nm)/8 inch Silicon (111).

The grown GaN HEMT structure exhibited an average two dimensional electron gas (2DEG) mobility (mH) of 1550 cm2V-1s-1, sheet carrier concentration (ns) of 0.84 × 1013 cm-2, and an average Rsh of < 400 Ω/sq. The product ns × mH is 1.26 × 1016 V-1s-1 and the surface RMS roughness of AlGaN/GaN HEMT structure is 0.25 nm. These figures are equivalent to previously reported data in the literature for AlGaN/GaN heterostructures grown on 4 inch to 6 inch diameter silicon substrates.

The submicron-gate devices were fabricated using a conventional process technique. The device dimensions are: gate length (Lg) = 0.3 µm; gate width (Wg) = (2 × 75) µm; gate-drain spacing (Lgd) = 2 µm.

Figure 2 shows the small-signal microwave characteristics of AlGaN/GaN HEMTs on 8 inch diameter Silicon (111) for VD = 10 V and Vg = -2.4 V.

Figure 2. Small-signal microwave characteristics of AlGaN/GaN HEMT structure on an 8 inch diameter silicon substrate

The device exhibited good pinch-off characteristics with IDmax = 853 mA/mm, gmmax = 180mS/mm, threshold voltage = -3.8 V, fT = 28 GHz and fmax = 64 GHz. The fLg is 8.4 GHz.μm, which is comparable to the AlGaN/GaN HEMTs fabricated on smaller diameter silicon substrates.

The observed fmax/fT > 2 to 2.66 is due to the occurrence of good quality buffer GaN with low buffer leakage current (4.8 × 10-3 mA/mm at 100 V).

The 2-µm-gate HEMT in the same device structure exhibited a BVgd of 188 V which is almost equivalent to the lateral buffer breakdown voltage (BVBuff) of 192 V. The presence of a higher screw- and edge- dislocation density at the hetero-interfaces usually leads to a lower BVBuff. 

In the future, the researchers aim to optimise the growth of GaN-on-silicon to improve the buffer quality and reduce the wafer bowing.

This research work was supported by SERC-A*STAR under the TSRP program grant Nos. 102-169-0126 and 102-169-030.


More details of this work has been published in the paper, "Direct Current and Microwave Characteristics of Sub-micron AlGaN/GaN High-Electron-Mobility Transistors on 8-Inch Si(111) Substrate," by S. Arulkumaran, G. I. Ng, S. Vicknesh et al in Japanese Journal of Applied Physics, 51, 111001 (2012). DOI: 10.1143/JJAP.51.111001
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