Normally-Off AlGaN/GaN HEMTs with Thin InGaN Cap Layer

Masafumi ITO  Shigeru KISHIMOTO  Fumihiko NAKAMURA  Takashi MIZUTANI  

IEICE TRANSACTIONS on Electronics   Vol.E91-C   No.7   pp.989-993
Publication Date: 2008/07/01
Online ISSN: 1745-1353
DOI: 10.1093/ietele/e91-c.7.989
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Section on Heterostructure Microelectronics with TWHM 2007)
Category: Nitride-based Devices
normally-off,  AlGaN/GaN,  HEMT,  InGaN cap,  

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We have fabricated AlGaN/GaN HEMTs with a thin InGaN cap layer to implement normally-off HEMTs with a small extrinsic source resistance. The key idea is to employ the polarization-induced field in the InGaN cap layer, by which the conduction band is raised leading to the normally-off operation. Fabricated HEMT with an In0.2Ga0.8N cap layer with a thickness of 5 nm showed normally-off operation with a threshold voltage of 0.4 V and a maximum transconductance of 85 mS/mm for the device with a 1.9-µm-long gate. By etching-off the In0.2Ga0.8N cap layer at the region except under the gate using gate and ohmic electrodes as etching masks, the sheet resistance has decreased from 2.7 to 0.75 kΩ/, and the maximum transconductance has increased from 85 to 130 mS/mm due to a reduction of the extrinsic source resistance. The transconductance was increased from 130 to 145 mS/mm by annealing the devices at 250 for 20 minutes in a N2 atmosphere.