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Advanced Processing Technology for GaAs Modulation-Doped Transistors and Lasers

Objective

The experiments carried out within this project were intended to identify the advantages and disadvantages of MBE/laser processed HEMTs, MESFETs and GaAs on silicon MESFETs.
The experiments carried out within this project were intended to identify the advantages and disadvantages of molecular beam epitaxy (MBE)/laser processed high electron mobility transistors (HEMT), metal semiconductor field effect transistors (MESFET) and gallium arsenide on silicon MESFETs. The study of the various levels of laser processing and MBE growth have been completed. The HEMT technology gave the most significant advantages. Laser processing eliminates DX centre problems, side gating effects, and leads to improved heterojunction properties.

The work done clearly shows that the performance enhancement was due to the laser assisted 2-dimensional growth technology.

Significant advantages with laser processing have also been achieved for gallium arsenide on silicon MESFETs. In this case both performance and reliability advantages have been identified and attributed to the 2-dimensional growth achieved by laser assisted growth techniques.

Laser processing of gallium arsenide on silicon was also found to enhance dopant activation, resulting in a high current saturation.

Since reliability is an important aspect of the technology developed, the consortium put emphasis on life testing and failure physics: for the laser processed HEMTs, a bulk type degradation mechanism has been observed.

The modelling effort concentrated on simulating parasitic effects. The results from the correlations are consistent with the experimental observation that thermally desorbed material is sensitive to backgating, wheras laser desorbed material does not give rise to backgating.
The study of the various levels of laser processing and MBE growth have been completed. The HEMT technology gave the most significant advantages. Laser processing eliminates DX centre problems, side-gating effects, and leads to improved heterojunction properties. The TiWSi gates allowed the consortium to develop HEMTs with an NF of less than 1.0 dB at 12 GHz, with a 0.75 um gate. This structure has resulted in a reliability improvement from the 105 HR range for conventional HEMT, to 1-2 x 107 HRS for laser-processed HEMTs. The work done by the characterisation group clearly shows that the performance enhancement was due to the laser-assisted 2-D growth technology.
Significant advantages with laser processing have also been achieved for GaAs-Si MESFETs. In this case both performance and reliability advantages have been identified and attributed to the 2-D growth achieved by laser assisted growth techniques. A transconductance of over 200 mS/mm was achieved for laser-processed GaAs/Si FETs, which is comparable to conventional FETs. The surprising result is the high reliability result of 6 x 105 - 1 x 106 HRS (110 C) achieved, indicating that the 4% lattice mismatchmay not hinder the reliability of the GaAs on silicon technology. Laser processing of GaAs on silicon was also found to enhance dopant activation, resulting in a high current saturation.
Since reliability is an important aspect of the technology developed, the consortium put emphasis on life testing and failure physics: for the laser-processed HEMTs, a bulk type degradation mechanism has been observed, without Al or Si diffusion from the superlattice and with no creation of DX centres during ageing. The activation energies for laser-processed devices range between 1.4 and 1.5 eV with an extrapolated median lifetime of 3 x 107 HRS at 100 C channel temperature. The gate sinking effect obse rved on ion implanted devices is not the dominant failure mechanism for MESFETs fabricated with MBE material. The classical ohmic contact resistance increase is the primary degradation mode. The GaAs on Si FETs (laser processed) without the piezo-electriceffect and substrate leakage behaved similarly to GaAs on GaAs MESFETs during ageing. Ohmic contact degradation was the primary degradation mode.
The modelling effort concentrated on simulating parasitic effects. The results from the correlations are consistent with the experimental observation that thermally desorbed material is sensitive to backgating, whereas laser desorbed material does not give rise to backgating.
Exploitation
FORTH has established a small company (MITOS SA) to further exploit low-noise packaged HEMTs and to develop custom MBE wafers. Furthermore, an impressive list of publications has resulted from this project.

Coordinator

RESEARCH CENTER OF CRETE - CRETAN COMPUTER INSTITUTE
Address
P.o. Box 1527
71110 Iraklion
Greece

Participants (4)

CNET France Télécom
France
Address
2 Route De Tregastel
22300 Lannion
ELLTEC SA
Greece
Address
Ardittou Street, 36
11636 Athinai
GEC Plessey Semiconductors plc
United Kingdom
Address
Caswell
NN12 8EQ Towcester
UNIV OF WALES
United Kingdom
Address
King Edward Vii Avenue
CF1 3NU Cardiff