Bell rings in GaAs MOSFETs

Bell rings in GaAs MOSFETsSteve Bush
Gadolinium oxide could be the key to making Mosfets on Gallium Arsenide, claims Bell Labs.
Mosfets are the transistor building blocks of CMOS and as such find their way into almost all digital chips and many analogue ones as well.
Gallium Arsenide (GaAs), due to its high carrier mobility and semi-insulating nature, is the semiconductor commonly used in high frequency (greater than 2GHz) chips.  
  The future of RF?… Perfect, stable, monocrystaline layers of gadolinium oxide. These could be the key to making GaAs MOSFETs in production.
GaAs Mosfets would enable extremely fast, low power digital circuits to be built.
So does anyone make GaAs Mosfets?, Ihear you ask.
The answer is no. Mosfets require an insulating layer in their structure and GaAs does not form a satisfactory insulating oxide, unlike silicon which does. Instead it forms a mixture of gallium and arsenic oxides and the arsenic oxide upsets device operation.
“In the last 35 years, since people realised gallium arsenide made faster semiconductors, thousands of PhD theses have been written about oxides for GaAs,” said Ming-Hwei Hong, a Bell Labs’ material scientist.
None have worked.
Hong has been investigating insulating oxides for GaAs for several years and was part of the team that produced both P- and N-channel Mosfets in GaAs late last year. In this case, the material used was an amorphous mixture of gallium and gadolinium oxides.
The breakthrough has come with realisation that gadolinium oxide, on its own, can form mono-crystalline oxide layers on GaAs, despite having a lattice constant twice that of GaAs (Gd2O3 10.8, GaAs 5.65).
This lattice mismatch makes the single crystal oxide growth an entirely unexpected result. “It is not a one-to-one relationship, we call it a super-cell,” said Hong. In a super-cell, every Nth molecule of one crystal matches with every Mth molecule of the other.
Exactly what is going on in the super-cell is not understood, however. “We don’t know on a fundamental physics level, but we have it way under control,” said Hong.
The Gd2O3 film has been grown using molecular beam epitaxy and as such can be moved to production, said Hong.
The dielectric constant of Gd2O3 is ten, twice as high as that of SiO2. “This is a benefit,”said Hong, “it generates more charge.”
So far, the gadolinium oxide has only been tested in MOS diodes. “The electrical measurements show that Gd2O3 is an excellent dielectric. Furthermore, both inversion and accumulation layers were observed in the Gd2O3/GaAs MOS diodes,”said Hong.
This bodes well for future work with MOSFETs and the work could be extended to other semiconductors. “We expect that epitaxial growth of this kind of structure can be extended to other rare earth oxides, and to other semiconductor substrates like silicon,” said Hong. “Our findings from this work thus suggest new opportunities of producing high dielectric constant gate insulators for silicon and GaAs-based MOSFETs.”


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