Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2003-02-26
2004-08-03
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S192000, C257S201000
Reexamination Certificate
active
06770922
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device composed of a group III-V nitride semiconductor represented by a general formula (In
X
Al
1−X
)
Y
Ga
1−Y
N (where 0≦X≦1 and 0≦Y≦1 are satisfied) and to a method for fabricating the same.
A group III-V nitride semiconductor such as GaN, AlGaN, InGaN, or InAlGaN, i.e., a so-called gallium nitride-based semiconductor is an important semiconductor for an optical device having a short oscillating wavelength such as a semiconductor laser device outputting, e.g., a blue laser beam. The applications of the gallium nitride-based semiconductor are not limited to the optical device. In recent years, attention has been focused on the gallium nitride-based semiconductor for its high dielectric breakdown field strength, high thermal conductivity, and high electron saturation velocity so that gallium nitride-based semiconductor is considered to be promising also as the material of an RF power device.
In an AlGaN/GaN heterojunction structure composed of aluminum gallium nitride (AlGaN) and gallium nitride (GaN) which are stacked in layers, in particular, electrons are accumulated at a high density in the vicinity of the heterojunction interface between AlGaN and GaN to form a so-called two-dimensional electron gas. The two-dimensional electron gas exhibits a high mobility since it exists spatially separated from a donor impurity used to dope AlGaN. Therefore, the AlGaN/GaN heterojunction structure achieves the effect of reducing a source resistance component when used in a field effect transistor (FET).
Since the distance d from a gate electrode to the two-dimensional gas is normally as small as several tens of nanometers, a ratio Lg/d between a gate length Lg and the distance d, which is termed an aspect ratio, can be held at a large value of 5 to 10 even if the gate length Lg is as small as about 100 nm. Hence, the use of the AlGaN/GaN heterojunction structure offers an advantage of easy fabrication of a FET with a reduced short channel effect and an excellent saturation characteristic.
The electron velocity of a two-dimensional electron in a high-field region of about 1×10
5
V/cm in the AlGaN/GaN-based heterojunction structure is double or more the electron velocity thereof in a gallium arsenide-based (GaAs-based) FET which is currently prevalent as an RF transistor, i.e., an AlGaAs/InGaAs heterostructure FET. In addition, the density of electrons accumulated at the heterointerface becomes as high as 1×10
13
/cm
2
when the composition of Al in AlGaN is 0.2 to 0.3, which is about three to five times as high as the density of electrons in the GaAs-based device.
Since the dielectric breakdown field strength of the GaN-based heterostructure FET is about ten times as high as that of the GaAs-based FET, the application of a drain voltage therein which is ideally about ten times a drain voltage applied in the GaAs-based FET becomes possible on the assumption that the GaN-based heterostructure FET and the GaAs-based FET have the same device patterns. This renders the GaN-based heterostructure FET promising as an RF power device capable of generating an output power which is at least 5 times, ideally about 10 times as high as or higher than an output power generated by the GaAs-based power device but it also has numerous problems to be solved.
One of the problems associated with the GaN-based heterostructure FET is a large surface leakage current between the gate and drain thereof A gate electrode composing a GaN-based heterostructure FET is a so-called Schottky gate electrode normally composed of a metal material with a relatively large work function coated directly on a semiconductor. As a metal material composing the Schottky gate electrode, a material having a large work function such as nickel (Ni), palladium (Pd), or platinum (Pt) is used appropriately.
If a current-voltage characteristic between the gate and drain is examined after the metal material is vapor-deposited, an abnormally large leakage current is observed frequently at a reverse voltage and a leakage current value cannot be reduced consistently. The large leakage current significantly increases an idle current component to the power device when a high negative voltage is applied to the gate electrode. As a result, the advantage of the GaN-based heterostructure FET that it can be driven at a high drain voltage cannot be used effectively any more, which presents a critical problem.
The occurrence of such a large gate leakage current may be attributed to a reaction between an oxide film (natural oxide film) formed on a surface of the GaN-based semiconductor and the metal material coated thereon and to a reaction between the surface of the GaN-based semiconductor and the metal material.
To prevent such reactions and thereby reduce the gate leakage current, there has been proposed a so-called MIS (Metal-Insulator-Semiconductor) structure or a MOS (Metal-Oxide-Semiconductor) structure in which an insulating film composed of a silicon nitride (SiN), a silicon dioxide (SiO
2
), or the like is deposited on the surface of the GaN-based semiconductor and a gate electrode is formed on the deposited insulating film.
However, since the oxide film mentioned above exists at the interface between the gate insulating film and the GaN-based semiconductor or a trap is easily introduced into a surface of the semiconductor by a surface treatment performed in the fabrication process, a GaN-based FET using the MIS structure or MOS structure described above is not necessarily stable because of the current-voltage characteristic thereof which varies depending on the operating frequency.
SUMMARY OF THE INVENTION
As a result of making various examinations, the present inventors have found that a FET having a MOS structure obtained by directly thermally oxidizing a GaN-based semiconductor to form a thermal oxide film on the surface of the semiconductor, not by depositing an insulating film on the GaN-based semiconductor, and forming a gate electrode on the formed thermal oxide film has a relatively stable operating characteristic. This indicates that the number of traps formed in the interface between the thermal oxide film and the GaN-based semiconductor is relatively small so that such a MOS structure is promising in terms of stabilizing the operation of the FET and reducing a gate leakage current.
The present inventors have also found that, if the thickness of the thermal oxide film is relatively small in the MOS structure, the effect of reducing the gate leakage current is not sufficient and therefore the thickness of the thermal oxide film should be increased to a certain extent. The presumed reasons for this are as follows: Since a thermal oxide film formed by oxidizing a semiconductor composed of AlGaN is composed of gallium oxide (Ga
2
O
3
) and aluminum oxide (Al
2
O
3
) and contains, as a main component, gallium oxide having a relatively small energy gap of about 4.2 eV, the energy barrier thereof against electrons is not sufficiently high compared with the energy barrier of silicon dioxide having a large energy gap of 10 eV In addition, the thermal oxide film composed of polycrystals of gallium oxide, aluminum oxide, and the like contains numerous grain boundaries in the polycrystals so that a leakage current flows through the grain boundaries.
Increasing the thickness of the gate oxide film to reduce the leakage current flowing through the grain boundaries reduces the transconductance of the FET disadvantageously so that the problem of a reduction in the performance of the FET occurs.
On the other hand, it has been reported that oxygen vacancies are formed readily at a high temperature in a MOS structure using a thermal oxide film formed by oxidizing a GaN-based semiconductor. Since the vacancies may function as an n-type impurity and change the conductivity of the thermal oxide film or the thermal oxide film may react with a metal material formed thereon, the long-term reliability cannot be obtained.
The present invention ha
Hirose Yutaka
Ikeda Yoshito
Inoue Kaoru
Nishii Katsunori
McDermott Will & Emery LLP
Ngo Ngan V.
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