Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Heterojunction
Reexamination Certificate
2000-06-29
2002-12-03
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Heterojunction
C257S190000, C257S191000, C257S192000, C257S201000
Reexamination Certificate
active
06489628
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-184647, filed Jun. 30, 1999; and No. 2000-179544, filed Jun. 15, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a high electron mobility transistor and power amplifier and, more particularly, to a group III nitride inverted high electron mobility transistor and a power amplifier using the same.
Since field effect transistors (FETS) using a group III nitride compound semiconductor material, especially, gallium nitride (GaN) can realize high output power in the high-frequency range, they are expected to be used as power elements. As such transistors, a metal-semiconductor field effect transistor (MESFET), high electron mobility transistor (HEMT), metal-insulator field effect transistor (MISFET), and the like have been proposed.
Among these transistors, according to a GaN-based HEMT which has an AlGaN layer as a donor layer and a GaN layer as a channel layer, higher density of two-dimensional electron gas can be realized in the channel layer compared to a GaAs-based HEMT which has an AlGaAs layer as a donor layer and a GaAs layer as a channel layer. Therefore, the GaN-based HEMT is considered as a very promising high-output element. However, in order to realize high-output characteristics by the GaN-based HEMT, some problems remain unsolved, as will be described below.
FIG. 1
is a schematic sectional view showing a conventional GaN-based normal HEMT. A GaN-based HEMT
110
a
shown in
FIG. 1
has the following structure. That is, a GaN underlayer
114
, a GaN channel layer
111
, and an Al&agr;Ga
(1−60)
N (0<&agr;<1) donor layer
113
are stacked in turn on a substrate
115
, and a gate electrode
116
, and source and drain electrodes
117
and
118
are formed on the AlGaN layer
113
.
In such structure, a good ohmic contact must be achieved between the source and drain electrodes
117
and
118
, and the AlGaN layer
113
. However, since the AlGaN layer
113
has a broad band gap, it is hard to achieved a good ohmic contact between the source and drain electrodes
117
and
118
, and the AlGaN layer
113
. For this reason, in the GaN-based HEMT
110
a
shown in
FIG. 1
, the contact resistance between the source and drain electrodes
117
and
118
, and the AlGaN layer
113
is large. Hence, the GaN-based normal HEMT structure shown in
FIG. 1
cannot realize sufficiently high output.
A structure shown in
FIG. 2
is known as the one for combating the problems that have been explained in association with the GaN-based normal HEMT structure shown in FIG.
1
.
FIG. 2
is a schematic sectional view showing a conventional GaN-based inverted HEMT (IHEMT). In this GaN-based inverted HEMT
110
b
, an Al
&agr;
Ga
(1−&agr;)
N (0<&agr;<1) donor layer
113
is formed on a GaN underlayer
114
as in a GaAs-based inverted HEMT. A GaN channel layer
111
is stacked on the AlGaN layer
113
, and a gate electrode
116
, and source and drain electrodes
117
and
118
are formed on the GaN layer
111
(O. Aktas, et al., IEEE Electron Device letters, Vol. 18, No. 6, pp. 293-295, 1997).
The GaN-based inverted HEMT
110
b
shown in
FIG. 2
is mainly different from the GaN-based normal HEMT
110
a
shown in
FIG. 1
in that the Al&agr;Ga
(1−&agr;)
N donor layer
113
is located between the GaN channel layer
111
and GaN underlayer
114
. In such inverted HEMT, the source and drain electrodes
117
and
118
are formed on the GaN layer
111
unlike the normal HEMT. Also, the band gap of the GaN layer is narrower than that of the AlGaN layer. Therefore, according to the inverted HEMT
110
b
shown in
FIG. 2
, the aforementioned problem of the contact resistance can be avoided.
However, the difference between the lattice constants of GaN and Al&agr;Ga
(1−&agr;)
N is one or more orders of magnitudes larger than that between GaAs and Al&agr;Ga
(1−&agr;)
As. For this reason, in the GaN-based inverted HEMT
110
b
shown in
FIG. 2
, larger piezoelectric charges are produced compared to the GaAs-based inverted HEMT. The piezoelectric charges act to reduce the density of two-dimensional electron gas in the GaN channel layer
111
with respect to the HEMT
110
b
having the structure shown in FIG.
2
. Therefore, even the GaN-based inverted HEMT structure shown in
FIG. 2
cannot realize sufficiently high output power.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to provide a group III nitride high electron mobility transistor which can realize high output power.
It is another object of the present invention to provide a power amplifier using a group III nitride high electron mobility transistor which can realize high output power.
According to the first aspect of the present invention, there is provided a group III nitride high electron mobility transistor comprising an underlayer comprising a first group III nitride compound semiconductor material, a donor layer formed on the underlayer and comprising a second group III nitride compound semiconductor material, a lattice constant of a bulk of the donor layer being larger than a lattice constant of the underlayer, a channel layer formed on the donor layer and comprising a third group III nitride compound semiconductor material, and gate, source, and drain electrodes formed on the channel layer.
According to the second aspect of the present invention, there is provided a group III nitride high electron mobility transistor comprising an underlayer comprising Al
x
Ga
(1−x)
N, a donor layer formed on the underlayer and comprising Al
y
Ga
(1−y)
N, x and y satisfying an inequality 0≦y<x≦1, a channel layer formed on the donor layer and comprising a nitrogen compound, and gate, source, and drain electrodes formed on the channel layer.
According to the third aspect of the present invention, there is provided a power amplifier comprising a group III nitride high electron mobility transistor. which comprises an underlayer comprising a first group III nitride compound semiconductor material, a donor layer formed on the underlayer and comprising a second group III nitride compound semiconductor material, a lattice constant of a bulk of the donor layer being larger than a lattice constant of the underlayer, a channel layer formed on the donor layer and comprising a third group III nitride compound semiconductor material, and gate, source, and drain electrodes formed on the channel layer, an input terminal receiving an input signal and is connected to the gate electrode, an output terminal outputting an output signal and connected to the drain electrode, and a power source connected to the drain electrode via a choke coil.
According to the fourth aspect of the present invention, there is provided a power amplifier comprising group III nitride high electron mobility transistor comprising an underlayer comprising Al
x
Ga
(1−x)
N, a donor layer formed on the underlayer and comprising Al
y
Ga
(1−y)
N, x and y satisfying an inequality 0≦y<x≦1, a channel layer formed on the donor layer and comprising a nitrogen compound, and gate, source, and drain electrodes formed on the channel layer, an input terminal receiving an input signal and connected to the gate electrode, an output terminal outputting an output signal and connected to the drain electrode, and a power source connected to the drain electrode via a choke coil.
As described above, the conventional GaN-based IHEMT has a structure in which an Al&agr;Ga
(1−&agr;)
N (0<&agr;<1) donor layer having a smaller lattice constant than that of GaN is stacked on a GaN underlayer. In such structure, since positive piezoelectric charges are produced near the interface of the donor layer with the underlayer, and negative piezoelectric charges are produced near the interface of the donor layer with the channel layer, the density of two-dimensional electron gas in the chan
Nelms David
Tran Mai-Huong
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