Semiconductor device

Details Semiconductor device Semiconductor device

2001-01-12

2003-03-11

Lee, Eddie (Department: 2815)

Active solid-state devices (e.g., transistors, solid-state diode

Heterojunction device

Light responsive structure

C257S190000, C257S191000, C257S192000, C257S187000, C257S020000, C257S024000

Reexamination Certificate

active

06531718

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device such as a field-effect transistor (FET) having a heterostructure of gallium nitride-based semiconductor which is generally represented as (In
X
Al
1-X
)
Y
Ga
1-Y
N (where 0≦X≦1, 0≦Y≦1).
2. Description of the Related Art
A gallium nitride-based semiconductor such as GaN, AlGaN, InGaN, InAlGaN or the like has a high dielectric breakdown field, high thermal conductivity and a high electron saturation velocity, and thus is promising as a material for a high-frequency power device. Particularly in a semiconductor device having an AlGaN/GaN heterojunction structure, electrons accumulate with high density in the close vicinity of a heterojunction interface between AlGaN and GaN, and a so-called two-dimensional electron gas is formed. This two-dimensional electron gas exists in a spatially separated state from donor impurities added to AlGaN, and thus shows high electron mobility. A field-effect transistor having such a heterostructure is produced so that a source resistance can be reduced. Moreover, a distance d from a gate electrode to the two-dimensional electron gas is typically as short as tens of nanometers, and thus, even if a gate length Lg is as short as about 100 nm, the ratio of the gate length Lg to the distance d (i.e., aspect ratio) Lg/d, can be increased from 5 to about 10. Accordingly, semiconductor devices having an AlGaN/GaN heterostructure have a superior feature in that a field-effect transistor which has an insignificant short-channel effect and satisfactory saturation property can be readily produced. Moreover, a two-dimensional electron of the AlGaN/GaN-based heterostructure has an electron velocity in a high field region of about 1×10
5
V/cm, which is twice or more than the electron velocity in AlGaAs/InGaAs-based heterostructure currently prevalent as a high-frequency transistor, and thus, is expected to be applied to high-frequency power devices.
FIG. 8
shows an exemplary cross-sectional view of a conventional FET
800
having an AlGaN/GaN-based heterostructure. The AlGaN/GaN-based heterostructure of the FET
800
is typically formed on a substrate
801
composed of a [0001] facet (c facet), through a crystal growth process using a metal-organic chemical vapor deposition method or a molecular beam epitaxy method. Typically, a sapphire substrate or SiC substrate is used as the substrate
801
. In the FET
800
, a buffer layer
802
including GaN and an electron supply layer
805
including AlGaN are sequentially provided on the sapphire or SiC substrate
801
. On the electron supply layer
805
, a source electrode
806
, a gate electrode
807
, and a drain electrode
808
are provided separately from one another. In the case of forming the buffer layer
802
including GaN on the sapphire or SiC substrate
801
, it is necessary to thickly form the buffer layer
802
in order to account for a great lattice constant difference between the substrate
801
and the buffer layer
802
. This is because the strain due to a lattice mismatch between the buffer layer
802
and the substrate
801
is sufficiently relaxed by forming the buffer layer
802
so as to have a relatively large thickness. By forming the electron supply layer
805
containing AlGaN to which n-type impurities, such as Si or the like, are added so as to have a thickness on the order of tens of nanometers on the thick buffer layer
802
, a two-dimensional electron gas (i.e., electron channel) is formed in the buffer layer
802
which has a great electron affinity in the heterointerface between the buffer layer
802
and the electron supply layer
805
(i.e., between AlGaN and GaN) due to the effects of selective doping. The crystal facet of a heterostructure formed by an MOCVD (Metal-Organic Chemical Vapor Deposition) method, is typically composed of a facet of Ga, which is a group III element. This two-dimensional electron gas is susceptible to the effects of piezo-polarization in a c axis direction due to tensile stress imposed on AlGaN, in addition to a difference in spontaneous polarization between AlGaN (included in the electron supply layer
805
) and GaN (included in the buffer layer
802
). Thus, electrons accumulate at a density which is higher than a value which would be expected from the density of the n-type impurities added to the electron supply layer
805
. When the Al composition of AlGaN of the electron supply layer
805
is 0.2 to 0.3 with respect to AlGaN, electron density of the channel layer formed in the buffer layer
802
is about 1×10
13
/cm
2
, which is about 3 times the density of a GaAs-based device. Since the two-dimensional electron gas at such a high density is accumulated, the semiconductor device
800
used as a GaN-based heterostructure field-effect transistor (FET) is considered as a highly promising power device.
However, such a conventional FET
800
has several disadvantages. The first of which is that due to the immaturity of crystal growth techniques, a crystal with satisfactory quality cannot be obtained.
One of the problems related to the crystal growth is associated with the fact that the undoped GaN included in the buffer layer
802
typically is an n-type and the carrier density may be as high as about 10
16
/cm
3
or more. This is presumably because constituent nitrogen (N) atoms are released during crystal growth, and thus, vacancies are liable to be formed. When there are such residual carriers, the leakage current component via the buffer layer
802
of the device becomes greater. In particular, when operating the device at a high temperature, deteriorations in the element properties, such as aggravation of pinch-off characteristics, may occur. As for an isolation problem, when forming a plurality of GaN-based heterostructure FETs on the same substrate, the FETs interfere with each other, hindering normal operation. When the gate electrode
807
is further provided above this buffer layer
802
, a problem such as an increase of a gate leakage current, or the like, may arise.
The second disadvantage of a conventional FET
800
is ascribed to the effects of polarization as described above. In a conventional FET having an AlGaAs/InGaAs-based heterostructure, a channel layer is composed of InGaAs, and an electron (carrier) supply layer is composed of AlGaAs and is doped with Si. In general, when such a FET is applied as a power device, an AlGaAs/InGaAs/AlGaAs structure, in which an InGaAs (channel) layer is sandwiched by two n-type AlGaAs layers, is employed. In this structure, the electron density of the channel layer is about 2 times the electron density of a channel layer in a non-sandwich type structure.
FIG. 9
schematically shows a distribution of the potential energy of conduction band along the depth direction of such a semiconductor device. As shown in
FIG. 9
, electrons are supplied from the AlGaAs layers to the InGaAs layer whose potential is lower than those of the AlGaAs layers. Such a structure which has two Si-doped AlGaAs layers is called a double-doped structure or a double-doped, double-heterostructure.
FIG. 10
shows a structure of an n-type AlGaN/GaN
-type AlGaN device
1000
, which is a GaN-based device having a double-doped structure.
FIG. 11
shows a distribution of the potential energy along the depth direction in the semiconductor device
1000
.
The conventional FET
1000
shown in
FIG. 10
sequentially includes the following layers on a sapphire or SiC substrate
1001
: a first channel layer
1002
, including GaN; a first electron supply layer
1013
, including AlGaN; a second channel layer
1004
, including GaN; and a second electron supply layer
1005
, including AlGaN. On the second electron supply layer
1005
, a source electrode
1006
, a gate electrode
1007
and a drain electrode
1008
are provided separately from one another. In the GaN-based double-doped structure shown in
FIG. 10
, doping is performed only on the second ele

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