Field-effect transistor

Details Field-effect transistor Field-effect transistor

2000-02-17

2003-04-29

Smith, Matthew (Department: 2825)

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

Heterojunction device

Field effect transistor

C257S183000, C257S189000, C257S190000, C257S194000, C257S200000, C438S172000, C438S174000, C438S191000, C438S194000, C438S197000, C438S285000

Reexamination Certificate

active

06555850

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a III-V compound semiconductor field-effect transistor.
2. Related Background Art
In semiconductor device art, field-effect transistors made of III-V compound semiconductor materials such as GaAs and InP are known. Since GaAs and InP have smaller electron effective masses than that of silicon (Si), they have high electron mobility. These materials are suitable for high-frequency devices.
Available high-frequency devices include voltage-controlled elements such as a field-effect transistor (FET) having an active layer of GaAs semiconductor and a high-electron-mobility transistor (HEMT), and current-controlled elements such as a hetero-bipolar transistor (HBT).
SUMMARY OF THE INVENTION
The inventors have found the following problem in the course of studying high-frequency devices.
FIG. 1
shows the dependence of the electron drift velocities on electric field strength in GaAs, InP, and Si semiconductors. The slopes of characteristic curves shown in
FIG. 1
indicate the electron mobility of these materials. GaAs and InP have mobility three to five times as large as that of Si.
GaAs and InP, however, indicate the characteristics as follows. As the field strength increases, their electron drift velocities rise to the respective maximum values and decrease. When the electric field strength further increases, the carriers lose the kinetic energy by optical phonon scattering. Due to this scattering, the electron drift velocity approaches a certain value, e.g., approximately 1.0×10
7
cm/s, which is independent of the electric field strength. The advantage of the higher mobility in such materials is lost in the following two cases: (i) the GaAs and InP devices operate under a high applied voltage; and (ii) the device dimensions are reduced, so that the internal electric field increases.
In GaAs, the avalanche breakdown phenomenon occurs at relatively low voltage. The avalanche breakdown voltage relates to the internal electric field in semiconductor, and the voltage is inversely proportional to the impurity concentration. If the impurity concentration of a GaAs active layer is about 1.0×10
18
cm
−3
, only an avalanche breakdown voltage of about 30 V can be achieved in commercially available devices. The inventors have found the following problem: when device dimensions decrease, not only the electron velocity but also the breakdown voltage of the device lowers under device operation.
On the other hand, InP is a material having smaller electron mobility than that of GaAs, but its maximum electron drift velocity is higher than that of GaAs semiconductor. InP semiconductor FETs also has been developed. InP semiconductor can not attain Schottky barrier height enough to be applied to MESFET, and also can not be applied to MIS FET or MOS FET because no semiconductor material and insulating film suitable for these FET are not found.
To avoid this problem, an attempt has been made to apply semiconductor having a composition of Ga
0.51
In
0.49
P to an active layer. In this semiconductor, a number of In cites in an InP semiconductor crystal are replaced with Ga atoms. This Ga
0.51
In
0.49
P has the following characteristics: (a) an energy gap value of 1.9 eV; (b) an electron effective mass larger than that of GaAs semiconductor. Avalanche breakdown voltages depend upon the energy gap values of semiconductors. Under the same impurity concentration, the larger the energy gap is, the higher the avalanche breakdown voltage becomes. A Ga
0.51
In
0.49
P semiconductor FET has an avalanche breakdown voltage of 50 V or more. With this semiconductor, the device can achieve a high avalanche breakdown voltage. Ga
0.51
In
0.49
P semiconductor, however, has smaller mobility than that of GaAs semiconductor because of its effective mass. The inventors have found that, under high electric field, GaInP semiconductor attains electron drift velocity nearly equal to that of GaAs. Therefore, in the high-frequency performance, the Ga
0.51
In
0.49
P device is about as high as a GaAs semiconductor device.
It is an object of the present invention to provide a device having a high avalanche breakdown voltage and high performance in high-frequency regions.
A field-effect transistor according to the present invention includes a channel layer and a gate electrode. The channel layer has a first Ga
x
In
1−x
As
y
P
1−y
layer (0<x<1, 0≦y<1) and a Ga
z
In
1−z
As layer (0<z≦1). The gate electrode is provided so as to control a channel current flowing in the channel layer.
Since the channel layer includes the first Ga
x
In
1−x
As
y
P
1−y
layer and the Ga
z
In
1−z
As layer, this Ga
z
In
1−z
As layer serves as a main channel when the applied voltage is low. As the applied voltage is increased, carriers begin moving to the &Ggr;-valley energy level of the first Ga
x
In
1−x
As
y
P
1−y
layer, not to the L-valley energy level of the GaInAs layer. When the applied voltage is high, the first Ga
x
In
1−x
As
y
P
1−y
layer serves as a main channel.
Additionally, the first Ga
x
In
1−x
As
y
P
1−y
layer has a larger energy gap than that of the Ga
z
In
1−z
As layer. When the applied voltage is high, the carriers conduct in the first Ga
x
In
1−x
As
y
P
1−y
layer having a relatively large avalanche breakdown voltage.
In the present invention, any features in accordance with the present invention as described below can be arbitrarily combined with each other.
The field-effect transistor according to the present invention can further include a first Al
u
Ga
1−u
As layer (0≦u<1). The first Al
u
Ga
1−u
As layer (0≦u<1) can be provided between a substrate and the channel layer.
The field-effect transistor according to the present invention can further include a second Al
v
Ga
1−v
As layer (0≦v<1). The second Al
v
Ga
1−v
As layer (0≦v<1) can be provided between the gate electrode and the channel layer. This can increase the barrier height against the gate electrode.
In the field-effect transistor according to the present invention, the first Ga
x
In
1−x
As
y
P
1−y
layer can contain donor impurities. The Ga
z
In
1−z
As layer has smaller donor impurity concentration than that of said first Ga
x
In
1−x
As
y
P
1−y
layer.
In the field-effect transistor according to the present invention, the first Ga
x
In
1−x
As
y
P
1−y
layer can be in contact with the Ga
z
In
1−z
As layer (0<z≦1). The first Ga
x
In
1−x
As
y
P
1−y
layer can also have a lattice constant different from that of GaAs semiconductor. This is the case where the Ga
z
In
1−z
As layer preferably has a thickness equal to or smaller than the critical thickness.
In the field-effect transistor according to the present invention, the first Ga
x
In
1−x
As
y
P
1−y
layer can have substantially the same lattice constant as a GaAs layer.
In the field-effect transistor according to the present invention, the channel layer can further have a second Ga
p
In
1−p
As
q
P
1−q
layer (0<p<1, 0≦q<1). The Ga
z
In
1−z
As layer (0<z≦1) can be provided between the first Ga
x
In
1−x
As
y
P
1−y
layer and the second Ga
p
In
1−p
As
q
P
1−q
layer.
In the field-effect transistor according to the present invention, the second Ga
p
In
1−p
As
q
P
1−q
layer can have substantially the same lattice constant as GaAs semiconductor.
In the field-effect transistor according to the present invention, the channel layer can include a Ga
t
In
1−t
P layer (0<t<1) and a GaAs layer.
In the field-effect transistor according to the present invention, the substrate can be a GaAs substrate. The first Ga
x
In
1−x
As
y
P
1−y
layer can also have substantially the same lattice constant as GaAs semiconductor.
The present invention will become more fully understood from the detailed description given

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