Active solid-state devices (e.g. – transistors – solid-state diode – Schottky barrier – To compound semiconductor
Reexamination Certificate
2000-12-26
2004-11-23
Baumeister, Bradley (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Schottky barrier
To compound semiconductor
C257S192000, C257S194000, C257S472000, C257S745000, C257S751000, C257S763000, C257S768000, C257S769000, C257S770000
Reexamination Certificate
active
06822307
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
The present application is based on Japanese priority application No. 2000-095895 filed on Mar. 30, 2000, the entire contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The present invention generally relates to semiconductor devices and more particularly to a high-speed semiconductor triode having a compound-semiconductor channel layer.
Compound-semiconductor triodes, typical examples being a MESFET or a HEMT, is characterized by high operational speed due to high electron mobility of compound-semiconductor material used for the active layer thereof. Thus, such compound-semiconductor triodes are used extensively for high-frequency or ultra high-frequency applications including GHz band application.
In such compound-semiconductor triodes, too, there holds the scaling law, and efforts are made to reduce the gate length as much as possible for maximizing the operational speed.
A high-speed semiconductor triode having a short gate length is designed, in order to suppress the short-channel effect as much as possible, such that carriers are transported through a shallow, limited surface region of a compound-semiconductor layer used for the active layer of the semiconductor triode.
Thus, the quality of the crystal of the compound-semiconductor layer, particularly the quality of the surface part of the compound-semiconductor layer used for the active layer is extremely important for the operational characteristic of the semiconductor triode.
FIG. 1
shows the construction of a HEMT
10
according to a related art.
Referring to
FIG. 1
, the HEMT
10
is constructed on a semi-insulating InP substrate
11
and includes a channel layer
12
of undoped InGaAs formed epitaxially on the InP substrate
11
and an electron-supplying layer
13
of n-type InAlAs formed also epitaxially on the channel layer
12
. A cap layer
14
of n
+
-type InGaAs is formed on the electron-supplying layer
13
epitaxially, and an opening
14
A exposing the surface of the electron-supplying layer
13
is formed in the cap layer
14
. Further, a gate electrode
15
Is formed on the exposed surface of the electron-supplying layer
13
in the opening
14
A.
In the illustrated example, the gate electrode
15
is a so-called mushroom type or T-type electrode and includes a Ti layer
15
A making a Schottky contact with the exposed electron-supplying layer
13
, a Pt diffusion-barrier layer
15
B formed on the Ti layer
15
A, and a low-resistance Au electrode layer
15
C having the mushroom-shape and formed on the Pt layer
15
B.
By using the Au electrode
15
C with such a mushroom-shape, it becomes possible to reduce the resistance of the gate electrode
15
while minimizing the gate-length of the gate electrode
15
simultaneously. The Pt diffusion barrier layer
15
B, on the other hand, blocks the diffusion of Au atoms from the Au electrode into the electron-supplying layer
13
. Further, the Ti layer
15
A provided between the electron-supplying layer
13
and the Pt layer
15
B improves the adherence of the Pt layer
15
B to the electron-supplying layer
13
.
In the HEMT
10
of
FIG. 1
, it should further be noted that ohmic electrodes
16
and
17
are formed on the InGaAs cap layer
14
in correspondence to contact regions
14
B and
14
C respectively. The ohmic electrode
16
constitutes a non-alloy ohmic electrode and includes a Ti layer
16
A forming an ohmic contact with the n
+
-type cap layer
14
, a Pt diffusion barrier layer
16
B formed on the Ti layer
16
A and a low-resistance Au electrode layer
16
C formed on the Pt diffusion barrier layer
16
B. The ohmic electrode
17
has a similar construction.
Further, the HEMT of
FIG. 1
includes an SiN passivation film
18
covering the exposed part of the electron-supplying layer
13
and the contact regions
14
B and
14
C.
In such a conventional compound-semiconductor triodes, including also MESFETs in addition to HEMTs, the gate electrode
15
makes a direct contact with the semiconductor layer, and thus, there is a substantial risk that Ti atoms may cause a diffusion from the Ti adhesion layer
15
A of the gate electrode
15
into the n-type electron-supplying layer
13
and further into the channel layer
12
underneath the electron-supplying layer
13
. When such a diffusion of Ti is caused in the semiconductor layers constituting the channel of the triode, the threshold characteristic of the device is deteriorated seriously.
FIG. 2
shows such a change of the threshold voltage Vth for the case such a diffusion of Ti is caused from a gate electrode into a channel layer in the case of a conventional MESFET.
Referring to
FIG. 2
, it can be seen that the threshold voltage Vth increases generally linearly with the depth of penetration of the Ti atoms, and that the threshold voltage Vth changes as much as 0.1V with the penetration of only 1 nm in depth. Thus, there is a need for a structure, in compound-semiconductor triodes such as HEMTs or MESFETs, which is effective for suppressing the diffusion of TI atoms from the electrode into the compound-semiconductor layer.
Conventionally, it has been practiced in the art of compound-semiconductor Schottky diode to interpose a metal oxide layer between the Schottky electrode and the compound-semiconductor layer for suppressing the diffusion of metal elements from the Schottky electrode to the compound-semiconductor layer, and hence to suppress the change of Schottky barrier height. In relation to this, reference may be made to Japanese Laid-Open Patent Publication 4-69974.
In this prior art reference, the use of TiOx formed as a result of oxidation of the surface of the metallic Ti layer is described as an example of such a metal oxide layer.
FIG. 3
shows the effect of Ti diffusion on the Schottky barrier height &phgr;
B
of a Schottky diode.
Referring to
FIG. 3
, it can be seen that there occurs no substantial change of Schottky barrier height &phgr;
B
even when the Ti atoms have penetrated into the semiconductor layer with the thickness of several nanometers. Thus, it is concluded that, in the case of a semiconductor Schottky diode, the use of such a metal oxide layer between the semiconductor layer and the Schottky electrode causes no substantial change of diode characteristic.
In the case of a compound-semiconductor triodes such as a HEMT or a MESFET, on the other hand, the situation is different, and penetration Ti of only 1 nm depth in the channel region causes a serious change of the threshold voltage Vth.
In the fabrication process of a semiconductor triode, various annealing steps are applied after a Schottky electrode is formed on a channel layer as a gate electrode. Thus, the foregoing variation of the threshold voltage Vth, caused as a result of Ti penetration, remains a substantial problem in the art of compound-semiconductor triodes.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide a novel and useful compound-semiconductor triode wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a high-speed compound-semiconductor triode stable against thermal annealing process.
Another object of the preset invention is to provide a semiconductor triode, comprising:
a semiconductor layer including a channel layer;
a first ohmic electrode supplying carriers into said channel layer;
a second ohmic electrode collecting carriers from said channel layer; and
a gate electrode controlling a flow of said carriers through said channel layer from said first ohmic electrode to said second ohmic electrode,
said gate electrode including an insulating metal oxide film formed at an interface to a surface of said semiconductor layer.
According to the present invention, the threshold characteristic of the semiconductor triode is stabilized substantially by interposing the metal oxide film. Further, such a structure is advantageous for improving the yield of production of the device.
Preferably, the metal oxide film is formed of an
Nihei Mizuhisa
Watanabe Yuu
Baumeister Bradley
Fujitsu Limited
Westerman Hattori Daniels & Adrian LLP
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