JPH02226733A - Semiconductor device - Google Patents

Abstract

PURPOSE:To prevent electrons from exuding while maintaining the high crystalline quality for enhancing the noise characteristics by a method wherein the first undoped layer in contact with a semi-insulating semiconductor substrate is thickly formed while the second undoped layer formed into an electron feeding layer and a hetero junction is thinly formed. CONSTITUTION:Within a HEMT element 10, a super lattice layer 12, the second undoped layer 13 and an AlGaAs layer 104 as an electron feeding layer are successively laminated on the first undoped layer 11 formed on a semi-insulating GaAs substrate 101. At this time, the layer 11 in thickness of e.g. 0.35mum and sufficiently high resistance while the super lattice layer 12 is formed of respective ten layers of AlGaAs layers e.g. 50Angstrom thick and GaAs layers 50Angstrom thick alternately laminated. Furthermore, the second undoped layer 13 as a thin layer e.g. 500Angstrom thick in sufficiently high resistance becomes an electron running layer. Through these procedures, the title semiconductor device can prevent electrons from exuding while maintaining the high crystalline quality and high gm.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) This invention relates to a semiconductor device, and in particular to a high electron mobility transistor (llighElec) using two-dimensional electron gas.
tron Mobility Transistor,
(hereinafter abbreviated as HEMT).

(Prior art) HEHT is a method of heterojunction of two types of semiconductors with different band gaps, and by controlling the electron concentration of a two-dimensional electron gas formed at the heterojunction interface by applying a voltage to a control electrode. , is known as a three-terminal active semiconductor device that controls the amount of two-dimensional electron gas traveling as a carrier.

An example of the structure of a conventional) IEMT device is shown in a cross-sectional view in FIG. 4. As shown in FIG.
is an undoped GaAs layer 102 which is a buffer layer formed on a semi-insulating GaAs substrate lot, and an AlGaAs layer with a high impurity concentration which is stacked on this undoped GaAs layer 102 and forms a heterojunction! 04, and the buffer layer (undoped GaAs layer 10
A two-dimensional electron gas 103 serving as an electron transit layer is formed on the 2) side. Furthermore, an IEMτ element is constructed by comprising a gate electrode 105G which is a control electrode provided on the AlGaAs layer 104, and a source electrode iqss and a drain electrode 1050 provided on the AlGaAs layer 104 so as to sandwich the gate electrode 105G. has been done.

The characteristics of the HE (MT) described above are that the electron supply layer 104 and the electron transport layer 103 are separated by a heterojunction, and the two-dimensional electron gas as a carrier travels on the buffer layer side with fewer impurities, so impurity scattering is reduced. Therefore, HEMT is expected to be used as a low-noise device at high frequencies.In order to obtain such a high electron mobility μ, it is necessary to At least the upper part of the layer 102 needs to be a high-quality crystal. However, when the thickness of the buffer layer 102 is less than 0.4 μm, residual impurities on the semi-insulating substrate 101 and the interface Due to the adverse effects, the crystal quality of the buffer layer deteriorated and the electron mobility μ decreased.Therefore, the buffer layer 102 is made sufficiently thicker than the required thickness for the electron transport layer 103, for example, a thickness of about 0.5μ. At least the upper layer portion of the buffer layer 102 is made of high-quality crystal.

(Problems to be Solved by the Invention) In order to further improve the high frequency characteristics of HEMT, shortening the gate length is considered to be an effective means.

By shortening the gate length, the source-gate capacitance C
This is because it is expected to reduce gs and increase mutual conductance g, and contribute to improving noise characteristics through increasing cutoff frequency f7. However, as the gate length is shortened, the electric field becomes high at the end of the gate electrode on the drain side in the operating state, so in this part the electrons as carriers no longer flow through the heterojunction interface as secondary electron gas. g in the static characteristics of the IEMT.

A problem arises in that the bottom jamming becomes noticeable. Specifically, this is a problem in HEMTs with gate lengths of 0.3 μm or less; for example, HEMs with gate lengths of 0.25 μm
At T, static characteristics as shown in FIG. 5 are exhibited.

Such a decrease in g, that is, a reduction in g at low drain current degrades the noise characteristics. Therefore, the effect of improving noise characteristics by shortening the gate length was not necessarily sufficient. For example, set the IEMT gate length to 0.4
Even when shortening from μ-Feng to 0.25μ-Feng, the NF (Noise Figure) value hardly changes at 18 GHz, with an average value of 1
．． It was 0dB.

SUMMARY OF THE INVENTION In view of the above problems, it is an object of the present invention to provide a HUNT structure with significantly improved low noise characteristics.

[Structure of the invention]

(Means for Solving the Problem) A semiconductor device according to the present invention includes a first undoped semiconductor layer on a semi-insulating semiconductor substrate, a superlattice layer on the first undoped semiconductor layer, and a second undoped semiconductor layer on the superlattice layer. An undoped semiconductor layer, a highly impurity-concentrated electron supply layer having a wider forbidden band width is sequentially laminated on the second undoped semiconductor layer, and gate, source, and drain electrodes are formed on the electron supply layer. Of the two undoped layers provided, the first undoped layer in contact with the semi-insulating semiconductor substrate is formed sufficiently thick, and the superlattice layer has a steep potential for electrons.
The second undoped layer is formed as a barrier and further forms a heterojunction with the electron supply layer, which is formed sufficiently thin. In particular, the thickness of the second undoped layer is 20
00 Å or less. In particular, the thickness of the first undoped layer was such that the sum of this, the thickness of the superlattice layer, and the thickness of the second undoped layer was 0.5 μm or more.

(Function) The HEMT according to the present invention uses the above-mentioned means and basically includes a superlattice layer between the first undoped layer and the second undoped layer. The feature of this invention is that the second undoped layer is formed sufficiently thin to effectively prevent electron seepage in order to utilize the superlattice layer as a steep potential barrier against electrons.

(Example) An embodiment of the present invention will be described below with reference to the drawings.

In addition, in the description, parts that are the same as in the conventional example are shown in the drawings with the same reference numerals as in the conventional example, and the description thereof will be omitted.

The structure of a HEMT element according to an embodiment of the present invention is shown in cross-sectional view in FIG.

As shown in FIG. 1, H1l! MT Element Publishing describes the first undoped Ga formed on the element edge GaAs substrate 101.
An As layer 11 and a superlattice layer 12 . Second undoped GaAs
A layer 13° and an AlGaAs layer 104 as an electron supply layer are formed. The first undoped GaAs layer 11 has a thickness of 0.35 .mu.m and has a sufficiently high resistance, and the superlattice layer 12 has a thickness of 50 μm, for example.
Ten As layers and 50 GaAs layers each having a thickness of 10 layers are stacked alternately. Further, the second undoped G
The aAs layer 13 is, for example, a 500-layer thin layer with sufficiently high resistance to serve as an electron transit layer. Furthermore, the AlGaAs layer 104 is formed by doping Si to 2.times.10"cm" and continuously growing the layer to a thickness of 300 cm using, for example, a molecular beam epitaxy (MBE) apparatus. The side where the second undoped GaAs layer 13 is in contact with this AlGaAs layer 104 becomes a two-dimensional electron gas 103.

Next, a control electrode gate electrode 105G formed of Al on the AlGaAs layer 104, and a source electrode 1055 and a drain electrode 105D formed of Ni/AuGe on the AlGaAs layer 104 sandwiching this gate electrode 105G are provided. , HE-D elements are constructed.

The following experiment revealed that the layer thickness of the second undoped GaAsJllll13 is important.

The experiment was conducted using an IEMT (with a gate length of 0.25μ).
The total thickness of the first undoped layer, the superlattice layer, and the second undoped layer is 5,000 layers, and the thickness of the first undoped layer and the 27th undoped layer is changed to +1EM.
The effect on the characteristics of T was investigated. Figure 3 shows how the characteristics of the intrinsic transconductance g, , change at this time.
High intrinsic transconductance g, by forming 2μ or less. It can be seen that it is possible to maintain This high gso
speculated that this occurs because the superlattice effectively acts as a barrier to prevent electrons from seeping out.

For example, a first undoped GaAs layer 11 with a layer thickness of 0.35 μm and a second undoped GaAs layer 13 with a layer thickness of 500 μm.
A high gwao can be maintained by inserting a superlattice layer 12 with a thickness of 1000 between these two layers.

In addition, a remarkable effect was obtained regarding the lower clogging of g. For example, a HEMT formed with a gate length of 0.25μ
Figure 2 shows the static characteristics of HEMT, and Figure 5 shows the static characteristics of HEMT with conventional structure.
is 16m5 in the conventional structure, whereas in the present invention, it is 35@S
As a result, the effect of improving g.

Furthermore, looking at the low noise characteristics at high frequencies, which is a feature of ) IEMT, at 18 GHz, IDS=
The NF (noise figure) value of 10 mA was 0.8 dB compared to 1.0 dB in the conventional example, and a remarkable effect was recognized.

Next, although the above embodiment describes a superlattice made of AlGaAs/GaAs, other materials such as AlA
It goes without saying that it is also effective in superlattices made of s/GaAs, and also in recessed structure) IEMT.
It is also effective in forming undoped AlGaAs between the AlGaAs electron supply layer and the undoped GaAs layer.
It is also effective to have a structure in which a spacer layer made of S is provided.

(Effects of the Invention) According to the present invention, two undoped layers and a superlattice layer inserted between them are provided in place of the conventional buffer layer, and the first undoped layer in contact with the semi-insulating semiconductor substrate is sufficiently By forming the second undoped layer thickly and forming a sufficiently thin second undoped layer that forms a heterojunction with the electron supply layer,
Prevents electron leakage while maintaining high crystal quality, and
It is possible to improve the bottom jam and maintain high g. As a result, the low noise characteristics can be significantly improved compared to HEMT.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing the structure of an IEMT according to an embodiment of the present invention, and FIG. 2 is a cross-sectional view showing the structure of an IEMT according to an embodiment of the present invention.
A diagram showing the static characteristics of EMT, Figure 3 shows the thickness of the second undoped layer and the intrinsic transconductance ga shown in Figure 1.
FIG. 4 is a cross-sectional view showing the structure of a conventional HEMT, and FIG. 5 is a diagram showing the static characteristics of a conventional IIIEMT. 10------ HEMT element 17th n-doped GaAs layer superlattice layer 27th n-doped GaAs layer

Claims (2)

[Claims] (1) A sufficiently thick first undoped semiconductor layer and a superlattice layer formed by sequentially stacking layers on a semi-insulating semiconductor substrate, and a sufficiently thin second undoped semiconductor layer on which a two-dimensional electron gas is formed on the superlattice layer. , a highly impurity-concentrated electron supply layer having a wider forbidden band width than the second undoped semiconductor layer, and a gate, source, and drain electrode formed on the electron supply layer. (2) The semiconductor device according to claim 1, wherein the second undoped semiconductor layer has a thickness of 2000 Å or less.