JPH05121455A - Compound semiconductor device - Google Patents

Abstract

PURPOSE:To provide a compound semiconductor device, having a HEMT structure, wherein its source resistance is reduced and its noise characteristic has been improved by the reduced source resistance. CONSTITUTION:A compound semiconductor device is provided with the following: an undoped semiconductor layer 2; a doped semiconductor layer 3 which has been formed on the undoped semiconductor layer 2, whose electron affinity is smaller than that of the undoped semiconductor layer 2 and which has been doped with impurities; a gate electrode 4 formed on the doped semiconductor layer 3; and a source electrode 6 and a drain electrode 7 which have been formed respectively on the doped semiconductor layer 3. An undoped substance layer 9 whose electron affinity is larger than that of the doped semiconductor layer 3 and a cap layer 5 is formed between the doped semiconductor layer 3 and the cap layer 5.

Description

Detailed Description of the Invention

[0001]

The present invention relates to HEMT (High Electr)
onMobility Transistor) and the like.

[0002]

2. Description of the Related Art A HEMT structure has been attracting attention as a field effect transistor utilizing a two-dimensional electron gas accumulated at a heterojunction interface. As shown in FIG. 4, this HEMT structure is formed on the substrate 1 by an undoped semiconductor layer 2, a doped semiconductor layer 3 having a smaller electron affinity and doped with impurities, and an impurity doped region 3 formed thereon. The gate electrode 4 and the source electrode 6 and the drain electrode 7 formed on the cap layer 5 on both sides of the gate electrode 4 are added to the doped semiconductor layer 3 having a small electron affinity in this HEMT structure. The formed donor impurities are all ionized, and the electrons generated by this ionization are accumulated at the hetero interface with the AND semiconductor layer 2 having a large electron affinity to form a two-dimensional electron gas 8.

The two-dimensional electron gas 8 can be controlled by applying a voltage to the gate electrode 4, and thus the current flowing between the source and drain can be controlled. At this time, the current flowing from the source flows into the two-dimensional electron gas 8 through a plurality of paths as indicated by I1 and I2.

FIG. 5 shows energy bands of the HEMT shown in FIG. 4, of which (a) is a cross section taken along line XX 'and (b).
Are energy band diagrams of the YY 'cross section. In these figures, EF indicates Fermi level.

[0005]

By the way, since the doped semiconductor layer 3 is in a completely ionized state, electrons are between the cap layer 5 and the two-dimensional electron gas 8, that is, a completely ionized doped region. It has to pass through the semiconductor layer 3 by the tunnel effect. Therefore, the source resistance becomes very large, and the noise characteristic is deteriorated particularly in the high frequency band.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a compound semiconductor device having a HEMT structure in which the source resistance is reduced and thereby the noise characteristics are improved.

[0007]

In order to achieve the above object, the present invention provides an AND semiconductor layer and an electron affinity smaller than that of the AND semiconductor layer formed on the AND semiconductor layer. A doped semiconductor layer doped with impurities, a gate electrode formed on the doped semiconductor layer, and source and drain electrodes formed on the doped semiconductor layer, respectively. In the compound semiconductor device having the above-mentioned, a material layer having an electron affinity higher than that of the doping semiconductor layer or the cap layer is formed between the doping semiconductor layer and the cap layer.

[0008]

According to this structure, the energy band of the cross section including the cap layer on the source side has a lower potential as a whole. Therefore, the doped semiconductor layer is not completely ionized, and electrons remain in the central portion of the doped semiconductor layer. As a result, electrons easily pass through the doped semiconductor layers, and the source resistance becomes low.

[0009]

FIG. 1 in which the present invention is embodied is significantly different from the conventional example of FIG. 4 in that a new layer 9 is provided between the cap layer 5 and the doped semiconductor layer 3. . This layer 9
Is a material having a higher electron affinity than the doped semiconductor layer 3 and the cap layer 5, and is formed to be very thin, and is an anddope layer. The layer 9 is formed of, for example, an undoped InGaAs layer having a thickness of several tens of angstroms.

Here, the layer 9 is formed thin because it is difficult to form because the crystal lattice constant becomes large if the layer 9 is thick. Therefore, if it is relatively thick and easy to make, it need not be thinned. In FIG. 1, the materials of the other layers will be specifically described. The undoped semiconductor layer 2 is GaAs or InGaAs, the doped semiconductor layer 3 is n + AlGaAs, and the cap layer 5 is GaAs.

When the above-mentioned layer 9 is provided, the entire potential of the energy band in the XX 'section of FIG. 1 is lowered as shown in FIG. 2, and the doped semiconductor layer 3 is not completely ionized. There are electrons near its center. Therefore, electrons easily pass through the doped semiconductor layer 3, and the resistance on the source side is reduced accordingly. The noise index is represented by NF = 1 + K (f / fT) √ {gm (Rs + Rg)}. In this equation, K is a fitting constant, f is a frequency, fT is a cutoff frequency, gm is a mutual inductance, Rs is a source resistance, and Rg is a gate resistance. As can be seen from the above, noise is reduced in the HEMT having a small source resistance Rs as in the first embodiment.

Next, FIG. 3 shows a HEMT having two undoped semiconductor layers provided with the above-mentioned layer 9, and in this case, a similar effect is produced. In FIG. 3, the undoped semiconductor layer 2a is made of, for example, InGaAs, and 2b is made of AlGaAs. Further, a buffer layer made of GaAs may be interposed between the substrate 1 and the undoped semiconductor layer 2a.

[0013]

As described above, the compound semiconductor device of the present invention has the effects of reducing the source resistance and improving the noise index.

[Brief description of drawings]

FIG. 1 is a structural diagram of a HEMT structure compound semiconductor device embodying the present invention.

FIG. 2 is an energy band diagram in the XX ′ cross section.

FIG. 3 is a structural diagram of another embodiment of the present invention.

FIG. 4 is a structural diagram of a conventional example.

FIG. 5 is an energy band diagram in the XX ′ cross section and the YY ′ cross section.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2, 2a, 2b AND semiconductor layer 3 Dope semiconductor layer 4 Gate electrode 5 Cap layer 6 Source electrode 7 Drain electrode 8 Two-dimensional electron gas 9 AND material layer.

Claims (3)

[Claims] 1. An undoped semiconductor layer, a doped semiconductor layer formed on the undoped semiconductor layer, having an electron affinity smaller than that of the undoped semiconductor layer, and doped with impurities. A gate electrode formed on the doped semiconductor layer,
A compound semiconductor device having a cap layer formed on the doped semiconductor layer, and a source and drain electrode respectively formed on the cap layer, wherein the doped semiconductor layer and the cap layer are provided. Between the
A compound semiconductor device, wherein an undoped material layer having a higher electron affinity than the semiconductor layer and the cap layer is formed. 2. The compound semiconductor device according to claim 1, wherein the layer thickness of the undoped material layer is several tens of angstroms. 3. The undoped semiconductor layer is composed of either an InGaAs layer or a GaAs layer, the doped semiconductor layer is an n + AlGaAs layer, and the cap layer is GaA.
3. The compound semiconductor device according to claim 1, wherein the s layer and the material layer of the undope are InGaAs layers.