JPH0368143A - Heterojunction field-effect transistor - Google Patents

Abstract

PURPOSE:To augment a two-dimensional electron gas concentration and to increase an electron mobility as well by a method wherein the forbidden band width of a semiconductor layer in the center in the thickness direction of channel layers is made narrower than those of semiconductor layers on the sides of both end parts of the channel layers. CONSTITUTION:A non-doped GaAs buffer layer 2, non-doped InyGa1-yAs channel layers 3, 4 and 5, a non-doped AlxGa1-xAs spacer layer 6, an Si-doped AlxGa1-xAs electron feeding layer 7 and an Si-doped GaAs contact layer 8 are formed in order on a semi-insulative GaAs substrate 1. Moreover, in respect to an energy difference in a conduction band, the layer 4 among the layers 3 to 5 is larger than the layers 3 and 5 and the forbidden band width of the layer 4 is narrower than those of the layers 3 and 5. As a result, a potential in the vicinity of the center of the thickness direction of the channel layers is reduced, a two-dimensional electron gas concentration is increased and an electron mobility is also augmented. Moreover, a mutual conductance is increased and a high efficiency can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a field effect transistor that utilizes two-dimensional electron gas at a heterojunction interface.

[Conventional technology]

Heterojunction field effect transistors that utilize two-dimensional electron gas generated at the heterojunction interface, such as high electron mobility transistors ()IEMTs, are attracting attention as ultra-high-speed semiconductor devices.
Soomorphic (Pseudomorphic) using yAs strained layer
It is known that (IEEE
Electron Device Lett, EDL
-6+ 453.1985).

This means that the electron mobility of Ir+y Gap-yAs is GaA
This is because C is larger than s, the forbidden band width of InyGa and -yAs is smaller than that of GaAs, and the secondary electron gas concentration can be increased, and furthermore, the secondary electron gas is well confined and the mutual conductance can be increased.

Figure 3 shows the conventional Aj! xGa+-xAs-InyGa
1 is a cross-sectional structural diagram of a conventional sootmorphic HEMT having a +-yAs-GaAs heterojunction, in which each layer shown in Table 1 is sequentially laminated on a semi-insulating GaAs substrate 11 by molecular beam epitaxy technology.

(Margin below) Then, on such a heteroepitaxial substrate, Au-
The source electrode 19 is formed using ohmic metal such as Ge/Ni.
a, forming a drain electrode 19b and forming a source electrode Fli19;
a. Etch and remove the contact layer between the drain electrodes 19b to expose the electron supply layer 17, and form the gate electrode 20 made of ^l or Ti/Pt/Au on the surface of the exposed electron supply layer 17. Figure 4 shows the soomorphic IIE structure as described above.
It is a conduction band energy diagram under the gate electrode 20 in MT, for example, non-doped I constituting the channel layer 13.
The y value of nyGa+-yAs is set to 0.15. Electronic supply Fi1
5 of Sj-doped Al xGaI-xAs comprising 7
iffj, degree 2×10”am-', x value 0.2
0, the conduction band energy difference ΔEc at the InyGaAs-^1GaAs heterojunction interface is approximately 0.35 eV. By the way, the conventional general AI GaAs-Ga
In As heterojunction FIEMT, if the X value, which is the AIMi composition of the electron supply layer, is 0.3, the conduction band energy difference ΔE
c is approximately 0.3 eV.

By the way, in general, if the X value, which is the A1 composition of GaAs, becomes large (0.25 or more), the conduction band energy difference ΔE
Although c can be increased, it is known that the Si impurity level becomes deeper, ionized impurities decrease, photoresponsiveness is observed, and in extreme cases, the two-dimensional electron gas concentration n decreases.

In this respect, in soomorphic HEFjT, as mentioned above, A
I Lower the X value, which is the A1 composition of GaAs (0,25
(below), a sufficient conduction band energy difference ΔPc can be obtained, and the photoresponsivity, which has been a problem in conventional HEMTs, can be suppressed.

In addition, in a normal HEMT of A I GaAs-GaAs heterojunction, the two-dimensional electron gas concentration n is 0.8
IOth (J-”electron mobility μ=4000aJV-'5
While it is ec-, soomorph 4yri IIHMT
Then ns □1.4X10"am-"electron mobility μ=5
000ajV-'5ec-bow. Furthermore, the mutual conductance g, which is one of the characteristic values that indicates the high speed performance of a transistor, is 50m5 in a conventional HEMT, whereas the soomorph ink H1l! MT has the advantage of being able to obtain a value of about 60*S.

[Problem to be solved by the invention]

By the way, such a soomorphic HE? IT-
One possible method for improving the performance of the layer is to increase the y value, which is the In composition in the Iny Ga, -yAs constituting the channel layer.

This is because when the y value is increased, the forbidden band width of the channel layer becomes narrower, the conduction band energy difference ΔEc can be increased accordingly, the two-dimensional electron gas concentration n increases, and the electron mobility also increases.

However, on the other hand, when the y value is increased, the lattice constant of 1nGaAs increases, and GaAs or A j! A problem of lattice mismatch with GaAs occurs. In general, the lattice constant of GaAs is approximately 5.6535.
The lattice misalignment (Δa/a) of GaAs and A1^S is approximately 1.5 Xl0-', while that of GaAs and In
The degree of lattice misalignment with As is as large as approximately 7×10−”.Therefore, AlGaA, which is a mixed crystal of GaAs and As,
The lattice mismatch of s with GaAs is small and poses no problem, but I
The mismatch between nGaAs and GaAs is at a level that cannot be ignored.

For this reason, conventionally, in soomorphic HEMTs, the influence of lattice mismatch has been suppressed by reducing the thickness of the InGaAs layer, but as mentioned above, when the y value is increased to improve performance, the lattice mismatch is The InGaAs layer would need to be made thinner to avoid mismatch. Moreover, in HEMT, when the channel layer thickness is reduced to less than 100 Å, the two-dimensional electron gas concentration n becomes smaller, which is a disadvantage.In conventional sootmorph ink, the y value is about 0.15 in IEMT. has been selected.

The present invention was made in view of the above circumstances, and its purpose is to increase the y value of In&u;
Moreover, it is an object of the present invention to provide a heterojunction field effect transistor that can eliminate the disadvantages caused by lattice mismatch.

[Means to solve the problem]

The heterojunction field effect transistor according to the present invention has a semiconductor channel layer having a narrower band gap than the semiconductor substrate and a semiconductor electron supply layer having a wider band gap than the semiconductor substrate on a semi-insulating semiconductor substrate, in this order. In the heterojunction field effect transistor having a stacked structure, the semiconductor channel layer is characterized in that the forbidden band width at the center in the thickness direction is narrower than at both sides.

[Effect]

According to the present invention, the two-dimensional electron gas concentration can be increased while suppressing lattice mismatch, and the electron mobility can also be increased.

[Example 1] The present invention will be specifically described below based on drawings showing examples thereof.

FIG. 1 is a cross-sectional structural diagram of a sootmorph ink (IEMT) according to the present invention, and numeral 1 in the figure is a semi-insulating GaAs substrate. On the surface of this semi-insulating substrate 1, a plurality of layers as shown in Table 2 are laminated in the following order by molecular beam epitaxy (MBB) technology.

(Margin below) Figure 2 is a diagram of the conduction band energy under the gate electrode of the sootmorph ink HEJIT as described above, with the horizontal axis showing the position in the thickness direction and the vertical axis showing the energy. . As is clear from this energy diagram, the conduction band energy difference ΔEc is such that the channel layer 4, which is the center in the thickness direction of the channel layer 3.4.5, has the channel N on both sides.
3.5, and the forbidden band width of channel layer 4 is narrower than that of channel layer 3.5. The conduction band energy difference ΔEc is GaAs-1nG
Approximately 0.15 eV at the aAs heterojunction, and A f!
Approximately 0.3 at the GaAs-TnGaAs heterojunction
It is 5eV.

In this example, Ina near the center of the channel layer
As a result of increasing the y value to 0.4, the potential near the center in the thickness direction decreases, and the two-dimensional electron gas concentration n
, and the electron mobility also increased. In addition, the mutual conductance g1 was 70 m5, and it was found that higher performance than the conventional soomorphic HEMT could be obtained.

In addition, as a result of measuring the two-dimensional electron gas concentration n3 and electron mobility μ of a heteroepitaxial substrate having a structure similar to that of the pseudomorphic IIEMT described above, it was found that ns −2XI
O"(J-"%μ=7500coi"V-'5ec
-', indicating that properties superior to conventional soomorphic HEMTs can be obtained.

[Example 2] Non-doped InyGa+-y constituting the channel layer 3 in a soomorphic HEMT as shown in FIG.
The y value of As is 0.15 on the buffer layer 2 side and 0.40 on the boundary with the channel layer 4, making the set tc slope type.
Furthermore, non-doped InyGa constituting the channel layer 5,
-ylls is 0.40 on the channel layer 4 side and 0.15 on the spacer layer 6 side. Even in this structure, substantially the same excellent effects as in Example 1 were obtained.

In addition, if the y value of channel N4 is further increased, it is desirable to use a U or inclined type in order to suppress the misalignment, so the y value is 10M1 near the center in the thickness direction.
A composition gradient type may be used over the entire region so that the value is maximized, and the same effect can be obtained.

In addition, other heterojunctions, such as InA RAs-1nG
Of course, the present invention can also be applied to other soomorph ink+IEMTs such as aAs or TnGaA It As-1nGaAs heterojunction.

[Gain fruit]

As described above, in the present invention, in order to suppress the lattice mismatch in the channel layer containing the two-dimensional electron gas, the forbidden band width in the semiconductor layer at the center in the thickness direction of the channel layer is made narrowest. High-performance soomorphic l that can increase the dimensional electron gas concentration and increase the amount of electron transfer.
(If EMT can be obtained, the present invention has excellent effects.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional structural diagram of the product of the present invention, FIG. 2 is a conduction band energy diagram thereof, FIG. 8 is a cross-sectional structural diagram of a conventional product, and FIG. 4 is a conductive band energy diagram thereof. 1... Semi-insulating substrate 2... Buffer layer 3.4
．． 5... Channel layer 6... Spacer layer 7...
Electron supply layer 9a...Source electrode 10...Gate electrode 8...Contact layer 9b...Drain electrode Patent Applicant: Sanyo Electric Co., Ltd.

Claims (1)

[Claims] 1. A semiconductor channel layer having a narrower band gap than the semiconductor substrate and a semiconductor electron supply layer having a wider band gap than the semiconductor substrate are laminated in this order on a semi-insulating semiconductor substrate. 1. A heterojunction field effect transistor having a structure, wherein the semiconductor channel layer has a forbidden band width at a central portion in the thickness direction thereof narrower than at both side portions.