JP2921835B2 - Heterojunction field effect transistor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a field effect transistor using a heterojunction structure.

2. Description of the Related Art A heterojunction field effect transistor (FET) using a heterojunction of AlGaAs and GaAs is widely used as a high-speed switching element or a microwave transistor. A typical heterojunction FET is a high electron mobility transistor (HEMT) that forms an N-type AlGaAs layer on a non-doped GaAs layer and uses a high mobility two-dimensional electron gas that accumulates on the GaAs side of the heterojunction interface. N-type AlGaAs
Since there is a difficulty in obtaining ohmic contact with the two-dimensional electron gas through the layer, an improved inverted HEMT structure has been considered. 2a and 2b show a cross-sectional structure diagram and an energy band diagram of the inverted HEMT structure. In FIG. 2a, 1 is a semi-insulating GaAs substrate, 2 is a non-doped GaAs buffer layer having a thickness of about 100 nm, and 3 is a thickness of 100 to 200 nm.
Non-doped AlGaAs layer 4 is an N-type AlGa
As layer, 5 is a non-doped AlGaAs spacer layer, 6 is a non-doped GaAs channel layer, 7 is an N-type GaAs layer doped with an N-type impurity at a low concentration (濃度 10 ¹⁷ / cm ³ ), and 8 is a high-concentration (> 10 ¹⁸⁾ / c
m ³ ) is an N ⁺ -type GaAs layer in which an N-type impurity is added. In this structure, two-dimensional electron gas accumulates on the GaAs side of the heterojunction 2 between the layers 5 and 6, but it is necessary to prevent the two-dimensional electrons from accumulating at the heterojunction 1 between the layers 2 and 3. The thickness of the GaAs layer of layer 2 is as thin as about 100 nm,
The thickness of the aAs layer must be relatively thick, 100 to 200 nm, and the bottom of the GaAs conduction band in the heterojunction 1 must be higher than the Fermi level as shown in the energy band diagram of FIG. No.

As described above, in the inverted HEMT structure, since the layer above the two-dimensional electron gas is a GaAs layer, the ohmic contact resistance can be reduced when the ohmic contact with the two-dimensional electron gas is obtained from the surface. However, there is a disadvantage in that some restrictions are imposed on the layer structure. In particular, when the GaAs buffer layer 2 is thin, the entire epitaxial layer becomes thin, and the interface between the substrate and the layer 2 is easily affected. Since the non-doped AlGaAs layer 3 is as thick as 100 nm to 200 nm, the heterojunction 2
Since the interface unevenness becomes large, the crystal quality of GaAs in the channel layer is reduced, and the mobility of the two-dimensional electron gas is reduced.

Further, when a high electric field is applied in a direction parallel to the heterojunction interface 2 and electrons are caused to travel along the channel layer, the electrons become hot due to the electric field and overflow into the AlGaAs layers 3, 4, and 5, but the AlGaAs layers 3, 4, Electrons are longer due to thicker AlGaA
There is a problem that the probability of traveling in the s layer increases, which may reduce the average speed of electrons.

Means for Solving the Problems The present invention uses the following means in order to solve the problems associated with the above-described inverted HEMT structure. That is, semi-insulating
GaAs substrate and GaAs formed on the semi-insulating GaAs substrate
A buffer layer, formed on the GaAs buffer layer,
Thin Al _X Ga _1-X As partially doped with N-type impurities
An electron supply layer having a thickness of 20 nm or less, a channel layer composed of an In _y Ga _1-y As layer (0 <y ≦ 0.2), and a GaAs layer formed on the channel layer. Heterojunction field effect transistor having a heterojunction structure.

The reverse effect is achieved by using the In _y Ga _1-y As layer for the channel layer.
The structural constraints in the HEMT structure are solved. That is,
When the In _y Ga _1-y As layer is used as the layer 6 in FIG. 2, the bottom of the conduction band energy of the In _y Ga _1-y As layer is always lower in energy than that of GaAs. Most of the electrons supplied from the GaAs buffer layer 2 enter the In _y Ga _1-y As layer.
Even if the thickness is large or the AlGaAs layer 3 is thin, almost no electrons are supplied to the heterojunction 1.

Taking this into consideration, the adverse effect of the interface between the substrate and the epitaxial layer is reduced by forming the GaAs buffer layer thick. Also, by reducing the overall thickness of the AlGaAs layers 3, 4, and 5, it is possible to improve the crystallinity of the epitaxial layer on the AlGaAs layer and reduce the unevenness at the interface. Furthermore, it became hot when a high electric field was applied.
The two-dimensional electron gas quickly passes through the thin AlGaAs layer and increases the probability of traveling through the low energy GaAs buffer layer, thereby improving the average electron velocity.

Embodiment An embodiment of the present invention will be described in detail with reference to FIGS. FIG. 1a is a cross-sectional view of an inverted HEMT structure according to the present invention, and FIG. 1b is an energy band diagram thereof. First
In FIG. 1A, 1 is a semi-insulating GaAs substrate, and 2 is a non-doped GaAs buffer layer. The thickness of the non-doped GaAs buffer layer 2 needs to be increased to some extent in order to sufficiently supply electrons to the In _y Ga _1-y As channel layer 13, and at least 300 nm is required. In the embodiment, the thickness is set to 500 nm. Although there is no particular upper limit, about 1000 nm is appropriate in consideration of the time required for epitaxial growth. On the non-doped GaAs buffer layer 2, Al serving as an electron supply layer is formed.
GaAs layers 10, 11 and 12 are formed. Reference numerals 10 and 12 denote non-doped AlGaAs spacer layers, whose thickness is suitably about 2 to 5 nm. 11 the film thickness is an N-type AlGaAs layer is somewhat varied depending on the amount by doping amount of N-type impurity, the product of the impurity concentration N _D and the thickness _{d, N D · d1.2~1.6 × 10} 12 / cm ^What is necessary is just to determine so that it may become ^two . In the embodiment, N _D = 2 × 10 ¹⁸ / cm ³ . Therefore, the film thickness is 6 to 8 nm. Reference numeral 13 denotes a non-doped In _y Ga _1-y As layer, which serves as a channel layer of the FET and serves as a layer in which a two-dimensional electron gas accumulates. The In _y Ga _1-y As layer 13 includes an AlGaAs layer and a GaAs layer. Although lattice matching cannot be achieved originally,
A good crystal is obtained while keeping the strain inside. Since the thickness of the channel layer is considered to be appropriately 10 nm to 20 nm, the composition ratio y of In may be set to 0.2 or less in consideration of the critical thickness of the In _y Ga _1-y As layer. 14 is a non-doped or GaAs layer doped with about 10 ¹⁷ / cm ³ of N-type impurity,
Is an N ⁺ -type GaAs layer for obtaining a good ohmic contact. In the embodiment, 14 is a non-doped GaAs layer.

FIG. 1B shows the conduction band energy band diagram of the heterostructure of this embodiment. The conduction band energy discontinuity of the In _y Ga _1-y As layer 13 AlGaAs layer 12 and that of the GaAs layer 2 and AlGaAs layer 10 are shown. Therefore, most of the electrons supplied from the N-type AlGaAs layer 11 accumulate toward the In _y Ga _1-y As layer. As this electron concentration, a high value of about 1.5 × 10 ¹² / cm ² or more was obtained. Note that the Al composition ratio in AlGaAs of this embodiment was 0.15 to 0.3.

According to the present invention, the constraint on the thickness of the buffer layer, which is a problem in the inverted HEMT structure, is solved, and the GaAs buffer layer can be formed to be as thick as about 500 nm or more.
The adverse effect of the interface between the semi-insulating GaAs substrate and the epitaxial layer can be reduced. In addition, the entire thickness of the AlGaAs layer serving as the electron supply layer can be made extremely thin as 10 nm to 20 nm as compared with the conventional one, so that the crystallinity of the epitaxial layer on the AlGaAs layer can be improved, and unevenness at the interface between them can be reduced. Can be reduced. Also, since the AlGaAs layer is thin, hot electrons that overflow to the AlGaAs layer side when a high electric field is applied pass through the AlGaAs layer immediately and move to the GaAs buffer layer, so that the average electron speed degradation can be reduced compared to the conventional method. The inverted HEMT structure according to the present invention not only has less restrictions on the design as compared with the conventional one, but is also very useful for improving the characteristics of the heterojunction FET.

[Brief description of the drawings]

FIG. 1a is a sectional view of a field-effect transistor according to one embodiment of the present invention, FIG. 1b is a conduction band energy band diagram of the transistor, FIG. 2a is a sectional view of a conventional field-effect transistor, FIG. 2b is a conduction band energy band diagram of the transistor. 1 ... Semi-insulating GaAs substrate, 2 ... Non-doped GaAs buffer layer, 10,12 ... Non-doped AlGaAs spacer layer, 11 ...
... N-type AlGaAs layer, 13 ... Non-doped In _y Ga _1-y As layer, 14 ...
... Non-doped GaAs layer, 15 ... N ⁺ type GaAs layer.

 ──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-59-100577 (JP, A) JP-A-60-231366 (JP, A) JP-A-60-12773 (JP, A) JP-A-61-100 3464 (JP, A) JP-A-62-256477 (JP, A) JP-A-62-276882 (JP, A)

Claims (1)

(57) [Claims] A semi-insulating GaAs substrate, a GaAs buffer layer formed on the semi-insulating GaAs substrate, a thin GaAs buffer layer formed on the GaAs buffer layer and partially doped with N-type impurities. 20 thickness including Al _X Ga _1-X As layer
nm, a channel layer composed of an In _y Ga _1-y As layer (0 <y ≦ 0.2), and a GaAs layer formed on the channel layer. Having a heterojunction field effect transistor.