JP2621854B2 - High mobility transistor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a semiconductor device having a heterojunction, and more particularly, to a high mobility transistor having a heterojunction structure.

[Technical background of the invention and its problems]

As a transistor that operates at a high speed and a transistor that operates in a microwave region, a high-mobility transistor is regarded as promising and is being developed.

In this device, a low impurity concentration GaAs layer is grown as an active layer on a semi-insulating substrate, for example, a GaAs substrate having a specific resistance of 10 ⁶ Ωcm or more, and then an n-type AlGaAs layer is grown to form a heterointerface (GaAs / GaAs). A two-dimensional electron gas is formed on the GaAs side of (AlGaAs) and used as a wafer for a high mobility transistor. or,
To increase mobility, a low impurity GaAs layer and n-type AlGaAs
Very thin, low impurity AlGaAs layers may be grown between the layers. In this case, the mobility of the two-dimensional electrons accumulated on the GaAs side increases, but the concentration decreases.

The transistor may be operated at room temperature or may be operated at low temperature. When operating at low temperature, low impurity Al
The rate of increase in mobility that will grow the GaAs layer is greater than the rate of decrease in concentration. However, at room temperature, the rate of increase in mobility is smaller than the rate of decrease in concentration. Therefore, when fabricating a transistor operating at room temperature, a low impurity AlGaAs layer is often not grown. However, it is desired to further increase the two-dimensional electron gas without lowering the mobility of the two-dimensional electron gas.

[Object of the invention]

An object of the present invention is to provide a high-mobility transistor having a structure capable of increasing the concentration of the two-dimensional electron gas without significantly reducing the mobility of the two-dimensional electron gas described above.

[Summary of the Invention]

The structure of the present invention is, for example, a high resistance low impurity GaAs layer, 10 ¹⁷
an extremely thin GaAs layer containing a high-concentration n-type impurity of not less than cm ^{-3 and} forming a two-dimensional electron gas; n having a high concentration of not less than 10 ¹⁷ cm ^-3
It is composed of an AlGaAs layer as a current supply layer containing a type impurity. That is, according to the present invention, an extremely thin layer is formed in a portion adjacent to the current supply layer to form a heterojunction with the current supply layer, and furthermore, this layer is doped with impurities of the same conductivity type as the current supply layer at a high concentration. ing. Then, a two-dimensional electron gas is formed in this layer. This structural diagram is shown in FIG.

FIG. 2 shows the two-dimensional electron concentration ns and the mobility μ at room temperature when there is no extremely thin GaAs layer containing a high concentration of n-type impurity and when there is a GaAs layer containing the n-type impurity at a high concentration (100 A, 200 A). The n-type impurity concentration of the GaAs layer containing a high concentration n-type impurity is 1.5 × 10 ¹⁷ cm ⁻³ , and the n-type impurity concentration of the AlGaAs layer containing a high concentration n-type impurity is 1 ×
70 ¹⁷ cm ^-3 Its thickness is 2000mm. High resistance low impurity GaAs
The impurity concentration of the layer is 1 × 10 ¹³ cm ⁻³ of P type.

As the GaAs layer containing a high concentration of n-type impurity becomes thicker, ns increases and μ decreases. FIG. 3 shows ns.times..mu. in order to examine which effect, the increase in ns or the decrease in .mu., is greater. ns × μ is the thickness d of the GaAs layer containing a high concentration of n-type impurities.
It can be seen that the larger the is, the larger it is.

That is, it can be seen that the effect of increasing ns is greater than the effect of decreasing μ.

〔The invention's effect〕

In a high mobility transistor that usually has a heterojunction,
Its two-dimensional electron concentration cannot exceed 2 × 10 ¹² cm ⁻² . On the other hand, with the structure according to the present invention, a two-dimensional electron concentration exceeding 2 × 10 ¹² cm ⁻² can be obtained by increasing the thickness of the GaAs layer containing a high concentration of impurities. As a result, the gain of the transistor increases, and the transistor can be applied to a power transistor by increasing the power.

(Example of the invention)

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 4 is a structural view of an epitaxial wafer for explaining the present invention. 11 is a semi-insulating (100) GaAs substrate, and 12 to 15 are epitaxial layers grown thereon by molecular beam epitaxy. Reference numeral 12 denotes an undoped GaAs layer having a thickness of 1 μm. 13 is GaAs obtained by doping 1.5 × 10 ¹⁷ cm ^{−3 of} Si, and has a thickness of 200 °. 14 is an electron supply layer made of GaAlAs, in which Si is doped at 1 × 10 ¹⁷ cm ^{−3, and} has a thickness of
There are 2000Å. 15 is a cap layer, which is undoped GaAs (200
Å). Using the wafer having this structure, the electron concentration and the electron mobility were measured at room temperature by the Hall effect. As a result, two-dimensional electron concentration was obtained ^{ns = 7.9 × 10 11 cm -2} , the electron mobility ^{μ = 4622cm 2 V -1 sec -1} . In addition, (S
Using the wafer without the i-doped GaAs layer 3), the concentration and mobility of the two-dimensional electron gas were measured. As a ^{result, ns = 4.6 × 10 11 cm} -2, to obtain a ^{μ = 6513cm 2 · V -1 sec} -1, compared with the structure of the present invention, ns is the poor indicated.

[Another embodiment of the invention]

In the present invention, a hetero junction of GaAs and AlGaAs is used, but another hetero junction may be used. Also, as a dopant,
Although Si was selected, it may be another n-type dopant or another p-type dopant.

[Brief description of the drawings]

FIG. 1 is a sectional structural view for explaining the basic structure of the present invention, and FIGS. 2 and 3 illustrate the difference between the effects obtained when the conventional method is adopted and when the present method is adopted. FIG. 4 is a diagram for explaining an embodiment according to the present invention. 1 GaAs layer containing high resistance and low impurity, 2 GaAs layer containing high concentration of n-type impurity, 3 AlGaAs layer containing high concentration of n-type impurity.

Claims (1)

(57) [Claims] In a high mobility transistor having a heterojunction formed of two layers, an impurity of the same conductivity type as that of the electron supply layer is formed in a portion where a two-dimensional electron gas is formed adjacent to the electron supply layer. A high mobility transistor characterized by being highly doped.