JP3010919B2 - Field effect type compound semiconductor device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a field effect compound semiconductor device for high frequency power using a heterojunction in a channel.

[0002]

2. Description of the Related Art A conventional field-effect compound semiconductor device using a heterojunction as a channel usually includes GaAs and InG which are heterojunction with n-AlGaAs as an electron supply layer.
The device is operated using a two-dimensional electron gas generated at the interface of aAs. In particular, since it is spatially separated from the donor level in the electron supply layer, it is less susceptible to electrical scattering, enabling high-speed operation. Has reached.

Recently, there has been a movement to develop field-effect compound semiconductor devices using these heterojunctions as channels as field-effect semiconductor devices for high-frequency power.
In order to make a device for high-frequency power, the method used in the development of a low-noise device must be modified. For this purpose, first, a method of increasing the two-dimensional electron gas concentration to obtain a large current has been adopted.

FIG. 4 shows a cross-sectional view of the conventional device. FIG. 4 shows an undoped AlG on a semi-insulating GaAs substrate 11.
aAs buffer layer 12, on which n-
An AlGaAs layer 13, an undoped GaAs or InGaAs channel layer 14, an n-AlGaAs layer 15 as an electron supply layer, a gate electrode 18, a source electrode 1 on an n-GaAs layer as a cap layer.
7, a drain electrode 19 is provided. Normally, the electron supply layer is provided only above the channel layer. However, due to the double hetero junction, the n-type layer provided below the channel layer is used.
Because of the two-dimensional electron gas generated from the AlGaAs layer 13,
The two-dimensional electron gas concentration in the channel is doubled.

FIG. 5 shows a conventional n-AlGaAs / GaAs.
1 shows a band diagram of an s-system heterojunction FET. An increase in current can be expected due to the two-dimensional electron gas concentration generated at both interfaces of the channel. However, the n-A provided below the channel layer
When the concentration of the lGaAs layer 13 is high, the activation rate is remarkably reduced, and electrons flow as dark current through the substrate, thereby deteriorating the device characteristics. In addition, when a high-frequency large-signal operation is performed, the depletion layer in the upper n-AlGaAs layer 15 between the electrodes other than the gate cannot affect the two-dimensional electron gas, and cannot be ignored. Occurs.

[0006]

As described above, in order to develop a field-effect compound semiconductor device having a heterojunction as a channel as a field-effect semiconductor device for high-frequency power, a two-dimensional electron gas concentration is required by a double heterojunction. But the high concentration n-
Since the AlGaAs layer significantly lowers the activation rate for generating a two-dimensional electron gas, the pinch-off characteristics of the device are degraded, and electrons flow as dark currents through the substrate, deteriorating the device characteristics. Was. When a high-frequency large-signal operation is actually performed for power, the depletion layer in the upper n-AlGaAs layer between the electrodes other than the gate becomes 2
Since this affects the two-dimensional electron gas, the state becomes different from that during the static operation, and the concentration of the two-dimensional electron gas immediately below the electrodes is remarkably reduced.

An object of the present invention is to provide an n-AlGaAs layer provided below a channel layer having a high activation rate and, during high-frequency large-signal operation, an upper layer n located between electrodes other than a gate.
-To provide a field-effect compound semiconductor device in which a depletion layer in an AlGaAs layer does not affect a two-dimensional electron gas.

[0008]

[0009]

SUMMARY OF THE INVENTION The present invention provides an n-AlGaAs / GaAs-based or n-AlGaAs / InGaAs-based field-effect compound semiconductor device using a double heterojunction for a channel, in an electron supply layer below a channel layer. Or doping layer with a doping concentration of 5 × 10 ¹⁷ cm
^-3 or more and less than 1 × 10 ¹⁸ cm ⁻³ n-GaAs.

[0010]

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a sectional view of an embodiment of a field-effect compound semiconductor device according to the present invention, and FIG. 2 is a band diagram thereof. As shown in FIG. 1, undoped AlGa is formed on a semi-insulating GaAs substrate 1 by epitaxial growth.
As buffer layer 2, and n-G having a doping concentration of 5 × 10 ¹⁷ cm ⁻³ to 1 × 10 ¹⁸ cm ^{−3 as} an electron supply layer thereon
aAs layer 3, on which undoped GaAs or I
A channel layer 4 of nGaAs, an n-AlGaAs layer 5 as an electron supply layer thereon, and a doping concentration of 2 × 10 ¹⁷ cm ⁻³ to 5 × 10 ¹⁷ c other than the gate electrode 9
m ^-3, a low concentration n-GaAs layer of 100 Angstroms to about 500 Angstroms thick 6, it gate electrode 9, the source electrode 8 in n-GaAs layer is a cap layer, the drain electrode 10 are provided.

In this embodiment, an n-AlGaAs / GaAs heterojunction FET or an n-AlGaAs / InGaAs heterojunction FET having a heterostructure in the channel is used.
The n-AlGaAs layer 5 which is an electron supply layer and n
A doping concentration of 2 × 10 ¹⁷ cm ⁻³ to 5 × 10 ¹⁷ cm ⁻³ between the GaAs ohmic contact layer 7 and a thickness of about 5
A low-concentration doping layer of 00 to 100 Å is provided. With this, the low-concentration n-GaAs layer 6 other than the gate electrode on the front side allows the two-dimensional electron gas to be supplied to the n-AlGaAs layer during high-frequency large-signal operation. 5 is not affected by the depletion layer, and the secondary electron gas concentration is not deteriorated.

The reason why the low-concentration doping layer is provided between the electron supply layer and the ohmic contact layer is as follows.

The doping concentration between the electron supply layer and the ohmic contact layer is 2 × 10 ¹⁷ cm ^-3 to 5 × 10 ¹⁷ cm.
^-3 , by providing a lightly doped layer having a thickness of about 500 Å to 100 Å,
The two-dimensional electron gas and the n-AlGaAs layer are not generated, and the depletion is completely caused by the state of the interface with the protective film. This depletion layer does not reach the n-AlGaAs layer. The width of this depletion layer approximately corresponds to the thickness of the lightly doped layer. As a result, the depletion layer in the upper n-AlGaAs layer between the electrodes other than the gate does not affect the two-dimensional electron gas due to the modulation of the potential immediately below the gate during high-frequency large signal operation. Therefore, a large output is obtained for electric power.

In this embodiment, an n-AlGaAs / GaAs heterojunction FET or n-AlGaAs / InGaAs heterojunction FE having a double heterostructure is used.
In T, only the electron supply layer below the channel layer is doped with n-type with a doping concentration of 5 × 10 ¹⁷ cm ⁻³ to 1 × 10 ¹⁸ cm ⁻³ .
Since GaAs is used, a large amount of two-dimensional electron gas is generated in the lower layer of the channel layer. At the same time, those that contribute to dark current as three-dimensional electrons in the substrate are drastically reduced.

Only the electron supply layer below the channel layer is doped with an n-type impurity having a doping concentration of 5 × 10 ¹⁷ cm ⁻³ to 1 × 10 ¹⁸ cm ⁻³ .
The reason for using GaAs is as follows.

FIG. 3 is a graph showing the activation rates of the concentration (horizontal axis) of n-GaAs doping and the concentration of generated carriers (vertical axis). Doping concentration is 5 × 10 ¹⁷
When the cm ^-3 of 1 × 10 ¹⁸ cm ^-3, 60~ about activation rate
It can be seen that the ratio is as high as 80%. However, it can be seen that the activation rate is reduced to 45% or less at the time of high doping of 2 × 10 ¹⁸ cm ⁻³ or more as in the conventional case.
That is, the two-dimensional electron gas generated from the donor level in n-GaAs exists at the channel layer interface as much as possible, so that it does not behave as three-dimensional bulk electrons in n-GaAs or in the substrate. Therefore, the existence probability of the dark current is drastically reduced, and the pinch-off characteristics of the element are not deteriorated. In addition, since as much two-dimensional electron gas as possible exists at the channel lower layer interface, it contributes to an increase in current for high-power applications.

[0018]

As is clear from the above embodiments, according to the present invention, the doping concentration between the electron supply layer and the ohmic contact layer is 2 × 10 ¹⁷ cm ^-3 to 5 × 10 ¹⁷ cm.
^-3 , by providing a lightly doped layer having a thickness of about 500 Å to 100 Å,
Eliminates the effect of deep levels generated at the interface between the low-concentration doping layer and the protective film, completely depletes the low-concentration doping layer, and reduces the concentration of two-dimensional electron gas generated at the interface above the channel layer Therefore, it is not affected by the depletion layer in the n-AlGaAs layer at the time of high frequency large signal operation.

Further, regarding a double hetero structure conventionally used to increase the two-dimensional electron gas concentration, an n-AlGaAs / GaAs heterojunction FET or an n-AlGaAs / InGaAs heterojunction FE is used.
Only the electron supply layer below the channel layer of T is doped with n-GaAs having a doping concentration of 5 × 10 ¹⁷ cm ⁻³ to 1 × 10 ¹⁸ cm ^−3.
By doing so, the dark current flowing through the substrate is drastically reduced while increasing the two-dimensional electron gas concentration particularly in the lower layer of the channel layer. This is to improve the pinch-off characteristics of the device.

According to the present invention, there is provided a field-effect type compound semiconductor device for high-frequency power using a heterojunction in a channel,
It is extremely large that contributes to higher efficiency.

[Brief description of the drawings]

FIG. 1 is a sectional view of an element of a field-effect compound semiconductor device of the present invention.

FIG. 2 is a band diagram of the field-effect compound semiconductor device of the present invention.

FIG. 3 is a diagram showing the activation rates of the concentration of carriers doped into n-GaAs and the concentration of generated free electrons.

FIG. 4 is an element cross-sectional view of a conventional field-effect compound semiconductor device.

FIG. 5 is a band diagram of a conventional field-effect compound semiconductor device.

[Explanation of symbols]

 Reference Signs List 1,11 semi-insulating GaAs substrate 2,12 undoped AlGaAs buffer layer 3 n-GaAs layer 13 n-AlGaAs layer 4,14 channel layer 5,15 n-AlGaAs layer 6 low concentration n-GaAs layer 7,16 n-GaAs Ohmic contact layer 8,17 Source electrode 9,18 Gate electrode 10,19 Drain electrode

Claims (2)

(57) [Claims] 1. An n-type transistor using a double heterojunction in a channel.
In an AlGaAs / InGaAs-based field-effect compound semiconductor device, only the electron supply layer below the channel layer has a doping concentration of 5 × 10 ¹⁷ cm ⁻³ or more and 1 × 10 ¹⁸ cm ^−3.
A field-effect compound semiconductor device characterized by having n-GaAs of less than. 2. An n-type transistor using a double heterojunction for a channel.
In the AlGaAs / GaAs field effect compound semiconductor device, the doping layer below the channel layer is made of n-GaAs having a doping concentration of 5 × 10 ¹⁷ cm ⁻³ or more and less than 1 × 10 ¹⁸ cm ^−3. Field-effect compound semiconductor device.