JP2012104738A - Compound semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a compound semiconductor device capable of suppressing variation in the characteristics while maintaining high performance of planar dope.SOLUTION: In a compound semiconductor device of an HEMT structure composed of a compound semiconductor, electron supply layers 14, 16, and 18 supplying electrons to a channel layer are formed by planar-doped layers of n-type impurity, and planar-doped layers 15 and 17, which are in the same electron supply layer, are divided into two or more layers.

Description

  The present invention relates to a compound semiconductor device made of a compound semiconductor.

  High Electron Mobility Transistor (HEMT) is capable of ultra-high-speed operation with low distortion, mainly switching low-noise amplifiers in receivers such as mobile phones and wireless LANs, power amplifiers in transmitters, and transmission / reception It is used for antenna switches that switch communication methods.

  As a structural feature of the HEMT, an electron supply region (electron supply layer) and a region where electrons travel so as to be able to operate at a higher speed than a field effect transistor (FET) used conventionally. The channel layer or the electron transit layer is separated. Furthermore, since the channel layer and the electron supply layer are made of different materials, they have different band gaps and electron affinities. A conduction band discontinuity occurs at the heterojunction interface, and a kind of quantum well is formed. Here, electrons are effectively confined, and so-called two-dimensional atomic gas is formed. Since there are no ionized impurities on the channel side where the two-dimensional atomic gas exists, higher mobility can be exhibited from low temperature to room temperature.

  As shown in FIG. 6, the basic structure of the HEMT includes a buffer layer 2 for preventing current leakage and buffering strain, a channel layer (electron traveling layer) 3 for traveling electrons, and an electron on a semi-insulating substrate 1. The electron supply layer 4 for supplying the Schottky electrode, the Schottky layer 5 for contacting the Schottky electrode to obtain a withstand voltage, and the n-type carriers are doped at a high concentration in order to reduce the contact resistance with the metal serving as the electrode on the Schottky layer The contact layer 6 is laminated in order.

  To be more specific, the following structure can be given as a main example.

The buffer layer 2 is formed by alternately laminating a non-doped GaAs layer and an Al _x Ga _(1-x) As layer (where 0 <x <1) by several nm to several tens of nm. The channel layer 3 is composed of an In _x Ga _(1-x) As layer (where 0 <x <1). However, since the InGaAs layer cannot be lattice-matched with GaAs, a composition and a film thickness that do not cause lattice relaxation are used. The electron supply layer 4 includes a high-concentration n-type Al _x Ga _(1-x) As layer (where 0 <x <1) of several tens of nm, or a Si planar doped layer, and the Schottky layer 5 A non-doped or low-concentration n-type Al _x Ga _(1-x) As layer (where 0 <x <1) is laminated by several tens of nm. As the contact layer 6, an n-type GaAs layer doped with Si at a high concentration, or an n-type In _x Ga _(1-x) As layer doped with Te or Se to lower the contact resistance in recent years (where 0 < x <1) may be used.

  Such a thin-film multilayer structure is formed by metal organic vapor phase epitaxy (MOVPE: Metal Organic Vapor Epitaxy, MOCVD: Metal Organic Chemical Vapor Deposition) or molecular beam epitaxy (MBE). Can do.

  The metalorganic vapor phase epitaxy method is a method in which a solid or liquid organometallic raw material is gasified and supplied, thermally decomposed and chemically reacted on a heated substrate, and a thin film crystal is epitaxially grown thereon. The molecular beam growth method is a method in which constituent elements of a crystal are evaporated from individual crucibles in an ultra vacuum, supplied onto a substrate heated in the form of a molecular beam, and a thin film crystal is epitaxially grown thereon.

JP-A-8-316461

  When the planar structure is used instead of the uniform doped structure as the above-described electron supply layer, the Schottky layer (distance from the gate electrode to the electron supply layer is effectively obtained even when the same threshold voltage (Vth) as the uniform doped structure is used. ) Can be made thicker, the gate breakdown voltage, which is important as a device characteristic, is improved, and the capacitance between the gate and the source can be lowered, so that the maximum cutoff frequency (Ft), maximum oscillation frequency (Fmax), and minimum noise figure (NFmin) Is improved.

  However, continuous improvement in characteristics is required every day, and in order to achieve further improvements, it has become important to increase the saturation current (Idss) and reduce the on-resistance. This improvement requires an increase in electron mobility and an increase in the amount of planar doping.

  Improvement of electron mobility mainly leads to reduction of on-resistance, but at the present time, it is approaching the mobility in undoped bulk crystals, and physical limitations are visible.

  On the other hand, the planar doping amount can be increased relatively easily, but the problem is that if the doping amount is increased too much, dopant diffusion occurs due to the electric field and heat during device operation, resulting in characteristic fluctuations and reliability. It was causing problems such as decline.

  Therefore, an object of the present invention is to provide a compound semiconductor device capable of suppressing characteristic fluctuations while maintaining the high performance of the planar dope.

  The present invention created to achieve this object is a compound semiconductor device having a HEMT structure made of a compound semiconductor, wherein an electron supply layer for supplying electrons to the channel layer is formed of a planar doped layer of an n-type impurity. A compound semiconductor device in which the planar doped layer in the same electron supply layer is divided into two or more.

  The n-type impurity is preferably Si.

The donor concentration per divided planar doped layer is preferably 1.5E + 12 cm ⁻² or less.

The donor concentration per divided planar doped layer may be 1E + 12 cm ⁻² or less.

  The planar doped layer in the same electron supply layer is preferably divided into four or less.

  The planar doped layer in the same electron supply layer may be divided into two.

  Of the planar doped layers divided into two or more parts, the distance between the farthest planar doped layers is preferably 5 molecular layers or less.

  The distance between the planar dope layers divided into two is preferably not more than two molecular layers.

  The distance between the planar dope layers divided into two may be one molecular layer or less.

  According to the present invention, characteristic variation can be suppressed while maintaining the high performance of the planar dope.

It is a figure which shows the relationship between the planar dope amount and the normalized value of mobility. It is a figure which shows the relationship between the division | segmentation number of a planar dope layer, and a frequency. It is a figure which shows the relationship between the distance between planar dope layers, and a frequency. It is a figure which shows an example of the compound semiconductor device to which this invention is applied. It is a figure which shows an example of the compound semiconductor device to which this invention is applied. It is a figure explaining the basic structure of HEMT.

  Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

As shown in FIG. 1, the problem that occurs when the planar doping is performed at a high concentration described above starts to appear when the doping amount exceeds 1E + 12 cm ⁻² , and becomes prominent when the doping amount exceeds 1.5E + 12 cm ⁻² . In FIG. 1, a value obtained by dividing the mobility value after device operation by the mobility value before operation is used as the standard value.

  As a result of repeated investigations by the present inventors, it has been found from analysis such as SIMS (Secondary Ion Mass Spectrometry) that if an donor with a density higher than the same level is gathered, anomalous diffusion occurs only by giving a little energy. It was.

  As a solution, it is conceivable to reduce the amount of planar dope to a concentration at which abnormal diffusion does not occur. However, sufficient electrons cannot be supplied. In order to solve this problem, the inventors of the present invention have further studied and divided the planar doped layer into a plurality of parts in the same electron supply layer so as to suppress the concentration of each layer to a concentration at which abnormal diffusion does not occur. Thus, it has been found that it is possible to provide an electron supply layer having a planar doped layer in which both the supply of electrons and the suppression of abnormal diffusion are compatible.

  That is, in the compound semiconductor device proposed by the present inventors, in the compound semiconductor device having a HEMT structure made of a compound semiconductor, the electron supply layer for supplying electrons to the channel layer is formed of a planar doped layer of n-type impurities. The planar doped layer in the same electron supply layer is divided into two or more parts. Thereby, an increase in the amount of planar dope necessary for improving the characteristics can be achieved without abnormal diffusion of the donor.

  Here, as the compound semiconductor, gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs), indium gallium arsenide (InGaAs), indium gallium arsenide phosphorus (InGaAsP), indium aluminum arsenic phosphorus (InAlAsP), indium aluminum gallium arsenide phosphorus ( InAlGaAsP) can be used which includes any or all of them, and the effect of the present invention can be obtained in a HEMT using a material that is lattice-matched or pseudo-lattice-matched to a GaAs substrate generally called a GaAs system.

  As compound semiconductors, indium phosphide (InP), indium gallium arsenide (InGaAs), indium aluminum arsenide (InAlAs), indium aluminum gallium arsenide (InAlGaAs), indium gallium arsenide phosphorus (InGaAsP), indium aluminum arsenic phosphorus (InAlAsP) Indium oxide gallium arsenide phosphide (InAlGaAsP), which includes any or all of them, can be used in a HEMT using a material that is lattice-matched or pseudo-lattice-matched to an InP substrate generally called an InP system. The effects of the invention can be obtained.

  As the n-type impurity in the planar doped layer, one or more of silicon (Si), tin (Sn), sulfur (S), selenium (Se), and tellurium (Te) can be used. When Si, which is difficult, is used, the effect is great. On the other hand, Sn, S, Se, and Te are originally easily diffused, so the effect is greatly inferior compared to Si.

As shown in FIG. 1, since the mobility after operation decreases as the planar doping amount increases, the donor concentration per planar doped layer divided in the same electron supply layer is 1.5E + 12 cm ⁻² or less. It is preferable that Although the effect can be found with this donor concentration, ideally, it is desirable that the mobility does not change even when the device operation is performed, so the donor concentration per one divided planar doped layer is 1E + 12 cm ^−2. The following is more preferable. If the donor concentration is lowered so far, depending on the evaluation conditions, characteristic fluctuations are hardly seen.

  Further, a larger effect can be obtained by limiting the number of divisions of the planar doped layer in the same electron supply layer. In practice, as shown in FIG. 2, as the number of divisions of the planar doped layer is increased, the characteristics of the uniform doped structure are approached and the maximum cutoff frequency (Ft) is lowered. The difference is invisible and ineffective. Preferably, unless a large amount of electron supply is required, a large effect can be obtained by setting the number of divisions of the planar doped layer to 2 in the normal use range.

  Further, as shown in FIG. 3, the characteristics of the uniform doped structure are approached as the distance between the planar doped layers (the farthest among the divided planar doped layers) at both ends divided in the same electron supply layer increases. At the same time, the maximum cutoff frequency (Ft) becomes low, and unless the distance between the planar dope layers is 5 molecular layers (ML) or less, electrons cannot be supplied sufficiently, and there is no difference in characteristics from the uniform dope structure. Of course, if it is further expanded, the characteristics will be worse than those of the uniformly doped structure. Note that when the number of divisions of the planar doped layer is 2, the distance between the planar doped layers at both ends divided in the same electron supply layer is from 5 molecular layers or less to 2 molecular layers or less, more preferably 1 If it is limited to a molecular layer or less (because division is assumed, the minimum division unit is up to one molecular layer), a greater effect can be obtained.

  Specific examples of the compound semiconductor device to which the present invention is applied will be described below.

  FIG. 4 is a diagram showing a cross-sectional structure of a compound semiconductor device grown on a GaAs substrate.

This compound semiconductor device includes a semi-insulating GaAs substrate 11, a buffer layer 12 made of undoped GaAs, a channel layer 13 made of undoped In _x Ga _(1-x) As (0 <x <1), an undoped Al _x. Spacer layer 14 made of Ga _(1-x) As (0 <x <1), n-type impurity planar doped layer 15, spacer layer made of undoped Al _x Ga _(1-x) As (0 <x <1) 16, a planar doped layer 17 of n-type impurity, a Schottky layer 18 made of undoped Al _x Ga _(1-x) As (0 <x <1), and a contact layer 19 made of n-type high-concentration GaAs are sequentially laminated. It becomes.

The planar doped layer has a two-part structure in which a spacer layer 16 is inserted between the planar doped layers 15 and 17 of n-type impurities. Si is applied to the n-type impurity planar doped layers 15 and 17, and the donor concentration is set to 1E + 12 cm ⁻² . Si impurities are also applied to the impurity dopant of the contact layer 19. The film thickness of the spacer layer 16 is a single molecular layer.

  FIG. 5 is a diagram showing a cross-sectional structure of a compound semiconductor device grown on an InP substrate.

This compound semiconductor device includes a semi-insulating InP substrate 21, a buffer layer 22 made of undoped In _x Al _(1-x) As (0 <x <1), an undoped In _x Ga _(1-x) As ( A channel layer 23 made of 0 <x <1), a spacer layer 24 made of undoped In _x Al _(1-x) As (0 <x <1), an n-type impurity planar doped layer 25, an undoped In _x Al _{(1 -x)} Spacer layer 26 made of As (0 <x <1), planar doped layer 27 of n-type impurity, barrier layer 28 made of undoped In _x Al _(1-x) As (0 <x <1), n A contact layer 29 made of type In _x Ga _(1-x) As (0 <x <1) is sequentially laminated.

The planar doped layer has a two-part structure in which a spacer layer 26 is inserted between the planar doped layers 25 and 27 of n-type impurities. Si is applied to the n-type impurity planar doped layers 25 and 27 and the donor concentration is set to 1E + 12 cm ⁻² . Si impurities are also applied to the impurity dopant of the contact layer 29. The thickness of the spacer layer 26 is a single molecular layer.

  According to the compound semiconductor device having such a configuration, in a HEMT structure made of a compound semiconductor, a characteristic change that occurs when a planar dope layer is used as an electron supply layer and the planar dope amount is increased to a certain amount or more to improve characteristics. By dividing the planar doped layer in a very small area, it is possible to suppress the characteristic fluctuation while maintaining the high performance of the planar doping.

  In addition, according to the compound semiconductor device of the present invention, if the same performance as the conventional one is sufficient, the element can be reduced in size by improving the characteristics. Therefore, it is very advantageous in a high-performance portable terminal such as a smartphone that has a multi-function but has a limited volume.

11 GaAs substrate 12 buffer layer 13 channel layer 14 spacer layer 15 planar doped layer 16 spacer layer 17 planar doped layer 18 Schottky layer 19 contact layer

Claims (9)

In a compound semiconductor device having a HEMT structure made of a compound semiconductor,
An electron supply layer for supplying electrons to a channel layer is formed of a planar doped layer of n-type impurities, and the planar doped layer in the same electron supply layer is divided into two or more parts. apparatus.   The compound semiconductor device according to claim 1, wherein the n-type impurity is Si. 3. The compound semiconductor device according to claim 1, wherein a donor concentration per one divided planar doped layer is 1.5E + 12 cm ⁻² or less. 3. The compound semiconductor device according to claim 1, wherein a donor concentration per one divided planar doped layer is 1E + 12 cm ⁻² or less.   The compound semiconductor device according to claim 1, wherein the planar doped layer in the same electron supply layer is divided into four or less.   The compound semiconductor device according to claim 1, wherein the planar doped layer in the same electron supply layer is divided into two parts.   The compound semiconductor device according to any one of claims 1 to 6, wherein a distance between the most distant planar doped layers among the divided planar doped layers is 5 molecular layers or less.   The compound semiconductor device according to claim 6, wherein a distance between the divided planar dope layers is two molecular layers or less.   The compound semiconductor device according to claim 6, wherein a distance between the divided planar dope layers is one molecular layer or less.

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