JP2004179359A - Heterojunction bipolar transistor - Google Patents

Abstract

A heterojunction bipolar transistor having a lower base resistance than conventional uniform base structures and compositionally graded base structures.
A heterojunction bipolar transistor in which a collector layer 3, a base layer 4 made of p-type Ga _x In _1-x As, and an emitter layer 5 are sequentially deposited on a semi-insulating InP substrate 1, The value of the GaAs composition ratio x in Ga _x In _1-x As in the entire region in the base layer 4 enables lattice matching between the semi-insulating InP substrate 1 and Ga _x In _1-x As of the base layer 4. Thus, a heterojunction bipolar transistor characterized in that the value is larger than the threshold value.
[Selection diagram] Fig. 1

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a heterojunction bipolar transistor.
[0002]
[Prior art]
[Non-patent document _{"paper" Effects of a Compositionally-Graded In} x Ga 1 -x As Base in Abrupt-Emitter InP / InGaAs Heterojunction Bipolar Transistors, "Jpn. J. Appl. Phys. Vol. 34, Part 1, No 2B, pp. 1221-1227, 1995. K. Kurishima, H. Nakajima, S. Yamahata, and T. Kobayashi "
A heterojunction bipolar transistor (hereinafter abbreviated as HBT) uses a semiconductor having a band gap larger than that of a base layer for an emitter layer, so that electrons from the base layer to the emitter layer can be formed even when the base layer is doped at a high concentration. Leakage can be suppressed, and high-speed operation can be performed as compared with a so-called homojunction bipolar transistor.
[0003]
The HBT has been applied to various integrated circuits due to its high speed, but reducing the base resistance is one of the most important issues in order to further improve the operation speed.
[0004]
Particularly, in the case of an HBT for an ultra-high speed, the base layer thickness must be reduced, and in order to realize a low base resistance in a thin-layer base HBT having a base layer thickness of 30 nm or less, the impurity concentration of the base layer should be as high as possible. There is a need to. However, the impurity concentration has an upper limit depending on the crystal material and the growth method, and the concentration cannot be increased arbitrarily.
[0005]
The most common structure of the base layer of the HBT is a so-called "uniform base" structure in which the composition of the crystal and the impurity concentration are constant throughout the base layer. As an example of a uniform base structure, in FIG. 4 the base layer distribution of _Ga x _{In 1-x} As GaAs composition ratio x of the base one scan layer in HBT using an InP substrate, shown by the horizontal dashed line. FIG. 4 shows an example in which the thickness of the base layer is 30 nm. Therefore, in the figure, a location at a distance of 30 nm from the emitter-base junction interface is the base-collector junction interface.
[0006]
In the uniform base structure according to the prior art shown in FIG. 4, the GaAs composition ratio x in Ga _x In _1-x As in the entire region in the base layer enables the lattice matching of Ga _x In _1-x As to the InP substrate. That is, the value is 0.47, which is constant over the entire area in the base layer. It is one of the reasons that such a composition is generally used because it is easier to obtain good crystal quality by using a composition that enables lattice matching.
[0007]
As described in the above-mentioned non-patent document, as a base structure capable of reducing the transit time of electrons, which are minority carriers in the base layer, as compared with the uniform base structure, mixed crystal Ga _x in the base layer There is a "composition gradient base" structure in which the GaAs composition ratio x of In1 _- xAs is changed from the collector layer side to the emitter layer side so as to increase the band gap. In FIG. 4, an example of the GaAs composition ratio x of the Ga _x In _1-x As base layer in the composition gradient base structure HBT according to the related art manufactured on the InP substrate is indicated by a broken line to the right. At the interface between the base and the collector, x = 0.47 so as to enable lattice matching with InP, as in the uniform base structure, but the value of x increases toward the emitter layer side. The larger the GaAs composition ratio x is, the larger the band gap is. This causes the conduction band potential in the base layer to tilt down to the right, so that a so-called built-in electric field is formed. By accelerating the electrons as minority carriers by the built-in electric field, the transit time is shortened. As a result, the operation speed is improved and the current gain is increased. It is also known that, as an accompanying effect, as the GaAs composition ratio x is increased, more P-type impurities are incorporated into the base layer during the epitaxial growth, thereby lowering the base resistance. ing.
[0008]
[Problems to be solved by the invention]
Since the composition gradient base structure in the above-described conventional method aims at shortening the transit time of minority carriers in the base, the GaAs composition ratio x changes in the gradient as shown by the broken line to the right in FIG. As the absolute value of the GaAs composition ratio x, a value that enables lattice matching with InP at the base-collector interface is used so that good crystal quality can be easily obtained.
[0009]
However, from the viewpoint of reducing the base resistance, the GaAs composition ratio x is higher only on the emitter layer side than in the uniform base structure, and an increase in the impurity concentration in the entire base layer cannot be expected. There's a problem.
[0010]
An object of the present invention is to solve the above-mentioned problems and to provide a heterojunction bipolar transistor having a lower base resistance than conventional uniform base structures and compositionally graded base structures.
[0011]
[Means for Solving the Problems]
In order to solve the above problem, in the present invention, as described in claim 1,
On an InP substrate, a collector layer, a base layer made of p-type _Ga x _{In 1-x} As, in the heterojunction bipolar transistor and the emitter layer are sequentially deposited, _Ga x an In ₁ in the entire area of the base layer the value of GaAs composition ratio x in the _-x as constitutes a heterojunction bipolar transistor, wherein the greater than the value which allows lattice matching between the InP substrate and the _Ga x _{in 1-x} as .
[0012]
Further, in the present invention, as described in claim 2,
2. The heterojunction bipolar transistor according to claim 1, wherein a value of the GaAs composition ratio x in the base layer increases from the collector layer side toward the emitter layer side. Constitute.
[0013]
In the present invention, as described in claim 3,
3. The heterojunction bipolar transistor according to claim 1, wherein said base layer has a thickness of 30 nm or less.
[0014]
In the present invention, as described in claim 4,
4. The heterojunction bipolar transistor according to claim 3, wherein a value of the GaAs composition ratio x in the base layer at an interface between the base layer and the collector layer is between the InP substrate and the Ga _x In _1-x As. A heterojunction bipolar transistor characterized in that the value is 0.05 or more larger than the value that allows the lattice matching.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
According to the present invention, in a heterojunction bipolar transistor in which a collector layer, a base layer made of a p-type mixed crystal semiconductor Ga _x In _1-x As, and an emitter layer are sequentially deposited on an InP substrate, The value of the GaAs composition ratio x in the mixed crystal semiconductor Ga _x In _1-x As in the entire region is a value that enables lattice matching between the InP substrate and the Ga _x In _1-x As, that is, 0.47. (Hereinafter, simply referred to as a value that enables lattice matching) to constitute a heterojunction bipolar transistor.
[0016]
When the thickness of the base layer is relatively large, if the lattice mismatch of the base layer is large, a lattice defect occurs, which deteriorates the crystal quality and consequently the device characteristics. However, in a very thin base layer such as a base layer having a thickness of 30 nm or less, good crystal quality is likely to be obtained even if there is a certain degree of lattice mismatch. Therefore, in the thin-layer base structure, as shown in FIG. 2, the GaAs composition ratio in the entire region of the base layer from the base-collector interface (at a distance of 30 nm from the emitter-base junction interface) to the emitter-base junction interface. Even if x is larger than a value that enables lattice matching with InP, good crystal quality can be easily obtained.
[0017]
In the present invention, unlike the conventional uniform base structure or the compositionally graded base structure, the average impurity in the entire base layer is increased by increasing the GaAs composition ratio x in Ga _x In _1-x As in the entire region in the base layer. The concentration also increases, resulting in a significant reduction in base resistance.
[0018]
The heterojunction bipolar transistor according to the present invention is also characterized in that a lattice mismatch occurs at the base-collector interface because the GaAs composition ratio x is larger than a value that enables lattice matching. .
[0019]
As shown in FIG. 2, in the heterojunction bipolar transistor according to the present invention, a uniform base structure (indicated by a horizontal broken line in the figure) can be manufactured, and a composition gradient structure (downward in the figure). (Indicated by a broken line) can also be produced.
[0020]
Further, in the heterojunction bipolar transistor according to the present invention, unlike the conventional structure, as shown by the solid line in FIG. 5, the conduction band due to the discontinuous GaAs composition ratio x at the base-collector interface. The end 10 also becomes discontinuous, and a conduction band end discontinuity 11 occurs. At this time, since the band gap of the base layer is wider than that of the collector layer, as shown by the solid line in FIG. 5, a negative conduction band edge discontinuity 11 is generated from the base layer toward the collector layer. It is known that when the negative discontinuity is formed at the conduction band edge, the electrons 12 passing through the discontinuity are accelerated, which causes the current gain cutoff frequency Ft of the HBT to decrease. The value also improves. If the GaAs composition ratio x is larger than the value that allows lattice matching by 0.05 or more, the electron acceleration effect due to the discontinuity appears remarkably, and therefore, the value of the current gain cutoff frequency Ft can be improved. Such an electron acceleration effect is caused by the discontinuity of the conduction band edge at the interface between the base and the collector in the case of the base layer having the composition gradient structure according to the prior art (the conduction band edge is exemplified by a dashed line falling to the right in FIG. 5). Since it has not occurred, it does not appear.
[0021]
As described above, the base structure of the heterojunction bipolar transistor according to the present invention has a lower base resistance and a higher Ft value than the uniform base structure and the composition gradient base structure of the conventional heterojunction bipolar transistor. .
[0022]
(Embodiment example)
Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
[0023]
FIG. 1 is an example of a cross-sectional view of an HBT structure according to the present invention. In the figure, 1 is a semi-insulating InP substrate, 2 is a sub-collector layer of n ⁺ -InP, 3 is a collector layer of n ^-- GaInAs, 4 is a base layer of p ⁺ -GaInAs, and 5 is an emitter of n ^-- InP. The layer 6 is a cap layer of n ⁺ -GaInAs. 7 is a collector electrode, 8 is a base electrode, and 9 is an emitter electrode.
[0024]
Here, the GaInAs base layer has a thickness of 20 nm, and the GaAs composition ratio x is the composition ratio x shown in FIG. The sheet resistance of the base layer obtained by this configuration is 749 Ω / □. On the other hand, the sheet resistance value obtained with a uniform base structure (corresponding to the uniform base structure according to the prior art) using a GaInAs layer lattice-matched with the same layer thickness is 974 Ω / □, and the lattice resistance at the base-collector interface is high. In the composition gradient base structure according to the prior art which enables matching, the value is 885 Ω / □. Thus, it is clear that by employing the base structure according to the present invention, the base sheet resistance is reduced as compared with the prior art. Due to the reduction of the base sheet resistance, the base resistance is also reduced.
[0025]
Here, the example in which the emitter layer is InP is shown, but the emitter layer may be AlInAs. Further, the example of the collector layer is made of GaInAs, but InP, AlInAs, GaInAsP, and a stacked structure thereof may be used. Further, it goes without saying that the layer configuration may be changed, for example, by using an n ⁺ -GaInAs layer for the sub-collector layer, without departing from the spirit of the present invention.
[0026]
As is apparent from the above description, the heterojunction bipolar transistor according to the present invention can have a lower base resistance than the conventional heterojunction bipolar transistor. Further, electrons are accelerated by the conduction band discontinuity at the base-collector interface, and the high-frequency characteristics are also improved.
[0027]
【The invention's effect】
By implementing the present invention, it is possible to provide a heterojunction bipolar transistor having a lower base resistance than conventional uniform base structures and compositionally graded base structures.
[Brief description of the drawings]
FIG. 1 shows an example of a cross-sectional view of a heterojunction bipolar transistor according to the present invention.
FIG. 2 is a diagram showing an example of a GaAs composition ratio x in a Ga _x In _1-x As base layer of a heterojunction bipolar transistor according to the present invention.
FIG. 3 is a diagram showing an example of a GaAs composition ratio x in a Ga _x In _1-x As base layer of a heterojunction bipolar transistor according to the present invention.
FIG. 4 is a diagram illustrating an example of a GaAs composition ratio x in a Ga _x In _1-x As base layer of a heterojunction bipolar transistor according to the related art.
FIG. 5 is a diagram showing an energy band structure at a conduction band edge of a base layer of a heterojunction bipolar transistor according to the present invention and a heterojunction bipolar transistor according to the related art.
[Explanation of symbols]
1 ... semi-insulating InP substrate, 2 ... subcollector layer ⁽ⁿ + -InP), 3 ... a collector layer ⁽ⁿ - -GaInAs), 4 ... base layer ^(p + -GaInAs), 5 ... emitter layer (n ^- - InP), 6 cap layer (n ⁺ -GaInAs), 7 collector electrode, 8 base electrode, 9 emitter electrode, 10 conduction band end, 11 discontinuous conduction band end, 12 electrons.

Claims (4)

On an InP substrate, a collector layer, a base layer made of p-type _Ga x _{In 1-x} As, in the heterojunction bipolar transistor and the emitter layer are sequentially deposited, _Ga x an In ₁ in the entire area of the base layer heterojunction bipolar transistor being larger than a value that allows lattice matching between the values of GaAs composition ratio x in the _-x as is with the InP substrate and the _Ga x _{in 1-x} as. 2. The heterojunction bipolar transistor according to claim 1, wherein the value of the GaAs composition ratio x in the base layer increases from the collector layer side toward the emitter layer side. 3. The heterojunction bipolar transistor according to claim 1, wherein said base layer has a thickness of 30 nm or less. 4. The heterojunction bipolar transistor according to claim 3, wherein a value of the GaAs composition ratio x in the base layer at an interface between the base layer and the collector layer is between the InP substrate and the Ga _x In _1-x As. A heterojunction bipolar transistor characterized in that the value is 0.05 or more larger than a value that allows lattice matching of:

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