JP2557613B2 - Heterojunction bipolar transistor - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heterojunction bipolar transistor having an emitter region formed of a semiconductor material having a bandgap larger than that of a semiconductor material forming a base region.

[0002]

2. Description of the Related Art In recent years, molecular beam epitaxy (MBE)
With the development of metal organic chemical vapor deposition (MOCVD) and the like, development of semiconductor devices utilizing so-called heterojunction, in which different kinds of semiconductor materials are joined, has been actively conducted. It is known that the heterojunction bipolar transistor which employs the heterojunction between the emitter and the base of the bipolar transistor has many advantages as compared with the conventional homojunction transistor made of a single semiconductor material.
These advantages are as follows: (1) The semiconductor material forming the emitter region has a bandgap larger than that of the semiconductor material forming the base region, so that the ratio of the impurity concentration of the emitter region and the impurity concentration of the base region is small. In both cases, high emitter injection efficiency can be obtained. (2) As a result of (1), since the impurity concentration in the base region can be increased, the base resistance can be reduced. (3) Similarly, since the impurity concentration in the emitter region can be reduced, the emitter capacitance can be reduced. Etc. Due to these advantages, the heterojunction bipolar transistor has a possibility of being far superior to the conventional homojunction bipolar transistor in high frequency characteristics and switching characteristics.

However, considering a combination of semiconductor materials that can actually manufacture such a heterojunction bipolar transistor, a material having a wide band gap generally has a small electron affinity. As a result, when an npn transistor in which an emitter made of a wide bandgap material and a base made of a narrow bandgap material are directly joined in a stepwise manner is manufactured, the injection efficiency is lowered as described below, which is an advantage of the heterojunction bipolar transistor. The problem arises that you can't make the most of it.

FIG. 6 shows a conduction band in the vicinity of the base-emitter junction of a staircase type heterojunction bipolar transistor when the electron affinity of the semiconductor material forming the emitter region is smaller than that of the semiconductor material forming the base region by ΔEc. Is shown. As is clear from this figure, the conduction band has a height ΔE due to the difference in electron affinity.
A spike-shaped barrier of c is formed, which impedes injection of electrons from the emitter to the base. This causes a decrease in emitter injection efficiency and impairs transistor characteristics.

This problem can be avoided by making the heterojunction between the emitter and the base not a step type but a so-called inclined type in which the band gap changes smoothly from the emitter to the base. FIG. 7 shows the state of the conduction band when a region where the band gap changes linearly is provided between the emitter and the base. As can be seen from this figure, the barrier generated in the conduction band can be removed by using the graded heterojunction.

The region where the band gap changes between the emitter region and the base region (hereinafter referred to as the transition region) is
This is realized by setting the composition of the material forming the region to an intermediate composition between the semiconductor material forming the base region and the semiconductor material forming the emitter region. Conventionally, in the transition region, the material composition has been gradually changed so that the band gap changes linearly or smoothly as a quadratic function.

However, in order to realize such a transition region, the molecular beam flux in the MBE method is
In the VD method, it is necessary to control the gas flow rate precisely and continuously. Since such control is difficult with the current technology, there arises a problem that the cost of the control device increases and the growth rate when growing the transition region needs to be sufficiently slowed down.

Further, when the band gap is smoothly changed in the transition region, holes are likely to be injected from the base into this region, and carrier recombination becomes large especially when the carrier lifetime is small. This causes a decrease in DC amplification factor and deterioration in high frequency characteristics and switching characteristics due to an increase in accumulated carriers. That is, there is a problem that the advantage of using the heterojunction is lost.

[0009]

As described above, when the junction between the emitter and the base in the heterojunction bipolar transistor is of the conventional type, the spike-like barrier generated in the conduction band is eliminated to improve the emitter injection efficiency. Although it is possible, holes are likely to be injected from the base into the transition region, causing a decrease in DC amplification factor and
High frequency characteristics and switching characteristics are deteriorated due to an increase in accumulated carriers. That is, there is a problem that the advantage of using an emitter with a wide band gap cannot be utilized.

[0010] The present invention has been made in view of the above points,
An emitter region made of a material with a large band gap,
It has a transition region between base regions made of a material with a small band gap, and can improve the emitter injection efficiency in the same way as when the junction between the emitter and the base is graded, and from the base to the transition region. The present invention aims to provide a heterojunction bipolar transistor which can suppress a decrease in DC amplification factor and deterioration of high frequency characteristics and switching characteristics due to the injection of positive holes, and which has an advantage of using a wide bandgap emitter. And

[0011]

In order to solve the above problems, the present invention employs the following configurations. That is, the present invention has a p-type base region made of a first-type semiconductor material and an n-type emitter region made of a second-type semiconductor material having a band gap larger than that of the first-type semiconductor material and a small electron affinity, and In a heterojunction bipolar transistor having a transition region composed of a semiconductor material having an intermediate composition of a first-type semiconductor material and a second-type semiconductor material between the base region and the emitter region, a plurality of semiconductor materials having different transition regions in composition. It is characterized in that it is formed of a layer and a barrier for suppressing injection of holes from the base region to the emitter region is formed in the valence band of the transition region.

[0012]

The present invention is based on the following knowledge and consideration. As described above, in a so-called staircase type heterojunction bipolar transistor in which an emitter region with a large bandgap is directly joined to a base region with a small bandgap in a stepwise manner, the bandgap from the base region to the emitter region is also increased. Even in a graded heterojunction bipolar transistor that introduces a transition region in which the value changes smoothly, each has its drawbacks.

In order to solve both of these difficulties, it is necessary to suppress the injection of holes from the base region to the emitter region while completely reducing the barrier generated in the conduction band to a certain value or less. It is desirable to have a structure in which the barrier can be formed in the valence band. Such a structure can be realized by forming a transition region composed of a plurality of semiconductor layers in which the material composition changes stepwise between the emitter and the base.

[0014]

EXAMPLES Examples of the present invention will be described below with reference to Ga _0.7 Al _0.3 As as a material constituting a wide band gap emitter.
GaA as a material forming the base of a narrow band gap
The case of using s will be described with reference to FIG.

To manufacture a heterojunction bipolar transistor, it is necessary to epitaxially grow collector, base and emitter layers on a conductive or semi-insulating substrate. If the MBE method or the MOCVD method is used, the thickness, doping concentration and material composition of each of these layers can be controlled.
It is easy to create the structure of the present invention.

In FIG. 1, an n ⁺ type GaAs substrate 2 is used, and an n type G containing, for example, Si as an impurity on the substrate 2.
aAs collector region 3, p-type GaAs base region 4 containing Be as an impurity, and S as an impurity, for example.
It shows a case where a transition region 6 composed of a plurality of layers containing i and having a stepwise change in composition, and a Ga _0.7 Al _0.3 As emitter region 7 having a wide bandgap containing Si as an impurity are sequentially grown. .

The transistor is formed by forming electrodes in electrical contact with the thus formed emitter, base and collector regions. In FIG. 1, the emitter electrode 8,
The collector electrode 1 is formed directly on both surfaces of the wafer, and the base electrode 5 is formed after etching from the wafer surface to the base region 4.

FIG. 2 schematically shows the band shape in the vicinity of the emitter / base when the transition region 6 is composed of two layers having an intermediate composition of the emitter region 7 and the base region 4. At the interface of each layer, a barrier is formed in the conduction band, but its height is sufficiently smaller than when a direct heterojunction is formed between the emitter and the base. On the other hand, it is expected that there is an effect of preventing holes from being deeply injected from the base into the transition region due to the barrier generated in the valence band. Even when the carrier recombination in the transition region is not significant, even if the composition change in the transition region is formed in multiple steps instead of the linear shape, the device characteristics are deteriorated as shown in more specific data later. It has become clear that a device having a small size and having practically sufficient characteristics can be obtained.

This gives a great advantage in forming a wafer for a heterojunction bipolar transistor by the MBE method or the MOCVD method. That is, in order to smoothly change the material composition, it is necessary to precisely control the growth conditions, and it is necessary to sufficiently reduce the growth rate, whereas in the case of changing the composition stepwise, Much simpler control is required, and as a result, the wafer growth efficiency can be significantly improved.

It should be noted here that if the transition region inserted between the emitter and the base is to be formed by a single layer having an intermediate composition of the semiconductor material forming each of the emitter and the base, the composition of this layer is not enough. Unless accurately controlled, the height of the barrier on the conduction band generated on the base region side or the emitter region side cannot be reduced to 1/2 of the height without the transition region, and the effect of introducing the transition region can be fully utilized. That is not the case.

To avoid this problem, the transition region is composed of a plurality of layers having different material compositions, and the composition of these layers is closer to that of the base region from the base region toward the emitter region. It is necessary to sequentially change the composition close to the composition of the emitter region. Further, by doing so, the strictness of composition control for the semiconductor layer inserted as the transition region can be relaxed.

The effects of the present invention will be clarified below based on concrete data. FIG. 3 shows changes in transistor characteristics when the method of manufacturing the transition region is changed when the doping concentration of the emitter and the transition region is 3 × 10 ¹⁷ cm ⁻³ and the doping concentration of the base is 3 × 10 ¹⁸ cm ⁻³ . It is shown up to FIG. In these figures, A is the case where the transition region is not provided, and B is the thickness of a single layer having an Al molar ratio of 0.15.
When a transition region of nm is provided, C has an Al molar ratio of 0.1,
When the transition region is formed by two layers each having a thickness of 0.2 and a thickness of 10 nm, D has an Al molar ratio of 0.05, 0.1,
When the transition region is formed by 5 layers each having a thickness of 0.15.0.25 and a thickness of 4 nm, E has a linear Al mole ratio change from 0 to 0.3 in a thickness range of 20 nm. It corresponds to the case where the transition region is formed. That is, C and D are examples of the present invention.

FIG. 3 shows the current-voltage characteristics of the transistor,
FIG. 4 shows the dependence of the DC amplification factor on the collector current, and FIG. 5 shows the dependence of the cutoff frequency on the collector current. Since these data are for the case where carrier recombination in the transition region is not remarkable, the element (E) having the transition region in which the composition ratio changes linearly shows the best characteristics. As can be seen from the figure, in the element (C, D) in which two or more layers of stepwise transition regions are formed, the deterioration of the characteristics is not so large as P in practical use as compared with E. This indicates that the structure of the present invention is sufficiently effective in consideration of the ease of manufacturing the device.

In the embodiment described above, the base is GaAs.
In the above, the case where the emitter is made of Ga _0.7 Al _0.3 As is shown, but not only when the Al molar ratio of the emitter is other than 0.3, but also when other combinations are used as the material forming the emitter and the base, For example, InP and InGaAs,
It goes without saying that the present invention can be similarly applied to the case of GaAs and Ge, GaP and Si, and the like.

[0025]

According to the present invention, by providing a transition region composed of a plurality of semiconductor layers having different compositions between the emitter and the base, the barrier against electrons from the emitter to the base can be made sufficiently small, and further, from the base to the emitter. It is possible to obtain a heterojunction bipolar transistor that takes advantage of the advantage of using a wide bandgap emitter by leaving some barriers to holes of the above. Further, in the present invention,
Since a smooth composition change is not given to the transition region, it is easy to control the manufacturing process.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a transistor configuration of an embodiment of the present invention.

FIG. 2 is a diagram showing a band structure near the emitter / base of the device of FIG.

FIG. 3 is a diagram showing current-voltage characteristics of an example transistor in comparison with a conventional example.

FIG. 4 is a diagram showing the collector current dependency of the DC amplification factor in the example transistor in comparison with the conventional example.

FIG. 5 is a diagram showing a collector current dependence characteristic of a cutoff frequency in an example transistor in comparison with a conventional example.

FIG. 6 is a diagram showing a conduction band shape in the vicinity of an emitter / base of a staircase type heterojunction bipolar transistor.

FIG. 7 is a diagram showing a conduction band shape in the vicinity of an emitter / base junction of a graded heterojunction bipolar transistor.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Collector electrode 2 ... n ⁺ type GaAs substrate 3 ... n type GaAs collector region 4 ... p type GaAs base region 5 ... base electrode 6 ... transition region 7 ... n composed of a plurality of layers having different material compositions Type Ga _0.7 Al _0.3 As emitter region 8 ... Emitter electrode.

Claims (2)

(57) [Claims] 1. A p-type base region made of a first-type semiconductor material, and an n-type emitter region made of a second-type semiconductor material having a band gap larger than that of the first-type semiconductor material and a small electron affinity. A heterojunction bipolar transistor having a transition region between a base region and an emitter region, the transition region being made of a semiconductor material having an intermediate composition of a first-type semiconductor material and a second-type semiconductor material, wherein the transition region has a plurality of semiconductors having different compositions. A heterojunction bipolar transistor comprising a material layer, and a barrier for suppressing injection of holes from the base region to the emitter region is formed in a valence band of the transition region. 2. The heterojunction bipolar transistor according to claim 1, wherein the composition of the plurality of semiconductor material layers in the transition region changes stepwise in the stacking direction of the layers.