JP2003115495A - Epitaxial wafer for hetero junction bipolar transistor and hetero junction bipolar transistor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an HBT structure in which a current amplification factor βis not lowered even when a sub-collector layer of the HBT is doped by a high concentration of dopant exceeding 3×10<18> cm<-3> . SOLUTION: The epitaxial wafer for an AlGaAs or InGaP HBT is provided. The HBT using the epitaxial layer comprises a GaAs spacer layer 12 filled with a triethyl gallium (TEG) as a Ga source between a semi-insulating GaAs substrate 1 and GaAs sub-collector layers 2, 12.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an epitaxial wafer for a heterojunction bipolar transistor (HBT) and improvement of the current amplification factor β of the heterojunction bipolar transistor.

[0002]

2. Description of the Related Art A heterojunction bipolar transistor (HBT) using a heterojunction as an emitter-base junction is
Since the bandgap of the emitter layer is wider than the bandgap of the base layer, the emitter injection efficiency can be increased, and therefore it is expected to be used as an ultrahigh-speed, high-power device. In particular, HBTs made of AlGaAs / GaAs are excellent in high-speed and high-current driving capability, and therefore have been actively developed as high-speed electronic devices for mobile phones and optical communications.

Conventionally, in the HBT, the reduction in the contact area of the emitter electrode due to the miniaturization leads to the increase in the contact resistance, and there is a problem that the expected high speed cannot be achieved. Therefore, in order to reduce the emitter contact resistance. In addition, an InGaAs layer having a narrow band gap is doped at a high concentration and is epitaxially grown on the upper portion of the emitter region to form an emitter contact layer, so that the barrier height between the metal and the emitter electrode is substantially eliminated.

Conventional AlGaAs / G using this technique
An example of the wafer structure of the aAs-based HBT is shown in FIG. Figure 5
On a semi-insulating (SI) GaAs substrate 1 at⁺
Type GaAs subcollector layer 2 (thickness 500 nm, carrier
Oh concentration 5 × 10¹⁸cm^-3), N ^-Type GaAs collector layer 3
(Thickness 500 nm, carrier concentration 2 × 10¹⁶cm^-3), P
⁺Type GaAs base layer 4 (thickness 70 nm, carrier concentration
Is 4 × 10¹⁹cm^-3), The emitter layer 5 and the emitter contour
The cut layer 6 is laminated. In addition, S. I is semi-insulating
(Semi insulation). The emitter layer 5 is n
Type Al_0.3Ga_{0. 7}As layer 5a (carrying by Si doping)
Oh concentration 5 × 10¹⁷cm^-3, Thickness 100 nm) and n-type Al
_xGa_1-xAs (x = 0.3 → 0) graded layer 5b
(AlAs mixed crystal ratio x is gradually decreased from 0.3 to 0
Carried by Si-doping with a thickness of about 50 nm.
Oh concentration 5 × 10¹⁷cm^-3From 5 × 10¹⁸cm^-3) And consist of
Has been done. The emitter contact layer 6 is n⁺Type GaA
s layer 6a (thickness 100 nm, Si-doped carrier)
Concentration 5 × 10¹⁸cm^-3) And n⁺Type In_yGa_1-yAs (y
= 0 → 0.5) Graded layer 6b (thickness 50 nm, S
Carrier concentration by e-doping is 1 × 10¹⁹cm^-3From 4x
10¹⁹cm^-3) And n⁺Type In_0.5Ga_0.5As layer 6c (no
Alloy ohmic contact layer) (thickness 50 nm, S
Carrier concentration of 4 × 10 by e-doping¹⁹cm^-3) And
Is made. This emitter contact layer 6 is
Resistance between the contact layer 5 and the emitter electrode (not shown)
Since it reduces the
The contact resistance with the electrode is low and the sheet resistance is low.
nGaAs is used. Above n⁺Type In_yGa_1-yAs
The graded layer 6b has an InAs mixed crystal ratio y of from the bottom to the top.
The structure gradually increases from 0 to 0.5 over the surface.
ing.

As described above, in the related art, the heterojunction bipolar transistor (HB) is obtained by focusing on the emitter layer and the base layer and improving the crystallinity and the interface thereof.
Attempts have been mainly made to improve the current amplification factor β of T). Therefore, the relationship between the crystallinity and the current amplification factor β of the collector layer and the subcollector layer below the base layer has not been well studied.

[0006]

Recently, it has been found that the doping concentration of the subcollector layer has a great influence on β. In vertical devices such as HBT,
The area of the electrode for drawing out the wiring is considerably limited.
Therefore, in order to obtain a sufficiently low contact resistance, it is necessary to dope the sub-collector layer with a considerably high concentration.

However, it has become clear that when the carrier concentration of the subcollector layer exceeds 3 × 10 ¹⁸ cm ^-3 , the current amplification factor β extremely decreases.

As a solution to this, for example, Japanese Patent Laid-Open No. 11-6
Japanese Patent No. 7785 proposes that the subcollector layer has a laminated structure of first to third subcollector layers. That is, the carrier concentration of the first subcollector layer on the GaAs substrate side is increased and the collector electrode is formed thereon. A second subcollector layer, which is a single layer having a lattice constant different from that of the GaAs substrate or a laminated structure including a layer having a lattice constant different from that of the GaAs substrate, is formed on the first subcollector layer. The crystal defects of the layer are prevented from propagating to the third subcollector layer or the base layer. Further, a third subcollector layer is formed on the second subcollector layer, and its carrier concentration is made lower than that of the first subcollector layer to reduce crystal defects, and a base layer is formed thereon. To do.

According to the sub-collector layer having such a three-layer structure, it is possible to reduce the contact resistance of the collector electrode, prevent the propagation of crystal defects to the base layer, and prevent the decrease of the current gain. Can be expected.

However, according to this method, the subcollector layer has a laminated structure of first to third subcollector layers, and
The carrier concentration must be controlled.

Therefore, an object of the present invention is to solve the above problems and to make the sub-collector layer exceed 3 × 10 ¹⁸ cm ^-3 without requiring the sub-collector layer itself to have a laminated structure of three layers. It is an object of the present invention to provide an epitaxial wafer for a heterojunction bipolar transistor and a heterojunction bipolar transistor in which a structure in which the current amplification factor β does not decrease even when doped is obtained.

[0012]

In order to achieve the above object, the present invention is configured as follows.

The invention according to claim 1 specifies an AlGaAs-based epitaxial wafer for HBT.
That is, on a GaAs (gallium arsenide) substrate, a subcollector layer made of a GaAs crystal that exhibits ohmic contact with a metal electrode and showing n-type conduction, and a GaAs crystal showing n-type conduction that draws electrons from the base layer are formed. And a base layer made of a GaAs crystal, an InGaAs (indium gallium arsenide) crystal, or an AlGaAs (aluminum gallium arsenide) crystal that exhibits p-type conduction that controls the flow of electrons, and a heterojunction to the base layer. AlGaA that forms and injects electrons into the base layer and suppresses the injection of holes from the base layer and exhibits n-type conduction
An epitaxial wafer for a heterojunction bipolar transistor, which is formed by sequentially stacking an emitter layer made of s crystal and an emitter contact layer made of InGaAs crystal showing an n-type conduction forming an ohmic contact with a metal electrode. GaAs using triethylgallium (TEG) as a Ga source between the subcollector layer
A feature is that a spacer layer is provided.

The invention according to claim 2 is the InGaP-based H
The BT epitaxial wafer is specified. That is, on a GaAs substrate, a subcollector layer made of a GaAs crystal exhibiting n-type conduction that forms an ohmic contact with a metal electrode, and an electron n is extracted from the base layer.
A collector layer made of a GaAs crystal exhibiting p-type conductivity, a base layer made of a GaAs crystal, an InGaAs crystal, or an AlGaAs crystal exhibiting p-type conductivity for controlling electron flow, and a heterojunction is formed with respect to the base layer. , An emitter layer made of InGaP (indium gallium phosphide) crystal exhibiting n-type conduction for injecting electrons into the base layer and suppressing injection of holes from the base layer, and n-type conduction for forming ohmic contact with the metal electrode. In an epitaxial wafer for a heterojunction bipolar transistor, which is formed by sequentially stacking an emitter contact layer made of InGaAs crystal shown in FIG. 1, GaA using triethylgallium (TEG) as a Ga source between the substrate and the subcollector layer.
An s spacer layer is provided.

For AlGaAs HBT or InGa
In the P-based HBT epitaxial wafer, it is preferable that the GaAs spacer layer using triethylgallium (TEG) as a Ga source has a thickness of at least 2 nm or more (claim 3).

The spacer layer, the subcollector layer, the collector layer, the base layer, the emitter layer and the emitter contact layer can be formed by the MOVPE method (claim 5).

<Main Points of the Invention> The main point of the present invention is that AlGa
In the epitaxial wafer for As-based HBT or InGaP-based HBT and the HBT using the same, semi-insulating G
This is to have a structure in which a GaAs spacer layer using triethylgallium (TEG) as a Ga source is inserted between the aAs substrate and the GaAs subcollector layer. With such a structure, as shown in FIG. 2, the sub-collector layer is 3 × 10 ¹⁸ cm 2.
^Even if the doping exceeds ^-3 , the current amplification factor β does not decrease. Further, this structure can be formed relatively easily by the MOVPE method.

The thickness of the GaAs spacer layer is at least 2 nm or more. The reason is that, as shown in FIG. 3, when the GaAs spacer is not provided, the current amplification factor β
This is because the current amplification factor β is improved when the GaAs spacer is 1 nm, and the current amplification factor β can be reliably improved when the thickness is 2 nm. Also, 5n
The current amplification factor β becomes 150 at m, and the current amplification factor β does not change even if the thickness increases thereafter. Therefore, the thickness of the GaAs spacer layer is optimally 5 nm or more.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below based on the illustrated embodiments.

<Embodiment 1> FIG. 1 shows an example of the structure of an AlGaAs / GaAs-based HBT epitaxial wafer according to a first embodiment of the present invention. This is semi-insulating Ga
The conventional HBT epitaxial wafer shown in FIG. 5 has a structure in which a GaAs spacer layer 12 using triethylgallium (TEG) as a Ga source is inserted to a thickness of 5 nm between the As substrate 1 and the GaAs subcollector layer. And the structure is different.

In FIG. 1, a GaAs layer 12 having a thickness of 5 nm is formed on a semi-insulating GaAs substrate 1 using triethylgallium (TEG) as a Ga source. On top of that, n with a thickness of 500 nm and a carrier concentration of 5 × 10 ¹⁸ cm ^{−3
A +} -type GaAs subcollector layer 2 and an n ⁻ -type GaAs collector layer 3 having a thickness of 500 nm and a carrier concentration of 2 × 10 ¹⁶ cm ⁻³ are sequentially stacked, and a C-doped carrier concentration of 4 × 10 ¹⁹ cm 3 is formed thereon. ^-3 , 70 nm thick p ⁺ type GaAs
The base layer 4 is laminated.

An emitter layer 5 and an emitter layer 5 are formed on the base layer 4.
Then, the carrier concentration by Si doping is 5 × 10.¹⁷cm^-3,
100 nm thick n-type Al_0.3Ga_0.7As layer 5a
The AlAs mixed crystal ratio x is gradually changed from 0.3 to 0 on the layer 5a of
About 50 nm thick, Si doped
Carrier concentration by 5 × 10¹⁷cm^-3From 5 × 10¹⁸cm ^-3
N⁺Type Al_xGa_1-xLayered with As graded layer 5b
To be done.

Further, on the emitter layer 5, an n ⁺ type GaAs layer 6a having a thickness of 100 nm and a carrier concentration of 5 × 10 ¹⁸ cm ^-3 by Si doping is formed as an emitter contact layer 6, and on this layer 6a. 50nm thick, S
Carrier concentration by e-doping is from 1 × 10 ¹⁹ cm ^-3 to 4 ×
10 ¹⁹ cm ⁻³ n ⁺ type In _y Ga _1-y As (y = 0 → 0.
5) Graded layer 6b and n ⁺ type In _0.5 Ga _0.5 As layer 6 formed on the graded layer 6b and having a carrier concentration of 4 × 10 ¹⁹ cm ⁻³ by Se doping having a thickness of 50 nm.
c (non-alloy layer) are sequentially stacked. Above n
^{The +} type In _y Ga _1-y As graded layer 6b is made of In As
The structure is such that the mixed crystal ratio y is gradually increased from 0 to 0.5 from the lower surface to the upper surface.

The spacer layer 12, the subcollector layer 2, the collector layer 3, the base layer 4, the emitter layer 5 and the emitter contact layer 6 are grown by the MOVPE method.

As described above, the epitaxial wafer for the heterojunction bipolar transistor of this embodiment has the Ga between the semi-insulating GaAs substrate 1 and the subcollector layer 2.
The structure is such that the As spacer layer 12 is 5 nm.

To compare the dependency of the current amplification factor β on the doping concentration of the sub-collector layer between the structure of this embodiment (FIG. 1) and the conventional structure (FIG. 5: same as FIG. 1 except that the GaAs spacer layer 12 is not provided). Samples were prepared in which the carrier concentration of the sub-collector layer 2 was changed in both cases.

An evaluation HBT having an emitter size of 100 μm was manufactured by a process, and the emitter current density was 10
The current amplification factors β at ³ A / cm ² were compared. FIG. 2 shows the relationship between the carrier concentration of the subcollector and β.

In the conventional product, the current amplification factor β decreases from when the carrier concentration of the subcollector exceeds 3 × 10 ¹⁸ cm ^-3 . On the other hand, in the product of this embodiment, the carrier concentration of the subcollector is reduced. Even if it exceeds 3 × 10 ¹⁸ cm ⁻³ , the current amplification factor β remains almost constant and maintains a high current amplification factor β.

It has been found that the current amplification factor β and the reliability of the HBT are related to each other, and that the reliability is improved as the current amplification factor β is higher in the same structure. Therefore, according to the present invention, it is possible to significantly improve the current amplification factor β even in the structure having the highly-doped subcollector layer.
It can be said that the reliability of the BT element can be considerably improved.

Next, the proper range of the thickness of the GaAs spacer layer 12 will be described. In FIG. 3, the GaAs spacer layer 1
2 shows the result of examining the relationship between the thickness of No. 2 and the current amplification factor β. The carrier concentration of the subcollector layer 2 was 5 × 10 ¹⁸ cm ⁻³ . Without the GaAs spacer layer 12, the current amplification factor β is 1.
Although it is about 10, the current amplification factor β is improved when the GaAs spacer layer 12 is 1 nm, and the current amplification factor β is surely improved when the GaAs spacer layer 12 is 2 nm, and the current amplification factor β is increased at 5 nm.
Was 150, and the current amplification factor β was about 150 even if the thickness increased thereafter. In this experiment, the upper limit of the thickness of the GaAs spacer could not be obtained.

<Embodiment 2> FIG. 4 shows an example of the structure of an InGaP / GaAs HBT according to a second embodiment of the present invention. This HBT also includes triethylgallium (TEG) between the semi-insulating GaAs substrate 1 and the subcollector layer 2.
The structure is such that the GaAs spacer layer 12 using Ga as a Ga source is inserted to a thickness of 5 nm.

This InGaP / GaAs-based HBT has triethylgallium (TE) on a semi-insulating GaAs substrate 1.
G) with a Ga source of 5 nm thick GaAs spacer layer 1
N ² -type GaAs sub-collector layer 22 (thickness: 500 nm, carrier concentration: 5 × 1) on which an ohmic contact with a metal electrode (collector electrode 7) is formed.
0 ¹⁸ cm ^-3 ), and n ⁻ -type Ga that pulls out electrons from the base layer
As collector layer 23 (thickness 500 nm, carrier concentration 2
X 10 ¹⁶ cm ^-3 ) and a p ⁺ -type GaAs base layer 24 provided with a metal electrode (base electrode 8) to control the flow of electrons
And n-type In forming a heterojunction with the base layer
GaP emitter layer 25 and metal electrode (emitter electrode 9)
And an emitter contact layer 26 forming an ohmic contact are sequentially laminated. The emitter contact layer 26 is an n ⁺ type GaAs emitter contact layer 26b.
And an n ⁺ type InGaAs non-alloy layer 26a.

Also in the heterojunction bipolar transistor having such a structure, the semi-insulating GaAs substrate 1 and the GaAs
Between the subcollector layer 22 and triethylgallium (TE
Since the GaAs spacer layer 12 using G) as the Ga source is provided, a structure in which the current amplification factor β does not decrease even if the sub-collector layer is doped to exceed 3 × 10 ¹⁸ cm ^-3 can be obtained.

In the above first and second embodiments, GaA
As the carrier type of the s spacer layer 12, a non-intentionally doped carrier type was used, but an n-type or p-type doped carrier type was also effective. Further, in the above-described embodiment, as the dopant of the sub-collector layers 2 and 22, Si is used as the n-type dopant, but the same phenomenon occurs even when Se is used, and the current amplification factor β is improved by the GaAs spacer. I was seen.

In the first and second embodiments, the base layers 4 and 24 are made of p ⁺ type GaAs crystal.
Even if it is made of nGaAs crystal or AlGaAs crystal, the effect that the current amplification factor β does not decrease even if the sub-collector layer is doped to exceed 3 × 10 ¹⁸ cm ^-3 can be obtained.

[0036]

As described above, according to the present invention, A
In an epitaxial wafer for lGaAs-based HBT or InGaP-based HBT and an HBT using the same, a structure in which a GaAs spacer layer using triethylgallium (TEG) as a Ga source is inserted between a semi-insulating GaAs substrate and a GaAs subcollector layer. Therefore, even if the sub-collector layer is doped with a high concentration ^exceeding 3 × 10 ¹⁸ cm ⁻³ , the current amplification factor β does not decrease. This structure is also MO
It can be formed relatively easily by the VPE method.

Further, by setting the thickness of the GaAs spacer layer to be at least 2 nm or more, the reliable improvement of the current amplification factor β can be achieved.

Therefore, according to the present invention, in the HBT having the sub-collector layer which is doped with a high concentration and has a sufficiently low resistance, the current amplification factor β and the reliability of the HBT are improved.
It becomes possible to provide BT and its epitaxial wafer.

[Brief description of drawings]

FIG. 1 is a diagram showing a structural example of an HBT epitaxial wafer according to a first embodiment of the present invention.

FIG. 2 is a carrier concentration of the subcollector layer and a current amplification factor β.
It is a figure which shows the relationship with.

FIG. 3 is a diagram showing the relationship between the thickness of a GaAs spacer layer and the current amplification factor β.

FIG. 4 is a diagram showing a structural example of an HBT according to a second embodiment of the present invention.

FIG. 5 is a diagram showing a structural example of a conventional HBT epitaxial wafer.

[Explanation of symbols]

1 Semi-insulating GaAs substrate 2.22 Sub-collector layer 3, 23 Collector layer 4, 24 Base layer 5, 25 Emitter layer 6,26 Emitter contact layer 7 Collector electrode 8 base electrode 9 Emitter electrode 12 GaAs spacer layer

Claims (5)

[Claims] 1. A subcollector layer made of n-type conductivity which forms ohmic contact with a metal electrode on a GaAs substrate, and a n-type conduction GaAs crystal which extracts electrons from a base layer. A collector layer, a base layer made of GaAs crystal, InGaAs crystal, or AlGaAs crystal exhibiting p-type conduction for controlling electron flow, and a heterojunction are formed with respect to the base layer, and electrons are injected into the base layer to form a base. An emitter layer made of AlGaAs crystal exhibiting n-type conduction that suppresses the injection of holes from the layer and an emitter contact layer made of InGaAs crystal exhibiting n-type conduction forming an ohmic contact with the metal electrode are sequentially laminated. In a heterojunction bipolar transistor epitaxial wafer configured as An epitaxial wafer for a heterojunction bipolar transistor, comprising a GaAs spacer layer using triethylgallium (TEG) as a Ga source between the rectifier layer and the rectifier layer. 2. A subcollector layer made of a GaAs crystal showing an n-type conduction which forms an ohmic contact with a metal electrode, and a GaAs crystal showing an n-type conduction extracting an electron from a base layer on a GaAs substrate. A collector layer, a base layer made of GaAs crystal, InGaAs crystal, or AlGaAs crystal exhibiting p-type conduction for controlling electron flow, and a heterojunction are formed with respect to the base layer, and electrons are injected into the base layer to form a base. An emitter layer made of InGaP crystal exhibiting n-type conduction for suppressing injection of holes from the layer and an emitter contact layer made of InGaAs crystal exhibiting n-type conduction for forming ohmic contact with the metal electrode are sequentially laminated. A heterojunction bipolar transistor epitaxial wafer configured as An epitaxial wafer for a heterojunction bipolar transistor, characterized in that a GaAs spacer layer using triethylgallium (TEG) as a Ga source is provided between the epitaxial layer and the contact layer. 3. The triethylgallium (TEG) is Ga
The thickness of the GaAs spacer layer used as the source is at least 2 nm
The epitaxial wafer for a heterojunction bipolar transistor according to claim 1 or 2, which is as described above. 4. The heterojunction bipolar according to claim 1, 2 or 3, wherein the spacer layer, the subcollector layer, the collector layer, the base layer, the emitter layer and the emitter contact layer are formed by MOVPE. Epitaxial wafer for transistors. 5. An epitaxial wafer for a heterojunction bipolar transistor according to claim 1, wherein the subcollector layer, the base layer and the emitter layer are:
A heterojunction bipolar transistor characterized in that each is provided with an electrode.