JP2004039901A - Epitaxial wafer for hetero-junction bipolar transistor and hetero-junction bipolar transistor using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a novel epitaxial wafer for a hetero-junction bipolar transistor and the hetero-junction bipolar transistor that can prevent the leak of a current adjacent to an emitter layer and improve a current gain. <P>SOLUTION: In the epitaxial wafer for the hetero-junction bipolar transistor that is provided with at least a collector layer 4, a base layer 5, an emitter layer 6, and an emitter contact layer 7 on a compound semiconductor substrate 2, the emitter contact layer 7 is doped with carbon at a specified concentration. Thus, recombination adjacent to the emitter layer 6 can be greatly reduced while avoiding the deterioration of a current gain due to the leakage of a current beforehand. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a heterojunction bipolar transistor and an epitaxial wafer for obtaining the heterojunction bipolar transistor.
[0002]
[Prior art]
As transistors for next-generation mobile phones such as CDMA, “single power supply”, “high-speed operation”, “low distortion”, and “high reliability” have been demanded. Attention has been focused on junction bipolar transistors (hereinafter, referred to as HBTs).
[0003]
The HBT is formed, for example, on a semi-insulating substrate such as GaAs, on a sub-collector layer of n ⁺ -type GaAs, a collector layer of undoped GaAs or n-type GaAs, a base layer of p ⁺ -type GaAs, and an emitter of n-type InGaP. Layer, an n ⁺ -type GaAs emitter contact layer, and an n-type InGaAs non-alloy layer. The HBT epitaxial wafer is provided with a collector current by applying a voltage between the emitter layer and the base layer. It is known that Ic is amplified by the base current Ib, and an excellent amplification factor (current gain) β is obtained. Here, β is represented by the ratio (Ic / Ib) between the collector current and the base current.
[0004]
[Problems to be solved by the invention]
By the way, in such a conventional epitaxial wafer for HBT, the base current Ib and the emitter current Ie may leak, and the excellent current gain β, which is a feature thereof, may decrease over time.
[0005]
That is, it is considered that one of the biggest causes of the decrease in the current gain β is that, when InGaP or a mixed crystal containing phosphorous similar to InGaP is used as the emitter layer, the current gain β and the other layers based on GaAs are used. Substitution of phosphorus and arsenic occurs in the boundary layer, and the properties of the crystal deteriorate around the boundary layer, and the recombination at the interface between the base layer and the emitter layer or between the emitter layer and the emitter contact layer increases. It is considered that the current gain β is reduced due to the occurrence.
[0006]
Therefore, the present invention has been devised in order to effectively solve such a problem, and an object of the present invention is to prevent current leakage due to recombination in an emitter layer and a layer around the emitter layer, thereby increasing a current gain. An object of the present invention is to provide a novel epitaxial wafer for HBT which can be improved and an HBT using the same.
[0007]
[Means for Solving the Problems]
In order to solve the above problems, the present invention provides an epitaxial wafer for a heterojunction bipolar transistor having at least a collector layer, a base layer, an emitter layer, and an emitter contact layer on a compound semiconductor substrate. An emitter contact layer, for example, an emitter contact layer made of GaAs as described in claim 2, is doped with a predetermined concentration of carbon, for example, in a range of 5 × 10 ¹⁷ cm ⁻³ to 1 × 10 ¹⁸ cm ^−3. .
[0008]
Thereby, recombination in the vicinity of the emitter layer can be greatly reduced, so that a decrease in current gain β due to current leakage can be avoided.
[0009]
By using such an epitaxial wafer for HBT, a high-quality HBT (heterojunction bipolar transistor) having excellent current gain can be easily obtained.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, a preferred embodiment of the present invention will be described with reference to the accompanying drawings.
[0011]
FIG. 1 shows an embodiment of an HBT epitaxial wafer 1 according to the present invention.
[0012]
As shown in the figure, the HBT epitaxial wafer 1 has an n ⁺ -type GaAs (carrier concentration: 3 to 5 × 10 ¹⁸ cm ⁻³ ) sub-collector layer 3 on a semi-insulating substrate 2 such as a GaAs substrate. A collector layer 4 of GaAs or n-type GaAs (carrier concentration: 1 to 30 × 10 ¹⁷ cm ⁻³ ), a base layer 5 of p ⁺ -type GaAs (carrier concentration: 2 to 5 × 10 ¹⁹ cm ⁻³ ), and n type InGaP: an emitter layer 6 of (carrier concentration ^{3~5 × 10 17 cm -3),} n + -type GaAs: the emitter contact layer 7 of (carrier concentration ^{3~5 × 10 18 cm -3),} n -type InGaAs (Carrier concentration: 1 to 5 × 10 ¹⁹ cm ⁻³ ) and a non-alloy layer 8 formed by successive epitaxial growth.
[0013]
In the epitaxial wafer 1 for HBT according to the present invention, 5 × 10 ¹⁷ cm ⁻³ to 1 × 10 ¹⁸ cm ⁻³ is formed in the emitter contact layer 7 located between the emitter layer 6 and the non-alloy layer 8. Carbon (C) is doped in the concentration range, whereby the recombination in the vicinity of the emitter layer 6 can be greatly reduced, and the current gain β is further reduced as well as the current gain β due to current leakage. It is possible to improve.
[0014]
Here, in the present invention, the carbon doping concentration is limited to the range of 5 × 10 ¹⁷ cm ⁻³ to 1 × 10 ¹⁸ cm ⁻³ , as described later, when the current gain β is less than 5 × 10 ¹⁷ cm ^−3. This is because it is difficult to effectively prevent the decrease of the thickness. On the other hand, if it exceeds 1 × 10 ¹⁸ cm ⁻³ , the surface of the layer becomes cloudy, and the improvement of the current gain β cannot be obtained.
[0015]
As a method of doping carbon into the emitter contact layer 7, when the emitter contact layer 7 is epitaxially grown by MOVPE (metal organic chemical vapor deposition), it is easily doped by an auto-doping method from a TMG (trimethylgallium) material. In addition, it is also possible to supply by using an auto-doping from TEG (triethyl gallium) or a doping material such as CBr ₄ or CCl ₃ Br.
[0016]
It is necessary to control the carbon-doped emitter contact layer 7 so that Si serving as a carrier supply medium is not excessively supplied, and its concentration is 1 × 10 ¹⁸ cm ⁻³ or less in volume density and 1 × 10 ¹⁸ cm ⁻³ in area density. It is desirable to control it to × 10 ¹³ cm ⁻² or less.
[0017]
【Example】
The thickness of the InGaP emitter layer was 40 nm, the In composition was 0.48, the thickness of the emitter contact layer was 100 nm, the carrier concentration was 3 × 10 ¹⁸ cm ⁻³ , and the carbon concentration was 1 × 10 ¹⁸ cm ⁻³ . The growth was performed using the MOVPE method, and carbon contained in the TMG raw material was used as the carbon and supplied as auto doping.
[0018]
Then, when the current gain β of the thus-obtained epitaxial wafer for HBT was measured, as shown in FIG. 2, an improvement in current gain β of about 5% as compared with the conventional case was recognized. In FIG. 2, the horizontal axis represents the carbon concentration (cm ⁻³ ), and the vertical axis represents the current gain β / base resistance Rb (Ω / sq [resistance per unit area]).
[0019]
【The invention's effect】
In short, according to the present invention, since the emitter contact layer made of GaAs or the like is doped with a predetermined concentration of carbon, an HBT epitaxial wafer having an excellent current gain can be obtained. As a result, the use of such an HBT epitaxial wafer can exhibit excellent effects such as easy provision of a high-quality HBT (heterojunction bipolar transistor) having an excellent current gain.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing one embodiment of an epitaxial wafer for HBT according to the present invention.
FIG. 2 is a graph showing the relationship between carbon concentration and current gain / base resistance (β / Rb) according to the present embodiment.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 epitaxial wafer for HBT 2 semi-insulating substrate 3 sub-collector layer 4 collector layer 5 base layer 6 emitter layer 7 emitter contact layer 8 non-alloy layer

Claims (4)

A heterojunction bipolar transistor epitaxial wafer having at least a collector layer, a base layer, an emitter layer, and an emitter contact layer on a compound semiconductor substrate, wherein the emitter contact layer is doped with a predetermined concentration of carbon. Epitaxial wafer for junction bipolar transistor. 2. The epitaxial wafer for a hetero-junction bipolar transistor according to claim 1, wherein the emitter contact layer is made of GaAs. 3. The epitaxial wafer for a heterojunction bipolar transistor according to claim 1, wherein the carbon is doped in a concentration range of 5 × 10 ¹⁷ cm ⁻³ to 1 × 10 ¹⁸ cm ⁻³ . 4. 4. A hetero-junction bipolar transistor produced using the epitaxial wafer for a hetero-junction bipolar transistor according to claim 1.

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