JP2005109368A - Heterojunction bipolar transistor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an HBT having a high output by increasing the breakdown voltage of a collector layer therein. <P>SOLUTION: Oxygen-doped AlGaAs having a band gap wider than that of GaAs is used in whole or part of a collector layer 16, or one or both of first and second collector layers 16b and 16a are made of an AalGaAs layer having a band gap wider than that of GaAs and doped with oxygen. The doped oxygen concentration of the AlGaAs layer is set to be preferably ≥2×10<SP>17</SP>cm<SP>-3</SP>. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a heterojunction bipolar transistor (HBT) using a III-V group compound semiconductor, and more particularly to a structure for increasing the breakdown voltage of its collector layer.

  A heterojunction bipolar transistor (HBT) using a group III-V compound semiconductor has characteristics such as low loss, low distortion, and a single power supply, such as CDMA (code-division multiple access). It is attracting attention as an output amplifier for digital mobile phones.

  The HBT generally has an emitter / base junction formed of an AlGaAs / GaAs heterojunction. However, recently, from the viewpoint of improving device characteristics or reliability, it has been studied to replace the emitter layer from an AlGaAs emitter layer to an InGaP emitter layer, and some have been fabricated. This is because, when an AlGaAs layer containing Al, which is an active atom, is used as an emitter layer, many non-radiative recombination centers due to deep levels are formed in the AlGaAs layer. This is because the degradation of the HBT proceeds through the center, and an attempt is made to solve the degradation problem by using an InGaP layer that does not contain Al as the emitter layer.

A conventional GaAs HBT will be described with reference to FIG. The HBT shown in the figure is roughly divided into three layers, ie, an emitter layer 3, a collector layer 5, and a base layer 4. That is, a collector layer 5 composed of an n ⁺ -type GaAs subcollector layer 5 b and an n ⁻ -type GaAs collector layer 5 a is formed on a semi-insulating GaAs substrate 6, and a p-type GaAs base layer 4, AlGaAs or An n-type emitter layer 3 made of InGaP, an n ⁺ -type GaAs emitter contact layer 2 and an n ⁺ -type InGaAs non-alloy layer 1 are sequentially stacked. In other words, the first conductivity type collector layer (second collector layer) having a low impurity concentration is provided between the base layer 4 and the first conductivity type sub-collector layer (first collector layer) 5b having a high impurity concentration. ) 5a is sandwiched. The emitter layer 3 is a layer for injecting electrons and is made of n-type AlGaAs or InGaP. The collector layer 5 is a layer that collects electrons injected from the emitter, and is mainly made of n-type GaAs. The base layer 4 is a layer for adjusting the emitter-collector current, and is made of p-type GaAs. The collector layer 5 includes a first collector layer (subcollector layer) 5b having a high carrier concentration and a second collector layer (collector layer) 5a having a low carrier concentration.

In the HBT, an emitter electrode 7 is formed on the InGaAs non-alloy layer 1, a base electrode 8 is formed on the GaAs base layer 4, and a collector electrode 9 is formed on the subcollector layer 5b. In the case of grounded emitter, the positive voltage V _C of the power source 10 is applied to the collector electrode 9, the voltage V _{B of the} power source 11 is applied, and the base current A is caused to flow from the base electrode 8 as a signal input. The current B is controlled (FIG. 3).

The operation of the HBT described above will be described with reference to FIG. When the base current A does not flow, the reverse voltage V _C by the power source 10 is applied between the base and the collector, so that a depletion layer is generated and no current flows between the emitter and the collector. Accordingly, the transistor is turned off.

  When the base current A is passed, a forward voltage is applied between the emitter and the base, so that a current flows between the emitter and the base. However, when electrons in the emitter region flow into the base, since the base is thin, there are few coupling partner holes, and most of the electrons pass through the base and enter the collector region. As a result, a current (collector current B) flows between the emitter and the collector, and thus the transistor is turned on.

When the transistor is off, as described above, the voltage V _{C of the} power supply 10 is applied between the emitter, the base and the collector. Since the voltage drop between the base and the emitter is small, most of the power supply voltage V _C is applied to the base layer and the collector layer. Accordingly, the setting of the power supply voltage is limited by the breakdown voltage (= withstand voltage) at the time of reverse bias between the base and collector of the transistor. As a performance of the transistor, it is desired that the withstand voltage is high.

  With regard to increasing the breakdown voltage of an HBT, conventionally, in an HBT having a structure in which a second collector layer having a low impurity concentration is sandwiched between a base layer and a first collector layer, impurities (such as Si or the like) are contained in the second collector layer. Donor ions) are intentionally added according to a predetermined concentration to electrically compensate the space charge in the second collector layer and eliminate the effect of electrolytic concentration due to band bending that promotes impact ionization. The technique to make is proposed (for example, refer patent document 1).

Although not related to the high breakdown voltage of the HBT, a technique is known in which oxygen is added to the subcollector layer to act as an electron trap in view of the problem that the current gain β decreases as the subcollector concentration increases. (For example, refer to Patent Document 2).
Japanese Patent Laid-Open No. 05-041388 JP 2001-319937 A

  However, the withstand voltage technology disclosed in Patent Document 1 has a purposeful collector structure that suppresses an avalanche breakdown phenomenon due to impact ionization at the time of high collector current injection. Therefore, it is added, and Patent Document 1 does not describe a specific structure (a constituent material) of each layer of the HBT.

  Further, Patent Document 2 adds oxygen to the subcollector layer, but does not directly increase the breakdown voltage of the collector layer or discuss the application of AlGaAs, which is a material handled by the present invention.

As described above, when the transistor is off, the voltage V _{C of the} power supply 10 is applied between the emitter, the base and the collector. Since the voltage drop between the base and the emitter is small, most of the power supply voltage is applied to the base layer and the collector layer. Accordingly, the setting of the power supply voltage is limited by the breakdown voltage (= withstand voltage) at the time of reverse bias between the base and collector of the transistor. As a performance of the transistor, it is desired that the withstand voltage is high.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a heterojunction bipolar transistor having a collector layer structure that solves the above problems and has a high breakdown voltage and can be used at a high output.

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

  The heterojunction bipolar transistor according to the invention of claim 1 is a heterojunction bipolar transistor using a group III-V compound semiconductor in which a collector layer, a base layer, and an emitter layer are sequentially stacked on a semi-insulating substrate. All or part of the collector layer is composed of an oxygen-doped AlGaAs layer.

  A heterojunction bipolar transistor according to a second aspect of the present invention is a first semiconductor having a lower impurity concentration than a first collector layer between a base layer and a first collector layer of a first conductivity type on a semi-insulating substrate. In a heterojunction bipolar transistor using a group III-V compound semiconductor having a structure in which a second collector layer of the above conductivity type is sandwiched, one or both of the first collector layer and the second collector layer is oxygen-doped AlGaAs. It is composed of layers.

The invention according to claim 3 is the heterojunction bipolar transistor according to claim 1 or 2, wherein the oxygen doping concentration of the AlGaAs layer is 2 × 10 ¹⁷ cm ⁻³ or more.

<Key points of the invention>
The heterojunction bipolar transistor of the present invention uses oxygen-doped AlGaAs for the whole or a part of the collector layer. Specifically, in a heterojunction bipolar transistor using a III-V group compound semiconductor, a collector layer having an oxygen-doped (2 × 10 ¹⁷ cm ⁻³ or more) AlGaAs region is used.

  When an AlGaAs layer is grown by a metal organic chemical vapor deposition method at a lower V / III ratio (group V / III molar flow rate ratio) than usual, oxygen is easily taken into the AlGaAs layer. Similarly, even when the growth temperature is lower than usual, oxygen is easily taken into the AlGaAs layer.

  By doping with oxygen, the band gap of AlGaAs becomes wider than that of GaAs, and a deeper n-type is realized. By using AlGaAs for the whole or a part of the collector layer, the breakdown voltage is increased.

  In the bipolar transistor of the present invention, oxygen collector AlGaAs having a wider band gap than GaAs is used for the whole or a part of the collector layer, or one or both of the first collector layer and the second collector layer are made to be more than GaAs. Since it is composed of an oxygen-doped AlGaAs layer having a wide band gap, it is possible to obtain a heterojunction bipolar transistor that can increase the breakdown voltage of the collector and can be used at high output.

The effect of improving the breakdown voltage can be remarkably enhanced by setting the oxygen doping concentration of the AlGaAs layer to 2 × 10 ¹⁷ cm ⁻³ or more.

  The embodiment of the present invention will be described below with reference to FIGS.

  FIG. 1 shows the structure of an InGaP / GaAs HBT according to this embodiment. This has a structure in which a collector layer 16, a base layer 15, an emitter layer 14, an emitter contact layer 13, and a non-alloy layer 12 are sequentially grown on a semi-insulating GaAs substrate 17 by metal organic vapor phase epitaxy (MOVPE). .

As shown in FIG. 1, the collector layer 16 is an n ⁺ -type AlGaAs having a thickness of 600 nm as a first conductivity type first collector layer (subcollector layer) 16b having a high impurity concentration on a semi-insulating GaAs substrate 17. (Carrier concentration 5 × 10 ¹⁸ cm ⁻³ ) and n ^- type AlGaAs (carrier concentration 1 × 10 ¹⁶ cm ⁻³ ) of 500 nm is grown as the first conductivity type second collector layer 16a having a low impurity concentration. Yes. The AlGaAs collector layers 16a and 16b can be grown at a low V / III ratio, thereby reducing the V group material cost. In addition, the crystallinity of the collector layer is improved by lowering the growth temperature of the AlGaAs collector layers 16a and 16b than usual.

On the second collector layer 16a, p ⁺ type GaAs (carrier concentration 4 × 10 ¹⁹ cm ⁻³ ) of 70 nm is grown as the base layer 15. Therefore, the first conductivity type second collector layer 16a having a low impurity concentration is sandwiched between the base layer 15 and the first conductivity type first collector layer 16b.

Further, on the base layer 15, an n-type InGaP of 100 nm as the emitter layer 14 (In mixed crystal ratio 0.49, carrier concentration 5 × 10 ¹⁷ cm ⁻³ ), and an n ⁺ -type GaAs of 100 nm as the emitter contact layer 13. As the non-alloy layer 12, n ⁺ -type InGaAs (In mixed crystal ratio: 0.5, carrier concentration: 1 × 10 ¹⁹ cm ⁻³ ) is grown as the non-alloy layer 12 (carrier concentration: 5 × 10 ¹⁸ cm ⁻³ ).

An emitter electrode 18, a base electrode 19 and a collector electrode 20 are formed on the non-alloy layer 12, the base layer 15 and the subcollector layer 16b, respectively, thereby forming a heterojunction bipolar transistor. In the case of grounded emitter, the positive voltage V _C of the power source 21 is applied to the collector electrode 20, the voltage V _{B of the} power source 22 is applied to the base electrode 19, and the base current A is input from the base electrode 19 as a signal input. The collector current B that is output is controlled.

  By using the HBT having such a configuration, the breakdown voltage of the collector layer of the HBT can be increased, thereby realizing a bipolar transistor that can be used at a high output.

Next, in order to confirm the effect of the present invention, the I-V _B characteristics (input characteristics) of the HBT having the conventional structure and the HBT having the present structure were measured and compared. FIG. 2 shows the measurement results. V _{B on the} horizontal axis is the base voltage, and I on the vertical axis is the base current. The unit of the vertical axis is, for example, “8.0E-02” and represents 8.0 × 10 ⁻² .

In FIG. 2, a curve a is an I-V _B characteristic (input characteristic) in an HBT having a conventional structure, and a curve b is an HBT having a structure according to the present embodiment in which the oxygen concentration in the AlGaAs collector layer 16 is 2 × 10 ¹⁷ cm ⁻³ . The I-V _B characteristic (input characteristic) in the case of the above, and the curve c is the I-V _B characteristic (input characteristic) in the case of 1 × 10 ¹⁸ cm ⁻³ .

  As can be seen from the comparison between the curves a and b and c, the base-collector breakdown voltage is higher in the structure in which the oxygen concentration in the AlGaAs collector layer 16 is higher. This means that the collector breakdown voltage of the HBT has been improved by applying the present invention.

<Other embodiments and modifications>
In the above embodiment, both of the two layers (second collector layer 16a and first collector layer 16b) constituting the collector layer 16 are used for oxygen-doped AlGaAs, but only one of the two layers is oxygen-doped. It is also possible to use AlGaAs or oxygen doped AlGaAs for a part of each layer.

  Moreover, in the said embodiment, although the emitter layer was InGaP, it can also be comprised by AlGaAs.

<Usage method, application system, etc.>
The oxygen-doped AlGaAs used in the present invention has a higher breakdown voltage than GaAs. The present invention can be applied not only to the collector layer of the HBT but also to the high breakdown voltage of each layer of the field effect transistor FET and the high electron mobility transistor HEMT.

It is a schematic sectional drawing which shows the structure of the heterojunction bipolar transistor which concerns on embodiment of this invention. I-V _B characteristics of the heterojunction bipolar transistor of the present invention (input characteristics) is a diagram showing, in comparison with the conventional example. It is a schematic sectional drawing which shows the structure of the conventional heterojunction bipolar transistor.

Explanation of symbols

12 Non-alloy layer 13 Emitter contact layer 14 Emitter layer 15 Base layer 16 Collector layer 16a Second collector layer (collector layer)
16b First collector layer (sub-collector layer)
17 Semi-insulating GaAs substrate 18 Emitter electrode 19 Base electrode 20 Collector electrode A Base current B Collector current

Claims (3)

In a heterojunction bipolar transistor using a group III-V compound semiconductor in which a collector layer, a base layer, and an emitter layer are sequentially stacked on a semi-insulating substrate,
A heterojunction bipolar transistor characterized in that all or part of the collector layer is composed of an oxygen-doped AlGaAs layer. A structure in which a second collector layer of the first conductivity type having a lower impurity concentration than the first collector layer is sandwiched between a base layer and a first collector layer of the first conductivity type on a semi-insulating substrate. In a heterojunction bipolar transistor using a III-V group compound semiconductor,
A heterojunction bipolar transistor, wherein one or both of the first collector layer and the second collector layer is formed of an oxygen-doped AlGaAs layer. The heterojunction bipolar transistor according to claim 1 or 2,
A heterojunction bipolar transistor, wherein the AlGaAs layer has an oxygen doping concentration of 2 × 10 ¹⁷ cm ⁻³ or more.

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