JPH04206833A - Meterojunction bipolar transistor and its manufacture - Google Patents

Abstract

PURPOSE:To remarkably stabilize characteristic of a device, by forming, on substrate material, at least, a highly doped N-type semiconductor layer for forming a first emitter, and a second emitter whose energy band gap is larger than that of the first emitter. CONSTITUTION:Oxide layers 50 and 60 are formed on the side wall of a mesa 30 and on a first emitter 2c, respectively. Said layers are irradiated with As in a high vacuum and heated at 600 deg.C, thereby eliminating the oxide layer on the mesa side surface and exposing the semiconductor surface. Only the oxide layers on the side surface of a first base P<+>-GaAs 4a and on the first emitter layer 2c are selectively eliminated, and an oxide layer 70 on the side surface of a second emitter 3a is left. A second base layer P<+>-Al0.4Ga0.6As 7 is formed by molecular beam epitaxy 40 from above. In this process, the epitaxial growth of the first base 4a and the first emitter 2c becomes nucleus, and the second base layer 7 of P<+>-Al0.4Ga0.6As is formed on also the oxide layer 70. An SiO2 mask 10 is eliminated together with P<+>-Al0.4Ga0.6As 11 stuck on the mask 10, and a collector electrode 8 and a base electrode 9 are formed.

Description

[Detailed Description of the Invention] Industrial Application Fields of the Invention The present invention is directed to heterojunction bipolar transistors (hereinafter referred to as HBTs), which are promising as ultra-high speed and ultra-high frequency devices.
. ) and its manufacturing method.

Conventional technology In order to improve the high speed and high frequency characteristics of HBT (friend
It is important to increase the current gain cutoff frequency ft and the maximum oscillation frequency f.

f2 is approximately expressed as f--Jft/8πRbCbo (1). Here, Rb is the base resistance KCbo is the base-collector capacitance. In order to increase f, it is important to increase ft and decrease Rb and Cbe, as shown by equation +1 (1). HBTs that use a semiconductor material with a larger energy bandgap as the emitter than the base essentially have a larger +Qft and a smaller LRb, making them extremely promising as high-speed/high-frequency devices. In order to reduce the Cbo of the HBT, it is important to reduce the collector size.In an HBT where the collector is placed on the top (collector top HBT), it is easy to reduce the collector size.AtsushiCbQ
This has the advantage of making it possible to make f smaller, thereby increasing f.This has the problem that electrons injected from the emitter at the bottom of the plate leak toward the base electrode, resulting in a significant drop in the current amplification factor. As a method to prevent this, by introducing p-type impurities into the emitter layer with a wide energy band gap below the external base layer and converting part of the emitter layer to the same type as the base,
By forming a pn junction made of a material with a larger energy bandgap than the base, the on-voltage of the diode in this part is made thicker than the on-voltage of the emitter-base junction in the intrinsic part (by doing so, electrons move toward the base electrode). Attempts have been made to suppress leakage.However, it is difficult to form a steep pn junction with this method, and high-performance devices have not yet been realized.Figure 4,
Figure 5 shows an example of the structure of this type of HBT and its manufacturing method.

In FIG. 5, semi-insulating GaAs (s
, 1-GaAs) on the substrate 1 of highly doped n-type G
aAs(n”-GaAs)2, n-type Al113Gai+ with large bandgap, ?As(n-Al@
,5Ga11. vAs) 3. Highly doped p-type GaA
s(p''-GaAs) 4, n-type GaAs (n-G
aAs) Using the multilayer structure material shown in FIG. 5(a) in which 5,n''-GaAs 6 is formed, a collector mask 10 made of 8102 is formed on the part that will become the collector, and using this as a mask, dry etching is performed to obtain η -GaAs5
A collector mesa 90 made of a and n'''-GaAs 6a is formed as shown in FIG. 5(c).
As in I), using the mask 10 as a mask, p-type impurities are introduced into n-A15.5Gai+, vAsABO3 by ion implantation 140, and that part is converted to p-type.
type Al11.3Ga11.7AS (p-Al11.3
Gai+, 7AS) layer 3b is formed. This results in
A1.1 with a large bandgap! ,3Ga11.7
A pn junction 120 made of As is formed. With this structure (
The barrier force of the conduction band for electrons formed by the junction 120<, n-Ale, 3Gae
, yAs5a and p'-GaAs4, which is larger than the barrier of the conduction band for electrons at the junction that constitutes the intrinsic part of the transistor.
When forward bias is applied between the bases, electrons 130 + & junction 12 are about to be injected from the emitter to the base.
At 0, it is effectively injected into the base only in the eight-intrinsic part, which is blocked by a high barrier to electrons. Hereinafter, the problems to be solved by the invention of forming the collector electrode 8 and the base electrode 9 by photolithography and vapor deposition to produce the structure shown in FIG.
Introducing impurities into the wide bandgap emitter layer through the external base layer and converting part of the emitter layer into a semiconductor layer with majority carriers of the same type as the base.The introduced impurities are distributed in the depth direction.Wide band There is a problem in that it is difficult to form a steep pn junction made of the gap material. Another problem is that when the external base is damaged, the resistance of the fitted sheet increases and the base resistance increases. In addition, in this method, the height of the barrier of the energy conduction band of the pn junction of the wide bandgap semiconductor formed is determined by the semiconductor material forming the emitter and the magnitude of the forward bias voltage applied to the emitter base. There is a limit, and if this limit is exceeded, electrons will leak from the emitter to the base beyond this barrier, resulting in a problem that the current amplification factor will decrease.

Means for Solving the Problems In order to solve the above problems, the collector top of the present invention is placed on a highly doped n-type semiconductor layer forming an emitter of HB T U th) with a higher energy than the first emitter. a second emitter made of an n-type semiconductor with a large band gap; a mesa made of a first base layer and a collector layer made of a p-type semiconductor with a smaller energy band gap than the second emitter; a second emitter that is equal to or greater than the first emitter layer that is bonded to the side surface of the first base layer constituting a portion of the mesa and the first emitter layer and that is formed to cover the side surface of the first emitter that constitutes the mesa; It has a structure including a second base layer made of a p-type semiconductor layer with a large energy band gap. The manufacturing method for this structure is to
At least a highly doped n-type semiconductor layer for forming a first emitter. An n-type semiconductor layer for forming a second emitter having a larger energy bandgap than the first emitter. A mask is formed on the part that will become the collector on a multilayer structure material in which a p-type semiconductor layer to form a first base with a small energy band gap and a semiconductor layer to form a collector are epitaxially formed in this order. Using this as a mask, etching is performed to form a mesa consisting of the second emitter layer, first base layer, and collector layer in the shape 151. ．． and exposing the first emitter layer; removing at least the exposed side surfaces of the first base layer and the oxide layer on the first emitter layer in vacuum; and applying energy using the mask. A step of epitaxially forming a p-type semiconductor layer whose band gap is equal to or larger than that of the second emitter is used.

Effect When the above structure is used, an np junction is formed outside the mesa between the first emitter with a small energy bandgap and the second base made of a semiconductor layer with a larger energy bandgap than the second emitter. Tashiki 2nd
The conduction band barrier to electrons is higher than that of a conventional type np junction made of a material that forms the emitter, and electrons that are about to be injected from the emitter into the second base (external base) are blocked by the high barrier of this junction. The current amplification factor does not decrease because it is no longer blocked and enters the external base. Since a junction is formed between the first base and the second base having a larger energy band gap than the first base layer on the side surface of the first base layer constituting the Mana mesa, the first base and the second base are During this period, a conduction band barrier is formed against electrons, and leakage of electrons to the external base is suppressed. Also, if there is an insulating film made of oxide on the side of the second emitter layer (translation), electrons may be blocked by this barrier, or the oxide layer may be thin and electrons may tunnel. Due to this phenomenon, electrons do not leak from the second emitter to the second base due to the high conduction band barrier of the second base even if they pass through the oxide layer. , if a junction is formed between the second emitter and the second base, electrons will not leak to the second base because they are blocked by the high barrier of the conduction pan of this junction. x0 Using the above manufacturing method
The side surfaces of the first base layer forming the mesa and the exposed surface of the first emitter layer, on which the natural oxide film can be easily removed in vacuum, are cleaned and epitaxy can be performed thereon, which facilitates crystal growth. It is also possible to grow a single crystal through the natural oxide film on the side surface of the second emitter layer, which forms a nucleus and forms a mesa whose natural oxide film is difficult to remove. For this reason, the p-type material with a large band gap that is bonded to the side surface of the first base layer constituting the mesa and the first emitter layer on the outside of the mesa and covers the side surface of the second emitter layer of the mesa. Epitaxial formation of a semiconductor layer becomes possible.

In addition, the collector mask used for forming the mesa consisting of the second emitter layer, the first base layer, and the collector layer and for exposing the first emitter layer is connected to the first base layer and the collector layer constituting the mesa. It can be used to epitaxy form a second base layer made of a p-type semiconductor that covers the side surfaces of the second emitter layer and the first emitter layer. Furthermore, since no impurities are introduced through the external base, high-temperature annealing is not required. Therefore, impurity diffusion between the layers of the multilayer structure material hardly occurs, so that the steepness of each junction is not impaired. In addition, in the conventional method, exposing a thin base uniformly was a major problem in the process, but in the method of the present invention, since the external base layer is formed again, it is necessary to uniformly form the thickness of the external base to a desired value. , and the characteristics of the device are significantly stabilized.

Embodiment FIG. 1 shows a first embodiment of the structure of the HBT of the present invention.On the first emitter layer 2 made of highly doped n-type GaAs (n"-GaAs), an n-type Al layer is formed. [1, aG
ae, 3As(n-A], a, 5Gal], vAs).

First base made of highly doped p-type GaAs (p”-GaAs) 4aX n-type GaAs (n-GaAs)
Collector 5a, highly doped n-type GaAs (n″
a first emitter n''- on the side of the side of the mesa and the outside of the mesa; p'''-AIll, +Ga11. connected to GaAs2c. It has a second base layer made of an eAS layer 7, a collector electrode 8, and a base electrode 9. This structure is fabricated as shown in FIG. Using the same multilayer structure material as in FIG. 5(a), a mask 10 made of 5iCh is formed in the portion that will become the collector, with the stripe direction [110]. Using this as a mask, a forward mesa 30 is formed. Then, the first emitter layer 2c is exposed (FIG. 2(a)). Next, by water washing treatment, oxide layers 50 and 60 are formed on the side surfaces of the mesa 30 and the first emitter 2c, respectively.
Figure (b)). Then, while irradiating As in a high vacuum, 6
The oxide layer on the side surface of the mesa is removed by heating to 00° C. and the semiconductor surface is exposed (FIG. 2(C)). In this step, only the oxide layer on the side surface of the first base p''-GaAs4a and the first emitter layer 2c is selectively removed, leaving the oxide layer 70 on the side surface of the second emitter 3a. .Then,
The second layer is removed from above by molecular beam epitaxy (MBE) 40
A base layer p'''-All14Ga@, eAs7 is formed (FIG. 2(d)).

In this process, the first base 4a that constitutes the mesa and = 13
- The epitaxial growth on the first emitter 2c becomes the core,
Also on the oxide layer 70 are p”-Al!!, 4Gae, eAs
A second base layer 7 is formed. Then, p'''-AI-94Ga11 with 5iCh mask 1o attached thereto.
．． It is removed together with θAs1l, and a collector electrode 8 and a base electrode 9 are formed using photolithography and vapor deposition to form the structure shown in FIG. However, the formation of the emitter electrode is omitted because it is not an important part for this invention.

In the example (i.e., after removing the mask 10 and forming the collector electrode using conventional methods, the force
Al11. After forming aGall and eAs7, the surface is flattened with a resist, dry etched from above, and the mask 10 is located at the beginning of the mask 1°. The mask 10 is chemically removed, and then electrodes are formed by vapor deposition and lift-off. Can be applied. Using this method, it is extremely easy to form electrodes even in the case of a microscopic collector. The base electrode 9 can also be formed in the vicinity of the mesa 30 by self-alignment if the base electrode 9 is formed by vapor deposition from above using a mask 10 formed in a parapet shape or a parapet shape electrode formed by the above method as a mask. It is effective in reducing base resistance.

In the embodiment, an oxide layer is formed both on the exposed surface of the mesa and on the first emitter layer 2c. It is not necessary if it can be introduced into the epitaxy growth chamber while maintaining the However, generally
The formation of a natural oxide film is unavoidable when forming a mesa.It is necessary to remove the natural oxide film layer and expose the cleaned surface while irradiating As in a high vacuum. In this case, GaA
In AlGaAs with a low s or Al content, the natural oxide layer can be removed at a relatively low temperature, exposing the cleaned surface and allowing epitaxy to be formed thereon. There is a problem that it is not easy to remove the oxide film layer and epitaxy is not possible.
The side of Al5, 3Gas, TAs3a is p-A
Epitaxy on the sides of the first base 4a and on the first emitter layer 2c, even if a native oxide layer remains on it, forms only a part of the region forming the eAS7. A single-crystal p
”-AI@40a8θAs7 formation of Ebiquxie is the second
This is also possible on the side surface of the emitter 3a. It is also possible to expose the cleaned surface outside the epitaxy growth chamber, cover the surface with a protective film other than oxide, introduce it into the epitaxy chamber, and remove it. In this case, (i) it becomes possible to bond the second emitter part to the second base even on the side surface of the second emitter part.
The p-Alθ, 4Ga11. eAs
and is formed by joining n-All, aG
It is sufficient that they are formed continuously on the side surface of the second emitter consisting of all and vAS3a. In the structure of the example (from n′″-GaAs2c of the first emitter to p of the second base)
”-A Is AGaseAs7 electrons i; t, n”-GaAs2c-A2B, aG
Since the barrier of the conduction band formed by the ae and aAs junction is higher than the intrinsic part, it is blocked by this barrier and does not enter the second base.

In addition, electrons injected from the second emitter to the first base (
yo first base p"-GaAs and second base p"-Al
ll.

At the junction with 4Gaθ and eAs, a conduction band barrier is formed that cuts electrons, and the electrons are reflected at this junction and do not leak to the second base. Also, the second emitter n-Al1! , 3GaB, 7As to second base Goo A] 1! , 4Ga11. Electron leakage to eAS
j n-A19.3Ga11゜7AS The force that prevents leakage to the second base because it is blocked by the insulating film layer made of the oxide layer on the surface＼ Or, this oxide layer is thin and electrons pass through the oxide layer due to tunneling phenomenon. Mop”-Allll, 4Gae
Since there is a high conduction band barrier caused by eAS, the electrons are reflected and do not leak to the second base. From these facts, electron injection from the emitter to the base is possible due to the n-A1[1,3Ga
11. A collector-top HBT with a large current amplification factor can be fabricated by effectively performing the operation at the junction between the tAS second emitter 3a and the p''-GaAs first base 4a.The material of the embodiment is the only material that can exhibit these characteristics. Instead, the following materials can be used: n-Aly Ga+ with a large energy band gap as the second emitter
-X As is used as 1, k as the first base and p” -AlGaA with a smaller band gap than the first emitter.
szAs (Z < The barrier of the conduction band formed by the junction with IY Ga+ -Y As is Φa
The barrier of the conduction band formed by the junction of y, n"-〇aAstI)"-AlxGa+-xAs is Φox
, n-AlxGa+-yAS and 1)"-AIXGat-
If the barrier of the conduction band formed by the xAS junction is ΦXX, then as shown in Figure 3, ΦXX×Φoz
The relationship <Φll'l' is obtained. In the conventional structure shown in FIGS. 4 and 5, ΦXX is formed in the external base portion.

In the structure of the present invention (p”-A1xG as the second base)
Even if a+-MAS is used, Φox is larger than Φxx, so leakage of electrons to the external base region is suppressed to a greater extent than in the conventional case. In addition, p''-A ly Ga+ -YAs, which has a larger band gap than p''-AIXGat-XAS, can be used as the second base, so ΦOY can be made much larger than 08M, which reduces the leakage of electrons to the external base. can be suppressed even more than before. Furthermore, in the conventional structure, the intrinsic base and the extrinsic base are made of the same material, which causes a slight leakage of electrons from the intrinsic base to the extrinsic base. Since a duction band barrier is formed, leakage of electrons is suppressed. Nayo 1st base and 2nd base
At the junction with the base, the barrier of the energy valence spunt is small and both bases are highly doped, so the thickness of the barrier to holes is extremely small, and holes can freely pass between the two bases. There is no problem for the diode.Also, since the second emitter is made of n-AlGaAs and p''-AlGaAsvAs, the electrons are blocked by the high barrier of the insulating layer made of oxide film.
Or, even if the oxide film is thin and electrons pass through the oxide film layer due to tunneling, p" -A lv Ga+ -Y
It is blocked by the high barrier of the As conduction band. Also, generally (Table 2) nJ1xGa+- of the second emitter
Removal of the oxide layer on XAS is difficult (the oxide layer is removed by some method and n-Al)! A junction of AlYGa1-yAs is formed on Ga+-xAsjl. 3rd
As shown in the figure, the high barrier Φ formed by this junction
X\ suppresses leakage of electrons from the second emitter to the second base. In the example (translation: n- as a collector)
Although GaAs is used, the invention is not limited to this. As a collector, n-AlGa has a large energy band gap.
As can be used, and undoped or p-type materials can also be used to help electrons travel at high speed. Also, in the example (with n4- as the first emitter)
Although GaAs is used, the present invention is not limited to this. In the structure of the present invention, the first emitter serves as an emitter contact layer and forms a high conduction band barrier for electrons between it and the second base to suppress leakage of electrons to the external base. It plays the role of easily removing the native oxide film layer in an ultra-high vacuum and facilitating the formation of epitaxy thereon.

Therefore, other materials may be used as long as they fulfill this role. In addition, although an HBT made of an AlGaAs/GaAs-based material is used in the embodiment, it is of course applicable to an HBT made of a material in which the above-mentioned effect can occur. In the embodiment, the second base layer 7 is the collector 5a.
(There is no problem since the base-collector is reverse biased.Also, in the example, the force is formed on the side surface of the mask 10 using SiO2 (other materials such as SiNx may also be used. In short, any material may be used as long as it does not react with the collector layer at the temperature at which the p-type material is epitaxially formed.

Effects of the Invention In the structure of the HBT of the present invention (mu) a second emitter layer with a large band gap formed on a highly doped first emitter layer with a small energy band gap, a first base layer with a small energy band gap, and a collector. A tanu emitter having a wide bandgap second base that covers the outer surfaces of the first base layer and second emitter layer constituting the mesa and the upper surface of the first emitter layer outside the mesa. Electrons that are about to be injected into the base from the 1st emitter and the 2nd base have a high barrier to electrons formed by the junction between the 1st emitter and the 2nd base, and a high conductivity caused by the oxide insulating film or the 2nd base between the 1st emitter and the 2nd base. Almost no leakage of electrons to the external base occurs due to the band barrier and the conduction band barrier to electrons formed by the junction between the first base and the second base. This occurs intensively only in the intrinsic part, and a collector top HBT with a large current amplification factor can be obtained.In addition, in the HBT manufacturing method of the present invention, the natural oxide film can be easily removed in a high vacuum, and the clean surface can be exposed. H
The H of the present invention can be formed without damaging other parts of the BT.
A structure of BT is created.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional diagram showing the structure of the HBT of the collector top of the present invention.
Figure 3 shows an example of the manufacturing method for the emitter.
FIGS. 4 and 5 are cross-sectional views showing the structure of a conventional collector top HBT and its manufacturing method. 1... Semi-insulating GaAs substrate, 2... First
n”-GaAs to form the emitter, 2c
... n"-GaAs 2, 3 outside of mesa 3o...
...n-A1a3Ga1 for forming the second emitter
1. vAa, 3 a = 2nd emitter 4...
p”-GaAs forming the first base, 4
a...First base member 5...n-GaAs for forming the collector, 5-a-...n-GaAs
) Refuku 6 ・-・：l n”-GaA＼ 6a...-n”-GaA for forming rector contact
s subcollection 7 “p”-Gae, aAle, aA
s Second base layer 8...Collector electrode 9...
・P'''-A attached on base electric h 10...sj, 02 collector mass 11...510210
ll! , 4Gag, eAS, ao+-+ Mesa consisting of first emitter 3a, first base 4a, collectors 5a and 6a, 40... Molecular beam epitaxy (MBE), 5
0...Oxide layer 60 on the outer 2c of mesa 30...
...Oxide layer 70 on the side surface of mesa 30...Oxide seed 90 on the side surface of the second emitter constituting mesa 30...Collector mesa consisting of collectors 5a and 6a, 120.--Illusion- =.n -Al13Gae1. vAs/p-Al113G
ae, vAs junction, 130---Electrons reflected by the conduction band barrier of junction 12o, 140...
...p-type ion implantation, 150...n-AlxGa+
-xAs/p-AlxGa+-xAs junction conduction band barrier ΦXX, 160...n''-G
aAs substrate "-A], vGa+-vAs junction conduction band barrier ΦOX, 170...n"-Ga
Barrier Φov of conduction band of As/p''''-AlYGa1-yAs junction, 180...-n-Al
xGa+-xAs/p''-AlYGa1-Y As8 conduction band barrier Φ×Y0 Name of agent
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Claims (3)

[Claims] (1) On a first emitter layer made of a highly doped n-type semiconductor, a second emitter layer made of an n-type semiconductor with a larger energy bandgap than the first emitter, and a second emitter layer made of an n-type semiconductor with a larger energy bandgap than the first emitter.
p, which has a smaller energy bandgap than the emitter of
a mesa comprising a first base layer and a collector layer made of a type semiconductor, at least a first emitter layer on the outside of the mesa and a first base layer and a second emitter constituting the mesa; On the sides of the layer there is a p
A heterojunction bipolar transistor, characterized in that it has a second base layer made of a type semiconductor layer. (2) The first emitter is highly doped n-type GaAs, the second emitter is n-type Al_XGa_1_-_XAs,
Al_YGa_1_-_YAs whose first base is p-type,
2. The heterojunction bipolar transistor according to claim 1, wherein the second base is made of p-type Al_ZGa_1_-_ZAS and has a relationship of Y<X≦Z. (3) a highly doped n-type semiconductor layer for forming a first emitter; an n-type semiconductor layer for forming a second emitter having a larger energy bandgap than the first emitter; A p-type semiconductor layer for forming a first base having a smaller energy band gap than an emitter and a semiconductor layer for forming a collector are epitaxially formed on a substrate in this order on a multilayer structure material,
A mask is formed on a portion that will become a collector, and a mesa consisting of a second emitter layer, a first base layer, and a collector layer is formed by etching, and the first emitter layer is exposed, and at least the exposed first base is removing the oxide layer on the side surfaces of the layer and the first emitter layer in a vacuum, and using the mask to form a p-type semiconductor layer with an energy bandgap equal to or larger than that of the second emitter layer; A method for manufacturing a heterojunction bipolar transistor, comprising the step of epitaxially forming at least the side surfaces of the first base layer and the second emitter layer constituting the mesa and the first emitter layer. (4) First emitter is highly doped n-type GaAs, second emitter is n-type Al_XGa_1_-_XAs, first base is p-type Al_YGa_1_-_YAs, second base is p-type Al_ZGa_1_- The method for manufacturing a heterojunction bipolar transistor according to claim 3, characterized in that the transistor is made of _ZAS and has a relationship of Y<X≦Z.