JP2805864B2 - Double heterojunction bipolar transistor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a double heterojunction bipolar transistor.

[Summary of the Invention]

In the present invention, the emitter, base and collector are each
In a double heterojunction bipolar transistor formed of InP, GaInAs and InP, a composition gradient layer composed of AlGaInAs and having a continuously changing Al composition is formed between the base and the collector. Thus, the base / collector to be able to remove the step difference Delta] E _c of conventional present are the conduction band edge E _c, thus it is possible to increase the current amplification factor beta.

[Conventional technology]

A heterojunction bipolar transistor (HBT) using a wide-gap semiconductor for the emitter is a conventional silicon (S
i) A transistor that can overcome the drawbacks of a bipolar transistor. The principle of this HBT is well known, but the main points are as follows.
That is, holes, which are majority carriers in the base, are hardly diffused into the emitter due to the barrier due to the band gap difference ΔE _g between the emitter and the base. As a result, the base current decreases, and the efficiency of electron injection from the emitter to the base increases. Therefore, the base impurity concentration is increased,
Even if the impurity concentration of the emitter is reduced, the current amplification factor β can be increased. This means that the base resistance and the emitter-base capacitance related to the high speed of the HBT can be reduced. And this HBT is Si
It has already been demonstrated to be much faster than bipolar transistors.

Conventionally, the material system of this HBT has been AlGaAs / GaAs (Al:
Aluminum, Ga: gallium, As: arsenic) are the most studied. As other material systems, i) AlInAs (E) / GaInAs (B) / GaInAs (C) ii) InP (E) / GaInAs (B) lattice-matched to an InP (In: indium, P: phosphorus) substrate / GaInAs (C) iii) InP (E) / GaInAs (B) / InP (C) are known. Here, E, B and C indicate an emitter, a base and a collector, respectively. These i) to ii
The material system of i) can emit and absorb light in the wavelength band of 1.3 to 1.5 μm, so optoelectronic integrated circuits (OEIC)
It is well studied from the point of view. These i) to ii
The feature of the material system i) is that the band gap difference ΔE _g between AlInAs / GaInAs or InP / GaInAs is larger than that between AlGaAs / GaAs, and the saturation speed of electrons is several times higher than that of GaAs. Therefore, it is considered that HBTs using the material systems i) to iii) are far superior in principle to AlGaAs / GaAs HBTs in terms of high-speed performance.

The HBT using InP / GaInAs / InP in iii) has a heterojunction between the emitter / base and between the base / collector, and such an HBT is known as a double heterojunction bipolar transistor (DHBT). Where InP
Γ-X valley energy in its energy band structure is 0.9 eV, which is larger than the Γ-X valley energy of GaInAs 谷 0.5 eV, and the electron saturation speed is also GaInAs
Greater than. Therefore, when the collector is formed of InP, the collector traveling time can be shorter than that when the collector is formed of GaInAs. In addition, as can be seen from the energy band diagram shown in FIG. 6, since a barrier for holes in the base is also formed on the collector side, the base spreading effect (kirk effect) at the time of high implantation is suppressed. For this reason, this InP / GaInAs / InP DHBT is superior to a single heterojunction HBT in which the collector is formed of GaInAs as an ultra-high-speed transistor.

Incidentally, in the heterojunction between InP and GaInAs, the conduction band edge E _c is discontinuous, it is believed to step Delta] E _c is present. The value of ΔE _c has not yet been established at present, but is generally considered to be 0.1 to 0.2 eV. Therefore, the energy band diagram of the InP / GaInAs / InP DHBT is as shown in FIG.

In the InP / GaInAs / InP DHBT shown in FIG.
The emitter-base ΔE _c can be used to provide the initial velocity of the electrons. However, Delta] E _c between the base / collector electrons injected from the emitter to the base hinders the flow of the collector. Therefore, the probability that the electrons injected into the base recombine with the holes in the base increases, so that a large current amplification factor β cannot be obtained. In particular, in the operation in the circuit, when the base-collector voltage _Vbc is reduced, the current amplification factor β is reduced at the time of ON, and the operating voltage range is limited.

In order to solve this problem, an n-type GaInAs layer is conventionally formed between the base and the collector as shown in FIG.
When an n-type GaInAs layer is formed between the base and the collector in this manner, electrons injected from the emitter into the base will
Gaining energy in the GaInAs layer to overcome the base-collector ΔE _c ,
By overflowing from the InAs layer, it becomes possible to easily reach the collector.

[Problems to be solved by the invention]

However, as described above, n-type GaIn
Even if the As layer is formed, the probability that the electrons accumulated in the n-type GaInAs layer recombine with the holes is still large, and therefore, it is necessary to avoid a decrease in the current amplification factor β compared to the ideal state. Have difficulty.

Accordingly, an object of the present invention, the step of InP / GaInAs / InP DHBT base / collector to the prior present are the conduction band edge E _c delta
Remove the E _c, it is to provide a double-heterojunction bipolar transistor that can increase the current amplification factor beta.

[Means for solving the problem]

In order to achieve the above object, the present invention provides a double heterojunction bipolar transistor having an emitter, a base and a collector formed of InP, GaInAs and InP, respectively, wherein AlGaIn is provided between a base (6) and a collector (3).
A composition gradient layer (4) composed of As and having a continuously changing Al composition is formed.

[Action]

According to the above means, for example, the Al composition of the composition gradient layer (4) made of AlGaInAs is
Between the base and the collector by continuously increasing the value from 0 at the interface with the base (6) composed of the composition gradient layer (4) to the value at the interface with the collector (3) composed of InP. can be made to the conduction band edge E _c is continuous. That is, it is possible to eliminate the step Delta] E _c of conventional present are the conduction band edge E _c between the base / collector. Therefore, the electrons injected from the emitter (8) into the base (6) are not prevented from flowing toward the collector (3), so that the probability of recombination of electrons and holes in the base (6) is small. Become. As a result, the current amplification factor β can be increased.

Embodiment Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In all the drawings of the embodiments, the same parts are denoted by the same reference numerals.

FIG. 1 shows InP / GaInAs / InP DH according to one embodiment of the present invention.
Indicates BT.

As shown in FIG. 1, InP / GaInAs / I
In the nP DHBT, for example, an n ⁺ -type InP layer 2 constituting a subcollector layer is formed on an n ⁺ -type InP substrate 1, for example. The thickness of the n ⁺ -type InP layer 2 is, for example, about 0.3 μm, and the impurity concentration is, for example, about 3 × 10 ¹⁸ cm ⁻³ . this
On the n ⁺ -type InP layer 2, for example, an undoped InP layer 3 constituting a collector layer is formed. This undoped InP
The thickness of the layer 3 is, for example, about 0.5 μm. On this undoped InP layer 3, for example, an n ^- type Al _x Ga _{1 -x} InAs (for example, (Al _x Ga _{1 -x} ) _0.47 In _0.53 As) composition gradient layer 4 is formed. The thickness of the n ⁻ -type Al _x Ga _{1 -x} InAs composition gradient layer 4 is, for example, about 0.1 μm, and the impurity concentration is, for example, 5 × 10 5
It is about ¹⁶ cm ^-3 . This n ^- type Al _x Ga _1-x InAs composition gradient layer 4
The Al composition x, the n ^- -type Al _x Ga _1-x value 0 at the interface with the InAs composition gradient layer 4 and undoped InP layer 5 described later, the n ^-
The value at the interface between the type Al _x Ga _{1 -x} InAs composition gradient layer 4 and the undoped GaInAs layer 3, for example, changes continuously (for example, linearly) from 0.1 to 0.5 (for example, 0.25). This n ^- type Al
_{On the} xGa1 _- xInAs composition gradient layer 4, for example, an undoped GaInAs layer 5 constituting a spacer layer is formed. The thickness of the undoped GaInAs layer 5 is, for example, about 0.01 μm. On this undoped GaInAs layer 5, for example, ap ⁺ -type GaInAs layer 6 constituting a base layer is formed. This p ⁺
The thickness of the GaInAs layer 6 is, for example, about 0.2 μm, and the impurity concentration is, for example, about 3 × 10 ¹⁸ cm ⁻³ .

An undoped GaInAs layer 7 constituting a spacer layer is formed on the p ⁺ -type GaInAs layer 6. The thickness of the undoped GaInAs layer 7 is, for example, about 0.01 μm. On this undoped GaInAs layer 7, for example, an n-type InP layer 8 constituting an emitter layer is formed. This n-type InP layer 8
Has a thickness of, for example, about 0.5 μm and an impurity concentration of, for example, about 10 ¹⁷ cm ⁻³ . On this n-type InP layer 8, for example, an n ⁺ -type GaInAs layer 9 constituting a cap layer is formed. The thickness of the n ⁺ -type GaInAs layer 9 is, for example, about 0.1 μm, and the impurity concentration is, for example, about 5 × 10 ¹⁸ cm ⁻³ .
Here, the n ⁺ -type GaInAs layer 9, the n-type InP layer 8 and the undoped GaInAs layer 7 have, for example, a circular mesa shape having a diameter of about 120 μm.

An emitter electrode 10 made of, for example, AuGe / Au (Au: gold, Ge: germanium) is formed on the n ⁺ -type GaInAs layer 9 constituting the cap layer. Also make up the base layer
On the p ⁺ -type GaInAs layer 6, a base electrode 11 made of, for example, AuZn / Au (Zn: zinc) is formed. Furthermore, n ⁺ type In
On the back surface of the P substrate 1, a collector electrode 12 made of, for example, AuGe / Au is formed.

Next, the InP /
An example of a method for manufacturing GaInAs / InP DHBT will be described.

As shown in FIG. 1, an n ⁺ -type InP layer 2, an undoped InP layer 3, and an n ⁻ -type Al _x Ga are formed on an n ⁺ -type InP substrate 1 by, for example, a normal pressure metal organic chemical vapor deposition (MOCVD) method. _{The 1-x} InAs composition gradient layer 4, the undoped GaInAs layer 5, the p ⁺ -type GaInAs layer 6, the undoped GaInAs layer 7, the n-type InP layer 8, and the n ⁺ -type GaInAs layer 9 are sequentially epitaxially grown. This epitaxial growth is performed under conditions where the lattice constants match, and the growth temperature is, for example, 63
0 ° C. As the n-type dopant and the p-type dopant, for example, disilane (Si ₂ H ₆ , 5 ppm) and dimethyl zinc (DMZ) are used, respectively. Incidentally, the epitaxial when growth is, the p ⁺ type by the upper and lower undoped GaInAs layer 7,5 of GaInAs layer 6, the p ⁺ -type GaInAs layer 6 pn junction by diffusion doped Zn in the n-type InP layer 8 side or undoped InP Movement to the layer 3 side is prevented.

Next, for example, n ⁺ type GaInAs by photolithography
For example, a circular resist pattern (not shown) on the layer 9
After the n ⁺ -type GaInAs layer 9 is etched with a phosphoric acid-based etching solution using the resist pattern as a mask, the n-type InP layer 8 is etched with a hydrochloric acid-based etching solution.
Is etched. Next, the undoped GaInAs layer 7 is again etched with a phosphoric acid-based etchant to form a p ⁺ -type GaInAs layer.
The layer 6 is exposed. After that, the resist pattern is removed. Next, another resist pattern is formed, and the p ⁺ -type GaInAs layer 6, the undoped GaInAs layer 5, and the n ^-- type Al
_{The x} Ga _{1 -x} InAs composition gradient layer 4 is sequentially etched, and further etched halfway in the depth direction of the undoped InP layer 3. Thereby, base separation between elements is performed.

Next, after forming the emitter electrode 10, the base electrode 11, and the collector electrode 12, alloying is performed.

Thus, the target InP / GaInAs / InP DHBT is completed.

FIG. 2 shows an energy band diagram of the InP / GaInAs / InP DHBT according to this embodiment. However, in the energy band diagram shown in FIG. 2, undoped GaInAs5, 7 are omitted. As shown in FIG. 2, the p ⁺ -type GaInAs layer 6 forming the base layer and the undoped In forming the collector layer
The conduction band edge of the n ⁻ type Al _x Ga _{1 -x} InAs composition gradient layer 4 formed between the P layer 3 and the n ⁻ type Al _x Ga _{1 -x} InAs composition gradient layer 4 is continuous with the conduction band edge of the p ⁺ type GaInAs layer 6. That is, ΔE _c existing between the base and the collector of the conventional InP / GaInAs / InP DHBT is according to this embodiment.
It is not present in InP / GaInAs / InP DHBT.

FIG. 3 shows an example of the grounded IV characteristics of the InP / GaInAs / InP DHBT according to this embodiment. InP / GaIn which does not have the n ⁻ -type Al _x Ga _{1 -x} InAs composition gradient layer 4 manufactured for comparison with the InP / GaInAs / InP DHBT according to this embodiment.
FIG. 4 shows an example of the grounded IV characteristic of the As / InP DHBT. Here, the vertical axis of the graph shown in these FIGS. 3 and 4 the collector current I _c, the horizontal axis represents the voltage V _be between the base / emitter.

As shown in FIG. 4, the n ^- type Al _x Ga _{1 -x} InAs composition gradient layer 4
In the case of InP / GaInAs / InP DHBT having no, when V _bc = 0, the collector current I _c was small because the ΔE _c existing between the base and the collector was a barrier to electrons, and thus the current amplification factor β was small. . On the other hand, in the InP / GaInAs / InP DHBT according to this embodiment, since the ΔE _c between the base and the collector is removed as described above, electrons injected from the emitter to the base easily flow to the collector. , the collector current I _c as shown in FIG. 3 is increased, thus the current amplification factor β is increased.

Incidentally, be added a large reverse bias between the base / collector, a large collector current I _c and the current amplification factor β will be of course any conventional InP / GaInAs / InP DHBT is obtained.
However, V _bc is constantly changing in the circuit, and this InP / G
For aInAs / InP DHBT is small V _bc is turned on,
At this time, the deterioration of the transistor characteristics adversely affects the circuit operation.

As described above, according to this embodiment, the n ⁻ -type Al _x Ga _{1 -x} InAs composition gradient layer 4 in which the Al composition x changes continuously is the base /
Because it is formed between the collector, there is no level difference Delta] E _c of the base / collector to the prior present are the conduction band edge E _c, whereby a large current amplification factor even at low bias β
InP / GaInAs / InP DHBT that yields the following can be realized. Further, since the emitter of the InP / GaInAs / InP DHBT according to the embodiment is formed by the n-type InP layer 8 having a high electron saturation speed, the InP / GaInAs / InP DHBT according to the embodiment is also excellent in terms of high speed. .

 Next, FIG. 5 shows another embodiment of the present invention.

As shown in FIG. 5, the InP / GaInAs / I
In the nP DHBT, the base layer is formed of, for example, a p ⁺ type Al _y Ga _{1 -y} InAs (eg, (Al _y Ga _{1 -y} ) _0.47 In _0.53 As) composition gradient layer, and the subcollector layer is formed of, for example, an n ⁺ type GaInAs. InP / GaInAs / InP according to the above embodiment except that it is formed by a layer
It has the same configuration as DHBT. Here, this p ⁺ type Al _y Ga _1-y
Al composition y of the InAs composition gradient layer, the value at the interface between the p ⁺ -type Al _y Ga _1-y InAs composition gradient layer and an n-type InP layer 8, for example from 0.3, the p ⁺ -type Al _y Ga _{1- y} InAs composition gradient layer and n ^- type Al _x Ga _1-x In
It changes continuously (for example, linearly) to a value 0 at the interface with the As composition gradient layer 4.

According to this embodiment, since the base layer is formed by the p ⁺ type Al _y Ga _1-y InAs composition gradient layer, the p ⁺ type Al _y Ga _1-y
The conduction band edge E _c of InAs composition gradient layer has a shape that descends towards the junction between the base / collector as shown in Figure 5. Therefore, electrons injected from the emitter to the base flow to the collector and quickly. Further, since the subcollector layer is formed of an n ⁺ -type GaInAs layer having a smaller band gap than InP, a step at the conduction band edge occurs between the collector and the subcollector as shown in FIG. Electrons easily flow from the subcollector layer.

Although the embodiments of the present invention have been specifically described above, the present invention is not limited to the above-described embodiments, and various modifications based on the technical idea of the present invention are possible.

For example, in the above embodiment, the n ^- type Al _x Ga _{1 -x} InAs composition gradient layer 4 is formed between the base and the collector.
Instead of the n ⁻ -type Al _x Ga _{1 -x} InAs composition gradient layer 4, for example, an undoped Al _x Ga _{1 -x} InAs composition gradient layer may be formed.

〔The invention's effect〕

As described above, according to the present invention, since a composition gradient layer composed of AlGaInAs and having a continuously changing Al composition is formed between the base and the collector, the composition gradient layer conventionally exists between the base and the collector. The step ΔE _c at the conduction band edge E _c can be removed, and the current amplification factor β can be increased.

[Brief description of the drawings]

FIG. 1 shows an InP / GaInAs / InP DHBT according to an embodiment of the present invention.
FIG. 2 is a sectional view showing InP / GaInAs / InP shown in FIG.
Fig. 3 shows the energy band diagram of DHBT.
FIG. 4 is a graph showing an example of the grounded base IV characteristic of GaInAs / InP DHBT, and FIG. 4 shows the grounded base I- of a conventional InP / GaInAs / InP DHBT having no n ^- type Al _x Ga _{1 -x} InAs composition gradient layer. FIG. 5 is a graph showing an example of V characteristics, FIG. 5 is an energy band diagram of InP / GaInAs / InP DHBT according to another embodiment of the present invention, FIG. 6 is an energy band diagram of a conventional InP / GaInAs / InP DHBT,
FIG. 7 is an energy band diagram of another conventional InP / GaInAs / InP DHBT. Description of main reference numerals in the drawings: 1: n ⁺ type InP substrate, 2: n ⁺ type InP layer, 3: undoped InP layer, 4: n ^- type Al _x Ga _1-x InAs composition gradient layer,
6: p ⁺ -type GaInAs layer, 8: n-type InP layer, 10: emitter electrode, 11: base electrode, 12: collector electrode.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-62-139354 (JP, A) JP-A-63-318777 (JP, A) JP-A-1-136368 (JP, A) JP-A-1- 149465 (JP, A) JP-A-2-280338 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H01L 21/33-21/331 H01L 29/68-29/737

Claims (1)

(57) [Claims] An emitter, a base and a collector, respectively.
A double heterojunction bipolar transistor formed of InP, GaInAs and InP, comprising AlGaInAs between the base and the collector,
A double heterojunction bipolar transistor, wherein a composition gradient layer in which the Al composition changes continuously is formed.