JPH04315437A - Heterojunction bipolar transistor - Google Patents

Abstract

PURPOSE:To decrease threshold voltage and power consumption by forming an AlGaInAs base layer of one conductivity type, which is within a critical film thickness and has a specific composition in which the aluminum content decreases toward the collector layer from the emitter. CONSTITUTION:A bipolar transistor includes a semi-insulating substrate 1 such as GaAs, on which are formed a heavily doped GaAs sub-collector layer 2, an i-AlxGa1-xAs collector layer 3, a heavily doped base layer 4, an n-AlxGa1-xAs emitter layer 5, and an n-GaAs emitter cap layer 6. The base layer is composed of (AlxGa1-x)1-xInyAs within a critical film thickness, and the aluminum content decreases toward the collector layer 3 from the emitter 5. As a result, the base resistance is decreased, and thus threshold voltage and power consumption can be reduced.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a heterojunction bipolar transistor (Heterojunction bipolar transistor).
polar transistor (HBT)).

0002

[Prior Art] III-V compound semiconductors, especially AlGa
With advances in epitaxial growth technology for As/GaAs-based semiconductors, semiconductor elements having heterojunctions that could not be produced in the past have been realized. One example is a high electron mobility transistor (HEMT), and another example is a heterojunction bipolar transistor (HB) according to the present invention.
T). Among these, heterojunction bipolar transistors in particular are fabricated by growing materials with different forbidden band widths on a semi-insulating substrate, which allows for a greater degree of freedom in design. It is hoped that this will be realized. Generally, by using a semiconductor having a wider forbidden band width than at least the base layer for the emitter layer, majority carriers in the base layer are prevented from flowing into the emitter layer at the heterojunction between the emitter layer and the base layer. By increasing the number of majority carriers in the base layer to the extent that the ability to prevent the majority carriers in the base layer from flowing into the emitter layer increases, the resistance of the base layer is lowered and higher performance is achieved. There is.

When an integrated circuit is actually manufactured using the above-mentioned heterojunction bipolar transistor, the rise voltage Vt of the single heterojunction bipolar transistor
The threshold voltage that determines h is determined by the material constant of the emitter base material, so the uniformity σ within the wafer surface
A value of Vth that is so low that it does not pose a practical problem is achieved.

0004

However, the current gain β within the wafer plane and the current ION in the ON state of the heterojunction bipolar transistor vary due to process variations, particularly the base resistance. For example, if there is a variation of 100 angstroms due to emitter mesa etching for a base layer thickness of 1000 angstroms, this will cause a variation of about 10% in the sheet resistance of the external base part and about 3% in the resistance of the contact part. , which poses a major practical problem.

Furthermore, in AlGaAs/GaAs heterojunction bipolar transistors, when the impurity density of the base is increased to lower the base resistance, the current amplification factor decreases, and both, that is, the impurity density of the base and the current amplification factor There was a trade-off relationship between them. And, as a problem accompanying these, Alx Ga1-x As
/In GaAs graded-based HBT, the emitter
There were also problems such as an increase in threshold voltage and an increase in contact resistance due to the base junction being made of Alx Ga1-x As.

Furthermore, in order to increase the speed of a heterojunction bipolar transistor, it is essential to make the base layer thinner, but in the past, it has been difficult to improve the crystal structure of the base layer to make it thinner. , and no process approach was taken.

[0007] Therefore, the present invention has been made in consideration of the above circumstances, and its purpose is to eliminate variations in the current gain within the wafer plane and the current in the ON state due to the influence of the base resistance, and to reduce the impurity density. The object of the present invention is to provide a heterojunction bipolar transistor that can reduce the threshold voltage and power consumption because the base resistance can be lowered without any problems.

[0008]

[Means for Solving the Problems] In order to solve the above objects, the present invention comprises a sub-collector layer and a collector layer of a first conductivity type, a base layer of a second conductivity type, and an emitter layer of a first conductivity type. In a heterojunction bipolar transistor in which a sub-collector layer, a collector layer, and an emitter layer are stacked on a substrate with a base layer in between, each of the above-mentioned layers is the base layer of the second conductivity type described above. teeth,
(Alx Ga1-x )1-y Iny configured such that the Al composition ratio x decreases from the emitter layer side to the collector layer side, and whose film thickness is within the critical film thickness range.
The structure was made of an As layer.

[0009]

In the above structure, according to the present invention, by setting the thickness of the base layer of the second conductivity type to be less than or equal to the critical thickness, a heterojunction bipolar transistor can be fabricated without the problem of dislocation. The main feature of conventional heterojunction bipolar transistors is the reduction of in-plane variation, which reduces the variation in current gain and on-current within the wafer plane due to the influence of base resistance. It becomes possible. Second, if we design under the condition of constant current gain, In
By using the GaAs base layer, the energy difference in the forbidden band width between the base layer and the emitter layer increases, and it becomes possible to reduce the base resistivity by increasing the impurity density. Thirdly, by reducing the base forbidden band width, a reduction in threshold voltage and power consumption is expected.

0010

[Embodiments] Before explaining the embodiments, the principle of the present invention will be explained as follows. In the heterojunction bipolar transistor according to the present invention, (Alx Ga1
-x)1-y Iny As layer is used, and in this case, if the In composition ratio y is increased to a certain value or more, only the emitter layer can be selectively etched during emitter layer etching. As a result, the base layer remains at the same thickness as it was during crystal growth. That is, only the variations in crystal growth affect the variations in sheet resistance of the base layer, and the uniformity is further improved.

[0011] Here, variations in etching become noticeable when the base layer thickness is made thinner with the aim of increasing the speed of a heterojunction bipolar transistor, so that it becomes more useful in practice. In addition, the base layer can be graded by changing the In composition in addition to changing the Al composition ratio x, making it possible to achieve high speed and high current gain in the same way as conventional heterojunction bipolar transistors. can be utilized.

In terms of crystal growth, if the In composition ratio y is increased too much, dislocations occur, but this problem is caused by (Alx Ga1-
x) 1-y The problem can be solved by making the Iny As film thickness less than or equal to the critical film thickness.

Next, in order to lower the base resistance without impurity density problems, (1) improve the mobility of majority carriers, (2) increase the difference in forbidden band width between the emitter layer and the base layer, and A solution is to increase the carrier confinement in the layer and increase the impurity density. On the other hand, according to the method according to the present invention, the forbidden band width of the base becomes small, so that the difference in the forbidden band width becomes large. As a result, if the current gain is designed to be constant, the impurity density can be increased and the base resistance can be reduced.

Furthermore, if the base forbidden band width of the emitter layer/base layer junction surface is made smaller than that of GaAs, the threshold voltage can be reduced, and power consumption can also be reduced. As a result of the above, sufficient integration and performance improvement of the heterojunction bipolar transistor can be achieved.

FIG. 1 is a diagram showing a cross-sectional structure of a heterojunction bipolar transistor according to an embodiment of the present invention.

In FIG. 1, for example, semi-insulating GaA
For example, a high concentration GaAs
A sub-collector layer 2 consisting of is laminated. On this sub-collector layer 2, for example, i-type Alx Ga1
A collector layer 3 made of -xAs is laminated. For example, a highly concentrated base layer 4 is laminated on the collector layer 3. Here, the characteristic feature of one embodiment of the present invention is that the highly concentrated base layer 4 is (Alx Ga1-
x) 1-y Iny As. On the above-mentioned high concentration base layer 4, for example, n-type Alx
An emitter layer 5 made of Ga1-xAs is laminated, and on this emitter layer 5, for example, n-type GaAs is layered.
An emitter cap layer 6 consisting of is laminated. In the above structure, the collector electrode 71 is the sub-collector layer 2
The base electrode 72 is provided in ohmic contact on the highly doped base layer 4 , and the emitter electrode 73 is provided on the emitter cap layer 6 in ohmic contact.

[0017] In the above structure, Iny Ga1-y
An example of etching selectivity when changing the In composition ratio y of the As layer is given in IEE Electron Device Letters, Vol. 11, No. 10, P. 425(
IEEE Electron Device L
etters, VOL. 11. P. 425 (1990)
), etching of Al0.3 Ga0.7 As was performed using potassium ferrocyanide-based aqueous solution/hydrochloric acid solution.
It has been shown that the amount is about three times that of 0.05Ga0.95As. Also, ammonia/hydrogen peroxide solution 3:1
I up to about In0.5 Ga0.5 As with aqueous solution
The present inventor has confirmed that as the n composition is increased, an etching selectivity of 100 or more occurs. As described above, the In-added layer is effective as an etch stopper, and can reduce the influence of etching on variations in base resistance. FIG. 2 is a diagram showing the Al composition of the emitter layer 5 and base layer 4 according to one embodiment of the present invention. Actually, the features of an embodiment according to the present invention are exhibited by realizing a heterojunction bipolar transistor in combination with the In composition structure shown in FIG. 3, which will be described below. Figure (a) shows a tilted junction emitter; Figure (b) shows a tilted junction emitter.
) shows a structure in which the grading of AlGaAs in the base layer 4 is eliminated in FIG.

[0018] Here, x1 is about 0.3, x2 is 0
The composition is around 0.1. In particular, with respect to x2, if the structure is such that base grading is created depending on the In composition, sufficient grading can be formed even at a value of 0.1 or less.

In particular, although three examples of changes in the Al composition have been given here, the Al composition from the emitter end toward the collector end in the base may be reduced or kept constant. In other words, there are parts with a constant composition here and there, and as a whole, Al increases from the emitter toward the collector.
It is good if the composition tends to decrease.

FIG. 3 is a diagram showing the In composition in the base layer 4 according to an embodiment of the present invention. Figure (a) shows a structure in which the base layer 4 is sloped and graded, and the value of y2 is set to an In composition that allows the emitter layer 5 to be etched sufficiently selectively when etching the emitter layer 5. . The grading is performed to ensure a sufficient running time in the base layer 4. Also, the base thickness is 1000
If it is about angstrom, the In composition y1 can take a maximum value of about 0.15. Same figure (b)
is an example in which the In gradient layer thickness is equal to or less than the base layer thickness. I
As for the n composition, it is effective if it tends to increase from the emitter end of the base toward the collector end. To be precise, (Alx Ga1-x)1-y Iny
It is sufficient that the compositions of Al and In are changed so that an electric field in the form of As is applied in the positive direction from the emitter direction to the collector direction. In particular, grading of the base layer 4 is necessary for the base layer 4 near the emitter layer 5. This is because the base layer 4 is stronger than the emitter layer 5.
This is because the carriers entering the emitter layer 5 have a small initial velocity and have a certain velocity near the collector layer, but the carrier traveling velocity near the emitter layer 5 is particularly low. That is, in order to improve carrier transport characteristics, it is important to improve the vicinity of the emitter layer 5.

As explained above, the Al shown in FIG.
By adopting the In composition combination shown in FIG. 3, it is possible to not only improve the in-plane uniformity of the heterojunction bipolar transistor characteristics, but also to improve the high frequency characteristics of the device, which is desirable. A heterojunction bipolar transistor can be realized. Note that even if a potential barrier occurs at the interface between the base layer and the collector layer 3, it can be dealt with by making the collector layer 3 near the base layer 4 a highly concentrated layer.

The present invention can be applied not only to npn-type bipolar transistors but also to pnp-type heterojunction bipolar transistors. Furthermore, it is applicable not only to emitter-up type transistors but also to collector-up type heterojunction bipolar transistors.

[0023]

[Effects of the Invention] As explained above, according to the present invention,
In the base layer of the second conductivity type, the Al composition ratio x decreases from the emitter layer side to the collector layer side, and the film thickness is (Alx Ga1-x )1-y within the critical film thickness range.
By using the InyAs layer, it is possible to eliminate the current gain within the wafer plane due to the influence of the base resistance and the variation in the current in the ON state of the heterojunction bipolar transistor, and to lower the base resistance without problems with impurity density. Thus, it is possible to provide a heterojunction bipolar transistor that can reduce threshold voltage and power consumption.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a cross-sectional structure of a heterojunction bipolar transistor according to an embodiment of the present invention.

FIG. 2 is a diagram showing an Al composition in an emitter layer-base layer according to an embodiment of the present invention.

FIG. 3 is a diagram showing an In composition in a base layer according to an example of the present invention.

[Explanation of symbols]

1...Semi-insulating GaAs substrate 2...High concentration GaAs subcollector layer 3...Collector layer 4...High concentration base layer 5...Emitter layer 6...Emitter cap layer 71...Collector electrode 72...Base electrode 73...Emitter electrode

Claims (3)

[Claims] 1. A sub-collector layer and a collector layer of a first conductivity type, a base layer of a second conductivity type, and an emitter layer of a first conductivity type, each of which includes the sub-collector layer and collector layer, and the emitter layer. In the heterojunction bipolar transistor, the base layer of the second conductivity type is formed of aluminum from the emitter layer side toward the collector layer side. (Alx Ga1-x )1-y I
A heterojunction bipolar transistor characterized by being a ny As layer. 2. A sub-collector layer and a collector layer having a first conductivity type, a base layer having a second conductivity type, and an emitter layer having a first conductivity type are sequentially formed on a gallium arsenide substrate. In the heterojunction bipolar transistor, the crystal structure in the base layer shows that the Al composition ratio decreases or remains constant from the emitter end toward the collector, and
The composition ratio of In increases or remains constant from the emitter end toward the collector, and the In composition ratio is positive in the base layer at the emitter end (Alx Ga1-x )1-y In
A GaAs-based heterojunction bipolar transistor characterized in that the As base layer is formed within a critical thickness range. 3. A sub-collector layer and a collector layer having a first conductivity type, a base layer having a second conductivity type, and an emitter layer having a first conductivity type are sequentially formed on a gallium arsenide substrate. In a heterojunction bipolar transistor, the crystal structure in the base layer changes in relation to the Al and In composition ratios and the carrier concentration, and as a result, the electric field in the base layer increases from the emitter end to the collector end. (AlxGa1-x)1-yIny, which has a positive or zero value in the direction and has a positive In composition ratio in the base layer at the emitter end.
A GaAs-based heterojunction bipolar transistor characterized in that an As base layer is formed within a critical thickness range.