JP3228327B2 - Semiconductor device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor device, and more particularly, to reduction of parasitic resistance of a semiconductor device. More specifically, the present invention relates to a heterojunction bipolar transistor.

[0002]

2. Description of the Related Art For example, Japanese Patent Application Laid-Open No. 6-244195 discloses that the composition of Al is 0.3 → 0 from the n-type AlGaAs layer side.
It is described that an n-type gradient composition AlGaAs layer having a layer thickness of 100 to 400 angstroms laminated in this order can be used in this order. FIG. 1 shows an n-type AlGaAs graded composition emitter layer 7.

Conventionally, in order to reduce the emitter resistance of this type of heterojunction bipolar transistor (HBT), as disclosed in, for example, Japanese Patent Application Laid-Open No. 6-244195, a structure between an emitter cap layer and an emitter layer and between an emitter layer and a base layer is disclosed. In general, a method of suppressing an increase in emitter resistance due to conduction band discontinuity by inserting a composition gradient layer into the substrate is generally adopted.

[0004] First, the configuration of the prior art will be specifically described. FIG. 8 is an epitaxial layer structure diagram showing an example of a conventional HBT. Semi-insulating G as a semiconductor substrate
On an aAs substrate 1, an n-type GaAs subcollector layer 2,
An n-type GaAs collector layer 3 and a p-type InGaAs composition gradient base layer 4 are sequentially stacked. n-type GaAs sub-collector layer 2 and p-type InGaAs composition gradient base layer 4
A collector electrode 10 and a base electrode 11 are deposited on a part of the substrate. On the p-type InGaAs composition gradient base layer 4, there is an n-type AlGaAs composition gradient emitter layer 5 with a gradient Al composition. This layer is formed on top of n
It is adopted to prevent band discontinuity from occurring between the type AlGaAs emitter layer 6 and the P-type InGaAs composition gradient base layer 4. On the n-type AlGaAs emitter layer 6, an n-type AlGaAs composition gradient emitter layer 7 is laminated. This layer has a band discontinuity between the n-type GaAs cap layer 8 and the n-type AlGaAs emitter layer 6 stacked on the n-type AlGaAs composition-graded emitter layer 7 like the n-type AlGaAs composition-graded emitter layer 5. Is adopted to prevent the occurrence of

FIG. 9 shows an impurity concentration distribution and an Al composition distribution in a conventional HBT. The donor concentrations of the n-type AlGaAs composition gradient emitter layer 5 and the n-type AlGaAs composition gradient emitter layer 7 are the same as those of the n-type AlGaAs emitter layer 6, from 1 × 10 ¹⁷ cm ⁻³ to 1 × 10 ¹⁸ cm.
It is set to a value between ^-3 . Both the n-type GaAs cap layer 8 and the n-type InGaAs contact layer 9 are 1 × 10
The impurity concentration is set to ¹⁸ cm ^-3 or more. n-type Ga
The As cap layer 8 is an n-type AlGa
It functions as a protective layer for the As emitter layer 6. Further, the n-type InGaAs contact layer 9 is used to reduce the contact resistance with the emitter electrode.

Next, the reason why the emitter resistance is high in the prior art will be described. FIG. 10 is an energy diagram of a conduction band in a conventional HBT. When the base voltage is changed from 1.5 V to 1.8 V, as the potential of the P-type InGaAs composition gradient base layer 4 decreases, the n-type AlG
It can be seen that the potentials of the aAs composition gradient emitter layer 5 and the n-type AlGaAs emitter layer 6 also decrease. However, a part of the n-type AlGaAs composition gradient emitter layer 7
Even if the potential of the P-type InGaAs composition gradient base layer 4 is lowered, the potential hardly changes, and a triangular potential barrier is formed. This triangular potential controls the current between the base and the emitter and increases the emitter resistance significantly because the height of the potential hardly changes due to the base potential.

FIG. 11 shows a distribution of differential resistivity in an emitter layer in a conventional HBT. The area of the differential resistivity r (x) is the emitter resistance as shown in the following equation 1. The dotted line in FIG. 11 indicates the bulk resistivity calculated from the sheet resistance of the n-type AlGaAs emitter layer 6. As can be seen from the figure, two composition gradient layers 5 and 7,
That is, it can be seen that the emitter resistance is higher than the bulk resistance due to the n-type AlGaAs composition gradient emitter layer 5 and the n-type AlGaAs gradient emitter layer 7 (hereinafter, sometimes abbreviated as necessary). In particular, the resistivity increases from the layer 7 toward the layer 6 because the triangular potential in the layer 7 suppresses the current between the emitter and the base.

[0008]

(Equation 1)

[0009]

The first problem is shown in FIG.
In the conventional structure shown in FIG.
The resistance of the emitter (the resistance from the interface between the layer 5 and the base layer 4 to the emitter electrode 12) is considerably higher than the sum of the bulk resistance and the contact resistance between the emitter electrode 12 and the layer 9. The reason is that the band gap increases in the layer 7 toward the emitter side, so that a triangular potential is generated in the layer 7, which increases the emitter resistance.

An object of the present invention is to improve the above-mentioned characteristics and performance, and more specifically, to provide an HBT having an emitter layer structure with a low emitter resistance.

[0011]

The structure of the emitter resistance reduction according to the present invention is to reduce the resistance generated near the composition gradient layer, and more specifically, to the composition gradient layer having a high impurity concentration (n in FIG. 1). ⁺ -Type AlGaAs composition gradient emitter layer 1
According to 3), the generation of a triangular potential which causes high resistance in the vicinity of the composition gradient layer is suppressed.

[0012]

[0013]

According to the present invention , from the base layer toward the emitter electrode, the first composition gradient layer, the first emitter layer having a wider band gap than the base layer, and the second emitter layer having the same band gap as the first emitter layer. An emitter layer, a second composition gradient layer, and a semiconductor layer having a narrower band gap than the emitter layer and a higher impurity concentration than the emitter layer are sequentially formed .
Impurity concentration of the emitter layer is higher than that of the first emitter layer.
There is provided a heterojunction bipolar transistor, characterized in that brewing. The present invention also provides a
Toward the emitter electrode, a first composition gradient layer, a base layer
The first emitter layer having a wider band gap than the first
A second emitter layer having the same band gap as the emitter layer,
The band gap is higher than the second composition gradient layer and the emitter layer.
Narrow semiconductor layers with higher impurity concentration than emitter layer
And the second emitter layer has an impurity concentration of
That the gradient concentration layer increases toward the
Providing a unique heterojunction bipolar transistor
Is what you do.

[0015]

In the case of the present invention described above, by increasing the concentration of the composition gradient layer itself, the generation of a triangular potential generated near the composition gradient layer is suppressed, and the generation of a high-resistance portion is suppressed. . Thereby, the emitter resistance can be reduced.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the configuration will be described.

First, an embodiment of the present invention will be described in detail with reference to the drawings.

Referring to FIG. 1, the transistor structure of the present invention comprises an n-type AlGaAs composition gradient emitter layer 5 and an n-type AlGaAs emitter layer 6 for loosely connecting a band on a p-type InGaAs composition gradient base layer 4. And an n ⁺ -type AlGaAs composition gradient emitter layer 13 having a higher impurity concentration than the n-type AlGaAs emitter layer 6. In FIG. 1, n-type A
A high donor concentration emitter layer 14 of the same material as the emitter having a higher impurity concentration than the lGaAs emitter layer 6
Used to further reduce emitter resistance.

Next, the impurity concentration distribution near the n-type AlGaAs emitter layer 6 will be described. FIG. 2 shows an impurity concentration distribution and a composition distribution in the vicinity of the n-type AlGaAs emitter layer 6 of the present invention. The high donor concentration emitter layer 14 has only an impurity concentration higher than that of the n-type AlGaAs emitter layer 6. This layer is made of n-type AlGaAs
This is introduced to prevent a part of the potential of the n ⁺ -type AlGaAs composition gradient emitter layer 13 from being pulled up by the s emitter layer 6. High donor concentration emitter layer 1
If 4 is not present, the impurity concentration of the n-type AlGaAs emitter layer 6 are lower than the n ⁺ -type AlGaAs composition graded emitter layer 13, thereby pulling part the potential of the n ⁺ -type AlGaAs composition graded emitter layer 13.

On the high donor concentration emitter layer 14, n
An n ⁺ -type AlGaAs high-concentration composition gradient emitter layer 13 having an impurity concentration higher than that of the n-type AlGaAs emitter layer 6 and gradually changing the composition is laminated. This layer is higher than the impurity concentration of the n-type AlGaAs emitter layer 6, unlike the conventional n-type AlGaAs composition gradient emitter layer 7. As a result, n ⁺ type AlGaAs is
The potential position of the composition gradient emitter layer 13 can be lowered.

Next, the operation will be described.

FIG. 3 shows n-type AlGa in the structure of the present invention.
A band diagram near the As emitter layer 6 is shown. It can be seen that as the base potential increases, the potential of the base layer 4 decreases, and the potentials of the n-type AlGaAs composition gradient emitter layer 5 and the n-type AlGaAs emitter layer 6 also decrease. Further, it can be seen that the potentials of the high donor concentration emitter layer 14 and the n ⁺ -type AlGaAs composition gradient emitter layer 13 having a high impurity concentration also decrease in accordance with the base potential. Here, in FIG. 3, it appears that band discontinuity has occurred between the n-type AlGaAs emitter layer 6 and the high donor concentration emitter layer 14, but this is because the impurity concentration between the two layers is greatly different. Because of that. Unlike the triangular potential in the conventional HBT, the potential peak near the interface does not always limit the current and does not cause a resistance because the potential position also fluctuates depending on the base potential.

FIG. 4 shows the differential resistivity in the vicinity of the n-type AlGaAs emitter layer 6 in the present invention. From the figure,
It can be seen that the differential resistivity of the n ⁺ -type AlGaAs composition gradient emitter layer 13 and the high donor concentration emitter layer 14 having the composition gradient is greatly reduced. Furthermore, n-type AlG
The differential resistivity in the aAs emitter layer 5 is also reduced.
This is because the electric field, that is, the band inclination in FIG. 3 was concentrated in the n-type AlGaAs composition gradient emitter layer 5 in the conventional structure, but also spread in the n-type AlGaAs emitter layer 6 in the structure of the present invention. This is because concentration has become weak. As a result, in the conventional structure, the electron velocity is saturated due to the strong electric field, and the current is rate-limiting and the resistance is increased. In the structure of the present invention, it is possible to suppress.

[0025]

Next, the present invention will be described in detail with reference to examples.

Embodiment 1 Description of the Configuration FIG. 1 is a general structural diagram showing the configuration of the first embodiment. The transistor structure according to the present invention has a structure for connecting a band loosely on the p-type InGaAs composition gradient base layer 4.
0 Å n-type AlGaAs composition gradient emitter layer 5, 1000 Å n-type Al _0.25 Ga _0.75
As emitter layer 6, 500 Å n ⁺ having a higher impurity concentration than n-type AlGaAs emitter layer 6
It is characterized in that the type AlGaAs composition gradient emitter layer 13 is laminated. Al composition is n-type AlGaAs
In the compositionally graded emitter layer 5, the number increases from 0 to 0.25 from the p-type InGaAs compositionally graded base layer 4 toward the n-type AlGaAs emitter layer 6, and in the n ⁺ -type AlGaAs compositionally graded emitter layer 13, the n-type AlGaAs emitter layer 6
From 0.25 to 0 toward the n-type GaAs cap layer 8. Further, the n ⁺ -type AlGaAs composition gradient emitter layer 13 and the n-type AlGaAs emitter layer 6
If the 500 Å high donor concentration emitter layer 14 is inserted between them, the emitter resistance is further reduced.

Next, referring to FIG. 2, a high donor concentration emitter layer 14 and a high concentration n ⁺ type AlG
The impurity concentration of the aAs composition gradient emitter layer 13 will be specifically described. The impurity concentration of both the n-type AlGaAs composition gradient emitter layer 5 and the n-type AlGaAs emitter layer 6 is set to 3 × 10 ¹⁷ cm ⁻³ . The high donor concentration emitter layer 14 is set to have an impurity concentration equal to or higher than that of the n-type AlGaAs emitter layer 6 and at the same level as the n ⁺ -type AlGaAs composition gradient emitter layer 13. In FIG. 2, the impurity concentration of the high donor concentration emitter layer 14 is 4 × 1.
It is set to 0 ¹⁸ cm ^-3 . It is desirable that the impurity concentration of the n ⁺ -type AlGaAs composition gradient emitter layer 13 be as high as possible to eliminate the triangular potential seen in the conventional structure. In Figure 2, since n ⁺ -type AlGaAs composition graded emitter layer 13 is Al _0.25 Ga _0.75 As, Al _0.25
4 × 10 ¹⁸ c which is the maximum carrier concentration in Ga _0.75 As
m ^-3 was set. In order to suppress the triangular potential, it is desirable to set at least the impurity concentration of the n ⁺ -type AlGaAs composition gradient emitter layer 13 to 1 × 10 ¹⁸ cm ⁻³ or more.

FIG. 3 shows n-type AlGaAs in the structure of the present invention.
The band diagram near the s emitter layer 6 is shown. When the base potential is increased from 1.5 V to 1.8 V, the potential of the p-type InGaAs composition gradient base layer 4 is reduced accordingly, and the potentials of the n-type AlGaAs composition gradient emitter layer 5 and the n-type AlGaAs emitter layer 6 also decrease. You can see that Further, it can be seen that the potentials of the high donor concentration emitter layer 14 and the n ⁺ -type AlGaAs composition gradient emitter layer 13 also decrease according to the base potential. Here, in FIG. 3, n-type AlGa
It appears that band discontinuity occurs between the high donor concentration emitter layer 14 of the As emitter layer 6. This is because the impurity concentration of the high donor concentration emitter layer 14 is 4 × 10 ¹⁸ c
m ⁻³ , whereas the n-type AlGaAs emitter layer 6 has 3
Since there is a considerable density difference of × 10 ¹⁷ cm ⁻³ , this is caused by a shift in the position of the Fermi level. The peak of the potential seen near this interface is the conventional HBT
Unlike the triangular potential in, the potential position also moves due to the base potential. By the way, at the time of measurement by the open collector method, the emitter resistance r _E is multiplied by r _E = dV _B / dI _BE using the potential V _B and I _BE of the base layer 4, so that the emitter resistance changes the potential of the base layer. Sometimes the lower the current, the greater the change.
The height of the peak of the potential shown in FIG. 3 changes following the base potential and the I _BE also changes greatly, so that it does not become a high-resistance portion like the conventional triangular potential.

For the above reason, the emitter area is 1.6 μm.
In the case of m × 4.6 μm, the emitter resistance is reduced from 15Ω in the conventional structure to 7Ω in the structure of the present invention.

FIG. 4 shows the differential resistivity in the vicinity of the n-type AlGaAs emitter layer 6 in the present invention. From the figure,
It can be seen that the differential resistivity of the n ⁺ -type AlGaAs composition gradient emitter layer 13 and the high donor concentration emitter layer 14 having the composition gradient is greatly reduced. Comparing FIG. 11 with FIG. 4, the resistance between the position 1.2 and 1.3 μm is
When the emitter area is 1.6 μm × 4.6 μm, the resistance is reduced by 5Ω.

Further, the differential resistivity in the layer 5 is also reduced. The resistance from position 1.08 to 1.12 μm is reduced by 3Ω when the emitter area is 1.6 μm × 4.6 μm. This is because the electric field, that is, the inclination of the band in FIG. 3 was concentrated on the n-type AlGaAs composition gradient emitter layer 5 in the conventional structure, but was changed in the n-type AlGa
This is because the electric field concentrates as a result of spreading in the As emitter layer 6. As a result, in the structure of the present invention, the electron velocity is saturated due to the strong electric field, the current is rate-limiting, and the resistance is increased.

Embodiment 2 Next, a second embodiment will be described. FIG. 5 shows a composition distribution and an impurity concentration distribution in the vicinity of the emitter layer 6 according to the second embodiment. The difference from the first embodiment is that a donor concentration gradient emitter layer 15 having a gradual change in concentration distribution is employed instead of the high donor concentration emitter layer 14. In FIG. 2, the concentration gradient is from the middle of the n ⁺ type AlGaAs composition gradient emitter layer 13 to the n type A
lGaAs emitter layer 6 and donor concentration gradient emitter layer 1
Although the process is performed up to the interface 5, the process may be performed only with the donor concentration gradient emitter layer 15. The impurity concentration gradient of the donor concentration gradient emitter layer 15 is preferably performed between the impurity concentration of the n-type AlGaAs emitter layer 6 and the impurity concentration of the n ⁺ -type AlGaAs composition gradient emitter layer 13.

FIG. 6 shows a band diagram according to the second embodiment. As can be seen from the figure, the discontinuity between the n-type AlGaAs emitter layer 6 and the donor concentration gradient emitter layer 15 observed in the first embodiment is completely eliminated, and the potential peak is considerably reduced. .

FIG. 7 shows the differential resistivity in the form of the second embodiment. It can be seen that the resistance near the composition gradient layer is considerably reduced as compared with the conventional structure.

Embodiment 3 Next, a third embodiment will be described. The third embodiment is different from the first embodiment in that the high-concentration n ⁺ -type AlG
The aAs composition gradient emitter layer 13 is changed to a planar doped composition gradient layer. The planar dope position is the top of the triangular potential in the conventional structure.

Embodiment 4 Next, a fourth embodiment will be described. In the fourth embodiment, the layer 13 in the third embodiment is applied to the second embodiment.

The present invention can be used not only for reducing the emitter resistance of the HBT but also for reducing the resistance of a portion having a composition gradient.

[0038]

The first effect is that the emitter resistance of the HBT can be reduced.

The reason is that using a high impurity concentration layer,
This is because the triangular potential caused by the composition gradient is suppressed.

[Brief description of the drawings]

FIG. 1 is a layer structure diagram showing a first embodiment of the present invention.

FIG. 2 is an impurity concentration distribution and a composition distribution diagram showing the first embodiment of the present invention.

FIG. 3 is a band diagram for explaining the operation of the first exemplary embodiment of the present invention.

FIG. 4 is a distribution diagram of resistivity showing an effect of the first exemplary embodiment of the present invention.

FIG. 5 is an impurity concentration distribution and composition distribution diagram showing a second embodiment of the present invention.

FIG. 6 is a band diagram for explaining an operation of the second exemplary embodiment of the present invention.

FIG. 7 is a distribution diagram of resistivity showing an effect of the second exemplary embodiment of the present invention.

FIG. 8 is a layer structure diagram of a conventional HBT.

FIG. 9 is a diagram showing an impurity concentration distribution and a composition distribution of a conventional HBT.

FIG. 10 is a band diagram for explaining the operation of a conventional HBT.

FIG. 11 is a distribution diagram of resistivity in a conventional HBT structure.

[Explanation of symbols]

Reference Signs List 1 semi-insulating GaAs substrate 2 n-type GaAs sub-collector layer 3 n-type GaAs collector layer 4 p-type InGaAs composition gradient base layer 5 n-type AlGaAs composition gradient emitter layer 6 n-type AlGaAs emitter layer 7 n-type AlGaAs composition gradient emitter layer 8 n-type GaAs cap layer 9 n-type InGaAs contact layer 10 collector electrode 11 base electrode 12 emitter electrode 13 n ⁺ -type AlGaAs composition gradient emitter layer 14 high donor concentration emitter layer 15 donor concentration gradient emitter layer

Continuation of the front page (56) References JP-A-6-244195 (JP, A) JP-A-63-244679 (JP, A) JP-A-8-293505 (JP, A) JP-A-6-13315 (JP) JP-A-1-103869 (JP, A) JP-A-3-192727 (JP, A) JP-A-8-250509 (JP, A) JP-A-63-12165 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01L 21/33-21/331 H01L 29/00-29/267 H01L 29/30-29/38 H01L 29/68-29/737

Claims (2)

(57) [Claims] 1. From a base layer toward an emitter electrode,
A first composition gradient layer, a first emitter layer having a wider band gap than the base layer, a second emitter layer having the same band gap as the first emitter layer, a second composition gradient layer, and a band gap greater than the emitter layer A heterojunction bipolar transistor wherein a semiconductor layer having a smaller impurity concentration than the emitter layer is sequentially formed, and the impurity concentration of the second emitter layer is higher than that of the first emitter layer. 2. From the base layer to the emitter electrode,
A first composition gradient layer, a first emitter layer having a wider band gap than the base layer, a second emitter layer having the same band gap as the first emitter layer, a second composition gradient layer, and a band gap greater than the emitter layer A heterojunction bipolar layer characterized in that a semiconductor layer having a smaller impurity concentration and a higher impurity concentration than the emitter layer is sequentially formed, and the second emitter layer is an inclined concentration layer in which the impurity concentration increases toward the emitter electrode side. Transistor.