JPH11354531A - Bi-polar transistor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a bi-polar transistor in which a current amplification ratio, current gain cut-off frequencies, and maximum oscillation frequencies in a microwave band or the like can be made excellent. SOLUTION: A σ dope Si layer 10 whose carrier connection is locally high is provided at an Si emitter layer 5 side in the neighborhood of the base and emitter junction of a hetero bi-polar transistor. Thus, barrier height is validly increased so that the inverse injection of a carrier from an SiGe base layer 4 to an Si emitter layer 5 can be suppressed. As a result, even when a base doping concentration is increased, the inverse injection of the carrier can be suppressed by the σ dope Si layer 10, and a sufficient current amplification ratio β can be obtained, and maximum oscillation frequencies fmax can be improved. Also, even when the band discontinuous values of the emitter and band junction are decreased by integrating an inclined composition into a base, high maximum oscillation frequencies can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bipolar transistor and, more particularly, to a countermeasure for suppressing back injection of carriers from a base near an emitter-base junction of a hetero bipolar transistor.

[0002]

2. Description of the Related Art Conventionally, bipolar transistors have
Due to its excellent high frequency characteristics, it is being used as an active device in the microwave and millimeter wave bands. In particular, hetero-bipolar transistors (HBTs) using III-V compound semiconductors such as GaAs have been most actively researched and developed, but recently, IV-IV compounds that can be formed on inexpensive silicon substrates have been developed. HBTs using certain SiGe-based materials have attracted attention.

[0003] Regarding the speeding up of SiGe-based HBTs,
HBT (L) comprising a collector layer made of i, a base layer made of SiGe, and an emitter layer made of Si, and having a gradient composition base layer in which the Ge composition in the SiGe base layer is gradually increased from the emitter side toward the collector side. . H
arame et al., "Optimization of SiGe HBT Technology
for High Speed Analog and Mixed-Signal Applicatio
ns, "IEDM Tech. Dig. 1993, p. 71.), a collector layer made of Si, a base layer made of SiGe, and an emitter layer made of Si, and Ge in the SiGe base layer.
HBT (A. Schuppen et a) having a uniform composition base layer with a high composition and doping concentration and a very thin thickness
l., "Enhanced SiGe Heterojunction Bipolar Transist
ors with160 GHz-fmax, "IEDM Tech. Dig. 1995, p.74
The three types are typical structures.

FIG. 8 is a band diagram of a hetero bipolar transistor having a gradient composition base layer of the above two types. As shown in the figure, in a hetero bipolar transistor having a gradient composition base layer, SiG
The carriers injected into the e-base layer drift drift in the SiGe base layer toward the collector layer by the electric field due to the gradient composition. Since the traveling of the carrier by the drift electric field is faster than the traveling by the diffusion, the base traveling time is reduced, and good high-frequency characteristics are obtained.

On the other hand, FIG. 9 is a band diagram of a hetero bipolar transistor having a uniform composition base structure among the above two types. As shown in the figure, a hetero bipolar transistor having a uniform composition base layer is intended to shorten the base transit time by obtaining a very thin base layer and to obtain good high frequency characteristics. in this case,
Although there is a concern that the base resistance may increase as the thickness of the base layer is reduced, the resistance is reduced by doping the base layer with a high concentration of impurities. Further, in order to prevent carriers from being reversely injected from the heavily doped base layer to the emitter side, SiGe having a high Ge composition is used for the base layer, and the hetero barrier with the emitter Si layer is increased. Also in this structure, good high-frequency characteristics are obtained. In particular, by increasing the carrier concentration of the base, the maximum oscillation frequency is improved by reducing the base resistance.

[0006]

However, in the structure having the conventional gradient composition base layer shown in FIG. 8, the drift electric field due to the gradient composition is increased.
It is necessary to increase the composition gradient. That is, the Ge composition is small on the emitter side of the base layer, and Ge composition is small on the collector side.
It is necessary to increase the composition. For this reason, the emitter side is usually Si without Ge. At this time, the PN junction between the base and the emitter is a homo junction of silicon and silicon. Therefore, in order to improve the maximum oscillation frequency fmax of the HBT, it is effective to reduce the base resistance as shown in the following equation (1).

[0007]

(Equation 1)

F _T : current gain cut-off frequency R _B : base resistance C _BC:: base-collector junction capacitance However, if the base doping concentration is increased to lower the base resistance, it is naturally injected from the base layer to the emitter layer. The amount of holes increases.

That is, if the emitter-base junction is a homojunction, or if the composition of the base end is close to Si even if it is a heterojunction, there is no heterobarrier in the base layer, and it is extremely high Since the amount is small, the amount of reverse injection of carriers into the emitter side increases, and the current amplification factor β cannot be obtained.

[0010] It is derived from the between the band discontinuity value ΔEv and base layer doping concentration N _B of the valence band of the current amplification factor β and the emitter-base junction, there is a relationship of the following formula (2) .

[0011]

(Equation 2)

N _E : Doping concentration of emitter layer N _B : Doping concentration of base layer v _n : Diffusion speed of electrons in base layer v _p : Diffusion speed of holes in emitter layer k: Boltzmann constant T: Absolute temperature As a result, when such a gradient composition base is used, the base traveling time is reduced, and the current gain cutoff frequency f _T is reduced.
However, since the carrier concentration of the base layer cannot be increased, there is a problem that an increase in the maximum oscillation frequency fmax cannot be expected as a result.

On the other hand, in the conventional structure using a uniform composition base shown in FIG. 9, the back injection of carriers in the base layer is suppressed by the hetero barrier due to the high Ge composition ratio of the base layer. For shortening, a very thin base layer is required. However, there is a trade-off relationship that the base resistance increases when the base layer is thinned, and it is necessary to further increase the base doping concentration in order to improve the maximum oscillation frequency fmax. In this case, a higher Ge composition ratio is required, and a critical film thickness at which dislocation occurs due to a difference in lattice constant between the emitter layer and the base layer becomes a problem.

That is, it is possible to increase the function of suppressing the reverse injection of carriers from the base to the emitter by means of a hetero barrier by using the emitter-base junction as a heterojunction to a certain extent, but to improve the high frequency characteristics of the bipolar transistor. Has limitations.

An object of the present invention is to limit the increase in base doping concentration by providing a region having a function of suppressing back injection of carriers from the base layer in the emitter layer separately from the hetero barrier at the emitter-base junction. It is an object of the present invention to provide a bipolar transistor which can relax and thereby increase the current amplification factor even if the base doping concentration is increased in order to improve the maximum oscillation frequency fmax.

[0016]

In order to achieve the above-mentioned object, the present invention provides a high-concentration bipolar transistor having a high-concentration element which effectively enhances the function of suppressing back injection of carriers from the base layer into the emitter layer. The purpose is to direct the doped layer. Specifically, the following means for a bipolar transistor are provided.

According to the bipolar transistor of the present invention, the first
A bipolar transistor having an emitter layer into which a conductivity type impurity is introduced, a base layer into which a second conductivity type impurity is introduced, and a collector layer into which a first conductivity type impurity is introduced. A heavily doped layer provided in a region close to the base layer and doped with a first conductivity type impurity at a higher concentration than in the emitter layer;

Thus, not only the barrier due to the band discontinuity of the valence band at the emitter-base junction, but also the high-concentration doped layer causes the carrier of the base layer to try to be injected back into the base layer by the band modulation. Will be blocked.
Therefore, even if the carrier doping concentration of the base layer is increased, the reverse injection of carriers is suppressed, so that it is possible to increase the current amplification factor while improving high-frequency characteristics such as the maximum oscillation frequency fmax. .

In the above bipolar transistor, the highly doped layer is preferably a δ doped layer having a thickness of 10 nm or less.

In the above bipolar transistor, the concentration of the first conductivity type carrier in the highly doped layer is 1 × 1.
It is preferably at least 0 ¹⁹ cm ³ .

In the above bipolar transistor, the concentration of the first conductivity type carrier in the highly doped layer is preferably 10 times or more higher than the concentration of the first conductivity type carrier in the emitter layer.

In the bipolar transistor, it is preferable that the heavily doped layer is adjacent to a depletion region formed at an emitter-base junction.

As a result, the function of suppressing the reverse injection of carriers from the base to the emitter can be maximized.

In the bipolar transistor, it is preferable that the concentration of the second conductivity type carrier in the base layer is higher than the concentration of the first conductivity type carrier in the emitter layer.

In the above-mentioned bipolar transistor, the emitter layer and the base layer are made of two kinds of semiconductor materials having different band gaps, and the semiconductor material forming the emitter layer has a larger band gap. Preferably, a heterojunction is provided between the emitter layer and the base layer.

Thus, the function of suppressing the reverse injection of carriers from the base layer using the high barrier at the hetero junction is further enhanced.

In the bipolar transistor having the hetero junction, it is preferable that the base layer is strained.

[0028] Thereby, a particularly large effect can be exhibited when the difference between the lattice constants of the emitter layer and the base layer is large.

In this case, it is preferable that the base layer is inclined in a direction in which the band gap decreases from the emitter side to the collector side.

As a result, the traveling speed of carriers in the base layer is determined not by the diffusion speed but by the drift speed, and the base traveling time is shortened, so that the current gain cutoff frequency f _T is improved. In addition, despite the reduction of the band discontinuity due to the gradient composition base, the back-injection of carriers from the base layer to the emitter layer is suppressed by the highly doped layer. Further, since the base resistance can be reduced by increasing the concentration of the base doping or the thickness of the base layer can be increased, the maximum oscillation frequency fmax can be improved.

In the bipolar transistor having the hetero junction, it is preferable that the emitter layer is made of a semiconductor containing silicon and the base layer is made of a semiconductor containing at least silicon and germanium.

Thus, a hetero-bipolar transistor having good high-frequency characteristics can be obtained while using an inexpensive semiconductor material.

[0033]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present embodiment, a δ-doped layer having an extremely high impurity concentration is provided on the emitter side near the emitter-base junction, and the heterobarrier height (barrier height) is effectively increased.
Is characterized by suppressing back injection of carriers from the base layer by increasing the current gain, improving the current gain,
The present invention relates to a hetero-bipolar transistor with improved high-frequency characteristics.

FIG. 1 shows the structure of an NPN heterobipolar transistor according to the present embodiment in which a δ-doped layer is introduced into an emitter layer. As shown in FIG. 1, on a Si substrate 1, a high-concentration n-type Si subcollector layer 2, an n-type Si collector layer 3, a high-concentration p-type SiGe base layer 4, an n-type Si emitter layer 5, And a high concentration n-type Si emitter contact layer 6 is sequentially laminated by the MBE method. The collector electrode 20 is formed on the Si sub-collector layer 2.
However, on the SiGe base layer 4, a base electrode 21
An emitter electrode 22 is provided on each of the i-emitter contact layers 6.

However, the thickness of the high concentration n-type Si subcollector layer 2 is about 500 nm and the impurity concentration is about 2 × 10 ¹⁹ c
m ⁻³ and the thickness of the n-type Si collector layer 3 is about 650 n
m, the impurity concentration is about 1 × 10 ¹⁷ cm ⁻³ ,
The type SiGe base layer 4 has a thickness of about 50 nm, an impurity concentration of about 1 × 10 ¹⁹ cm ⁻³ , and an n-type Si emitter layer 5.
Has a thickness of about 100 nm and an impurity concentration of about 2 × 10 ¹⁸ cm.
^-3 , the high-concentration n-type Si emitter contact layer 6 has a thickness of about 50 nm and an impurity concentration of about 2 × 10 ¹⁹ cm ⁻³ , all of which are sequentially laminated by the MBE method.

Here, near the emitter-base junction in the n-type Si emitter layer 5, an n-type highly doped δ-doped Si layer 10 is provided. The δ-doped Si layer 10 has a thickness of about 5 nm and an impurity concentration of about 1 × 10 ²⁰ cm.
⁻³ , and the δ-doped Si layer 10 is provided at a position where it enters the emitter side by about 40 nm from the emitter-base junction. The SiGe base layer 4 has a gradient composition base structure in which the Ge composition increases almost continuously from 0% to 30% from the Si emitter layer 5 side to the Si collector layer 3 side. Note that boron is used as the p-type dopant and antimony is used as the n-type dopant.

FIG. 2 is a band diagram of the NPN hetero bipolar transistor according to the present embodiment in which a δ-doped Si layer is provided in the emitter layer. FIG. 3 shows a conventional NPN without a δ-doped Si layer in a Si emitter layer.
FIG. 3 is a band diagram of a hetero bipolar transistor.

As can be seen from FIGS. 2 and 3, when the n-type heavily doped δ-doped Si layer 10 is provided on the emitter side near the emitter-base junction,
The band is modulated by the δ-doped Si layer 10 to form a potential barrier for holes. That is,
When viewed from the holes of the SiGe base layer 4, δ-doped Si
The provision of the layer 10 effectively increases the barrier height. As a result, even if the hole concentration of the SiGe base layer 4 is increased, back injection of holes into the emitter is suppressed, and a sufficient current gain can be secured. Therefore, HB having a small base resistance and a very high maximum oscillation frequency fmax
T can be realized.

Here, the potential barrier height increased by the δ-doped Si layer 10 includes the doping concentration of the δ-doped Si layer 10, the doping concentration of the Si emitter layer 5 around the δ-doped Si layer 10, and the δ-doped Si layer 10. Changes depending on the impurity profile or the like.

FIG. 4 is a characteristic diagram showing a change in the barrier height with respect to the carrier concentration of the δ-doped Si layer. However,
FIG. 4 shows that the carrier concentration of the Si emitter layer is 1 × 10 ¹⁸ c
It is a characteristic diagram in case of m- ³ . In the present embodiment, the carrier concentration of the Si emitter layer 5 is 2 × 10 ¹⁸ cm ⁻³ , whereas the carrier concentration of the δ-doped Si layer 10 is 1 × 10 ²⁰ c.
m ⁻³ , and the carrier concentration ratio between the two is 1:50. That is, the carrier concentration of the δ-doped Si layer in FIG.
The barrier height corresponding to the barrier height at 10 ¹⁹ cm ⁻³ (that is, when the concentration ratio of both is 1:50), that is, 1
The barrier height is increased by about 00 meV.

In order to exhibit the effect of the present invention more effectively, the carrier concentration of the δ-doped Si layer 10 should be 1 ×
It is preferably at least 10 ¹⁹ cm ³ . The amount of increase in the barrier by the δ-doped Si layer 10 is preferably about 10 times or more. However, when the carrier concentration of the δ-doped Si layer 10 is 10 times the carrier concentration of the Si emitter layer 5, the barrier Is higher by 50 meV. Therefore, it is preferable that the carrier concentration of the δ-doped Si layer 10 be higher than the carrier concentration of the Si emitter layer 5 by 10 times or more.

Further, as can be seen from FIG.
It is preferable that the carrier concentration of the base layer 4 is higher than the carrier concentration of the Si emitter layer 5 because the barrier against reverse injection of holes is higher.

Next, FIG. 5 shows the relationship between the collector current and the current amplification factor β (see the solid line) of the NPN heterobipolar transistor provided with the δ-doped Si layer 10 in the emitter layer according to the present embodiment (see the solid line). FIG. 3 is a diagram showing a relationship between a collector current and a current amplification factor β (see a broken line) with a conventional NPN hetero-bipolar transistor having no δ-doped Si layer. As shown in the figure, in the present embodiment, the δ-doped S
It can be understood that the provision of the i-layer 10 improves the current amplification factor β. In particular, in a region where the collector current is large, the difference between the two current amplification factors β is remarkable. And
In the heterobipolar transistor according to the present embodiment, the high current amplification factor β is obtained even in the state where the collector current is large as described above, so that the maximum value f _Tmax of the current cutoff frequency is also improved, and there is no δ-doped Si layer. This is about 25% higher than that of the hetero bipolar transistor.

The G in the SiGe base layer 4
If the e composition is increased, dislocation may occur due to the critical thickness of SiGe. By the way, when the underlayer is Si, Si _0.2 Ge _0.8 , Si _0.3 Ge _0.7 , S
The critical film thickness for i _0.4 Ge _0.6 is 180 nm, 56
nm and 25 nm.

[0045] Therefore, in order to increase the critical film thickness, it is effective to the base layer is composed of _{Si 1-xy Ge x C y} . By reducing the composition of Ge to 40% or more and adding a little (about several%) of C to the composition, the lattice distortion can be reduced without significantly changing the magnitude of the band discontinuity value ΔEv of the emitter-base junction. As a result, the critical thickness of the base layer is improved. Therefore, by forming the base layer by _{Si 1-xy Ge x C y} / Si, it is possible to obtain a larger band discontinuity value ΔEv without exceeding the critical film thickness.

Next, δ-doped Si is
The effect of improving the high-frequency characteristics of an HBT having the SiGe base layer 4 as a gradient composition while increasing the effective barrier height by providing the layer 10 will be described.

FIG. 6 shows a conventional base having a uniform composition when the Ge composition in the SiGe base layer 4 is made to be a gradient composition base as shown in FIG. 8 while increasing the barrier height by the δ-doped Si layer 10. FIG. 10 is a diagram illustrating a result of calculating a degree of reduction in base transit time of a carrier (base transit time reduction factor) with respect to an HBT using a layer.

As can be seen from FIG. 6, when the base layer is formed into a graded composition with the same base layer thickness as the conventional uniform composition base layer, the base traveling time is shortened according to the degree of the slope. The bandgap gradient due to the gradient composition is 30
At 0 meV, it is reduced to about 20% compared to the uniform composition base.

[0049] Further, FIG. 7, [delta] doped Si layer 10 is provided with increasing barrier height, the base layer and the graded composition base of, and base resistance R increases the HBT of the thickness of the base layer to lower the _B FIG. 9 is a graph showing the results of calculating the degree of increase (fmax increase factor) of the maximum oscillation frequency fmax of the conventional HBT having a highly doped base layer having a uniform composition with a high concentration. Where Si
The thickness of the Ge base layer 4 is set to a thickness that makes the base transit time equal to the high-concentration doped uniform composition base layer in the conventional HBT. As shown in the figure, the base traveling time is reduced by forming the base layer with a gradient composition.
The larger the gradient of the band gap due to the gradient composition, the thicker the base layer can be. As a result, the base resistance R _B is reduced to improve the maximum oscillation frequency fmax.
As shown in the figure, when the gradient of the band gap due to the gradient composition is 300 meV, the maximum oscillation frequency fmax is improved by 1.5 times or more as compared with the uniform composition base.

From the above, the heterobipolar transistor (HBT) according to the present embodiment can exhibit the following effects.

First, the provision of the δ-doped Si layer 10 in the Si emitter layer 5 for effectively increasing the barrier height substantially reduces the band discontinuity ΔEv in the valence band of the emitter-base junction. Thus, the same effect as that of the current amplification can be exerted (see equation (2)), and the current amplification factor β can be improved. That is, by providing the δ-doped Si layer 10 for suppressing the reverse injection of holes from the SiGe base layer 4 into the Si emitter layer 5, the current Jp flowing from the base to the emitter represented by the formula (2) is obtained.
Is reduced, and the current amplification factor β is improved. This effect is obtained regardless of whether the emitter-base junction is a hetero junction or not. Therefore, the same effect can be exerted on a general bipolar transistor other than the HBT.

Second, the δ-doped S
In addition to providing the i-layer 10, the SiGe base layer 4 is made to have a gradient composition base, and the SiGe base layer 4 is formed so that the band gap in the SiGe base layer 4 decreases from the Si emitter layer 5 side to the Si collector layer 3 side. since changing the Ge composition, it is possible to improve the current gain cutoff frequency f _T. As described above, in the conventional HBT using a base layer having a uniform composition, when the base doping concentration is increased to reduce the resistance of the base layer, a sufficient current gain is obtained due to an increase in the amount of reverse injection of holes. Could not be secured. In contrast, the HB of the present invention
At T, the provision of the δ-doped Si layer 10 increases the effective barrier height at the heterojunction. Therefore, even if the SiGe base layer 4 is formed into a graded composition to reduce the band discontinuity of the hetero junction at the emitter-base junction and to increase the base doping concentration, the effective barrier height is sufficiently ensured. Reverse injection of holes can be suppressed. That is, a base layer doped at a high concentration, which has been impossible in the past, can be made into a gradient composition base. As a result, the base traveling time of the electrons is reduced, and the high-frequency characteristics are improved.

Third, since the base traveling time is reduced by forming the base layer with a gradient composition, the thickness of the base layer can be increased. As a result, the base resistance decreases, and the maximum oscillation frequency fmax can be improved.

Fourth, the fact that a sufficient current gain can be ensured even when the Ge composition ratio is low means that the occurrence of dislocation due to heat history in a post-process which becomes a problem when a high Ge composition ratio is adopted can be suppressed. That is, in the conventional HBT using the uniform composition base, when doping the SiGe base layer with a high concentration of impurities, it is necessary to increase the Ge composition ratio of the SiGe base layer. However, there is a problem that the critical film thickness at which dislocation occurs due to the lattice constant difference becomes small, and the effective barrier height is reduced by providing the δ-doped Si layer 10 as in the present embodiment. By increasing, the G of the SiGe base layer 4 is increased.
e Sufficient high-frequency characteristics can be obtained without increasing the composition ratio. Further, the fact that a sufficient current gain can be secured at a low Ge composition ratio means that the occurrence of dislocation due to heat history in a post-process, which becomes a problem when a high Ge composition ratio is used, can be suppressed.
In other words, the effect that the thermal budget can be increased can be obtained. In other words, the present invention is also effective for improving the device manufacturing process margin and improving the reliability of the device.

Fifth, the temperature characteristics of the bipolar transistor are improved. That is, the concentration distribution of holes in the valence band of the SiGe base layer 4 shifts downward at high temperatures, so that the current amplification factor β of the bipolar transistor
Indicates a characteristic that decreases as the temperature T increases. This tendency is particularly remarkable when the band discontinuity value ΔEv is small. On the other hand, in the case of the bipolar transistor of the present invention, a high current amplification factor β can be exhibited even at a high temperature due to the function of suppressing the reverse injection of holes in the δ-doped Si layer 10.

As described above, by providing the δ-doped Si layer 10 in the region near the emitter-base junction in the emitter layer 5 of the hetero-bipolar transistor, the current gain and the high-frequency characteristics can be improved.

In order to ensure the barrier function of the δ-doped Si layer 10, the entire δ-doped Si layer 10 must have a depletion region at the designed maximum voltage between the emitter and the base (the maximum designed voltage between the emitter and the base must be lower). It is preferable that the δ-doped Si layer 10 is provided outside of the depletion region (i.e., a region that is depleted when applied). This is because, if a part of the δ-doped Si layer 10 is in the depletion region, the barrier height may be reduced and the function of suppressing back injection of holes may be reduced. However, the δ-doped Si layer 10 is
It is preferable that the distance from the SiGe base layer 4 is not more than the diffusion length of holes.

In the present embodiment, the improvement of the characteristics of the hetero bipolar transistor alone has been described. However, it goes without saying that the HBT according to the present invention may be used for the bipolar portion of the BiCMOS device in which the bipolar transistor and the CMOS are integrated. .

In this embodiment, the NPN type SiGe
Although the HBT has been described as an example, it is needless to say that the present invention can be applied to a PNP-type bipolar transistor. Further, as already described, a general homojunction type bipolar transistor other than the HBT or a hetero bipolar transistor using a III-V group compound semiconductor may be used.

[0060]

According to the present invention, a high-concentration doped layer doped with a carrier having a higher concentration than the emitter layer is provided on the emitter side near the emitter-base junction of the bipolar transistor to modulate the band and provide a potential barrier. Is formed, the barrier height is effectively increased, so that the current amplification factor β can be improved by suppressing the reverse injection of carriers from the base layer, and the structure of the base layer can be improved. by restriction mitigation it can also be achieved an improvement of current gain _cut-off frequency f _T and maximum oscillation frequency fmax.

In addition, HB using a conventional uniform composition base
As compared with T, the high-concentration doped layer increases the effective barrier height, so that a sufficient high-frequency characteristic can be obtained without using a high Ge composition ratio, and in a later process which becomes a problem when a high Ge composition ratio is used. Generation of dislocations due to the thermal history of
It is possible to improve the manufacturing process margin of the device and the reliability of the device.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of an NPN heterobipolar transistor according to an embodiment in which a δ-doped Si layer is provided on an emitter layer.

FIG. 2 is a band diagram of an NPN heterobipolar transistor in which a δ-doped Si layer is provided on an emitter layer.

FIG. 3 shows an N-type in which a δ-doped Si layer is not provided in an emitter layer.
FIG. 3 is a band diagram of a PN hetero bipolar transistor.

FIG. 4 is a diagram showing a relationship between a carrier concentration in a δ-doped Si layer and an effective increase in barrier height.

FIG. 5 is a diagram showing the relationship between the current amplification factor β and the collector current of an NPN heterobipolar transistor when a δ-doped Si layer is provided in an emitter layer and when it is not provided.

FIG. 6 shows the H of the present invention in which the barrier height is increased by the δ-doped Si layer and the base layer is made to have a gradient composition.
FIG. 11 is a diagram illustrating a calculation result of a reduction degree of a base transit time for a conventional HBT having a uniform composition base layer by BT.

FIG. 7 shows the maximum oscillation with respect to the conventional gradient composition-based HBT according to the HBT of the present invention in which the barrier height is increased by the δ-doped Si layer, the base layer is made into a gradient composition base, and the base layer thickness is increased. Frequency fmax
FIG. 9 is a diagram showing a calculation result of a degree of improvement of the graph.

FIG. 8 shows a SiGe-based N using a conventional gradient composition base layer.
FIG. 3 is a sectional view of a PN hetero bipolar transistor.

FIG. 9 shows a conventional SiGe-based N using a uniform composition base layer.
FIG. 3 is a band diagram of a PN hetero bipolar transistor.

[Explanation of symbols]

 Reference Signs List 1 Si substrate 2 Si sub-collector layer 3 Si collector layer 4 SiGe base layer 5 Si emitter layer 6 Si emitter contact layer 10 δ-doped Si layer 20 Collector electrode 21 Base electrode 22 Emitter electrode

Claims (10)

[Claims] An emitter layer into which an impurity of a first conductivity type is introduced; a base layer into which an impurity of a second conductivity type is introduced;
A bipolar transistor having a collector layer into which a conductive impurity is introduced, wherein the bipolar transistor is provided in a region of the emitter layer adjacent to the base layer, and is doped with a higher concentration of the first conductive impurity than in the emitter layer. A bipolar transistor comprising a highly doped layer. 2. The bipolar transistor according to claim 1, wherein said highly doped layer is a δ-doped layer having a thickness of 10 nm or less. 3. The bipolar transistor according to claim 1, wherein the concentration of the first conductivity type carrier in the highly doped layer is 1 ×.
A bipolar transistor having a size of 10 ¹⁹ cm ³ or more. 4. The bipolar transistor according to claim 1, wherein the concentration of the first conductivity type carrier in the highly doped layer is the concentration of the first conductivity type carrier in the emitter layer. 10 than
A bipolar transistor characterized by being twice as high. 5. The bipolar transistor according to claim 1, wherein the heavily doped layer is adjacent to a depletion region formed at an emitter-base junction. And a bipolar transistor. 6. The bipolar transistor according to claim 1, wherein the concentration of the second conductivity type carrier in the base layer is higher than the concentration of the first conductivity type carrier in the emitter layer. A bipolar transistor, characterized in that: 7. The bipolar transistor according to claim 1, wherein the emitter layer and the base layer are made of two kinds of semiconductor materials having different band gaps from each other. A bipolar transistor, wherein a constituent semiconductor material has a larger band gap, and a heterojunction is provided between the emitter layer and the base layer. 8. The bipolar transistor according to claim 7, wherein said base layer is strained. 9. The bipolar transistor according to claim 7, wherein the base layer is inclined in a direction in which a band gap decreases from an emitter side to a collector side. 10. The bipolar transistor according to claim 7, wherein the base layer is made of a semiconductor containing at least silicon and germanium.