Hetero bipolar transistor

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Technical field to which the invention belongs]

The present invention relates to a heterobipolar transistor using a semiconductor layer containing silicon, and particularly relates to a countermeasure for lowering the drive voltage.

【0002】

[Conventional technology]

Conventionally, by changing the composition of the emitter region and the base region so that the band gap of the emitter region is larger than that of the base region, the injection efficiency of the emitter is significantly improved and the characteristics of the transistor are improved. (Hereinafter referred to as HBT) is attracting attention as a high-performance element. This HBT is being used as a device in the microwave / millimeter wave band because of its excellent high frequency characteristics. Conventionally, HBTs have been manufactured by combining GaAs and AlGaAs, which are group III-V compound semiconductors, but in recent years, research and development of SiGeHBTs utilizing the fact that the bandgap of the base layer composed of SiGe layers is smaller than Si. Is being actively promoted.

【0003】

SiGeHBT utilizes the fact that the Ge bandgap (0.66 eV at room temperature) is smaller than the Si bandgap (1.12 eV at room temperature), and the SiGe mixed crystal has a smaller bandgap than Si. Then, by using the Si layer as the emitter region and the SiGe layer as the base region and reducing the band gap of the base layer with respect to the emitter layer, the voltage is lower than the drive voltage (about 0.7V) of the homo Si bipolar transistor. It can be driven. The drive voltage here refers to a state in which the voltage between the base and the emitter becomes equal to the diffusion potential between the base and the emitter in the active region of the bipolar transistor. That is, in the NPN bipolar transistor, the energy difference at the valence band edge between the emitter layer and the base layer is increased to some extent to suppress the injection of holes from the base layer to the emitter layer, and the emitter layer and the base layer Since the energy difference at the end of the conduction band can be reduced, the drive voltage can be reduced.

【0004】

In HBT, the Ge content of the base region is gradually increased in the direction from the emitter region to the collector region, so that the band gap in the base region is gradually reduced in the direction from the emitter region to the collector region. It becomes possible to configure. The electric field generated by this tilt composition accelerates the carriers injected into the base layer and drifts. Due to this drift electric field, the speed can be made faster than the carrier speed due to diffusion, so that the base travel time can be shortened and the cutoff frequency (f)._T ) Can also be improved.

【0005】

However, since the lattice constant of Ge (5.65 Å) is different from the lattice constant of Si (5.43 Å), if the Ge content is increased, dislocations due to strain due to the difference in lattice constant occur and the electrical characteristics deteriorate. That is, in order to promote lower voltage drive, it is necessary to increase the Ge content in the SiGe layer, but as described above, if the Ge content in the SiGe layer is increased, the lattice constant difference from the Si layer There is an upper limit to the Ge content as it becomes larger. Therefore, paying attention to the fact that the lattice constant of the C crystal is smaller than the lattice constant of the Si crystal, it is possible to reduce the strain in the SiGeC mixed crystal containing C in the SiGe layer (LD Lanzerotti, A. St. . Amour, CW Liu, JC Strum, JK Watanabe and ND Theodore, IEEE Electron Device Letters, Vol.17 No.7 334 (1996)). Then, an HBT utilizing a heterojunction between the Si layer and the SiGeC layer can be considered. In this HBT, impurities contained in the base region during heat treatment diffuse to the collector region side, so that between the base and the collector. There is a problem that so-called parastic barriers are formed in (JW Slotboom, G. Streutker, A. Pruijmboom and DJ Gravesteijn, IEEE Electron Device Letters 12 pp 486 (1991)). Then, by forming this parastic barrier, the current multiplication factor (β) is lowered, the early voltage Va and the cutoff frequency f._T Deterioration occurs. To solve this, there is a method of interposing an undoped spacer layer between the base collector (EJ Prinz, PM Garone, PV Schwartz, X. Xiano and JC Strum, IEDM Technology Digital p.p.853 (1991)). .. C has the effect of suppressing impurity diffusion (LD Lanzerotti, JC Strum, E. Stach, R. Hull, T. Buyuklimanli and C. Magee, AppliedPhysics Letters 70 (23) 3125 (1997)). Due to this effect, the profile of boron, which is a p-type impurity in the base region, is maintained, and the early voltage Va and cutoff frequency f are maintained._T It is expected that such characteristics will be improved.

【0006】

[Problems to be Solved by the Invention]

However, the SiGeC-HBT using the conventional SiGeC / Si heterojunction has the following problems.

【0007】

In order to make the band gap of the SiGeC layer, which is the base region of the SiGeC-HBT, smaller in order to further improve the current multiplication factor, the Ge content must be increased. At this time, as described above, in order to reduce the lattice strain accompanying the increase in the Ge content, the C content may be increased. However, according to the experiments conducted by the present inventors, for example, in an HBT using a SiGeC layer having a C content of 0.8% or more as a base region, the n value of the base current is about 2, and so on. It was found that increasing the content rate deteriorates the high frequency characteristics of HBTs. The results of the experiments conducted by the present inventors will be described below.

【0008】

Figures 8 (a) and 8 (b) show SiGe in order._0.268 HBT, SiGe_0.26₈ C_0.0091It is a figure which shows the gunmel plot of HBT. Figures 9 (a) and 9 (b) show SiGe in that order._0.268 HBT, SiGe_0.268 C_0.0091It is a figure which shows the current multiplication factor (β) of HBT. However, in this specification, "SiGe"_0.268 HBT "," SiGe_0.268 C_0.0091When it is expressed as "HBT", it means that the composition ratio of Si is a value obtained by subtracting the content ratio of other materials (Ge, C, etc.) from 1.

【0009】

As you can see by comparing Fig. 8 (a) and (b), SiGe_0.268 C_0.0091The n value (slope) of the base current Ib of the HBT is SiGe._0.268 It is significantly deteriorated compared to the n value of HBT. Also, as can be seen by comparing FIGS. 9 (a) and 9 (b), SiGe_0.268 C_0.0091The maximum current multiplication factor β of HBT is only 50, and SiGe_0.268 It is deteriorated compared to the maximum value of the current multiplication factor β of HBT being 400. The cause of this is thought to be that when the C content in SiGeC-HBT approaches 1%, the recombination current increases and the n value deteriorates, and this deterioration in the n value reduces the current multiplication factor β. ..

【0010】

Figure 10 shows SiGe_0.268 HBT, SiGe_0.268 C_0.0091It is a figure for investigating the fitting of the measurement result of the forward current-voltage characteristic of the diode characteristic between the emitter and base of HBT, and the measurement result of the calculated value of the sum of the electron recombination current and the diffusion current. In the figure, the calculated value of the sum of the electron recombination current and the diffusion current of the diode is fitted to the measurement result with the recombination lifetime (τr) in the depletion layer between the emitter and the base as a parameter. As can be seen from the result of this diode characteristic, the recombination lifetime is about 100 nsec in the SiGeC layer having a C content of 0% (that is, the SiGe layer), whereas the SiGeC layer having a C content of 0.91%. The recombination lifetime is about 400 psec. As described above, when the C content is close to 1%, the recombination life is remarkably shortened and the recombination current becomes very large, and as a result, it is considered that the characteristics are deteriorated.

【0011】

Figures 11 (a) and 11 (b) show SiGe containing Ge uniformly in the base region, respectively._0.268 Recombination lifetime in the base region of HBT is 1 × 10^-51 × 10 from sec^-9It is a figure which shows the result of simulating the gunmel plot and the current multiplication by changing up to sec. As can be seen from FIG. 11 (a), when the recombination life becomes short, the collector current is not affected so much, but the n value deteriorates due to the recoupling current of the base current becoming very large. .. Further, as can be seen from FIG. 11B, when the recombination life becomes short, the current multiplication factor β decreases significantly due to the increase in the recombination current of the base current as described above. As described above, when the recombination life is shortened, it causes deterioration of the characteristics of the transistor.

【0012】

In SiGeC-HBT with a high C content, one of the causes of the reduced recombination lifetime is that in the case of a SiGeC crystal with a high C content, the amount of C present at the interstitial position in the crystal increases. Can be mentioned. It is considered that C existing at this interstitial position constitutes the recombination level and increases the recombination current.

【0013】

An object of the present invention is to provide a heterobipolar transistor that can realize a reduction in a recombination current between an emitter and a base, a low voltage drive, and an improvement in high frequency characteristics.

【0014】

[Means for solving problems]

The first heterobipolar transistor of the present invention is provided on a substrate and is provided on a first conductive type collector region made of a semiconductor material containing Si and a Si which is provided on the collector region and has a non-uniform C content._1-xy Ge_x C_y A semiconductor containing Si, which is provided on a second conductive type base region composed of layers (0 <x <1,0 y <1) and is provided on the base region and forms a heterojunction between the base region. It is provided with a first conductive type emitter region made of a material, and the portion of the base region having the maximum C content is separated from the region adjacent to the emitter region.

【0015】

As a result, the C content is relatively low in the region adjacent to the emitter region in the base region, so that the depletion layer formed at the emitter-base junction has less regions with a high C content, and the depletion layer is regenerated. The number of bond centers can be reduced. Therefore, the recombination current caused by the presence of the recombination center in the depletion layer can be suppressed. That is, by utilizing the heterojunction using the base region composed of the SiGeC layer, it is possible to improve the electrical characteristics such as the current multiplication factor and the high frequency characteristics while reducing the drive voltage.

【0016】

The C content in the region adjacent to the emitter region of the base region is preferably less than 0.8%.

【0017】

When the C content in the region adjacent to the emitter region of the base region is 0.01% or more, the band structure in the base region can be finely adjusted.

【0018】

Since the depletion layer formed at the emitter-base junction is contained in the region of the base region that is in contact with the emitter region, the recombination current can be suppressed more effectively.

【0019】

Since the Ge content of the region in contact with the emitter region of the base region is constant, the diffusion potential of the emitter-base junction is almost constant even if the depth position of the diffusion layer varies, so that the operating voltage is almost constant. Can be retained.

【0020】

Since at least the central portion of the base layer other than the region adjacent to the emitter region has a uniform Ge content, the epitaxial growth of the base region in the manufacturing process can be facilitated.

【0021】

The thickness of the region of the base layer adjacent to the emitter region is preferably 5 nm or more, and more preferably 10 nm or more.

【0022】

The region of the base region other than the region adjacent to the emitter region is configured so that the band gap becomes smaller in the direction from the emitter region to the collector region, thereby accelerating the traveling in the base region of the carrier. Therefore, the high frequency characteristics can be improved.

【0023】

The region of the base region other than the region adjacent to the emitter region has a composition in which the C content increases in the direction from the emitter region to the collector region, so that the C content is high and there are many recombination centers. There is an advantage that the region can be kept away from the emitter-base junction as much as possible to suppress the recombination current and reduce the drive voltage.

【0024】

The base region is divided into a first base region including a region adjacent to the collector region and a second base region including a region adjacent to the emitter region, and at least a second base region of the first base region. Since the band gap at the side end is equal to or smaller than the band gap in the second base region, it is possible to significantly reduce the drive voltage.

【0025】

In that case, the difference in Ge content between at least the second base region side end of the first base region and the second base region is Δx, and at least the second base region side end of the first base region and the second base When the difference in C content from the region is Δy, it is preferable that there is a relationship of Δx 4.288 Δy.

【0026】

Further, in the region of the first base region excluding the end on the side of the second base region, the band gap is configured to be smaller in the direction from the second base region to the collector region in the first base region. As a result, as described above, it is possible to improve the high frequency characteristics by improving the base traveling speed of the carrier.

【0027】

In that case, the difference in Ge content between at least the second base region side end of the first base region and the second base region is Δx, and at least the second base region side end of the first base region and the second base When the difference in C content from the region is Δy, it is preferable that there is a relationship of Δx 4.288 Δy.

【0028】

BEST MODE FOR CARRYING OUT THE INVENTION

Before explaining each embodiment, the basic advantages of the hetero-bipolar transistor in which the base layer of the hetero-bipolar transistor is formed by the SiGeC layer which is a ternary mixed crystal semiconductor containing Si, Ge and C will be described.

【0029】

FIG. 1 is a phase diagram showing the relationship between the Ge and C contents, the band gap, and the lattice strain in the SiGeC ternary mixed crystal semiconductor. In the figure, the horizontal axis represents the Ge content, the vertical axis represents the C content, and the composition conditions in which the strain amount (including compression strain and tensile strain) and the band gap are constant are shown by straight lines. .. In Fig. 1, the dot-hatched region has a bandgap of practical SiGe (Ge content is about 10%) with a lattice strain amount of 1.0% or less in the SiGeC layer on the Si layer and a conventional bandgap. It is an area that can be made smaller than. This area is Si_1-xy Ge_x C_y When the Ge content is x and the C content is y, the following four straight lines are used for SiGeC. Straight line: y = 0.122x-0.032 Straight line: y = 0.1245x + 0.028 Straight line: y = 0.2332x-0.0233 (Ge content is 22% or less) Straight line: y = 0.0622x + 0.0127 (Ge content is 22% or less) The area surrounded by. In the figure, the SiGeC layer having a linear composition with a lattice strain of 0% is lattice-matched with the underlying Si layer.

【0030】

Therefore, in a heterobipolar transistor composed of an emitter layer, a base layer, and a collector layer, by configuring the base layer with SiGeC having the composition of the region shown by the dot hatching in FIG. 1, problems due to lattice distortion are caused. It is possible to realize a narrow band gap base without.

【0031】

In other words, by selecting a SiGeC ternary mixed crystal semiconductor material as a material with a small bandgap and a small amount of lattice strain in the base layer, a heterobipolar transistor capable of high reliability, low voltage operation, and high speed operation can be obtained. It can be realized.

【0032】

Note that FIG. 1 is a phase diagram when the underlying layer of the SiGeC layer has a single Si composition, but even when the underlying layer contains some Ge and C in Si, the lattice strain of the SiGeC layer is 1.0. The same effect can be exhibited as long as it is less than% and a large difference between the base layer and the SiGeC layer and the band gap can be secured.

【0033】

FIG. 2 is a cross-sectional view of a heterobipolar transistor (HBT) common to each embodiment of the present invention. As shown in the figure, the HBT of the present embodiment includes a Si substrate 10 containing a p-type impurity, a Si collector embedded layer 11 formed by introducing an n-type impurity (for example, phosphorus) into the Si substrate 10, and Si. A first base region 12 composed of a SiGeC layer having a high C content provided on the collector embedded layer 11, and a SiGeC layer or a SiGe layer having a low C content provided on the first base region 12 It includes two base regions 13, a Si cap layer 14 provided on the second base region 13, and an emitter electrode 15 made of a silicon film provided on the Si cap layer 14.

【0034】

Next, a method for manufacturing this HBT will be described. First, the concentration of phosphorus (p), which is an n-type impurity, is about 2 × 10 on the surface of the Si substrate 10 by using an ion implantation method or the like.¹⁷/cm³ Introduced in to form the collector embedding layer 11. Then, on the collector-embedded layer 11, the first base region 12 composed of the SiGeC layer having a high C content and the SiGeC having a lower C content than the first base region 12 by the UHV-CVD method or the like are used. The second base region 13 composed of the layer or the SiGe layer is epitaxially grown in this order. Here, the C content is set to less than 0.8% at least at the emitter region side end portion (Si cap layer side end portion) of the second base region 13. At this time, as a source for epitaxial growth, silane or disilane is used as the raw material for Si, German is used as the raw material for Ge, and methylsilane or methylgerman is used as the raw material for C. In the first and second base regions 12 and 13, for example, about 4 × 10 boron (B), which is a p-type impurity, is contained.¹⁸/cm³ The film thickness of the first base region 12 is about 35 nm and the film thickness of the second base region 13 is about 25 nm (total film thickness is about 60 nm). Then, the Si cap layer 14 composed of the Si layer is epitaxially grown on the second base region 13. Impurities are not dopeed to the Si cap layer 14, and the film thickness of the Si cap layer 14 is about 10 nm. Further, a silicon oxide film 16 having only a part opened is formed on the Si cap layer 14, and an n-type such as arsenic (As) or phosphorus (P) is formed on the opening and the silicon oxide film 16. N containing impurities⁺ An emitter electrode 15 made of a polysilicon film is formed. Arsenic (or phosphorus) is about 1 × 10 in this emitter electrode 15.²⁰/cm³ It is doped with the above high concentration, and n-type impurities are diffused in the Si cap layer 14 by heat treatment to form an emitter region 14a in the Si cap layer 14.

【0035】

That is, the second base region 13 having a low C content is interposed between the first base region 12 having a high C content and the emitter layer 14a, and at least the emitter region side end of the second base region 13 By setting the C content in the first base region 12 to less than 0.8%, the recombination center generated by the high C content in the first base region 12 is located outside the depletion layer between the emitter and the base. It is configured. With this configuration, the n value of the base current can be improved and the leak current can be reduced, and the defects shown in FIGS. 8 (b), 9 (b), etc. can be suppressed. .. On the other hand, by providing the first base region 12 having a high C content, it is possible to reduce the voltage while suppressing the occurrence of lattice distortion, as in the case of HBT using the conventional Si / SiGeC heterojunction. .. This is the basic effect of the present invention.

【0036】

In FIG. 2, the first base region 12 and the second base region 13 are divided for convenience, but the present invention can also be applied to those that cannot be divided into the first base region and the second base region. .. For example, Si that constitutes the base layer_1-xy Ge_x C_y It may be the case that the component ratio of is continuously changed in the entire base layer. That is, if the C content in the region adjacent to the emitter layer of the base layer is smaller than the C content in the region adjacent to the collector layer of the base layer, the basic effect of the present invention can be exhibited. Is.

【0037】

(First Embodiment) FIGS. 3 (a) and 3 (b) show the C and Ge contents of the first base region and the second base region in the first embodiment and the concentration of boron (B) which is an impurity. It is a figure which shows, and the energy band figure of the emitter region-base region-collector region at the time of applying a voltage. In addition, in FIG. 3A, the illustration of the concentration of the n-type impurity is omitted.

【0038】

As shown in FIG. 3A, in the present embodiment, the Ge content is constant (for example, 26.8%) over the first base region 12 and the second base region 13. On the other hand, the C content is 0.91% in the first base region 12 and 0.35% in the second base region 13. That is, the first base region 12 is SiGe._0.268 C_0.0091Consisting of layers, the second base region 13 is SiGe_0.268 C_0.0035It consists of layers.

【0039】

At this time, SiGe_0.268 C_0.0091The bandgap of the layer is about 0.95 eV, Ge_0.268 C_0.0035The bandgap of the layers is about 0.92 eV. In this way, when two SiGeC layers with the same Ge content are laminated, the band gap with the higher C content becomes larger, so as shown in Fig. 3 (b), the emitter regions 14a and C By interposing the SiGeC layer (second base region 13) having a low C content between the first base region 12 having a high content, a barrier is less likely to occur at the emitter-base junction. Therefore, the presence of the second base region 13 having a low C content does not adversely affect the drive voltage of the HBT. On the other hand, as described above, the second base region 13 having a low C content is interposed between the emitter region 14a and the first base region 12, so that the depletion layer between the emitter and the base (Fig. 3 (b)). The recombination current in the region Rdp) shown in) can be reduced. That is, in HBT, it is possible to further reduce the voltage drive while suppressing the deterioration of the n value and the reduction of the current multiplication factor due to the increase in the recombination current.

【0040】

If the base layer cannot be divided into two layers because there is no boundary between the first base region 12 and the second base region 13, or if the base layer can be divided into three or more layers, for example, the base layer is configured. Si_1-xy Ge_x C_y If the C content of the base layer adjacent to the emitter layer is sufficiently small, the depletion formed at the emitter-base junction will occur even if the component ratio of The effect of suppressing the recombination current in the layer can be exhibited.

【0041】

-Experimental data on the first embodiment- FIG. 12 is a diagram showing each parameter of the sample used in the experiment for confirming the effect of the present invention in a table. In FIG. 12, the thickness of the Si cap layer 14 is indicated by S, the thickness of the first base layer 12 is indicated by D1, the thickness of the second base layer 13 is indicated by D2, and Ge in the first base layer 12 is indicated. Content rate, C content rate, boron concentration are N respectively_G1, N_C1, N_B1Is displayed, and the Ge content, C content, and boron concentration in the second base layer 13 are N, respectively._G2, N_C2, N_B2Is displayed.

【0042】

FIG. 13 is a diagram showing data of bias voltage-current characteristics measured for the sample shown in FIG. As shown in the figure, in the sample (No. 1) without the layer with low C content (second base region), the slope of the voltage-current characteristic is gentle, so the recombination current may be large. Understand. In addition, in the sample (No. 2) with a thickness of 10 nm in the second base region 13 with a low C concentration, the slope of the voltage-current characteristic rises slightly compared to the sample (No. 1), and the effect of reducing the recombination current is slightly increased. Although it can be seen, the effect is small. Further, in the sample (No. 3) having a thickness of 20 nm in the second base region 13 having a low C concentration, the slope of the current becomes slightly steep, and the effect of reducing the recombination current is clearly shown. Further, in the sample (No. 4) in which the thickness of the second base layer 13 is 30 nm, the slope of the voltage-current characteristic becomes steep, and the effect of reducing the recombination current becomes very large.

【0043】

In the sample used in this experiment, the concentration of impurities (boron) in the 1st and 2nd base regions 12 and 13 was set to 2 × 10.¹⁸cm^-3Impurity concentration in the base region of a standard heterobipolar transistor 1 × 10¹⁹cm^-3It is considerably lower than. Therefore, it is considered that the depletion layer in the emitter-based junction is expanding. That is, the impurity concentration in the base region is 1 × 10.¹⁹cm^-3In this case, the spread of the depletion layer in the emitter-base junction is narrower than that of the sample used in this experiment. Therefore, if the thickness of the second base region 13 is about 5 nm or more, the effect of reducing the recombination current can be obtained. can get.

【0044】

(Second Embodiment) FIGS. 4 (a) and 4 (b) show the C and Ge contents of the first base region and the second base region and the concentration of boron (B), which is an impurity, in the second embodiment. It is a figure which shows, and the energy band figure of the emitter region-base region-collector region at the time of applying a voltage. In addition, in FIG. 4A, the illustration of the concentration of the n-type impurity is omitted.

【0045】

The present embodiment is characterized in that the Ge and C contents of the two regions 12 and 13 are adjusted so that the band gaps between the first base region 12 and the second base region 13 are equal. For that purpose, the Ge content in the first base region 12 may be higher than that in the second base region 13 without making the Ge content the same value in the first and second base regions. Then, the composition in the SiGeC layer is expressed by the general formula Si._1-xy Ge_x C_y When the difference in C content between the first base region 12 and the second base region 13 is Δy, the difference in Ge content between the first base region 12 and the second base region 13 Δx is expressed by the following equation. (1) Δx = 4.288Δy (1) Determined based on. In both the first base region 12 and the second base region 13, the composition is such that the Si layer is subjected to compressive strain.

【0046】

As shown in FIG. 4A, in the present embodiment, the Ge content of the first base region 12 is set to a high constant value (for example, 31.3%), and the Ge content of the second base region 13 is set to a low constant value. (For example, 26.8%). On the other hand, the C content is 1.4% in the first base region 12 and 0.35% in the second base region 13. That is, the first base region 12 is SiGe._0.313 C_0.014 Consisting of layers, the second base region 13 is SiGe_0.268 C_0.0035It consists of layers.

【0047】

At this time, SiGe_0.313 C_0.014 The bandgap of the layer is about 0.92eV, Ge_0.268 C_0.0035The bandgap of the layers is also about 0.92 eV, and as shown in FIG. 4 (b), the conduction band ends in the two base regions 12 and 13 are flat. In this way, when two SiGeC layers having the same band gap are laminated, it is possible to further reduce the voltage drive. Then, as described above, the second base region 13 having a low C content is interposed between the emitter region 14a and the first base region 12, so that the depletion layer between the emitter base is formed (FIG. 4 (b). The recombination current in the region Rdp) shown in) can be reduced. That is, in HBT, it is possible to promote a particularly remarkable low voltage drive while suppressing deterioration of the n value and reduction of the current multiplication factor due to an increase in the recombination current.

【0048】

Further, since the conduction band ends in the two base regions 12 and 13 are flat, there is no heterobarrier that hinders the traveling of the carrier, so that the operation speed of the heterobipolar transistor can be increased.

【0049】

(Third Embodiment) FIGS. 5 (a) and 5 (b) show the C and Ge contents of the first base region and the second base region and the concentration of boron (B), which is an impurity, in the third embodiment. It is a figure which shows, and the energy band figure of the emitter region-base region-collector region at the time of applying a voltage. In addition, in FIG. 5A, the illustration of the concentration of the n-type impurity is omitted.

【0050】

In the present embodiment, the band gaps of the first base region 12 and the second base region 13 are made equal to each other so that the band gap of the first base region 12 changes in the direction of accelerating the base traveling electrons. , The feature is that the Ge and C contents of the 1st and 2nd base regions 12 and 13 are adjusted. Therefore, the composition in the SiGeC layer is expressed by the general formula Si._1-xy Ge_x C_y When the difference in C content between the 2nd base region side end of the 1st base region 12 and the 2nd base region 13 is Δy, it is the same as the 2nd base region side end of the 1st base region 12. The difference Δx in the Ge content from the second base region 13 is determined based on the above equation (1). Then, the Ge content in the first base region 12 is increased in the direction from the end on the side of the second base region toward the collector embedded layer 11.

【0051】

As shown in FIG. 5A, in the present embodiment, the Ge content at the end of the first base region 12 on the side of the second base region is set to a higher value (for example, 20.0%), and the first base region 12 The Ge content at the side end of the collector embedding layer is set to a higher value (for example, 30%), and the Ge content of the second base region 13 is set to a low constant value (for example, 15.2%). On the other hand, the C content is assumed to be a high constant value (for example, 1.4%) in the first base region 12 and a low constant value (for example, 0.3%) in the second base region 13. That is, the second base region side end of the first base region 12 is SiGe._0.20C_0.014 It consists of layers, and the side end of the collector embedded layer of the first base region 12 is SiGe._0.30C_0.014 Consisting of layers, the second base region 13 is SiGe_0.152 C_0.003 It consists of layers.

【0052】

At this time, Ge_0.20C_0.014 The bandgap of the layer is about 1.02 eV, SiGe_0.152 C_0.003 The bandgap of the layers is also about 1.02 eV, and as shown in FIG. 5 (b), the bandgap at the boundary between the two base regions 12 and 13 is equal. On the other hand, the band gap at the collector embedded layer side end of the first base region 12 is about 0.93 eV. Therefore, in the first base region 12, the band gap gradually decreases in the direction from the side end of the second base region toward the collector embedded layer 11, so that the electrons in the first base region 12 drift electric fields. Accelerates by shortening the traveling time of electrons and improving the high frequency characteristics of the heterobipolar transistor. Further, when two SiGeC layers having the same band gap are laminated at the boundary portion, it is possible to further reduce the voltage drive as in the second embodiment. Then, as described above, the second base region 13 having a low C content is interposed between the emitter region 14a and the first base region 12, so that the depletion layer between the emitter base is formed (FIG. 5 (b). The recombination current in the region Rdp) shown in) can be reduced.

【0053】

That is, in the present embodiment, in addition to the same effect as in the second embodiment, the high frequency characteristics of the heterobipolar transistor can be improved.

【0054】

(Fourth Embodiment) FIGS. 6 (a) and 6 (b) show the C and Ge contents of the first base region and the second base region and the concentration of boron (B), which is an impurity, in the fourth embodiment. It is a figure which shows, and the energy band figure of the emitter region-base region-collector region at the time of applying a voltage. In addition, in FIG. 6A, the illustration of the concentration of the n-type impurity is omitted.

【0055】

In the present embodiment, the band gaps of both the first base region 12 and the second base region 13 are equal to each other, and the lattice distortion at the boundary between the first and second base regions 12 and 13 is minimized. Another feature is that the Ge and C contents of the 1st and 2nd base regions 12 and 13 are adjusted. Therefore, the Ge and C contents at the side end of the second base region of the first base region 12 are the same as those of the second base region 13, and the Ge content and the C content in the first base region 12 are set to the second. Increase in the direction from the base region side end toward the collector embedding layer 11. At that time, the composition in the SiGeC layer is expressed by the general formula Si._1-xy Ge_x C_y When the difference in C content between the region excluding the 1st base region side end of the 1st base region 12 and the 2nd base region 13 is Δy, the 1st base region side of the 1st base region 12 The difference Δx in the Ge content between the region excluding the end and the second base region 13 is determined based on the above equation (1).

【0056】

As shown in FIG. 6A, in the present embodiment, the Ge content in the second base region 13 and the second base region side end of the first base region 12 is set to a common value (for example, 26.8%). , Set the Ge content at the end of the first base region 12 on the side of the collector embedding layer to a higher value (for example, 31.3%). On the other hand, the C content is a common value (for example, 0.35%) in the second base region 13 and the second base region side end of the first base region 12, and is on the collector embedding layer side of the first base region 12. It is assumed that the value is higher (for example, 1.4%) at the end. That is, the second base region 13 and the second base region side end of the first base region 12 are SiGe._0.268 C_0.₀₀₃₅It consists of layers, and the side end of the collector embedded layer of the first base region 12 is SiGe._0.₃₁₃ C_0.014 It consists of layers.

【0057】

At this time, SiGe_0.268 C_0.0035The bandgap of the layer is about 0.93eV, SiGe_0.313 C_0.014 The bandgap of the layers is about 0.93 eV, and as shown in Figure 6 (b), the bandgap in the two base regions 12, 13 is equal. Since the Ge and C contents at the boundaries of the first and second base regions 12 and 13 are both equal, the lattice distortion of the entire base region is minimized because there is no sudden change in the lattice constant at the boundaries. can do. Therefore, since the occurrence of defects such as dislocations due to lattice strain can be suppressed, the electrical characteristics of the heterobipolar transistor can be improved.

【0058】

On the other hand, when two SiGeC layers having the same band gap are laminated, it is possible to further reduce the voltage drive as in the second embodiment. Then, as described above, the second base region 13 having a low C content is interposed between the emitter region 14a and the first base region 12, so that the depletion layer between the emitter base is formed (FIG. 6 (b). The recombination current in the region Rdp) shown in) can be reduced.

【0059】

That is, in this embodiment, in addition to the same effect as in the second embodiment, it is possible to improve the electrical characteristics of the heterobipolar transistor by suppressing the occurrence of defects.

【0060】

(Fifth Embodiment) FIGS. 7 (a) and 7 (b) show the C and Ge contents of the first base region and the second base region and the concentration of the impurity boron (B) in the fifth embodiment. It is a figure which shows, and the energy band figure of the emitter region-base region-collector region at the time of applying a voltage. In addition, in FIG. 7A, the illustration of the concentration of the n-type impurity is omitted.

【0061】

In the present embodiment, the band gaps of both the first base region 12 and the second base region 13 are made equal to each other, and the band gap of the first base region 12 is changed in the direction of accelerating the base traveling electrons. The feature is that the Ge and C contents of the 1st and 2nd base regions 12 and 13 are adjusted so that the lattice strain at the boundary between the 1st and 2nd base regions 12 and 13 is adjusted as much as possible. Therefore, while the Ge and C content at the side end of the second base region of the first base region 12 is the same as that of the second base region 13, the C content and Ge content in the first base region 12 are set to the second. Increase in the direction from the base region side end toward the collector embedding layer 11.

【0062】

As shown in FIG. 7A, in the present embodiment, the Ge content in the second base region 13 and the second base region side end of the first base region 12 is set to a common value (for example, 15.2%). , Set the Ge content at the end of the first base region 12 on the side of the collector embedding layer to a higher value (for example, 30%). On the other hand, the C content is a common value (for example, 0.3%) in the second base region 13 and the second base region side end of the first base region 12, and is on the collector embedded layer side of the first base region 12. It is assumed that the value is higher (for example, 1.4%) at the end. That is, the second base region 13 and the second base region side end of the first base region 12 are SiGe._0.152 C_0.003It consists of layers, and the side end of the collector embedded layer of the first base region 12 is SiGe._0.30C_0.₀₁₄ It consists of layers.

【0063】

At this time, SiGe_0.152 C_0.003The bandgap of the layer is about 1.02 eV, SiGe_0.30C_0.014 The layer bandgap is about 0.93 eV. Therefore, in the first base region 12, the band gap gradually decreases in the direction from the side end of the second base region toward the collector embedded layer 11, so that the electrons in the first base region 12 drift electric fields. Accelerates by shortening the traveling time of electrons and improving the high frequency characteristics of the heterobipolar transistor. Since the Ge and C contents at the boundaries of the first and second base regions 12 and 13 are both equal, the lattice distortion of the entire base region is minimized because there is no sudden change in the lattice constant at the boundaries. can do. Therefore, since the occurrence of defects such as dislocations due to lattice strain can be suppressed, the electrical characteristics of the heterobipolar transistor can be improved.

【0064】

Further, when two SiGeC layers having the same band gap are laminated at the boundary portion, it is possible to further reduce the voltage drive as in the second embodiment. Then, as described above, the second base region 13 having a low C content is interposed between the emitter region 14a and the first base region 12, so that the depletion layer between the emitter base is formed (FIG. 7 (b). The recombination current in the region Rdp) shown in) can be reduced.

【0065】

That is, in the present embodiment, the effects of the third embodiment and the fourth embodiment can be exhibited together.

【0066】

(Other Embodiments) In each of the above embodiments, only the case where the second base region 13 is the SiGeC layer has been described, but in each of the above embodiments, the second base region 13 is composed of the SiGe layer. It can also be applied to those that are.

【0067】

[Effect of the invention]

According to the heterobipolar transistor of the present invention, the C content of the region adjacent to the emitter region in the base region composed of the SiGeC layer is made smaller than the C content of the region adjacent to the collector region, so that the recombination current is suppressed. As a result, it is possible to improve electrical characteristics such as current multiplication and high frequency characteristics while reducing the drive voltage.

[Simple explanation of drawings]

[Figure 1]

It is a phase diagram which shows the relationship between the content rate of Ge and C in a SiGeC ternary mixed crystal semiconductor, a band gap, and a lattice strain.

[Figure 2]

It is sectional drawing of the heterobipolar transistor (HBT) common to each embodiment of this invention.

[Fig. 3]

(a) and (b) are a diagram showing the C, Ge content and the boron concentration of HBT in the first embodiment, and an energy band diagram when a voltage is applied.

[Fig. 4]

(a) and (b) are a diagram showing the C, Ge content and the boron concentration of HBT in the second embodiment, and an energy band diagram when a voltage is applied.

[Fig. 5]

(a) and (b) are a diagram showing the C, Ge content and the boron concentration of HBT in the third embodiment, and an energy band diagram when a voltage is applied.

[Fig. 6]

(a) and (b) are a diagram showing the C, Ge content and the boron concentration of HBT in the fourth embodiment, and an energy band diagram when a voltage is applied.

[Fig. 7]

(a) and (b) are a diagram showing the C, Ge content and the boron concentration of HBT in the fifth embodiment, and an energy band diagram when a voltage is applied.

[Fig. 8]

(a) and (b) are SiGe in order._0.268 HBT, SiGe_0.268 C_0.0091It is a figure which shows the gunmel plot of HBT.

[Fig. 9]

(a) and (b) are SiGe in order._0.268 HBT, SiGe_0.268 C_0.₀₀₉₁It is a figure which shows the current multiplication factor (β) of HBT.

[Fig. 10]

SiGe_0.268 HBT, SiGe_0.268 C_0.0091It is a figure for investigating the fitting of the measurement result of the forward current-voltage characteristic of the diode characteristic between the emitter and base of HBT, and the measurement result of the calculated value of the sum of the electron recombination current and the diffusion current.

[Fig. 11]

(a) and (b) are SiGe containing Ge uniformly in the base region, respectively._0.268 It is a figure which shows the result of simulating the gammel plot and the current multiplication by changing the recombination lifetime in the base region of HBT.

[Fig. 12]

It is a figure which shows the parameter of the sample used in the experiment for confirming the effect of this invention in a table.

[Fig. 13]

It is a figure which shows the data of the bias voltage-current characteristic measured about the sample shown in FIG.

[Explanation of symbols]

10 Si board 11 Collector embedded layer 12 1st base area 13 Second base area 14 Si cap layer 14a Emitter region 15 Emitter electrode