Lateral heterojunction bipolar transistor

Abstract: A semiconductor device is provided with an emitter area and a collector area of a first conductive type and a base area of a second conductive type, arranged in a horizontal structure. The semiconductor device comprises an area constituting at least a part of the base area, being in contact with a part of the base area at least positioned between the emitter and collector areas, and having a narrower forbidden band than in the base area.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a horizontal bipolar transistors constituting integrated circuits. Related Background Art There is already known a horizontal bipolar transistor in which the current flows horizontally, namely along the surface of substrate. Such horizontal bipolar transistor is widely used because of the advantage that it can be easily compatible with a vertical bipolar transistor in which the current flows vertically, namely in the direction of depth of substrate. For example, a horizontal bipolar transistor of pnp structure can be loaded on a same substrate with a vertical bipolar transistor of opposite npn structure. However, the conventional horizontal bipolar transistor has been associated with a drawback that the current amplification factor h FE cannot be increased because of the following reasons. This is due to facts that, because of a low emitter-collector breakdown voltage resulting from symmetrical arrangement of the emitter area and the collector area in such horizontal bipolar transistor, the base width is apt to be influenced by the collector voltage resulting eventually in so-called early effect (spreading of depletion layer), and that, because of wide internal spreading of the current flowing from the emitter into the base, the recombined current in the base area governs the function and the increased base current reduces significantly the current amplification factor h FE . R.L. Thornton et al. disclose in IEEE Transactions on Electron Devices, vol. 36, no. 10 (1989), pages 2156-2164 a lateral bipolar transistor with a heterojunction base. SUMMARY OF THE INVENTION The present invention provides a lateral bipolar transistor comprising: an emitter region; a collector region laterally spaced from and facing said emitter region; and a base region; disposed in-between said emitter and collector regions which transistor is characterised in that: said base region has a first portion located beneath the underside of said emitter region at a depth shorter than the diffusion length of minority carriers in a second portion of said base region, said second portion lying immediately beneath said emitter region, in-between said emitter region and said first region, and forming, at its interface with said first region, a potential barrier against minority carriers in said second portion injected therein from said emitter region, thus preventing the diffusion of minority carriers into the base region. Furthermore, there may also be a third portion of the base region, which third portion is of an enhanced dopant concentration and is located immediately beneath the first portion. The barrier can be rendered sufficiently effective even at room temperature, by selecting the magnitude of potential of the barrier at least equal to the corresponding thermal energy at the desired temperature. Furthermore, by forming the area constituting said barrier in such a manner that it surrounds the emitter and collector areas, a drastic reduction in the recombination current of the minority carriers can be achieved, as well as a reduction in the contact resistance between the emitter, base and collector areas. Furthermore, the first and second portions of the base preferably have different respective band gaps, the band gap of the first portion being wider than the band gap of the second portion. The first and second portions of the base region may also be heterogeneous, a heterojunction thus being defined at the interface therebetween, thus creating the barrier preventing the diffusion of minority carriers into the base and thus increasing the current application factor h FE of the horizontal bipolar transistor. In the preferred embodiment the first and second portions of the base region are of silicon Si and silicon-germanium mixed composition Si 1-x Ge x , respectively. In a second embodiment the transistor may further comprise a polycrystalline silicon or polycrystalline silicon carbide contact layer, formed on the emitter and having an enhanced dopant concentration and a wider forbidden band than the emitter, in order to prevent the diffusion of electrons into the emitter. In the third embodiment, the transistor has the base region buried beneath a surface layer region of silicon. In the fourth embodiment, the barrier consists of a heterogeneous interface and above the emitter area, there is formed a thin tunnel film and a polycrystalline silicon layer of a wider forbidden band than in the emitter area, in order to prevent the diffusion of electrons injected into the emitter area. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic cross-sectional view of a first embodiment of the present invention; Fig. 2A is a schematic view showing the potential distribution along a line A-A' in Fig. 1; Fig. 2B is a schematic view showing the potential distribution along a line B-B' in Fig. 1; Fig. 3 is a chart showing the mobility, life and diffusion length of positive holes as a function of concentration of n-type impurity; Fig. 4 is a chart showing the change in the band gap as a function of mixed crystal ratio in Si 1-x Ge x ; Fig. 5 is a schematic cross-sectional view of a second embodiment; Fig. 6A is a schematic view showing the potential distribution along a line A-A′ in Fig. 5; Fig. 6B is a schematic view showing the potential distribution along a line B-B′ in Fig. 5; Fig. 7 is a chart showing the mobility life and diffusion length of electrons as a function of concentration of p-type inpurity; Fig. 8 is a schematic cross-sectional view of a third embodiment; Fig. 9 is a schematic cross-sectional view of a fourth embodiment; Fig. 10A is a schematic view showing the potential distribution along a line A-A′ in Fig. 9; Fig. 10B is a schematic view showing the potential distribution along a line B-B′ in Fig. 9; In the following the present invention will be clarified in detail by embodiments thereof shown in the attached drawings. [Embodiment 1] Fig. 1 illustrates a preferred first embodiment of the present invention. A silicon substrate 1 is composed, in the present embodiment, of a p-substrate doped with a p-type impurity capable of controlling the conductive type in the p-type, such as boron (B). An embedded area 2, doped with an impurity capable of controlling the conductive type, is doped in the present embodiment with an n-type impurity such as phosphor (P) with an impurity concentration of 10 16 - 10 20 [cm -3 ]. An n-area 3, constituting a part of the base area B R , is for example formed by epitaxial growth with a low impurity concentration (for example 10 14 - 5 x 10 7 [cm -3 ]). An n-area 4, constituting a feature of the present invention, is composed of a mixed crystal Si 1-x Ge x with a narrower forbidden band than in Si, and constitutes the base area B R . P + -areas 5, 5′ are formed in said n-area 4 and respectively constitute an emitter area E R and a collector area C R . An n + -area 6 connects said embedded area 2 with a metal base electrode 201, in order to reduce the base resistance of the horizontal bipolar transistor. An n-area 7 and a p-area 8 constitute channel stops. There are also shown an oxide film 30, an element separating area 100, an insulating area 110 for separating electrodes, a metal base electrode 201, and a metal collector electrode 202. Figs. 2A and 2B schematically show the potential distribution of the semiconductor device shown in Fig. 1, respectively along lines A-A′ and B-B′ shown therein. As shown in Fig. 2B, the potential distribution along the line B-B′ in Fig. 1 is similar to that in the usual bipolar transistor, and the collector current I C can be easily calculated as it is determined by the concentration N B of the base area B R and the base width W B . In the conventional horizontal bipolar transistor, the emitter area and the collector area mutually face only in a very small area, while they are extended quite widely along the surface of the substrate. The collector current flows only through the mutually facing area of the emitter and the collector. On the other hand, the base current is principally governed by a current generated by the recombination of the positive holes, injected from the emitter area E R into the base area B R in the direction of depth of the substrate, with the electrons constituting the majority carriers, and a diffusion current contributed by the electrons injected from the base area into the emitter area, and becomes large because of a wide area involved. These facts pose limitation on the increase of the current amplification factor h FE . In the present invention, in order to prevent the diffusion of the positive holes, injected from the emitter area, into the direction of depth of the substrate, a heterogeneous interface is formed at a depth X B , between the n-area 3 and the area 4. The potential distribution in the direction of depth is shown in Fig. 2A. In the following explained are current components in the horizontal bipolar transistor: (I) Collector current I C : The collector current I C between the mutually opposed emitter and collector can be substantially determined by the following equation. The magnitude of the emitter area opposed to the collector area in a horizontal bipolar transistor can be approximately represented by ϒ E ·L wherein ϒ E and L are respectively the depth and length of the emitter. I C = ϒ E ·L( q·D p ·n i L B ·L P ) 2 cosech( W B L P ) [exp( V BE KT )-1] wherein q is elementary charge [C], D P is diffusion coefficient [cm 2 /s] of positive holes, L P is diffusion length [cm] of positive holes n i is true carrier density [cm -3 ], V BE is emitter-base voltage, W B is base width [cm], N B is base impurity concentration [cm -3 ], ϒ E is emitter depth, L is emitter length, k is Boltzman constant [J/K], and T is absolute temperature [°K]. (II) Base current I B : The base current I B is principally composed of three components shown in the following equations (2) - (4). (a) Recombination current I B1 generated in the base area B R by the positive holes flowing horizontally from the emitter area E R : I B1 = ϒ E ·L·( q·D p ·n i 2 N B ·L P )· cosh(W B /L P )-1 sinh(W B /L P ) ·[exp( V BE kT )-1] (b) Recombination current I B2 generated by the positive holes flowing vertically from the emitter area E R : I B2 = (W E ·L) q·n i 2 ·D p N B ·L P ·tanh( ϒ B L P )·[exp( V BE kT )-1] wherein ϒ B is the distance in the direction of depth from the emitter area E R , and W E is emitter width. (c) Diffusion current I B3 generated by the electrons flowing from the base area B R to the emitter area E R : I B3 = (W E ·L) q·n i 2 ·D n N E ·L n ·cotanh( ϒ E L n )·[exp( V BE kT )-1] wherein N E is impurity concentration in the emitter area E R , L n is diffusion length of the electrons, and D n is diffusion coefficient of the electrons. In the present invention, particularly considered is the base current component I B2 represented by the equation (3). In the conventional horizontal bipolar transistor, most of the positive holes injected from the emitter area E B flows vertically and contributes to the base current by recombination. On the other hand, in the present invention, a potential barrier Δ B is formed at a distance ϒ B from the emitter area E R to stop the injected carriers. The probability of passing through said barrier is represented by exp(-Δ B /kT). Said probability, for example at room temperature is Δ B = 0.1 [eV] , which is about 1/54 of that in the conventional horizontal bipolar transistor. If ϒ B << L P in the equation (3), since tanh(ϒ B /L P ) ≃ ϒ B /L P , I B2 in the horizontal bipolar transistor of the present invention becomes significantly lower than that in the conventional one. In the following there will be explained the diffusion length L P of positive holes in the n-area. Following equation stands for the mobility µ P of positive holes, over a wide range of the n-impurity concentration N D : µ P = 370 1 + (N D /8 x 10 17 ) 1.25 + 130 [cm 2 /V·sec] As will be understood from this equation (5), the mobility µ P approaches to a constant value 500 [cm 2 /V·sec] when the impurity concentration N D becomes smaller, but becomes a function of said concentration N D when it exceeds 10 17 [cm -3 ]. On the other hand, the life τ p of the minority carriers is represented by the following equation when the impurity concentration exceeds 10 17 [cm -3 ]: 1 τ p = 7.8 x 10 -13 N D + 1.8 x 10 -31 N D 2 [sec -1 ] Based on said mobility µ p and life τ p , the diffusion length L p of positive holes is generally represented by: L p = kT q ·µ p ·τ p Fig. 3 illustrates the mobility µ p (curve F 3 ), life τ p (curve F 1 ) and diffusion length L p (curve F 2 ) of positive holes, constituting the minority carriers in n-type silicon, as calculated from the foregoing equations (5), (6) and (7) in a range of N D of 10 17 - 10 19 [cm -3 ]. As will be apparent from Fig. 3, the diffusion length of positive holes is extremely long and amounts to 120 [µm] at N D = 10 17 [cm -3 ]. It is still about 30 [µm] even at N D = 10 18 [cm -3 ]. In a horizontal bipolar transistor of pnp type formed on a same substrate together with a usual vertical bipolar transistor of npn type, the impurity concentration of the base area becomes same as that of the collector area of said vertical bipolar transistor of npn type and is as low as 10 14 - 5 x 10 17 [cm -3 ], so that the relation ϒ B << L p can be easily obtained. However a condition of about ϒ B ≦ L p /10 is desirable. Under such condition, said recombination current I B2 is drastically reduced so that the base current I B is principally governed by I B1 and I B3 and the current amplification factor h FE can be made close to that of the vertical bipolar transistor. In case of mixed crystal Si 1-x Ge x , the mobility of the carriers is reduced due to the alloy effect, but, in a high impurity concentration region (for example 10 18 [cm -3 ] or higher), the impurity effect becomes predominant and the mixed crystal and Si single crystal behave almost same manner. When said recombination current I B2 is decreased, as the components of the base current satisfy a relation I B3 << I B1 , an increase in the current amplification factor h FE of the horizontal bipolar transistor can be achieved, if W E << L p , by increasing the following ratio: I C I B3 ≃ ϒ E W E · D p D n · L n L p · N E N B In designing, ϒ E /W E and N E /N B can be made easily larger in said equation (8). A larger value of N E /N B is same as in the usual vertical bipolar transistor, but a larger value of ϒ E /W E is specific to the horizontal bipolar transistor. In the following there will be explained an example of the manufacturing process of the device of the first embodiment shown in Fig. 1: (1) On a p-type Si substrate 1, an n + -embedded area 2 with an impurity concentration for example of 10 15 - 10 19 [cm -3 ] was formed by implantation (or diffusion) of at least an element selected from the elements of group V, such as As, Sb, P etc. (2) An n-area 3 with an impurity concentration for example of 10 14 - 10 17 [cm -3 ] was formed for example by epitaxial growth. (3) An n + -area 6 (with an impurity concentration for example of 10 17 - 10 20 [cm -3 ]) was formed for reducing the base resistance. (4) An insulating film 100 for element separation was formed by selective oxidation or CVD, and a channel stop area 7 was formed under said insulating film 100. (5) Ge ions were selectively implanted into the Si substrate 1 for example with an impurity concentration of 1 x 10 16 - 1 x 10 17 [cm -2 ], and a heat treatment was conducted to form an n-area 4 consisting of Si 1-x Ge x . (6) B + ions were implanted with a concentration of 1 x 10 15 [cm -2 ], and a heat treatment was conducted to form P + -areas 5, 5′ constituting the emitter and the collector. (7) An insulation film 110 was deposited and annealed, and contact holes were opened. (8) Al-Si (1%) for forming the electrodes 200, 201, 202 was deposited by sputtering and was subsequently patterned. (9) The alloy formation of said Al-Si electrodes was conducted for example for 30 minutes at 450°C, and a passivation film was formed. The semiconductor device of the first embodiment was prepared through the above-explained procedure. Fig. 4 shows the change in the band gap of the mixed crystal Si 1-x Ge x , wherein the abscissa indicates the mixed crystal ratio of Ge (× · 100%) whereas the ordinate indicates decrease -ΔE in the band gap in comparison with the Si single crystal. A curve H O indicates a distortion-free state whereas a curve H U indicates a distorted state, which is selected in a semiconductor device. As shown in Fig. 4, a band gap decrease -ΔE ≃ 0.1 [eV] corresponds approximately to a mixed amount X = 0.12, and a barrier of 0.1 [eV] is formed in this state. When Ge ions were implanted with a concentration 1 x 10 16 [cm -2 ] and with an accelerating voltage 150 [keV], the peak concentration is about 1 x 10 20 [cm -3 ]. Since the density of monocrystalline Si is 5 x 10 23 [cm -3 ], the mixed amount of Ge becomes about 2%. X = 0.12 can be attained for example with a Ge concentration of 6 x 10 16 [cm -3 ]. [Embodiment 2] Fig. 5 illustrates a 2nd embodiment of the present invention. A silicon substrate 1 is composed, in the present embodiment, of a p-substrate doped with a p-type impurity capable of controlling the conductive type in the p-type, such as boron (B). An embedded area 2, doped with an impurity capable of controlling the conductive type, is doped in the present embodiment with an n-type impurity such as phosphor (P) with an impurity concentration of 10 16 - 10 20 [cm -3 ]. An n-area 3, constituting a part of the base area B R , is for example formed by epitaxial growth with a low impurity concentration (for example 10 14 - 5 x 10 17 [cm -3 ]). An n-area 4, constituting a feature of the present invention, is composed of a mixed crystal Si 1-x Ge x with a narrow forbidden band, and constitutes the base area B R . P + -areas 5, 5′ are formed in said n-area 4 and respectively constitute an emitter area E R and a collector area C R . An n + -area 6 connects said embedded area 2 with a metal base electrode 201, in order to reduce the base resistance of the horizontal bipolar transistor. An n-area 7 and a p-area 8 constitute channel stops. Semiconductor layers 730, 731 are composed of polycrystalline p-silicon doped with boron (B) at a high concentration (for example 10 18 - 10 20 [cm -3 ]). There are further shown an element separating area 100, an electrode separating insulating area 110, a metal base electrode 201, and a metal collector electrode 202. Figs. 6A and 6B schematically show the potential distribution of the semiconductor device shown in Fig. 5, respectively along lines A-A′ and B-B′ shown therein. As shown in Fig. 6B, the potential distribution along the line B-B′ in Fig. 5 is similar to that in the usual bipolar transistor, and the collector current I C can be easily calculated as it is determined by the concentration N B of the base area B R and the base width W B . In the conventional horizontal bipolar transistor, the emitter area and the collector area mutually face only in a very small area, while they are extended quite widely along the surface of the substrate. The collector current flows only through the mutually facing area of the emitter and collector. On the other hand, the base current is principally governed by a current generated by the recombination of the positive holes, injected from the emitter area E R into the base area B R in the direction of depth of the substrate, with the electrons constituting the majority carriers, and a diffusion current contributed by the electrons injected from the base area into the emitter area, and becomes large because of a wide area involved. These facts pose limitation on the increase of the current amplification factor h FE . In the present embodiment, therefore, a heterogeneous interface is formed at a depth ϒ B from the emitter, and a semiconductor layer 730 of a wider forbidden band than in the emitter is formed thereon. The potential distribution in the direction of depth is shown in Fig. 6A. In the following the current components in the horizontal bipolar transistor will be explained, though the explanation may partly overlap with the foregoing description. (I) Collector current I C : The collector current I C between the mutually opposed emitter and collector can be substantially determined by the following equation. The magnitude of the emitter area opposed to the collector area in a horizontal bipolar transistor can be approximately represented by ϒ E ·L wherein ϒ E and L are respectively the depth and length of the emitter. I C = ϒ E ·L( q·D p ·n i L B ·L P ) 2 cos ech( W B L P ) [exp( V BE kT )-1] wherein q is elementary change [C], D P is diffusion coefficient [cm 2 /s] of positive holes, L P is diffusion length [cm] of positive holes, n i is true carrier density [cm -3 ], V BE is emitter-base voltage, W B is base width [cm], N B is base impurity concentration [cm -3 ], ϒ E is emitter depth, L is emitter length, k is Boltzman constant [J/K], and T is absolute temperature [°K]. (II) Base current I B : The base current I B is principally composed of three components shown in the following equations (2) - (4). (a) Recombination circuit I B1 generated in the base area B R by the positive holes flowing horizontally from the emitter area E R : I B1 = ϒ E ·L·( q·D p ·n i 2 N B ·L P )· cosh(W B /L P )-1 sinh(W B /L P ) ·[exp( V BE kT )-1] (b) Recombination current I B2 generated by the positive holes flowing vertically from the emitter area E R : I B2 = (W E ·L) q·n i 2 ·D p N B ·L P ·tanh( ϒ B L P )·[exp( V BE kT )-1] wherein ϒ B is the distance in the direction of depth from the emitter area E R , and W E is emitter width. (c) Diffusion current I B3 generated by the electrons flowing from the base area B R to the emitter area E R : I B3 = (W E ·L) q·n i 2 ·D n N E ·L n ·tanh( ϒ E L n )·[exp( V BE kT )-1] wherein N E is impurity concentration in the emitter area E R , L n is diffusion length of the electrons, and D n is diffusion coefficient of the electrons. In the present invention, particular attention is paid to the base current component I B2 represented by the equation (3), and I BB represented by (9). In the conventional horizontal bipolar transistor, most of the positive holes injected from the emitter area E B flows vertically and contributes to the base current by recombination. On the other hand, in the present invention, a potential barrier Δ B is formed at a distance ϒ B from the emitter area E R to stop the injected carriers. The probability of passing through said barrier is represented by exp(-Δ B /kT), as already explained before. Said probability, for example at room temperature is Δ B = 0.1 [eV], which is about 1/54 of that in the conventional horizontal bipolar transistor. If ϒ B << L P in the equation (3), since tan (ϒ B /L P ) ≃ ϒ B /L P , I B2 in the horizontal bipolar transistor of the present invention becomes significantly lower than that in the conventional one. In the following there will be explained the diffusion length L P of positive holes in the n-area. Following equation stands for the mobility µ P of positive holes, over a wide range of the n-impurity concentration N D : µ P = 370 1 + (N D /8 x 10 17 ) 1.25 + 130 [cm 2 /V·sec] As will be understood from this equation (5), the mobility µ P approaches to a constant value 500 [cm 2 /V·sec] when the impurity concentration N D becomes smaller, but becomes a function of said concentration N D when it exceeds 10 17 [cm -3 ]. On the other hand, the life τ p of the minority carriers is represented by the following equation when the impurity concentration exceeds 10 17 [cm -3 ]: 1 τ p = 7.8 x 10 -13 N D + 1.8 x 10 -31 N D 2 [sec -1 ] Based on said mobility µ p and life τ p , the diffusion length L p of positive holes is generally represented by: L p = kT q ·µ p ·τ p As will be apparent from Fig. 3, the diffusion length of positive holes is extremely long and amounts to 120 [µm] at N D = 10 17 [cm -3 ]. It is still about 30 [µm] even at N D = 10 18 [cm -3 ]. In a horizontal bipolar transistor of pnp type formed on a same substrate together with a usual vertical bipolar transistor of npn type, the impurity concentration of the base area becomes same as that of the collector area of said vertical bipolar transistor of npn type and is as low as 10 14 - 5 x 10 17 [cm -3 ], so that the relation ϒ B << L p can be easily obtained. However a condition of about ϒ B ≦ L p /10 is desirable. Under such condition, said recombination current I B2 is drastically reduced, so that the base current I B is principally governed by I B1 and I B3 , and the current amplification factor h FE can be made close to that of the vertical bipolar transistor. With the decrease of said recombination current I B2 , the base current I B is predominantly governed by the recombination current I B1 and the diffusion current I B3 , whereas in the conventional horizontal bipolar transistor, it is predominantly governed by a current generated by the recombination of the minority carriers diffused from the emitter in the direction of depth, and the diffusion current I B3 . Thus, according to the present embodiment, above the emitter area there is provided a semiconductive polycrystalline Si layer 730 having a wider forbidden band than in said emitter area, as shown in Fig. 6A, in order to prevent the diffusion of electrons. In the conventional horizontal bipolar transistor, said diffusion current I B3 is considerably large, because a metal electrode is formed directly above the emitter area 5. When the electrons are injected from the base area into the emitter area under a condition ϒ B << L N , the diffusion current I B3 is proportional to ϒ E /L n 2 as will be understood from the equation (4), but, in the conventional horizontal bipolar transistor, it is proportional to 1/ϒ E 2 . Thus I B3 in the horizontal bipolar transistor of the present invention is smaller, by a ratio ϒ E 2 /L n 2 , than that in the conventional horizontal bipolar transistor. Then there will be explained the diffusion length of electrons in the p + -area 5. Following relation stands on the mobility µ n of electrons, over a wide range of the concentration Na of p-impurity: µ n = 232 + 1180 1 + (Na/8 x 10 16 ) 0.9 [cm 2 /V·sec] Consequently the mobility µ n approaches to 1412 [cm 2 /V·sec] for a lower impurity concentration Na, and becomes equal to 232 [cm 2 /V·sec] for an infinite impurity concentration Na. On the other hand, the life τ n of the minority carriers is represented by the following equation for an impurity concentration equal to or larger than 10 17 [cm -3 ]: 1 τ n = (3.45 x 10 -12 Na) + (9.5 x 10 -32 Na 2 ) sec -1 Based on said mobility µ n and life τ n , the diffusion length L n is generally represented by: L n = kT q · µ n · τ n Fig. 7 shows the mobility µ n (curve M 3 ), life τ n (curve M 1 ) and diffusion length L n (curve M 2 ) of the electrons, calculated by the equations (10), (11) and (12) in a range of the p-impurity concentration Na of 10 17 - 10 20 [cm -3 ]. Following fact will be understood from Fig. 7. The p-emitter area of the horizontal bipolar transistor is usually common with the base area of the vertical bipolar transistor and has therefore an impurity concentration not exceeding 10 18 [cm -3 ], and the diffusion length becomes as large as 15 - 70 [µm] in such state. Consequently the diffusion current I B3 in the emitter area can be sufficiently lowered if the emitter depth ϒ E is about L n /5 or less. Si 1-x Ge x constituting said n-area 4 shows a small carrier mobility due to the alloy effect, but, under a high impurity concentration for example equal to or higher than 10 18 [cm -3 ], it behaves like Si because the carrier effect by the impurities becomes predominant. In the structure of the present embodiment, as explained in the foregoing, the base current approaches to the value of the recombination current I B1 , and the current amplification factor h FE approaches to that of the vertical heterogeneous bipolar transistor. The current amplification factor in such state, if a condition W B << L p is satisfied, can be represented by: h FE ≅ I c I B1 ≅ 2( L p W B ) 2 As explained in the foregoing, the present embodiment is designed to block the positive holes, injected from the p + -emitter area 5, by the heterogeneous interface between the n-area 4 and the n-area 3 consisting of mixed crystal, thereby preventing the diffusion of said positive holes into the deeper area of the substrate, and to prevent the diffusion of electrons, injected into the emitter area, by the potential barrier formed at the heterogeneous interface between the Si-Ge mixed crystal and the monocrystalline silicon. In the following there will be explained an example of the manufacturing process of the device of the 2nd embodiment shown in Fig. 5: (1) On a p-type Si substrate 1, an n + -embedded area 2 with an impurity concentration for example of 10 15 - 10 19 [cm -3 ] was formed by implantation (or diffusion) of at least an element selected from the elements of group V, such as As, Sb, P etc. (2) An n-area 3 with an impurity concentration for example of 10 14 - 10 17 [cm -3 ] was formed for example by epitaxial growth. (3) An n + -area 6 (with an impurity concentration for example of 10 17 - 10 20 [cm -3 ]) was formed for reducing the base resistance. (4) An insulating film 100 for element separation was formed by selective oxidation or CVD, and a channel stop area 7 was formed under said insulating film 100. (5) Ge ions were selectively implanted into the Si substrate 1 for example with an impurity concentration of 1 x 10 16 - 1 x 10 17 [cm -2 ], and a heat treatment was conducted to form an n-area 4 consisting of Si 1-x Ge x . (6) B + ions were implanted with a concentration of 1 x 10 15 [cm -2 ], and a heat treatment was conducted to form p + -areas 5, 5′ constituting the emitter and the collector. (7) Semiconductor layers 730, 731 consisting of polycrystalline silicon were deposited for example with a thickness of 400 nm, then subjected to boron ion implantation with a concentration of 1 x 10 16 [cm -2 ], further subjected to a heat treatment at 800 - 900 °C, and were patterned. In case of monocrystalline silicon, epitaxial growth was conducted for example at about 800°C. (8) An insulation film 110 was deposited and annealed, and contact holes were opened. (9) Al-Si (1%) for forming the electrodes 200, 201, 202 was deposited by sputtering and was subsequently patterned. (10) The alloy formation of said Al-Si electrodes was conducted for example for 30 minutes at 450°C, and a passivation film was formed. The semiconductor device of the 2nd embodiment was prepared through the above-explained procedure. The areas 730, 731 may be formed not only with Si but also with a polycrystalline material such as SiC, or microcrystalline Si, or a compound semiconductor such as GaAs or GaAlAs. [Embodiment 3] Fig. 8 illustrates a 3rd embodiment. The present embodiment is most characterized by the presence of an n-Si layer 50 on the n-area 4 consisting of Si 1-x Ge x . In the foregone 1st embodiment, the contact electrode and the heterogeneous junctions are used in common, but, in the present embodiment, the n-areas 3, 4 and said n-Si layer 50 are continuously formed by epitaxy. Also the element separating area 100 has a groove structure and mutually separates the n-areas 2, 3, 4 and 50. 120 indicates an oxide film. Other structures and functions are same as those in the foregoing 2nd embodiment, and will not, therefore, be explained further. [Embodiment 4] Fig. 9 illustrates a 4th embodiment of the present invention. A silicon substrate 1 is composed, also in the present embodiment, of a p-substrate doped with a p-type impurity capable of controlling the conductive type in the p-type, such as boron (B) . An embedded area 2, doped with an impurity capable of controlling the conductive type, is doped in the present embodiment with an n-type impurity such as phosphor (P) with an impurity concentration of 10 16 - 10 20 [cm -3 ]. An n-area 3, constituting a part of the base area B R , is for example formed by epitaxial growth with a low impurity concentration (for example 10 14 - 5 x 10 17 [cm -3 ]). An n-area 4, constituting a feature of the present invention, is composed of a mixed crystal Si 1-x Ge x with a narrow forbidden band, and constitutes the base area B R . P + -areas 5, 5′ are formed in said n-area 4 and respectively constitute an emitter area E R and a collector area C R . An n + -area 6 connects said embedded area 2 with a metal base electrode 201, in order to reduce the base resistance of the horizontal bipolar transistor. An n-area 7 and a p-area 8 constitute channel stops. An ultra thin tunnel oxide film 920 (for example of a thickness not exceeding 2 nm) serves to pass both the electrons and the positive holes by tunnel effect. Polycrystalline semiconductive layers 930, 931 are composed of polycrystalline silicon doped with boron (B) at a high concentration (for example 10 18 - 10 20 [cm -3 |). There are further shown an element separating area 100, an electrode separating insulating area 110, a metal base electrode 201, and a metal collector electrode 202. Figs. 10A and 10B schematically show the potential distribution of the semiconductor device shown in Fig. 9, respectively along lines A-A′ and B-B′ shown therein. As shown in Fig. 10B, the potential distribution along the line B-B′ in Fig. 9 is similar to that in the usual bipolar transistor, and the collector current I C can be easily calculated as it is determined by the concentration N B of the base area B R and the base width W B . In the conventional horizontal bipolar transistor, the emitter area and the collector area mutually face only in a very small area, while they are extended quite widely along the surface of the substrate. Consequently a current generated by the positive holes injected from the emitter area E R into the base area B R along the direction of depth of substrate and a diffusion current generated by the electrons flowing from the base area to the emitter area predominantly govern the function, and a limitation is posed on the increase of the current amplification factor h FE . In the present embodiment, therefore, a barrier Δ B consisting of a heterogeneous interface between the emitter area and a semiconductor layer is formed at a depth ϒ B from said emitter area, and, above the emitter area, there are formed a thin tunnel film and a polycrystalline silicon layer 930 of a wider forbidden band than in said emitter area, in order to prevent the diffusion of the electrons injected into said emitter area. In the following the current components in the horizontal bipolar transistor will be explained, though the explanation may partly overlap with the foregoing description. (I) Collector current I C : The collector current I C between the mutually opposed emitter and collector can be substantially determined by the following equation. The magnitude of the emitter area opposed to the collector area in a horizontal bipolar transistor can be approximately represented by ϒ E ·L wherein ϒ E and L are respectively the depth and length of the emitter. I C = ϒ E ·L( q·D p ·n i N B ·L p ) 2 cosech( W B L p ) [exp( V BE k T )-1] wherein q is elementary charge [C], D p is diffusion coefficient [cm 2 /s] of positive holes, L P is diffusion length [cm] of positive holes, n i is true carrier density [cm -3 ], V BE is emitterbase voltage, W B is base width [cm], N B is base impurity concentration [cm -3 ], T E is emitter depth, L is emitter length, K is Boltzman constant [J/K], and T is absolute temperature [°K]. (II) Base current I B : The base current I B is principally composed of three components shown in the following equations (2) - (4). (a) Recombination current I B1 generated in the base area B R by the positive holes flowing horizontally from the emitter area E R : I B1 = ϒ E ·L·( q·D p ·n i 2 N B ·L P )· cosh(W B /L p )-1 sinh(W B /L p ) ·[exp( V BE kT )-1] (b) Recombination current I B2 generated by the positive holes flowing vertically from the emitter area E R : I B2 = (W E ·L) q·n i 2 ·D p N B ·L p ·tanh( ϒ B L p )·[exp( V BE kT )-1] wherein ϒ B is the distance in the direction of depth from the emitter area E R , and W E is emitter width. (c) Diffusion current I B3 generated by the electrons flowing from the base area B R to the emitter area E R : I B3 = (W E ·L) q·n i 2 ·D n N E ·L n ·tanh( ϒ E Ln )·[exp( V BE kT )- 1] wherein N E is impurity concentration in the emitter area E R , Ln is diffusion length of the electrons, and D n is diffusion coefficient of the electrons. In the present embodiment, particular attention is paid to the base current components I B2 represented by the equation (3) and I B3 represented by (4). In the conventional horizontal bipolar transistor, most of the positive holes injected from the emitter area E B flows vertically and contributes to the base current by recombination. On the other hand, in the present invention, a potential barrier Δ B is formed at a distance ϒ B from the emitter area E R to stop the injected carriers. The probability of passing through said barrier is represented by exp(-Δ B /kT). Said probability, for example at room temperature is Δ B = 0.1 [eV] , which is about 1/54 of that in the conventional horizontal bipolar transistor. If ϒ B << L p in the equation (3), since tanh (ϒ B /L p ) ≃ ϒ B /L p , I N2 in the horizontal bipolar transistor of the present invention becomes significantly lower than that in the conventional one. In the following there will be explained the diffusion lens L p of positive holes in the n-area. Following equation stands for the mobility µ p of positive holes, over a wide range of the n-impurity concentration N D : µ p = 370 1 + (N D /8 x 10 17 ) 1.25 + 130 [cm 2 /V·sec] As will be understood from this equation (5), the mobility µ p approaches to a constant value 500 [cm 2 /V·sec] when the impurity concentration N D becomes smaller, but becomes a function of said concentration N D when it exceeds 10 17 [cm -3 ]. On the other hand, the life τ p of the minority carriers is represented by the following equation when the impurity concentration exceeds 10 17 [cm -3 ]: 1 τ p = 7.8 x 10 -13 N D + 1.8 x 10 -31 N D 2 [sec -1 ] Based on said mobility µ p and life τ p , the diffusion length L p of positive holes is generally represented by: L p = kT q ·µ p ·τ p As will be apparent from Fig. 3, the diffusion length of positive holes is extremely long and amounts to 120 [µm] at N D = 10 17 [cm -3 ]. It is still about 30 [µm] even at N D = 10 18 [cm -3 ]. In a horizontal bipolar transistor of pnp type formed on a same substrate together with a usual vertical bipolar transistor of npn type, the impurity concentration of the base area becomes same as that of the collector area of said vertical bipolar transistor npn type and is as low as 10 14 - 5 x 10 17 [cm -3 ], so that the relation ϒ B << L p can be easily obtained. However a condition of about ϒ B << L p is desirable. Under such condition, said recombination current I B2 is drastically reduced, so that the base current I B is principally governed by I B1 and I B3 , and the current amplification factor h FE can be made close to that of the vertical bipolar transistor. With the decrease of said recombination current I B2 , the base current I B is predominantly governed by the recombination current I B1 and the diffusion current I B3, whereas in the conventional horizontal bipolar transistor, it is predominantly governed by the diffusion current I B3 . Thus, in the present embodiment, above the emitter area, there is provided a polycrystalline layer 930 of a wider forbidden band than in said emitter area as shown in Fig. 10A, thereby preventing the diffusion of electrons. In the conventional horizontal bipolar transistor, said diffusion current I B3 is considerably large, because a metal electrode is formed directly above the emitter area 5. When the electrons are injected from the base area into the emitter area under a condition W B << L n , the diffusion current I B3 is proportional to W E /L n 2 as will be understood from the equation (4), but, in the conventional horizontal bipolar transistor, it is proportional to 1/W E . Thus I B3 in the horizontal bipolar transistor of the present invention is smaller, by a ratio of W E 2 /L n 2 , than that in the conventional horizontal bipolar transistor. Then there will be explained the diffusion length of electrons in the p + -area 5. Following relation stands on the mobility µ n of electrons, over a wide range of the concentration Na of p-impurity: µ n = 232 + 1180 1 + (Na/8 x 10 16 ) 0.9 [cm 2 /V·sec] Consequently the mobility µ n approaches to 1412 [cm 2 /V·sec] for a lower impurity concentration Na, and becomes equal to 232 [cm 2 /V·sec] for an infinitely high impurity concentration Na. On the other hand, the life τ n of the minority carriers is represented by the following equation for an impurity concentration equal to or larger than 10 17 [cm -3 ]: 1 τ n = (3.45 x 10 -12 Na) + (9.5 x 10 -32 Na 2 ) sec -1 Based on said mobility µ n and life τ n , the diffusion length L n is generally represented by: L n = kT q · µ n · τ n From Fig. 7, following fact will be understood. The p-emitter area of the horizontal bipolar transistor is usually common with the base area of the vertical bipolar transistor and has therefore an impurity concentratio not exceeding 10 18 [cm -3 ], and the diffusion length becomes as large as 15 - 70 [µm] in such state. Consequently the diffusion current I B3 in the emitter area can be sufficiently lowered if the emitter width W E is about L n /r or less. Si 1-x Ge x constituting said n-area 4 shows a small carrier mobility due to the alloy effect, but, under a high impurity concentration for example equal to higher than 10 18 [cm -3 ], it behaves like Si because the carrier effect by the impurities becomes predominant. In the structure of the present embodiment, as explained in the foregoing, the base current approaches to the value of the recombination current I B1 , and the current amplification factor h FE approaches to that of the vertical heterogeneous bipolar transistor. The theoretical current amplification factor in such state, if a condition W B << L p is satisfied, can be represented by: h FE ≅ I c I B1 ≅ 2( L p W B ) 2 As explained in the foregoing, the present embodiment is designed to block the positive holes, injected from the p + -emitter area 5, by the heterogeneous interface between the n-area 4 and the n-area 3 of mixed crystal, thereby preventing the diffusion of said positive holes into the deeper area of the substrate, and to prevent the diffusion of electrons, injected into the emitter area, by the potential barrier formed at the heterogeneous interface between the Si-Ge mixed crystal and the monocrystalline silicon. In the following there will be explained an example of the manufacturing process of the device of the 4th embodiment shown in Fig. 9: (1) On a p-type substrate 1, an n + -embedded area 2 with an impurity concentration for example of 10 15 - 10 19 [cm -3 ] was formed by implantation (or diffusion) of at least an element selected from the elements of group V, such as As, Sb, P etc. (2) An n-area 3 with an impurity concentration for example of 10 14 - 10 17 [cm -3 ] was formed for example by epitaxial growth. (3) An n + -area 6 (with an impurity concentration for example of 10 17 - 10 20 [cm -3 ]) was formed for reducing the base resistance. (4) An insulating film 100 for element separation was formed by selective oxidation or CVD, and a channel stop area 7 was formed under said insulating film 100. (5) Ge ions were selectively implanted into the Si substrate 1 for example with an impurity concentration of 1 x 10 16 - 1 x 10 17 [cm -2 ], and a heat treatment was conducted to form an n-area 4 consisting of Si 1-x Ge x . (6) B + ions were implanted with a concentration of 1 x 10 15 [cm -2 ], and a heat treatment was conducted to form p + -areas 5, 5′ constituting the emitter and the collector. (7) An ultra thin oxide film 920, of a thickness for example not exceeding 2 nm , was formed by low-temperature oxidation for example at 500 - 650°C. (8) Polycrystalline silicon layers 930, 931 were deposited for example with a thickness of 400 nm, then subjected to boron ion implantation for example with a concentration of 1 x 10 16 [cm -2 ], further subjected to a heat treatment at 800 - 900°C, and were patterned. In case of monocrystalline silicon, epitaxial growth was conducted for example at about 800°C. (9) An insulation film 110 was deposited and annealed, and contact holes were opened. (10) Al-Si (1%) for forming the electrodes 200, 201, 202 was deposited by sputtering and was subsequently patterned. (11) The alloy formation of said Al-Si electrodes was conducted for example for 30 minutes at 450°C, and a passivation film was formed. The semiconductor device shown in Fig. 9 was prepared through the above-explained procedure. As explained in detail in the foregoing, the present invention can provide a semiconductor device including a horizontal bipolar transistor which has a high emitter-collector breakdown voltage, is capable of suppressing so-called early effect, and has a low base current, thereby capable of increasing the current amplification factor h FE . Also the present invention can provide a semiconductor device excellent in resistance to ambient conditions. Furthermore, the semiconductor device of the present invention can be provided inexpensively, since the mass production technology of the conventional integrated circuits can be utilized. Furthermore the semiconductor device of the present invention has a wide field of application, since other elements such as MOS transistors can be simultaneously integrated. The foregoing embodiments have been limited to a horizontal bipolar transistor of pnp type, but the present invention is naturally applicable to a horizontal bipolar transistor of npn type. In an operation at room temperature, the value x in Si 1-x Ge x is preferably at least equal to 0.03, because ΔE needs to be at least equal to kT. At room temperature, kT ≅ 25 meV, so that ΔE functions as a barrier if x is 0.03 or larger. Also in an operation at the temperature of liquid nitrogen, x is preferably at least equal to 0.008. Also the upper limit of the value x is preferably 1.

Claims: A lateral bipolar transistor comprising: an emitter region (5); a collector region (5′) laterally spaced from and facing said emitter region (5); and a base region (3,4) disposed in-between said emitter and collector regions (5,5′); said transistor is characterised in that: said base region (3,4) has a first portion (3) located beneath the underside of said emitter region (5) at a depth (X B ) shorter than the diffusion length (Lp or L N ) of minority carriers in a second portion (4) of said base region (3,4), said second portion (4) lying immediately beneath said emitter region (5), in-between said emitter region (5) and said first portion (3), and forming, at its interface with said first portion (3), a potential barrier (Δ B ) against minority carriers in said second portion (4) injected therein from said emitter region (5). A transistor according to claim 1 wherein said potential barrier (Δ B ) is at least equal to the thermal energy of said minority carriers at the operating temperature. A transistor according to claims 1 or 2 wherein said second portion (4) of said base (3,4) is disposed in-between said emitter and collector regions (5,5′) and immediately beneath said collector region (5′). A transistor according to any preceding claim wherein said first and second portions (3,4) of said base region (3,4) have different respective band-gaps, the band-gap of said first portion (3) being wider than the band-gap of said second portion (4). A transistor according to claim 4 wherein said first and second portions (3,4) of said base region (3,4) are heterogenous, a heterojunction thus being defined at the interface therebetween. A transistor according to claim 5 wherein said first and second portions (3,4) of said base region (3,4) are of silicon Si, and silicon-germanium mixed composition Si 1-x Ge x , respectively. A transistor according to claim 6 wherein said base region (3,4) is buried beneath a surface layer region of silicon (50). A transistor according to claim 6 wherein said emitter region (5) is of silicon-germanium mixed composition Si 1-x Ge x and has a contact (730;930) of polycrystalline silicon or of polycrystalline silicon carbide having an enhanced dopant concentration. A transistor according to claim 8 wherein a thin layer (920) of insulator material is interposed between said emitter region (5) and said contact (930) to define a tunnelling junction therebetween. A transistor according to any preceding claim, wherein said base region (3,4) has a third portion (2), which is of an enhanced dopant concentration and is located immediately beneath said first portion (3).

Inventors: MORISHITA MASAKAZU, MORISHITA, MASAKAZU
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: H01L29
IPC codes: H01L29