Lateral bipolar transistor

Abstract: A lateral bipolar transistor (fig.5) is provided with an emitter (5) and a collector (5') of a first conductivity type and a base (3,4) of a second conductivity type, arranged in a lateral structure. The transistor includes a region (4) constituting part of the base (3,4), in contact with a remainder part (3) of the base and which is positioned between the emitter (5) and collector (5'), and has a narrower energy band gap than the remainder part (3) of the base (3,4).

Description: The present invention relates to a lateral bipolar transistor. There is already known a lateral bipolar transistor in which the current flows horizontally, namely along or parallel to the surface of the substrate. Such lateral 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 the substrate. For example, a lateral bipolar transistor of pnp structure can be loaded on a same substrate with a vertical bipolar transistor of opposite npn structure. However, the conventional lateral 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 lateral 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 . SUMMARY OF THE INVENTION The present invention is intended to provide a semiconductor device of a structure capable of increasing lateral bipolar transistor current amplification factor. A solution to this problem has been described by R.L. Thornton et al : IEEE Transactions on Electron Devices, 36, No.10 (1989), pages 2156-2164. A heterojunction bipolar transistor with lateral current flow is described therein having features defined by the preamble of claim 1 appended hereto. In the structure described therein the emitter, base and collector regions have a common laminated heterostructure in which an active portion of the base is a layer of GaAs. This layer, which extends across the adjacent emitter and collector regions is sandwiched between wider band gap layers of AlGaAs. In forming the emitter and collector regions, Si is diffused to produce a disordered structure in the GaAs layer regions of the emitter and collector. In this step Al diffuses into the emitter and collector portions of the GaAs layer thus producing a graded composition structure having a graded band gap intermediate that of the remainder of the emitter and collector regions and the base region portion of the GaAs layer. A lateral bipolar transistor in accordance with the present invention is defined in claim 1 of the claims appended. In the preferred embodiment of this invention, the emitter region, the collector region and a portion of the base located beneath the underside of the emitter and collector regions, are of Si, whilst another portion of the base located only between the emitter and collector regions is of mixed crystal Si 1-x Ge x . It is mentioned that vertical bipolar transistors of Si/Si 1-x Ge x heterostructure are known. An example is given by G.L. Patton et al: 1989 Symposium on VLSI Technology, May 22 (1989), pages 95-96. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic cross-sectional view of a lateral bipolar transistor as comparative example; 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 ; and Fig. 5 is a schematic cross-sectional view of a preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As described below there is provided a lateral bipolar transistor as defined in claim 1. The interface between the first and second portions of the base region functions as a barrier in the direction of depth of the base region to the minority carriers injected from the emitter region into said base region. The depth of the second portion of the base region can be made shallower than the depth of the emitter and collector regions to concentrate the emitter current in the vicinity of the surface of the substrate. Such current concentration close to the surface of the substrate allows to make the active area of the device thinner, thereby responding to the requirements of higher level of integration and finer geometry. Furthermore, the depth of the emitter region may be selected to be shorter than the diffusion length of the minority carriers injected from the base region, thereby effectively reducing a component of the base current, namely the current resulting from recombination of the electrons with the positive holes in the emitter region. In the following, a preferred embodiment of the present invention will be described and will be compared with a comparative example, making reference to the attached drawings of which: Fig. 1 illustrates a lateral bipolar transistor as a comparative example. A silicon substrate 1 is composed, in this example 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 is composed of a mixed crystal Si 1-x Ge x with a narrower energy band gap than 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 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 χ 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 χ E ·L wherein χ E and L are respectively the depth and length of the emitter. I C = χ E ·L( q·D p ·n i N 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 ], χ 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 = χ E ·L·( q·D p · n 2 i 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 ( χ B L P )·[exp( V BE kT )-1] wherein χ 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( χ 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 this example, particularly considered is the base current component I B2 represented by the equation (3). In the conventional lateral 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 example , a potential barrier Δ B is formed at a distance χ 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 lateral bipolar transistor. If χ B << L P in the equation (3), since tanh(χ B /L P ) ≃ χ B /L P , I B2 in the lateral bipolar transistor 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 χ B << L p can be easily obtained. However a condition of about χ 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 ≃ χ E W E · D p D n · L n L p · N E N B In designing, χ 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 χ 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 (X · 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 ]. [Preferred [Embodiment] Fig. 5 illustrates a preferred embodiment, wherein the n-area 4 consisting of mixed crystal Si 1-x Ge x is limited to a smaller area than in the comparative example. Thus the depth of said n-area 4 is comparable to that of the P + emitter and collector areas 5, 5'. The present invention utilizes a phenomenon that the p-n injection efficiency varies in the p-n junction between the n-area 4 and the p + -area 5 because of a smaller band gap in comparison with the p-n junction between the p + -area 5 and the n-area 3. The preferred embodiment is different from the comparative example in that the hole recombination current I B2 is given by multiplying the equation (3) with a factor exp(-ΔE/kT), and that the diffusion current I B3 is also given by multiplying the equation (4) with a factor exp(-ΔE/kT). An increase in the current amplification factor h FE is attained by these features. What is essentially important in the present embodiment is the confinement of the collector current in the vicinity of the surface, and, for this reason, the area 4 may be made shallower than the areas 5,5'. Other structures and functions are the same as those in the foregoing comparative example, and will not, therefore, be explained further. Also in the foregoing embodiment, the contact of the base area may be composed of Si-Ge mixed crystal, and the contact resistances of the areas can be similarly reduced. As explained in detail in the foregoing, the present invention can provide a semiconductor device including a lateral 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 utilised. 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 embodiment has 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 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) which has a first portion (3) located immediately beneath the underside of, and having a respective interface with, said emitter and collector regions (5,5'), and which has a second portion (4) located only in-between said emitter and collector regions (5,5'); wherein the energy band gap of said second portion (4) of said base region (3,4) is narrower than the energy band gap of said first portion (3) of said base region (3,4); said lateral bipolar transistor being characterised in that: the energy band gap of said emitter region (5) and the energy band gap of said collector region (5'), each at said respective interfaces with said first portion (3) of said base region (3,4), and the energy band gap of said first portion (3) of said base region (3,4), are substantially the same; and the energy band gap of said emitter region (5), and the energy band gap of said collector region (5') each at and over said respective interfaces with said first portion (3) of said base region (3,4), have substantially no variation in the vertical direction of said emitter and collector regions (5,5'). A transistor according to claim 1, wherein said first portion (3) of said base region (3,4) is of Si, the material of said emitter and collector regions (5,5') at and over said respective interfaces is of Si, and the region (4) comprising said second portion (4) of said base region (3,4) is of mixed crystal silicon-germanium Si 1-x Ge x . A transistor according to either preceding claim wherein said second portion (4) of said base region (3,4) has a depth less than that of said emitter and collector regions (5,5'). A transistor according to claim 2 wherein said base region (3,4) is in contact with a contact region composed of mixed crystal silicon-germanium.

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