Bipolar transistor

Abstract: A homojunction bipolar transistor having a superlattice base region (16) comprising alternate layers (18, 20) of extrinsic and intrinsic layers, with extrinsic layers (18) being of the opposite conductivity of the emitter (22) and collector layers (12, 14) of the transistor. The alternate extrinsic and intrinsic layers (18, 20) have substantially different doping levels providing abrupt transitions in the valence and conduction bands between layers. The abrupt transitions result in the energy band gap in the base region (16) being effectively reduced with respect to the band gap in the emitter region (22).

Description: This invention relates to wide band gap emitter transistors and more particularly, to transistors having a superlattice base region providing an effective narrow gap base. It is well known that transistors having a wide band gap emitter have important advantages in bipolar transistor technology. The wide band gap emitter in an npn transistor provides high emitter injection efficiency by blocking the movement of holes from the base to the emitter for the same electron injection from the emitter to the base. Consequently, it allows relatively lighter emitter doping and heavier base doping which results in reduced emitter capacitance and lower base resistance. At present, the emitter efficiency of homojunction bipolar transistors having equal energy band gaps in the emitter, base and collector, is controlled by the doping levels in the emitter and base. In the prior art devices, the emitter is heavily doped to increase the injection efficiency. However, the amount of doping and hence the efficiency is limited since too heavy a doping reduces the effective band gap of the emitter and permits holes to easily move from the base to the emitter. Also in the prior art devices, the base resistance is reduced by heavily doping the base. However, this increases the inverse injection efficiency. The reduction in base resistance thus results in an increase in base currents. Another type of wide band gap emitter in the prior art is shown in U.S. Patent 4,119,994 directed to a heterojunction device formed with GaAs having a wide energy band gap for the emitter layer and using Ge having a narrow band gap for the base and collector layers. Alternatively, one can also use AlGeAs and GaAs as the emitter and the base, respectively. Another approach in the prior art is to fabricate the transistor having the base region formed from a superlattice. As shown in U.S. Patent 4,137,542, the superlattice is formed with alternate layers of different material, such as, GaAs and GaAlAs. Another example of the superlattice is disclosed in Surface Science 113(1982) 479-480 disclosing a periodic sequence of n and p doped GaAs layers with intrinsic layers inbetween. The use of a superlattice in a wide band gap emitter transistor is disclosed in Japanese Patent 58-114455 which discloses a superlattice base being formed of alternate layers of p+ doped GaAsSb and p- doped GaInAs sandwiched between emitter and collector layers of n doped GaInAs. The present invention is directed to a homojunction bipolar transistor comprising a semiconductor body having an emitter and a collector region of a first conductivity type separated by a superlattice base region. The superlattice is made of a plurality of alternate layers of an extrinsic layer of a second conductivity type and an intrinsic layer of the same semiconductor material as the extrinsic layer. The bipolar transistor, being a homojunction device, is fabricated with the emitter, base and collector regions also each being made of the same semiconductor material. The superlattice has abrupt transitions between the alternate layers such that the effective band gap of the semiconductor in the base region is reduced with respect to the band gap of the semiconductor in at least the emitter region. The bipolar transistor of the invention may be fabricated with any semiconductor material such as silicon, germanium, or gallium arsenide. The homojunction transistor of the invention provides a wide band gap emitter that is much easier to fabricate than the heterojunction devices of the prior art. In addition, the present invention contemplates the use of monocrystalline silicon and germanium so that the wide band gap emitter may be realized in conventional silicon and germanium bipolar transistor technology. The abrupt transitions in the base region superlattice structure of the present invention are formed with alternate layers of the same semiconductor material having substantially different doping levels. The base region is made of a plurality of pairs of alternate layers of heavily doped extrinsic semiconductor material and intrinsic or undoped semiconductor material. The thickness of each pair of doped and undoped layers, referred to as the period of the superlattice, is sufficiently small so that there is a resonant state for both electrons and holes across the base region. If the relative doping of the superlattice is sufficiently large, there will be large band bending which reduces the energy spacing of the resonant states between the conduction and valence bands. Thus, the effective band gap of the base region is narrowed. In addition, a novel method of implementing the bipolar transistor of the invention is also disclosed. A semiconductor body is having a collector layer of a first conductivity type and a superlattice region contiguous to the collector layer. The superlattice is formed as described above with a plurality of pairs of alternate layers of an extrinsic layer of a second conductivity type and an intrinsic layer of the same semiconductor material as the collector layer. A central lengthwise portion of several upper layers of the superlattice is converted to the first conductivity type. The converted region forms the emitter of the bipolar transistor and the remaining unconverted layers of the superlattice form the base region. The number of layers converted to the opposite conductivity of the superlattice is determined by the desired thickness of the emitter and base regions. The emitter region is converted by heavily doping the upper layers of the superlattice with the opposite conductivity of the base. The level of dopant introduced into the central portion must be greater than the dopant level of the extrinsic layers of the superlattice. The heavy doping of these upper layers causes the region to be converted from a superlattice to a homogeneous structure having uniform energy bands. As a consequence, the energy band gap returns to the level of the bulk material. Side portions of the superlattice layers on either side of the emitter region are converted to homogeneous regions of the second conductivity type thereby forming a pair of ohmic contacts to the base region. The band gap of the base ohmic contacts also returns to that of the bulk material. Thus, the emitter region and the base ohmic contacts are formed merely by doping upper layers of the superlattice eliminating the need to grow additional layers. Fig. 1 is a cross-sectional schematic view of the bipolar effective narrow band gap base transistor of the present invention. Fig. 2 is an energy band diagram of the transistor of the present invention showing the band bending and effective narrowed base band gap. Fig. 3 is a cross-sectional schematic view of the bipolar effective narrow gap base transistor formed by the novel implementation method of the present invention. The present invention is directed to a homojunction bipolar transistor having an effective narrow band gap base so that the effective band gap in the base region is less than the band gap in the emitter region. The wide band gap emitter bipolar transistor of the invention is implemented in a semiconductor body in which the emitter, base and collector are all made of the same material, such as silicon, germanium or gallium arsenide. The band gap of the base is effectively narrowed by forming the base of a superlattice structure. The superlattice is made of a plurality of alternate layers of an extrinsic layer of heavily doped semiconductor material and an intrinsic layer of undoped semiconductor material. Referring now to the drawings, Fig. 1 shows a cross-sectional view of one implementation of the bipolar transistor of the present invention. The transistor 10 of Fig. 1 is shown illustratively as an npn transistor. It should be obvious to those skilled in the art that the implementation of the present invention may also be provided in a pnp transistor. The transistor 10 of Fig. 1 includes a collector region 12 heavily doped n+ and a transistion collector region 14 having moderate n doping. A superlattice base region 16 includes an plurality of alternate layers of an extrinsic layer 18 of p+ conductivity and intrinsic layers 20 of undoped semiconductor material. An emitter region 22 is provided contiguous to the base region 16 of n conductivity. The emitter region 22 may be lightly doped or heavily doped depending on the particular application of the transistor. Heavily doped p+ ohmic contacts 24 and 26 are grown on the base region 16. In the base region 16 each pair of contiguous extrinsic and intrinsic layers 18, 20 forms a period of the superlattice. If the period of the superlattice is sufficiently small, resonant states will result for both electrons and holes across the base region. The range of thickness for each of the layers 18 and 20 has been found to be from 5 to 20 nm. In practice, the typical thickness for the period of the superlattice is 100 nm. Furthermore, if the relative doping of the superlattice is sufficiently large, there will be large band bending which reduces the energy spacing of the resonant states between the conduction and valence bands. Fig. 2 is a schematic energy band diagram for the npn transistor implementation. The conduction energy band is indicated at Ec and the valence energy band is indicated at Ev. The energy gap in the emitter and collector regions is shown as Eg. The ripple in the conduction band and valence band energy levels in the base region is due to the periodically varying doping in the region 16 of Fig. 1. The p+ doped regions are shown with the more positive energy peak and the intrinsic or undoped regions have the less positive energy peak. The lowest resonant states in the conduction band is shown as Ec' and the lowest resonant states in the valence band is shown as Ev'. The effective band gap Eg' is equal to Ec'-Ev'. The effective base band gap Eg' is less than the energy gap Eg for the emitter and collector. In the preferred embodiment, the p+ region of the superlattice will be heavily doped to provide abrupt transitions between the extrinsic and intrinsic layers. The p doping may be in the range of 10¹⁷ cm⁻³ to 10²⁰ cm⁻³ with the optimal amount being 10²⁰. In the fabrication of the superlattice, uniformity of doping levels and thickness of the regions is desirable to provide the maximum band gap reduction. In the preferred embodiment, the thickness of each of the extrinsic layers is substantially the same, and the thickness of each of intrinsic layers is also substantially the same to provide uniform periodic band bending. There is no requirement that the thickness of the extrinsic layers be equal to the thickness of the intrinsic layers. The relative thickness of the extrinsic layers with respect to the intrinsic layers is a design parameter determined by the particular application. Generally, forming the superlattice with thicker intrinsic layers than extrinsic layers will result in a larger band gap reduction. Other considerations include the degree of coupling of the resonant states and the amount of doping. In one embodiment of the invention the device is fabricated with all layers of the supperlattice having the same thickness. The reduction in the energy gap will be at least a tenth of an electron volt and, depending on the amount of doping and size of the layers, may be as much as several tenths of an electron volt. In principle, the maximum energy gap reduction will be slightly smaller than half of the band gap of the bulk material utilized for the semiconductor. Thus, by way of example, the nominal energy gap for silicon is 1.1 eV and the range to which the band gap may be reduced by the present invention is from 1.0 eV to 0.6 eV. In fabricating the effective narrow band gap base transistor of the present invention, standard semiconductor techniques may be used such as, molecular beam epitaxy and chemical vapor deposition. In addition, conventional diffusion techniques may be used for doping the emitter base and collector. However, it is preferred that ion implantation be used because of its low temperature environment to maintain the abruptness of the superlattice layers. The formation of the superlattice for the base of the present invention may be made in accordance with the chemical vapor deposition process described in copending U.S. application Serial No. 906,854 by B. S. Meyerson, a co-inventor herein, which application is incorporated herein by reference. Briefly, the aforementioned Meyerson process permits in-situ CVD doping of silicon epilayers at temperatures below 550° C. The process is especially useful for forming thin, high quality monocrystalline silicon layers on a substrate. The ability to dope the layers in-situ to any desired level allows the superlattice of the invention to be fabricated in a single CVD process. Referring now to Fig. 3, a novel implementation of the effective narrow band gap base transistor of the present invention is schematically shown therein. The structure of Fig. 3 includes collector regions 32 and 34 grown by any known technique. Superlattice region 36 is formed on layer 34 by, for example, the process described in the aforementioned Meyerson application. The superlattice includes alternate p+ doped layers 35 and undoped layers 37. An emitter region is formed in the superlattice region by converting a lengthwise central portion 38 of several of the upper layers 39 of the superlattice to the opposite conductivity of the extrinsic layers 35 of the superlattice. The emitter region is converted by heavily doping the region 38 by, for example, ion implantation. In order to convert to region 38 to n+ from p+, the amount of doping introduced into region 38 must be higher than the doping level of layers 35. Thus, if layers 35 have a doping level of about 10¹⁷ cm⁻³, then region 18 should have a doping level of above 10¹⁸ cm⁻³. Preferably, a portion 40 of the region 38 is heavily doped n+ and side portions 42 and 44 are moderately n doped to act as emitter-base isolation spacer layers. The remaining layers of the superlattice in region 46 then define the base of the transistor of the invention. The side portions 48 and 50 on either side of the emitter region 38 are converted to p+ regions to act as ohmic contacts to the base. The doping of the upper layers 39 of the superlattice removes the superlattice nature of those layers and converts those layers into a homogeneous region. The energy bands of the homogeneous regions become uniform and the band gap returns to the original band gap of the bulk material. The transistor is fabricated by forming the emitter region within the superlattice eliminating the need to grow additional emitter and ohmic contact regions on the superlattice. The effective narrow band gap base transistor of the present invention has an equivalent wide band gap emitter having a higher emitter efficiency. The transistor allows for the possibility of having a lightly doped emitter thereby providing a small emitter to base capacitance. This can be realized since there is no longer a need to heavily dope the emitter to prevent holes from moving from the base to the emitter. In addition, the heavy doping in the periodically doped base region reduces base resistance. The transistor of the present invention also provides two dimensional (2D) confinement in the base region that lowers the lateral base resistance. The 2D confinement results in carriers being essentially confined to flow laterally in the individual layers. Other advantages of the transistor of the present invention are realized in the long minority lifetime due to the spatial separation of the majority and minority carriers. In addition, the device eliminates current crowding. Moreover, since the base region can be fabricated with heavily doped P⁺IP⁺I... superlattice, it can thus be operated at low temperatures because there would be no carrier freeze-out effect. The transistor performance and packing density can therefore be improved.

Claims: A bipolar transistor comprising: a homojunction semiconductor body having; an emitter (22, 38) and collector region (12, 14; 32, 34) of a first conductivity type separated by a base region (16, 46) having a plurality of alternate layers of an extrinsic layer (18, 35) of a second conductivity type and an intrinsic layer (20, 37), each of said emitter, base and collector regions being made of the same semiconductor material; whereby the effective band gap of said semiconductor in said base region (16, 46) is reduced with respect to the band gap of said semiconductor at least in said emitter region (22, 38). The transistor of claim 1 wherein the base, collector and emitter semiconductor material is one of silicon, germanium and gallium arsenide. The transistor of claim 1 or 2 wherein the effective band gap of the base region is reduced by 0.1 eV or greater. The transistor of claim 1 or 2 wherein the effective band gap of the base region is reduced in the range between 0.1 eV to one-half the band gap of the bulk semiconductor material. The transistor of at least one of the claims 1 to 4 wherein thickness of the alternate layers of extrinsic and intrinsic layers is in the range between 5 to 20 nm. The transistor of claim 5 wherein thickness of the alternate layers is about 5 nm. The transistor of claim 5 wherein thickness of each of the extrinsic and intrinsic layers is substantially equal. The transistor in claim 5 wherein each of the extrinsic layers have a first thickness and each of the intrinsic layers have a second thickness different from said first thickness. The transistor of at least one of the claims 1 to 8 wherein the alternate layers of extrinsic and intrinsic layers have an abrupt transition in energy in both the conduction and valence bands. The transistor of claim 9 wherein the extrinsic layers of said second conductivity type have a doping level in the range between 10¹⁷ to 10²⁰ cm⁻³. A method of fabricating a bipolar transistor comprising: providing a homojunction semiconductor body having a collector layer (32, 34) of a first conductivity type and a superlattice region (36) contiguous to said collector layer, said superlattice region having a plurality of alternate layers of an extrinsic layer (35) of a second conductivity type and an intrinsic layer (37); converting a lengthwise central portion of a plurality of upper layers of said superlattice region (36) to a homogeneous region of said first conductivity type to form an emitter region (39), the remaining layers of said superlattice region (36) forming a base region (46); converting side portions of said plurality of upper layers on either side of said emitter region (38) to a homogeneous region (48, 50) of said second conductivity type thereby forming a pair of contacts to said base region (36). The method of claim 11 wherein the central portion (38) and the side portions (48, 50) of the upper layers of the superlattice region are converted by heavily doping the central portion with the first conductivity type and heavily doping the side portions with the second conductivity type, the doping level of said central and side portions being greater than the doping level of the extrinsic layers (35) of said superlattice. The method of claim 11 or 12 wherein the central and side portions (38, 48, 50) are heavily doped by ion implantation. The method of at least one of the claims 11 to 13 further including forming spacers (42, 44) in said emitter region (38) by moderately doping side portions of said emitter region with said first conductivity type.

Inventors: BRODSKY MARC H, FANG FRANK F, MEYERSON BERNARD S, BRODSKY, MARC H., FANG, FRANK F., MEYERSON, BERNARD S.
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1989
Main classification: H01L29
IPC codes: H01L21, H01L29