HETERO-JUNCTION BIPOLAR TRANSISTOR

Abstract: The transistor comprises an emitter (29), a base (27) and a collector (31) has at least one quantum well (32) in the base region. The width of the well is determined so that the energy difference between one of the quantum levels in the quantum well and the barrier layer (33) is smaller than kT/2, where k is Boltzmann constant and T is the absolute temperature. Since holes (28) are confined in the wells (32), the emitter injection efficiency is improved and the base running time and the base resistance are decreased without reducing the collector injection efficiency.

Description: Technical Field The present invention relates to a high-speed and high-gain transistor and, more particularly, to a hetero-junction bipolar transistor having a heterojunction. Background Art As a device having performance exceeding the limits of silicon bipolar transistors, there is a hetero-junction bipolar transistor (hereinafter referred to as "HBT"). The HBT has, as will be clear from FIG. 1 which shows the energy band diagram thereof, a hetero-junction 4 which is formed between an emitter region 2 and a base region 1 to thereby make the energy bandgap of the emitter wider than that of the base. In the figure, 3 denotes a collector region. Attempts have been vigorously made to realize HBTs using compound semiconductors, and it has become possible to realize HBTs as a result, particularly, of the recent progress in epitaxial growth technology or the like. The feature of HBTs resides in that, since there is substantially no injection of minority carriers from the base into the emitter, it is possible to increase the impurity concentration in the base while maintaining the emitter injection efficiency at a high level. Therefore, it is possible to narrow the base width and lower the internal base resistance. As a result, it is possible to improve the current gain and cut-off frequency of the transistor as compared with the conventional bipolar transistors. As one technique used to further improve various characteristics of the above-described HBT, for example, a proposition made by Ito et al. in Extended Abstracts (The 46th Autumn Meeting, 1985) 3a-B-9; The Japan Society of Applied Physics may be cited. It is described in this literature that it is possible to reduce the transit time of electrons and increase the current gain by sloping the Aℓ composition in the base layer of an AℓGaAs/GaAs HBT. However, in the above-described prior art, the base layer is a heavily-doped p-type layer and therefore it has the problem that the hole mobility is small. Accordingly, lowering in the base resistance is limited, and this leads to restrictions on the high-frequency characteristics. It is described in Japanese Patent Laid-Open No. 60-10775 that the base resistance is reduced by forming a base region having a superlattice structure and thus shortening the switching time and improving the high-frequency characteristics. This prior art suffers, however, from the problem that the collector injection efficiency is lowered as a result of recombination of carriers inside the base region. JP-A-61-229361 discloses a hetero-junction bipolar transistor with the features included in the first part of Claim 1. The known transistor is so designed that it has a negative resistance characteristic, which is achieved by interaction (i.e. resonant tunnelling) between adjacent GaAs well layers formed in a superlattice base region. Summary of the Invention It is an object of the invention to provide hetero-junction by polar transistor in which the ermitter injection efficiency is improved, and the base transit time and base resistance are reduced without lowering the collector injection efficiency. This object is met by the hetero-junction bipolar transistor defined in Claim 1. It is known that, in a double-hetero structure of semiconductors, if the thickness of a semiconductor layer having a relatively narrow energy bandgap which is sandwiched between two layers is reduced, a quantum size effect is produced. Such a structure is known as a quantum well which is formed from a semiconductor having a narrow energy bandgap which constitutes a well layer and a semiconductor barrier layer having a wide energy bandgap which is adjacent to the well layer. In other words, in this quantum well the electron energy is no longer continuous but split. More specifically, assuming that the well width is Lw, the quantum level ΔEn measured from the bottom of the energy band may be approximated as follows: ΔEn = (²/2m*) (nπ/Lw)² In the above formula, n is an integer, and the depth of the well is approximated to be sufficiently deep. Examination of the light-emitting efficiency from the quantum well in a GaAs/AℓGaAs light-emitting device has found that the light-emitting efficiency is periodically changed as shown in FIG. 3 by changing the well width. This shows that trapping of carriers formed in the barrier layer (AℓGaAs) into the quantum well depends on the well width. It has been found that the trapping of carriers reaches a minimum value when the quantum level ΔEn of the uppermost electrons in the quantum well is substantially coincident (about ±kT/2) with the conduction band energy level, hereinafter named "the energy", in the barrier layer. This condition is referred to as resonant condition. The present invention applies to an HBT the fact that the above-described carrier trapping efficiency depends on the quantum well width to a substantial extent. By making the quantum level ΔEn and the energy in the barrier layer coincident with each other within about ±kT/2, various characteristics of the HBT are improved as follows. FIG. 2 shows the energy band diagram of the HBT according to the present invention. The base region 27 has a quantum well 32 formed therein. First, as to the emitter injection efficiency, holes 28 are two-dimensionally trapped in the quantum well 32 and thus prevented from being injected into the emitter region 29, so that the emitter injection efficiency is improved. In the case where holes are needed in the base at a high density, it suffices to dope an acceptor into the barrier layer 33 by modulation dope. Next, as to the base transit time and the base resistance, since electrons 30 which are injected into the base region 27 from the emitter region 29 reach ballistically the collector region 31 without being trapped in the quantum well 32, the base transit time can be reduced without lowering in the collector injection efficiency which would otherwise be caused by recombination taking place in the base. In particular, when the above-described modulation dope is effected with respect to the barrier layer, holes in the base layer have mobility which is about 4 times as high as that of holes in the conventional base layer, so that the base resistance can be lowered by a large margin and it is therefore possible to reduce the charge and discharge time in relation to the base layer. Although it has been mentioned in the foregoing description that the quantum level and the energy in the barrier layer are made coincident with each other within a range of about ±kT/2, this is due to the following reasons. When the HBT is operated, a voltage is, of course, applied between electrodes, and since the application of a voltage causes a change in potential of the quantum well, the resonant condition also changes, although the extent to which it changes is slight. In the case where a quantum well is formed in the base region, there are unavoidably some variations in the thickness. However, it is a matter of course that electrons which are injected from the emitter also have an energy distribution, which is estimated to be about ±kT/2. Accordingly, in order that almost all the injected electrons are in a substantially resonant condition, it is necessary to make the quantum level and the energy in the barrier layer coincident with each other within a range of about ±kT/2. A plurality of quantum wells may be provided. In this case, the above-described relationship between the quantum well and the energy in the barrier layer must be satisfied for each of the quantum wells. Each pair of adjacent quantum wells are spaced apart from each other at such a distance that there is no interaction between the quantum wells. This is because any interaction would change the quantum level by a large margin, resulting in a change in the carrier trapping conditions considered in relation to a single well. The distance at which no interaction occurs is, specifically, a distance which is longer than the de Broglie wavelength of electrons. It should be noted that, when the present invention is carried out, it is preferable to use a direct bandgap semiconductor as a semiconductor constituting the barrier layer. Brief Description of Drawings FIG. 1 shows the energy band diagram of a conventional hetero-junction bipolar transistor; FIG. 2 shows the energy band diagram of the hetero-junction bipolar transistor according to the present invention; FIG. 3 shows the relationship between the quantum well width and the light-emitting efficiency; FIG. 4 shows a cross-sectional structure of a hetero-junction bipolar transistor according to one embodiment of the present invention; and FIG. 5 is an enlarged view of the base layer portion of the hetero-junction bipolar transistor shown in FIG. 4. Best Mode for Carrying Out the Invention One embodiment of the present invention will be described hereinunder. In this embodiment, Si-doped Aℓ 0.3 Ga 0.7 As was employed as an emitter layer 15, while Si-doped Aℓ 0.2 Ga 0.8 As was employed as a collector layer 13, and undoped GaAs and Be-doped Aℓ 0.2 Ga 0.8 As were respectively employed as a quantum well layer 21 and a barrier layer 22 constituting in combination a base layer 14. Four quantum wells were provided. In the quantum well structure of the base layer, when GaAs and Aℓ x Ga 1-x As are employed as the quantum well layer 21 and the barrier layer 22, respectively, if the relationship between the composition ratio x of Aℓ in Aℓ x Ga 1-x As and the width of the quantum well layer 21 is within the following range, the quantum level and the energy in the barrier layer 22 coincide with each other within a range of ±kT/2: When x = 0.1 ±0.05, the width of the quantum well layer is from 6.0 to 8.8 nm, from 10.8 to 13.8 nm, or from 15.8 to 20.0 nm, When x = 0.2±0.05, the width of the quantum well layer is from 5.3 to 7.7 nm, from 9.7 to 12.1 nm from 14.1 to 16.5 nm, or from 18.5 to 22.0 nm, When x = 0.3±0.05, the width of the quantum well layer is from 4.0 to 7.0 nm, from 8.5 to 11.5 nm, or from 13.0 to 17.0 nm, In this embodiment, since the barrier layer in the quantum well structure of the base layer was made of Aℓ 0.2 Ga 0.8 As (i.e., x = 0.2), the width of the quantum well was set at 6.5 nm in the above-described range. FIG. 4 is a sectional view of an HBT according to one embodiment of the present invention. This HBT was formed as follows. On a semi-insulating GaAs substrate 11 are successively grown by molecular beam epitaxy an n⁺-Sidoped GaAs buffer layer 12 (1.0 µm, 3 x 10¹⁸ cm⁻³), a collector layer 13 (Si-doped Aℓ 0.2 Ga 0.8 As, 0.25 µm, 2 x 10¹⁶ cm⁻³), a Be-modulation doped quantum well base layer 14, an emitter layer 15 (Si-doped Aℓ 0.3 Ga 0.7 As, 0.2 µm, 2 x 10¹⁷ cm⁻³) and an n⁺-Si-doped GaAs contact layer 16 (0.03 µm, 3 x 10¹⁸ cm⁻³). The above-described buffer layer 12 is usually formed on the substrate for the purpose of improving characteristics of the HBT and therefore it may be omitted. The base layer 14 is, as shown in FIG. 5 which is an enlarged view, formed by alternately stacking four undoped GaAs layers 21 (65 nm) and three p⁺-Aℓ 0.2 Ga 0.8 As layers 22 (Be-doped, 2 x 10¹⁹ cm⁻³, 20.0 nm), the GaAs quantum well layers and AℓGaAs barrier layers forming in combination a quantum well structure. If the AℓGaAs barrier layers have a thickness such as that described above, it is possible to ignore the interaction caused by electrons between each pair of adjacent quantum wells. Next, at the base electrode forming position only, diffusion of Zn is carried out to a depth of about 0.25 to 0.4 µm using as a mask an Si₃N₄ film of 300 nm which is formed by plasma CVD and then patterned. Next, mesa-etching is carried out until the n⁺-GaAs layer 12 is exposed, and Au/Ge/Ni is evaporated on all of the collector, base and emitter regions and then alloyed at 420°C for 20 seconds. Thereafter, the base and the emitter are subjected to recess-etching to reach completion. Measurement of characteristics of the device of the present invention found that the maximum value of the differential DC gain was 1200 when the collector current density was 1 kA/cm², the measured value being the highest among those in the existing HBTs. The cut-off frequency was 12 GHz; thus, it was possible to achieve a substantial improvement over the heretofore maximum value, i.e., 11 GHz. Further, since the mobility of holes in the base layer can be increased to about 4 times that in the case of the conventional HBTs, it is possible to lower the base resistance to about 1/4 for the same impurity concentration. The two-dimensional trapping of holes into the quantum well enables the hole current injected from the base to the emitter to be reduced to 1/10 or less of that in the prior art and therefore permits an improvement in the emitter injection efficiency. Since lowering in the base transport factor as a result of the provision of the quantum well is 1 to 2%, the current gain is increased by about 10 times that of the prior art. It should be noted that, although in the above-described embodiment GaAs/AℓGaAs is employed to form a quantum well structure, other materials, for example, InAℓAs/InGaAs or the like, may be employed to obtain similar effects by selecting a quantum well width so that the quantum level and the energy in the barrier layer are coincident with each other within a range of ±kT/2.

Claims: A hetero-junction bipolar transistor having at least one hetero-junction (34) and including an emitter region (15, 29), a base region (14, 27) and a collector region (13, 31), said bipolar transistor comprising a plurality of quantum wells (32) in said base region, each quantum well being formed from a well layer (21) defined by a semiconductor layer having a relatively small energy bandgap and a barrier layer (22) defined by a semiconductor layer having a relatively large energy bandgap and formed between adjacent well layers, characterized in that the width of each quantum well and the difference between the respective energy bandgaps of a well layer and a barrier layer are determined so that the energy difference between one of the quantum levels formed in a quantum well and the conduction band of an adjacent barrier layer is within kT/2 - where k is the Boltzmann constant and T is the absolute temperature - and that adjacent quantum wells (32) are spaced apart from each other at such a distance that no interaction occurs between the quantum wells, said distance being greater than the deBroglie wavelength. The transistor of claim 1, wherein said emitter region (15, 29) is defined by a semiconductor region having a higher conduction band energy level than said barrier (33). The transistor of claim 1 or 2, wherein said barrier layer (22) has an impurity selectively doped therein. The transistor of claim 1 of 2, wherein said barrier layer (22) is made of a direct bandgap semiconductor. The transistor of claim 4, wherein said well layer (21) is made of GaAs and said barrier layer (22) is made of Aℓ x Ga 1-x As.

Inventors: KASAI, JUNICHI, KATAYAMA, YOSHIFUMI, MISHIMA, TOMOYOSHI, MURAYAMA, YOSHIMASA, SHIRAKI, YASUHIRO
Applicants: HITACHI, LTD.
Application year: 1987
Main classification: H01L29
IPC codes: H01L21, H01L29