JPH0658914B2 - Heterojunction bipolar transistor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a heterojunction bipolar transistor.

[Conventional technology]

Nowadays, as a semiconductor material used for bipolar transistors, it is attracting attention that a compound semiconductor having high electron mobility such as GaAs is applied instead of conventional silicon. Particularly, in recent years, due to remarkable progress and development of manufacturing techniques such as molecular beam epitaxial (MBE) method and metalorganic CVD (MOCVD) method, a heterojunction bipolar transistor with good characteristics can be produced, which is currently the mainstream. It has become very promising as a next-generation high-frequency / high-speed device that will replace GaAs FETs and HEMTs.

The most important feature of such a heterojunction bipolar transistor is that by using a semiconducting material having a wider forbidden band width than the base layer for the emitter layer, minority carriers are transferred from the base layer to the emitter layer even when the concentration of the base layer is increased. That is, the injection efficiency of the emitter can be suppressed and the emitter injection efficiency can be kept large.

FIG. 3 is a band structure diagram of a first example of a conventional heterojunction bipolar transistor.

The heterojunction bipolar transistor of this example has a P-type base layer 5'and a base layer 5'on an N-type collector layer 3 '.
It has a structure in which an N-type emitter layer 6'having a smaller electron affinity and a wider forbidden band is sequentially laminated, and the electron affinity is increased at the step-type heterojunction portion between the base layer 5'and the emitter layer 6 '. An energy discontinuity corresponding to the difference occurs in the conduction band. As a result, the electrons 9'injected from the emitter layer 6'into the base layer 5'perform so-called ballistic flight by the initial kinetic energy corresponding to the energy difference, as shown schematically in FIG. Conducts faster than diffusion. Therefore, due to the high-speed conduction by this so-called ballistic flight, the base transit time of minority carriers (electrons in this case) in the base layer 5'is shortened, and a heterojunction bipolar transistor excellent in high-speed and high-frequency performance can be realized.

However, the distance in which this so-called ballistic flight is effective is at most about the mean free path of electrons, so that the conduction of electrons beyond that distance becomes low-speed conduction depending on the ordinary diffusion mechanism, and the base layer 5 ' However, in a heterojunction bipolar transistor that is usually very heavily doped, the recombination probability of electrons during traveling is high, which in turn lowers the base transmittance, impedes high-speed and high-frequency performance, and dramatically improves performance. I can't hope too much.

FIG. 4 is a band structure diagram of a second example of the conventional heterojunction bipolar transistor.

The heterojunction bipolar transistor of this example has an N-type collector layer 3 ″ having an N-type collector layer 3 ″ having a crystal composition changed to a P-type graded bandgap base layer 5 ″ and a wider forbidden band width than the base layer 5 ″. Electrons 9 ″ injected from the emitter layer 5 ″ by an internal electric field based on the conduction band of the inclined base layer 5 ″, which has a structure in which the emitter layer 6 ″ is sequentially stacked.
Is accelerated as shown by arrow 10 ″, and faster conduction than diffusion is expected.

However, when the internal electric field increases and exceeds a certain value, valley scattering becomes remarkable, and the average electron mobility decreases. For example, when the base layer 5 ″ made of Al _x Ga _1-x As has a layer thickness of 1500 Å and x: 0.15 → 0, the internal electric field becomes about 10 kV / cm, which is a condition that valley scattering easily occurs.

Although valley scattering also occurs in the case of the first example, the effect of valley scattering is small unless the energy gap due to the difference in electron affinity of the heterojunction portion is too large.

[Problems to be solved by the invention]

As described above, in the first example of the conventional heterojunction bipolar transistor, high-speed electron conduction based on so-called ballistic flight utilizing energy discontinuity of the heterojunction portion due to the difference in electron affinity between the base layer and the emitter layer. However, the effective distance of this ballistic conduction is at most about the mean free path of electrons, and the conduction of the base layer after that follows the normal diffusion mechanism. When the layer thickness is increased or the impurity concentration is increased to reduce the base resistance, the electron conduction speed in the base layer is decreased and the electron recombination rate is increased due to the increase in the recombination rate, resulting in a decrease in the speed of the base layer. There is a drawback that high frequency performance is impaired.

Further, in the second example, by adopting the base layer having the graded band gap, the internal electric field based on the inclined conduction band of the base layer accelerates the electrons to improve the high speed performance. If the intensity of exceeds a certain value, valley scattering becomes significant and the average electron mobility decreases, so there is a limit to the improvement of high-speed and high-frequency performance.

It is an object of the present invention to provide a low base resistance heterojunction bipolar transistor which has a low recombination probability of minority carriers in the base layer and a high conduction speed, which is further excellent in high-speed and high-frequency performance.

[Means for solving problems]

The heterojunction bipolar transistor of the present invention is a heterojunction bipolar transistor in which the emitter / base junction is a step type, the emitter has a wider forbidden band than the base, the emitter layer has a uniform impurity distribution, and the base layer is The side close to the emitter layer is a high-concentration layer, the side close to the collector is a low-concentration layer, and the high-concentration layer has a thickness substantially equal to the mean free path of minority carriers.

[Action]

According to such a heterojunction bipolar transistor of the present invention, the electrons injected from the emitter conduct high-speed conduction in a high-concentration base layer having a thickness of about mean free path by ballistic flight, and then, in the low-concentration base layer. By allowing the collector to travel relatively slowly by an ordinary diffusion mechanism, it is possible to prevent the average conduction velocity of carriers in the base layer from decreasing and to suppress the increase in recombination probability, resulting in a low base resistance and A high-performance heterojunction bipolar transistor with high injection efficiency can be realized.

〔Example〕

 FIG. 1 is a sectional view of an embodiment of the present invention.

In this embodiment, the semi-insulating substrate 1 is partitioned by an insulating region 1a having a predetermined pattern formed by ion implantation of protons on the surface of the semi-insulating substrate 1 and the dopant concentration is Si and the impurity concentration is 3 × 1.
0 ¹⁸ atom / cm ³ thickness at consists n ⁺ -GaAs layer of 4000Å high concentration layer 2 was formed by the MBE method, the high concentration layer 2 impurity concentration of Si dopants on the 5 × 10 ¹⁶ atom / cm
^The collector layer 3 is an n ⁻ -GaAs layer having a layer thickness of 5000 Å and the dopant concentration is Be and the impurity concentration is 1 × 10 ¹⁸ atom
/ cm ³ with a layer thickness of 1000Å, a low-concentration base layer 4 consisting of a p-GaAs layer and Be as a dopant with an impurity concentration of 1 × 1
A high-concentration base layer 5 consisting of a p ⁺ -GaAs layer having a layer thickness of 0 ²⁰ atom / cm ³ and a mean free path distance of electrons of approximately 500Å is sequentially formed by the MBE method, and a dopant is formed on the high-concentration base layer 5. Impurity concentration as Si is 3 × 10 ¹⁷ atom / cm
³ , the emitter layer 6 consisting of an n-Al _0.3 Ga _0.7 As layer with a layer thickness of 2000Å and Si as a dopant and an impurity concentration of 5 ×
After the high-concentration layer 7 composed of an n ⁺ -GaAs layer having a layer thickness of 10 ¹⁸ atom / cm ³ and 2000 Å is sequentially formed by the MBE method, each layer from the high-concentration layer 7 to the collector layer 3 is sequentially etched into a predetermined pattern A collector and an emitter and base electrodes 8c, 8e and 8b are formed on the high concentration layers 2 and 7 and the high concentration base layer 5, respectively.

FIG. 2 is a band structure diagram of an embodiment of the present invention.

In the band structure of this embodiment, since the high-concentration base layer 5 is doped much higher than the emitter layer 6, the influence of the depletion layer generated in the high-concentration base layer 5 can be neglected. Energy discontinuity δEc of the conduction band due to the staircase heterojunction with layer 5 is about 0.1 eV
The electrons 9a injected from the emitter layer 6 are generated by δ
Due to the initial kinetic energy equivalent to Ec, the high-concentration base layer 5 having a layer thickness almost equal to the mean free path distance is conducted by so-called ballistic flight to lose its kinetic energy, and thereafter the low-concentration base layer 4 travels by diffusion. Reach the collector layer 3.

By the way, empirically, the recombination lifetime τ _{n in} GaAs doped with P-type impurities is Can be expressed as Here, N _T is a P-type impurity concentration, and N _ref is a reference concentration for experiments. That is, the recombination lifetime τ _{n of} electrons increases as the impurity concentration N _T decreases.

Therefore, as in this embodiment, the electrons injected from the emitter layer 6 are conducted at high speed in the high-concentration base layer 5 having a high recombination probability by ballistic flight, and the low-concentration base layer 4 having a low recombination probability. Since the inside is conducted by normal diffusion running, the increase in recombination probability during diffusion running is suppressed to improve the injection efficiency, and the high-concentration base layer 5 reduces the base resistance to further improve the high-speed / high-frequency performance. A bipolar transistor can be realized.

However, in this embodiment, AlGaAs and GaAs which are lattice-matched to each other were used as the semiconductor material, but the invention is not particularly limited to this, and any material having a difference in electron affinity may be used.
Alternatively, a material having a lattice mismatch may be used.

〔The invention's effect〕

As described above, in the present invention, the base layer is composed of the low-concentration base layer and the high-concentration base layer whose layer thickness is about the mean free line of the minority carrier. This has the effect of realizing a heterojunction bipolar transistor with reduced recombination probability and improved high-speed / high-frequency performance.

[Brief description of drawings]

1 and 2 are cross-sectional views and band structure diagrams of one embodiment of the present invention, and FIGS. 3 and 4 are band structure diagrams of first and second examples of conventional heterojunction bipolar transistors, respectively. Is. 1 ... Semi-insulating substrate, 1a ... Insulating region, 2 ... High concentration layer, 3, 3 ', 3 "... Collector layer, 4 ... Low concentration base layer, 5 ... High concentration base layer, 5 ′, 5 ″ ... Base layer, 6, 6 ′, 6 ″ ... Emitter layer, 7 ... High concentration layer,
8b ... Base electrode, 8c ... Collector electrode, 8e ...
Emitter electrodes, 9a, 9b, 9 ', 9 "... Electrons, 1
1, 12, 13 ... Fermi level.

Claims (1)

[Claims] 1. In a heterojunction bipolar transistor in which the emitter-base junction has a stepped structure and the emitter has a wider band gap than the base, the emitter layer has a uniform impurity distribution, and the base layer has the emitter layer. A heterojunction bipolar transistor characterized in that the side close to is a high-concentration layer, the side close to the collector is a low-concentration layer, and the thickness of the high-concentration layer is approximately equal to the mean free path of minority carriers.