JPS63318776A - Heterojunction bipolar transistor - Google Patents

Abstract

PURPOSE:To obtain a heterojunction bipolar transistor which is more improved in high speed and high frequency performance thereof so as to have a base layer of low resistance in which recombination probability of minority carrier is low and conduction speed thereof is high, by for the base layer being composed of a lamination structure consisting of a lightly doped layer and a heavily doped layer whose thickness is approximately equal to the mean free path of a carrier of one conductivity type. CONSTITUTION:A base layer of the opposite conductivity type formed on a collector layer 3 of one conductivity type is composed of a lamination structure consisting of a lightly doped layer 4 and a heavily doped layer 5 whose thickness is approximately equal to the mean free path of a carrier of one conductivity type. For example, on a heavily doped layer 2 of an n<+> type GaAs layer formed on the surface of a semi-insulating substrate 1, a collector layer 3 of an n-type GaAa layer, a lightly doped base layer 4 of a p-type GaAs layer with a Be density of 1X10<18>atm/cm<3> and with a thickness of 1000Angstrom , and a heavily doped base layer 5 of a p<+> type GaAs layer with a Be density of 1X10<20>atm/cm<3> and with a thickness of 500Angstrom which is approximately equal to the mean free path of an electron are formed one after another. Moreover, on the base layer consisting of the layers 4 and 5, an emitter layer 6 of an n-type Al0.3Ga0.7As layer.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a heterojunction bipolar transistor.

[Conventional technology]

BACKGROUND ART Today, compound semiconductors with high electron mobility, such as GaAs, are attracting attention as semiconductor materials used in bipolar transistors, instead of conventional silicon. In particular, in recent years, with the remarkable progress and development of manufacturing technologies such as molecular beam epitaxial (MBE) method and metal organic CVD (MOCVD) method, it has become possible to produce heterojunction bipolar transistors with good characteristics, which are now mainstream. It has become highly anticipated as a next-generation high-frequency, high-speed device to replace GaAsFETs and HEMTs.

The most important feature of such a heterojunction bipolar transistor is that the emitter layer is made of a semiconductor material with a wider bandgap than the base layer, so even if the base layer is highly doped, there is no trace of a small amount of energy from the base layer to the emitter layer. It is possible to suppress carrier injection and maintain high emitter injection efficiency.

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

The heterojunction bipolar transistor of this example includes a P-type base layer 5' and a base layer 5' on an N-type collector layer 3'.
The base layer 5 has a structure in which an N-type emitter layer 6' having a smaller electron affinity and a wider forbidden band width is sequentially laminated.
An energy discontinuity corresponding to the difference in electron affinity occurs in the conduction band at the stepped heterojunction between the emitter 96' and the emitter 96'. As a result, the electrons 9' injected from the emitter layer 6' to the base layer 5' perform a so-called paristic flight due to the initial kinetic energy corresponding to the energy difference, as schematically shown in FIG. The conduction is faster than that by diffusion. Therefore, high-speed conduction due to so-called paristic flight shortens the base transit time of minority carriers (electrons in this case) in the base layer 5', making it possible to realize a beterojunction bipolar transistor with excellent high-speed and high-frequency performance.

However, since the distance over which this so-called pallitic flight is effective is approximately the mean free path of the electron, the conduction of electrons beyond that distance becomes low-speed conduction that depends on the normal diffusion mechanism, and the base layer 5 In beterojunction bipolar transistors, which are usually doped with extremely high concentrations of becomes less hopeful.

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

The heterojunction bipolar transistor of this example has a P-type graded bandgap base layer 5'' with a different crystal composition on an N-type collector layer 3'' and an N-type with a wider forbidden band width than the base layer 5''. It has a structure in which emitter layers 6'' of
is expected to be accelerated as shown by arrow 10'', resulting in faster conduction than diffusion.

However, when the internal electric field becomes high and exceeds a certain value, valley scattering becomes noticeable and the average mobility of electrons decreases.For example, AI! ,, assuming that the thickness of the base layer 5'' made of Ga1-NAs is 1500 layers, x: 0.1
When 5→O, the internal electric field becomes approximately 10 kV/1, a condition that easily causes valley scattering.

Although valley scattering also occurs in the first example, the influence of valley scattering is small if the energy gap due to the difference in electron affinity at the heterojunction is not too large.

[Problem that the invention seeks to solve]

As mentioned above, in the first example of the conventional heterojunction bipolar transistor, the high-speed electron flight is based on so-called pallitic flight, which takes advantage of the energy discontinuity of the heterojunction due to the difference in electron affinity between the base layer and the emitter layer. We aim to further improve high-speed and high-frequency performance through conduction, but the effective distance for this pallitic conduction is at most the mean free path of an electron, and subsequent conduction in the base layer follows the normal diffusion mechanism. If an attempt is made to reduce the base resistance by increasing the thickness of the base layer or increasing the impurity concentration, the transmittance of the electrons in the base layer will decrease due to a decrease in the conduction velocity of electrons in the base layer and an increase in the rate of recombination.・There is a drawback that high frequency performance is impaired.

In addition, in the second example, by adopting a graded bandgap base layer, electrons are accelerated by the internal electric field based on the inclined conduction band of the base layer to improve high-speed performance, but the internal electric field When the intensity exceeds a certain value, valley scattering becomes noticeable and the average mobility of electrons decreases, so there is a limit to the improvement of high-speed/high-frequency performance.

An object of the present invention is to provide a low base resistance heterojunction bipolar transistor with a low probability of recombination of minority carriers in the base layer, high conduction speed, and even better high-speed and high-frequency performance.

[Means for solving problems]

The heterojunction bipolar transistor of the present invention has a base layer of an opposite conductivity type formed on a collector layer of a negative conductivity type, and a base layer formed on the base layer that has a smaller electron affinity and a wider forbidden band width. In a heterojunction bipolar transistor having an emitter layer of one conductivity type, the base layer is composed of a laminated layer of a lightly doped impurity layer and a highly doped impurity layer having a thickness approximately equal to the mean free path of carriers of one conductivity type.

[Effect]

According to such a heterojunction bipolar transistor of the present invention, electrons injected from the emitter conduct at high speed through a high concentration base layer with a thickness of about the mean free path, and then travel through a low concentration base layer through a paristic flight. By allowing carriers to travel relatively slowly to the collector using a normal diffusion mechanism, it is possible to prevent a decrease in the average conduction velocity of carriers in the base layer and to suppress an increase in the probability of recombination, resulting in a low base resistance. Moreover, 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 surface of a semi-insulating substrate 1 is partitioned by insulating regions 1a having a predetermined pattern formed by ion implantation of protons, and the dopant is Si and the impurity concentration is 3.
A high-concentration layer 2 consisting of an n-GaAs layer with a thickness of 4,000 layers and a layer thickness of X 10 "atoms/Ca13 is formed by the MBE method, and Si is used as a dopant on the high-concentration layer 2 so that the impurity concentration is 5 X 1016 atoms/c11. a collector layer 3 consisting of an n--GaAs layer with a layer thickness of 5000,
The dopant is Be and the impurity concentration is 1×1018ato.
A low concentration base layer 4 consisting of a p-GaAs layer with a thickness of 1000 m/clI' and a dopant of Be, an impurity concentration of I x 10 "atom/ays" and a layer thickness approximately equal to the mean free path distance of electrons. equal 500 P
A high concentration base layer 5 made of a -GaAs layer is sequentially formed by the MBE method, and the impurity concentration is 3 x 10'' atoms/c with St as a dopant on the high concentration base layer 5.
The layer thickness at ta' is 2000 people nAf (1,3Ga
(The emitter layer 6 consists of a 1,7As layer and the dopant is Si, and the impurity concentration is 5 x 10" atoms/
A high concentration layer 7 consisting of an n''-GaAs layer having a layer thickness of 2000 cta' is sequentially formed by the MBE method, and each layer from the high concentration layer 7 to the collector layer 3 is sequentially etched into a predetermined pattern. 2 and 7 and high concentration base layer 5
The structure has a collector, an emitter, and base electrodes 8C, 8e, and 8b formed thereon, respectively.

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

In the band structure of this example, the base layer consists of low concentration and high concentration base layers 4 and 5, and the energy discontinuity δEc of the conduction band is caused by the step heterojunction between the emitter layer 6 and the high concentration base layer 5. The electron 9a injected from the emitter layer 6 is conducted in a so-called pallistic flight through the highly concentrated base layer 5 having a layer thickness approximately equal to the mean free path distance due to the initial kinetic energy corresponding to δEc. It loses its kinetic energy and then travels through the low concentration base layer 4 by diffusion to reach the collector layer 3.

By the way, the recombination lifetime τ in GaAs doped with P-type impurities. can be expressed empirically as Here, NT is a P-type impurity concentration, and N rat is a reference concentration from the experiment. That is, as the impurity concentration N decreases, the electron recombination lifetime τ increases. grows.

Therefore, as in this embodiment, electrons injected from the emitter layer 6 are conducted at high speed by pallitic flight in the high concentration base layer 5 where the recombination probability is high, and the low concentration base layer 4 where the recombination probability is low. Since conduction occurs through normal diffusion, the injection efficiency is improved by suppressing the increase in recombination probability during diffusion, and the high-concentration base layer 5 reduces base resistance, further improving high-speed and high-frequency performance. A junction bibolar transistor can be realized.

However, in this example, Aj7 GaAs and GaAs, which are lattice-matched to each other, were used as semiconductor materials, but the material is not limited to this, and any material with a difference in electron affinity may be used. Materials may be used.

〔Effect of the invention〕

As explained above, in the present invention, the base layer is composed of a low-concentration base layer and a high-concentration base layer whose layer thickness is about the same as that of paper with a mean free flow of minority carriers, thereby reducing the base resistance and the minority carriers in the base layer. This has the effect of reducing the recombination probability and realizing a beterojunction bipolar transistor with further improved high-speed and high-frequency performance.

1A and 1B are band structure diagrams of first and second examples of conventional heterojunction bipolar transistors, respectively; FIG.

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 1
．． 7... High concentration layer, 8b... Base electrode, 8C...
・Collector electrode, 8 e-... Emitter electrode, 9a, 9
b, 9'.

9″...electron, 11,12.13...Fermi level.

Hakudo edict Defense 4 figure

Claims (1)

[Claims] A heterojunction having a base layer of an opposite conductivity type formed on a collector layer of one conductivity type, and an emitter layer of one conductivity type formed on the base layer and having a smaller electron affinity and a wider forbidden band width than the base layer. 1. A heterojunction bipolar transistor, wherein the base layer is composed of a laminated layer of a lightly doped impurity layer and a highly doped impurity layer having a thickness approximately equal to the mean free path of carriers of one conductivity type.