JPH0656852B2 - Heterojunction bipolar transistor - Google Patents

Description

The present invention relates to a heterojunction bipolar transistor.

[Conventional technology]

In recent years, especially Ga has been used as a next-generation high-speed and high-frequency device.
Heterojunction bipolar transistors based on As / AlGaAs have come to the forefront, and research is actively undertaken from both basic and applied sides. The concept of the heterojunction bipolar transistor was very old, almost at the same time as the invention of the transistor, but while its theoretical superiority was evaluated, lack of knowledge of GaAs itself as a material or its periphery,
Until recently, it was rarely seen back due to underdeveloped crystal growth technology.

However, research and development have suddenly come to the forefront today by the molecular beam epitaxy (MBE) method and organometallic CV.
This is because a multilayer thin film forming technique such as the D (MOCVD) method has become possible, and a good heterojunction that can be used practically can be formed.

In order to further improve the high-speed and high-frequency performance of a heterojunction bipolar transistor in which a semiconductor material having a wider forbidden band than the base is used for the emitter and the emitter injection efficiency is improved, the base transit time of the minority carrier is increased. It is necessary to shorten it further.

Conventionally, therefore, a method of applying the phenomenon of ballistic conduction of minority carriers in the base region and a method of driving the minority carriers by the internal electric field of the base region have been tried.

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

This example has a structure in which a p-type base layer 5'and an n-type emitter layer 6 'having a smaller electron affinity and a wider forbidden band width than the base layer 5'are sequentially stacked on an n-type collector layer 3'. As a result, the energy discontinuity δE _c corresponding to the difference in electron affinity generated in the conduction band of the heterojunction becomes the initial kinetic energy of the electron 9 ′ poured from the emitter to the base, which is faster than ordinary diffusion. It enables conduction by so-called ballistic flight 10 '.

However, the effective distance of conduction of the electron 9'by the ballistic flight 10 'is about the mean free path of the electron,
After that, the collector reaches the collector in the base layer 5'by ordinary diffusion. Therefore, when the base layer 5'has a thickness of several thousand Å like a normal heterojunction bipolar transistor, the mean free path of electrons is increased. Is relatively short at several hundred Å, the distance that electrons are conducted by ballistic flight is only a small part in the base layer 5 '. Therefore, the base transit time is determined by the diffusion transit time, and it is not expected that the base transit time will be significantly shortened. In addition, due to the high hole concentration of the base layer 5 ′ during the diffusion transit, the recombination of electrons occurs. The probability becomes remarkable and the arrival rate to the collector is low,
The injection efficiency is not very good.

In fact, p-type doped GaAs or Al _1-x Ga _x A
The recombination lifetime of electrons at s can be empirically expressed by the following equation.

Here, N _T represents a p-type impurity concentration, and N _ref represents a reference concentration for experiments. That is, if the impurity concentration is lowered, the recombination life of electrons is extended and the recombination probability can be reduced.

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

In this example, the electron affinity is smaller and the band gap is wider than the p-type base layer 5 ″ and the p-type base layer 5 ″ having a graded bandgap structure in which the material composition ratio is changed on the n-type collector layer 3 ″. Since the n-type emitter layer 6 ″ and the n-type emitter layer 6 ″ are sequentially stacked, the electrons 9 ″ injected from the emitter layer 6 ″ to the base layer 5 ″ are indicated by the arrow 10 ″ due to the internal electric field based on the inclination of the conduction band. It is expected that the conduction will be accelerated and faster than diffusion driving.

However, when the electrons are sufficiently accelerated and exceed a certain kinetic energy, valley scattering becomes remarkable, and the effective speed of the electrons decreases. For example, if the base layer 5 ″ thickness is 1500 Å, the base material Al _x Ga _1-x As x is 0.15.
When → 0 is set, the internal electric field becomes about 10 kV / cm, and it is understood that valley scattering easily occurs.

As mentioned above, two typical examples of the conventional heterojunction bipolar transistor have been given. Recently, a simulation result has been reported that a base structure that enables ballistic flight is more advantageous than a graded bandgap structure. ing.

[Problems to be solved by the invention]

As described above, in the first example of the conventional heterojunction bipolar transistor, although the minority carriers (electrons in this case) injected from the emitter layer to the base layer perform high-speed conduction by ballistic flight, In comparison, the conduction distance is very short, such as the mean free path, so most of the conduction is due to low-speed diffusion travel, and the effective speed is determined by the low-speed diffusion speed, so high-speed / high-frequency performance does not improve much and Since the impurity concentration of the base layer is made high in order to reduce the base resistance, there is a drawback that the recombination probability of carriers becomes high, the arrival rate to the collector layer is lowered, and the injection efficiency is deteriorated.

Further, in the second example, since the base layer has a graded bandgap structure, minority carriers (electrons in this case) are accelerated by the internal electric field based on the structure, but when the internal electric field exceeds a certain value, it is effective due to valley scattering. It is not possible to expect high-speed / high-frequency performance improvement because the target mobility will decrease.

[Means for solving problems]

In the heterojunction bipolar transistor of the present invention, the emitter and the base form a staircase type heterojunction, and the base layer is divided into a plurality of sub-base layers whose forbidden band width increases toward the emitter layer. Is divided into a high-concentration sub-base layer and a low-concentration sub-base layer on the side close to the emitter layer.

[Action]

According to such a heterojunction bipolar transistor of the present invention, carriers (in this case, electrons) that have completed ballistic conduction in the base region repeat a slight diffusion conduction, and then restart ballistic conduction, and the diffusion conduction is also performed. Since the concentration of holes in the region is low, the conduction velocity of carriers is improved, and the loss due to recombination is also reduced, so that the high-speed / high-frequency performance is dramatically improved.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

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

This embodiment is partitioned by an insulating region 1a formed by ion-implanting protons on a semi-insulating substrate 1 and has Si as a dopant and an impurity concentration of 3 × 10 ¹⁸ atom / cm ³ Thickness 40
A high-concentration layer 2 made of an n ⁺ -GaAs layer of 00 Å is formed by the MBE method. Dopan is Si on the high-concentration layer 2 with an impurity concentration of 5 × 10 ¹⁶ atom / cm ³ and a thickness of 5000 Å n ^−.
-The collector layer 3 made of a GaAs layer, the dopant is Be, and the impurity concentrations are 1 × 10 ¹⁸ atom / cm ³ and 1 × 10 ^{20 respectively.}
p and p ⁺ -Ga with atom / cm ³ and thickness of 100Å and 400Å
Lamination of low-concentration and high-concentration base layers 4a and 4b composed of As layers, with Be as a dopant and an impurity concentration of 1 × 1
0 ¹⁸ atom / cm ³ and 1 × 10 ²⁰ atom / cm ³ and 100 Å and 400 Å of p and p ⁺ -Al _0.15 Ga _0.85 As layers of low-concentration and high-concentration base layers 5a and 5b, and a dopant. N-A with Si and impurity concentration of 3 × 10 ¹⁷ atom / cm ³
l _0.3 Ga _0.7 As layer emitter layer 6 and Si as a dopant, and n ⁺ -GaAs with an impurity concentration of 5 × 10 ¹⁸ atom / cm ³
A high-concentration layer 7 composed of layers is sequentially formed in a predetermined pattern by the MBE method or the like, and collectors and emitters and base electrodes 8c and 8e and 8b are provided on the high-concentration layers 2 and 7 and the high-concentration base layer 5b, respectively. It has a different structure.

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

In this embodiment, the base layer is formed by stacking the low-concentration and high-concentration base layers 4a and 4b having the same electron affinity and the low-concentration and high-concentration base layers 5a and 5b having the electron affinity smaller than and equal to each other. Consists of laminated,
Further, since the emitter layer has a smaller electron affinity and a wider band gap than the high-concentration base layer 5b, the interface between the high-concentration and low-concentration base layers 4b and 5a and the interface between the high-concentration base layer 5b and the emitter layer 6 are high. discontinuity &Dgr; E _c 'and &Dgr; E _c of each conduction band occurs about 0.1 eV. Here, the concentration of the high-concentration base layers 4b and 5b is about the mean free path of electrons.

Therefore, in this embodiment, the electrons 9 injected from the emitter layer 6 conduct as ballistic flight in the high-concentration base layer 5b with δE _c as an initial kinetic energy, as shown by an arrow 10, and then, Diffusive running through the low-concentration base layer 5a, then conducting again by ballistic flight in the high-concentration base layer 4b with δE _c ′ as the initial kinetic energy, and further diffusively running through the low-concentration base layer 4a Reach layer 3.

Here, in the embodiment of the present invention, AlGaAs and GaAs which are lattice-matched with each other are used as the semiconductor material, but the material is not limited to this and any material having a difference in electron affinity may be used. Further, the heterojunction used in the present invention is not limited to the lattice matching system, but may be a lattice mismatching system.

〔The invention's effect〕

As described above, according to the present invention, a base layer having a predetermined thickness is formed by stacking a low-concentration base layer and a high-concentration base layer so that the electron affinities of the base layers are sequentially reduced, and an emitter having a smaller electron affinity is further deposited thereon. By forming the layer, the injected carriers from the emitter layer reach the collector layer while repeating conduction by ballistic flight and diffusion traveling in the base layer a plurality of times, so that the diffusion traveling distance can be made smaller than the conventional one. Realization of a heterojunction bipolar transistor with a high proportion of the ballistic flight distance, a short proportion of the ballistic flight distance, a short base layer travel time, and a small recombination probability of minority carriers, high resistance and high frequency performance with a small base resistance. There is an effect that you can.

[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, 4a, 5a ... Low concentration base layer, 4b, 5b ... Base layer , 6, 6 ',
6 ″ ... Emitter layer, 7 ... High concentration layer, 8b ... Base electrode, 8c ... Collector electrode, 8e ... Emitter electrode,
11, 11 ', 11 ", 12, 12', 12", 13,
13 ', 13 "... Fermi level.

Claims (1)

[Claims] 1. In a heterojunction bipolar transistor in which an emitter and a base form a staircase type heterojunction, the base layer is divided into a plurality of sub-base layers whose forbidden band width increases toward the emitter layer, and each sub-base is further divided. A heterojunction bipolar transistor, wherein the layer is divided into a high-concentration sub-base layer and a low-concentration sub-base layer on the side close to the emitter layer.