KR20010054040A - Double heterojunction bipolar transistor - Google Patents

Abstract

PURPOSE: A double hetero-junction bipolar transistor(DHBT) is provided to solve a reduction in collector current caused by a potential barrier of a base/collector junction and further to solve a reduction in breakdown voltage. CONSTITUTION: The transistor is composed of a GaAs sub-collector layer formed on a semiconductor substrate, an InGaP collector layer, an Al(x)Ga(1-x)As grading layer(15), the first GaAs spacer layer(14), a GaAs base layer, the second GaAs spacer layer, an InGaP or AlGaAs emitter layer, and a GaAs emitter cap layer. In particular, the Al(x)Ga(1-x)As grading layer(15) reduces a potential barrier produced between the base layer and the collector layer. Preferably, the Al(x)Ga(1-x)As grading layer(15) may have an n-type impurity concentration of about 5x10¬16/cm¬3.

Description

Dual Heterojunction Bipolar Transistors {DOUBLE HETEROJUNCTION BIPOLAR TRANSISTOR}

FIELD OF THE INVENTION The present invention relates to double heterojunction bipolar transistors and, more particularly, to improved double heterojunction bipolar suitable for eliminating collector current reduction caused by potential barriers of base / collector junctions in double heterojunction bipolar transistors. It relates to the structure of a transistor.

As is well known, AlGaAs / GaAs or InGaP / GaAs (hereinafter referred to as InGaP (AlGaAs) / GaAs), a compound semiconductor device of the gallium arsenide (GaAs) heterojunction bipolar transistor (HBT: Heterojunction Bipolar Transistor, hereinafter HBT) The device is widely applied to device applications in the microwave or millimeter wave (10-100 GHz) band, and has been applied to the radio communication circuit in the high frequency band due to its excellent speed characteristics. In particular, because of the high power density characteristics, it is a very important high frequency device widely used in the fabrication of power amplifying transistors or power amplifiers.

InGaP (AlGaAs) / GaAs HBTs generally have an emitter / base junction of InGaP (AlGaAs) / GaAs heterojunctions and a base / collector junction of GaAs homogeneous junctions. In addition, in order to improve the power characteristics of InGaP / GaAs HBT, the base / collector junction is replaced with a heterojunction GaAs / InGaP, which is an InGaP (AlGaAs) / GaAs / InGaP double heterojunction transistor (DHBT: Double Heterojunction Bipolar). Transistor, hereinafter abbreviated as DHBT).

InGaP used as a collector in InGaP (AlGaAs) / GaAs / InGaP DHBT has a critical electric field, that is, a breakdown phenomenon compared to GaAs used as a collector in InGaP (AlGaAs) / GaAs HBT. Since the minimum field is much larger, higher collector / base voltages can be applied. As a result, InGaP (AlGaAs) / GaAs / InGaP DHBT can amplify higher power than InGaP (AlGaAs) / GaAs HBT.

Figure 2a is a view showing an example of the epi structure of the InGaP (AlGaAs) / GaAs / InGaP DHBT according to the prior art, Table 1 is a constituent material of each layer constituting the DHBT shown in Figure 2a and impurities of each layer The concentration and the thickness of each layer are shown.

layer matter Impurity concentration Layer thickness (nm) Emitter cap layer GaAs n to 1 × 10 ¹⁹ / cm 3 100 Emitter layer InGaP (AlGaAs) n to 5 x 10 ¹⁷ / cm 3 200 Second spacer layer GaAs undoped 2 Base layer GaAs p to 1 × 10 ²⁰ / cm 3 70 First spacer layer GaAs undoped 20 n ⁺ collector layer InGaP n to 5 × 10 ¹⁸ / cm 3 10 Collector layer InGaP n to 5 x 10 ¹⁶ / cm 3 500 Sub-collector layer GaAs n to 8 × 10 ¹⁸ / cm 3 400 Board GaAs Semi-insulating

The role of each of the crystal layers 21, 22, and 23 constituting the GaAs / InGaP base / collector junction shown in FIG. 2A is explained using the energy band diagram shown in FIG. 2B and the transistor current-voltage characteristics of FIG. 2C. Is as follows.

First, when looking at the energy band diagram without the first crystal layer 21 and the second crystal layer 22, that is, without the first spacer layer and the n ⁺ collector layer, the potential barrier higher than the base region The potential barrier prevents the movement of electrons from the base region to the collector. As a result, as shown in the current voltage characteristic of FIG. 2C, a very low collector current flows, thereby significantly degrading performance as a transistor. If the collector / base reverse voltage is increased here, the thickness of the potential barrier decreases, which increases the electrons transferred to tunneling, but does not serve as a high performance transistor.

In contrast, in the energy band diagram when there is no second crystal layer 22, that is, when there is no n ⁺ collector layer, as shown in FIG. 2B, the potential barrier is formed by the role of the second spacer layer. It is in a position lower than the energy level of. However, due to the reflection of electrons by the potential barrier, a slightly smaller current flows than without the potential barrier.

On the other hand, when both the first crystal layer 21 and the second crystal layer 22 are present, that is, when the first spacer layer and the n ⁺ collector layer are present, the position of the potential barrier is lower than the energy level of the base layer. At the same time, the thickness of this potential barrier is so thin that the electron current is very active even at low collector / base voltages as shown in the current voltage characteristics of FIG. Will flow.

However, in this case, the base / collector breakdown voltage is reduced due to the n ⁺ doped second crystal layer 22. This low breakdown voltage reduces the power capacity of the HBT and degrades the performance as a power amplifier. have.

Accordingly, the present invention is to solve the above problems of the prior art, the spacer layer used as a method for solving the problem of the reduction of current associated with the potential barrier generated in the conventional InGaP (AlGaAs) / GaAs / InGaP DHBT and The purpose is to provide a new structure of InGaP (AlGaAs) / GaAs / InGaP DHBT structure that can solve the reduction of the breakdown voltage associated with the n ⁺ doped layer.

In order to achieve the above object, the present invention, in the dual heterojunction bipolar transistor capable of preventing the collector current reduction phenomenon caused by the potential barrier of the base / collector junction, the double heterojunction bipolar transistor is a semiconductor substrate A GaAs subcollector layer, an InGaP collector layer, an Al _x Ga _1-x As grading layer, a GaAs first spacer layer, a GaAs base layer, a GaAs second spacer layer, and an InGaP or AlGaAs emitter layer formed thereon And a GaAs emitter cap layer, wherein the Al _x Ga _1-x As grading layer reduces the potential barrier generated between the base layer and the collector layer. do.

1A illustrates a hierarchical structure of a double heterojunction bipolar transistor according to a preferred embodiment of the present invention;

FIG. 1B shows an energy band diagram before forming a junction for the dual heterojunction bipolar transistor shown in FIG. 1A;

1C shows an energy band diagram after forming a junction for the dual heterojunction bipolar transistor shown in FIG. 1A;

2A is a diagram showing a hierarchical structure of a double heterojunction bipolar transistor according to the prior art;

FIG. 2B shows an energy band diagram after forming a junction for the conventional double heterojunction bipolar transistor shown in FIG. 2A;

FIG. 2C shows current-voltage characteristics for the conventional dual heterojunction bipolar transistor shown in FIG. 2A.

<Description of the code | symbol about the principal part of drawing>

14, 21: first spacer layer 15: grading layer

22: n ⁺ collector layer 23: collector layer

The above and other objects and various advantages of the present invention will become more apparent from the preferred embodiments of the present invention described below with reference to the accompanying drawings by those skilled in the art.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1A is a diagram illustrating an InGaP (AlGaAs) / GaAs / InGaP DHBT epi structure according to a preferred embodiment of the present invention.

In addition, Table 2 is a table showing the constituent material of each layer constituting the InGaP (AlGaAs) / GaAs / InGaP DHBT according to the present invention, the impurity concentration of each layer, and the thickness of each layer.

layer matter Impurity concentration Layer thickness (nm) Emitter cap layer GaAs n to 1 × 10 ¹⁹ / cm 3 100 Emitter layer InGaP (AlGaAs) n to 5 x 10 ¹⁷ / cm 3 200 Second spacer layer GaAs undoped 2 Base layer GaAs p to 1 × 10 ²⁰ / cm 3 70 First spacer layer GaAs undoped 5 Grading layer Al _x Ga _1-x As n to 5 x 10 ¹⁶ / cm 3 10 Collector layer InGaP n to 5 x 10 ¹⁶ / cm 3 500 Sub-collector layer GaAs n to 8 × 10 ¹⁸ / cm 3 400 Board GaAs Semi-insulating

First, compared with the conventional DHBT structure described above, the difference is that the thickness of the first spacer layer 14 is thinner than the thickness of the first spacer layer 21 shown in FIG. 2A. In detail, the first spacer layer 21 in the conventional DHBT structure is generally composed of a portion that buffers diffusion of the P-type impurities in the base toward the collector and a portion that serves to lower the potential barrier below the base energy level. Although the first spacer layer 14 in the DHBT structure according to the present invention has a relatively thin thickness, since the first spacer layer 14 is composed only of a portion that buffers diffusion of P-type impurities from the base layer toward the collector, Formed into layers.

And, the grading layer 15 shown in FIG. 1A is a graded Al _x Ga _1-x As (where x varies from x1 to 0 in the upward direction from below the epistructure). The band gap energy (Eg) of Al _x Ga _1-x As is changed according to the change of Al component (x). changes in eV, the energy difference in the conduction band changes to ΔEc = 0.7 (1.087x + 0.438x ² ) eV, and the graded Al _x Ga _1-x As layer 15 is shown in FIGS. 2b and 2c. As described above, it serves to remove the potential barrier of the conduction band resulting from the difference between 1.42 eV, which is the band gap energy of GaAs used as the base, and 1.88 eV, which is the band gap energy of In _0.5 Ga _0.5 P used as the collector.

Here, when x is the value of x1 at the point where the graded Al _x Ga _1-x As layer 15 meets the InGaP collector layer, the energy band at the grading layer and the InGaP collector interface depends on the size of the value of x1. The structure will be different. x1 is the value (x1 ') that makes Al _x Ga _1-x As and InGaP conduction band energy difference (ΔEc) equal to 0 (x1' becomes 0.22 when ΔEc of InGaP / GaAs is 0.2eV) In one case, as shown in Figs. 2b and 2c, an ideal pn junction is formed between the base and the collector with no potential barrier.

When the value of x1 is smaller than the value of x1 '(x1 &lt;x1'), a slight potential barrier occurs as shown in FIG. 1C, resulting in a decrease in collector current. In contrast, when x1 is larger than x1 '(x1>x1'), a dislocation fault occurs, but the collector current decrease does not occur because it does not interfere with the transfer of electrons. When using a graded Al _x Ga _1-x As layer (where x1 ≥ x1 '), there is no potential barrier between the base / collector voltages, making collector current characteristics ideal, while at the same time used in prior art epistructures It is not necessary to use a thick first spacer layer and an n ⁺ doped layer, whereby a high breakdown voltage can be obtained.

As described above, according to the present invention, the InGaP (AlGaAs) / GaAs / InGaP DHBT is implemented using a graded Al _x Ga _1-x As layer, thereby eliminating the potential barrier of the base / collector junction, thereby providing an ideal collector current characteristic and There is an effect to obtain a high breakdown voltage, it is possible to significantly improve the power characteristics. In addition, since the potential barrier of the base / collector junction is removed, electrons are transferred without interruption of the potential barrier, thereby achieving an improved rate characteristic compared to the conventional InGaP (AlGaAs) / GaAs / InGaP DHBT structure.

Claims (2)

In a double heterojunction bipolar transistor capable of preventing a collector current reduction caused by a potential barrier of a base / collector junction, The double heterojunction bipolar transistor comprises a GaAs subcollector layer formed on a semiconductor substrate, an InGaP collector layer, an Al _x Ga _1-x As grading layer, a GaAs first spacer layer, a GaAs base layer, and a GaAs agent. Two spacer layers, an InGaP or AlGaAs emitter layer, and a GaAs emitter cap layer, And wherein the Al _x Ga _1-x As grading layer reduces the potential barrier generated between the base layer and the collector layer. The dual heterojunction bipolar transistor according to claim 1, wherein Al _x Ga _1-x As of the grading layer has an impurity concentration of about 5 x 10 ¹⁶ / cm 3 of n-type.