JPH01149465A - Heterojunction bipolar transistor - Google Patents

Abstract

PURPOSE:To enable electrons to travel rapidly in a collector layer and to improve on the collector in its breakdown strength against a backward bias by a method wherein the collector layer is a semiconductor layer so doped as to satisfy some specified conditions and is in Schottky-type contact with a collector electrode. CONSTITUTION:A Wc-thick collector layer is a single-layer structure built of a semiconductor material 'a' which is not doped with any impurity or a semiconductor material 'b' which is doped with an impurity and satisfies a formula where impurity concentration is NA, collector impurity concentration is ND, semiconductor dielectric factor is epsilons, charge elementary quantity is 'q', and base.collector junction internal potential is Vbi. The collector layer forms a Schottky-type contact with a collector electrode. With the bend of the energy band in the collector being quite small and negligible, no intensive electric field will locally concentrate in the collector layer. Such a collector section is a simple structure built only of a thin semiconductor layer and a metal electrode, which eliminates the need for a high concentration semiconductor layer for an ohmic contact between the semiconductor and the electrode.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to heterojunction bipolar transistors.

(Prior Art) Heterojunction bipolar transistors (HBTs) are attracting attention as next-generation ultra-high-speed devices that have both high current drive capability and excellent high-frequency characteristics. By the way, HB
The high-speed performance of T is determined by the sum of three delay times: the parasitic capacitance charging time, the minority carrier transit time in the base layer, and the collector depletion layer transit time, and each delay time is approximately 173 times larger than the total. It has become. Recent improvements in technology have made it possible to reduce parasitic capacitance and resistance and shorten the transit time of the base layer, but the challenge is to shorten the transit time of the erector depletion layer, which is about the same or larger than the base layer thickness. It has become.

FIG. 6 is a diagram showing the band structure of the first conventional HBT, where 1 indicates the top of the valence band and 2 indicates the bottom of the conduction band. P-type AlxGa is formed on the erector layer 65 made of low concentration n-type GaAs.
1-NA5 (x=0-+0.1), and n-type A10.25GaO, 75AS.
It has a structure in which emitter layers 63 consisting of are sequentially laminated. Under the operating conditions of this conventional transistor, a depletion layer 5d extends in the pn junction region between the base and collector, and an extremely strong electric field exists in the depletion layer. For this purpose, the emitter layer 63 is injected into the base layer 64.
As soon as the electrons that have passed through the
' (position 101 in the figure), and thereafter travels through most of the section of the collector depletion layer at a relatively slow speed determined by the heavy effective mass of the L valley (position 102 in the figure). Therefore, in this conventional HBT, the collector depletion layer transit time is long, which significantly limits the high frequency and high speed performance of the device. In order to solve this problem, a second HBT structure described below has been proposed.

FIG. 7 is a diagram showing the band structure of a second conventional HBT, in which a thin layer 5P made of highly doped p-type GaAs is formed on a collector heavily doped layer 5n made of highly doped n-type GaAs, and a thin layer 5P made of impurity-free GaAs. Semiconductor layer 5i, p''-Alx
A compositionally graded base layer 74 made of Ga1-xAs (x = O→0.1), and n-A1.2. Gao, 7
It has a structure in which emitter layers 73 made of 5As are sequentially laminated. In this conventional HBT, most of the potential difference between the base layer 74 and the collector high concentration layer 5n is the diffusion potential 11 of the pn junction in the collector.
Therefore, a strong electric field is not applied to the collector 1 layer 51, which determines the collector layer travel time of electrons. Therefore, electrons traveling through the collector layer 51 hardly transition to the conduction band or valley, are driven by an electric field of moderate strength, and pass through the collector 1 layer 51 at high speed (710 in the figure).
(indicated by the arrow).

(Problem to be Solved by the Invention) However, in the HBT of the second conventional example, since a pn junction in which impurities are doped at a high concentration on both sides of the collector region is used, the withstand voltage against the collector reverse bias is low. The disadvantage is that it is weak. Furthermore, since the crystal layer structure is relatively complex, it is difficult to control the impurity concentration, layer thickness, etc. to obtain a desired energy band structure.

An object of the present invention is to provide a highly reliable ultra-high speed heterojunction bipolar transistor that has a collector structure using a Schottky junction in order to improve these points.

(Means for Solving the Problems) The present invention has been made in view of the above points, and includes a collector having a thickness We in a Peter junction bipolar transistor consisting of the main layers of an emitter layer, a base layer, and a collector layer. If the layer is (a) an undoped semiconductor material, or (b) the base impurity concentration is NA
, the impurity concentration of the collector is ND, and the dielectric constant of the semiconductor is ε9.
, a single semiconductor layer of semiconductor material doped such that the charge element is q and the built-in potential of the base-collector junction is vbi, and the collector layer makes a Schottky-type contact with the collector electrode. A Peter junction bipolar transistor is obtained.

(Function) In a beterojunction bipolar transistor with such a collector structure, the energy of the Schottky barrier is lower than in the case of a normal collector structure that uses ohmic contact between a heavily doped semiconductor and a metal electrode. The potential drop within the collector layer becomes smaller accordingly. Moreover, since there are no or very few impurity ions in the collector layer, the bending of the energy band in the collector layer is so small that it can be ignored, resulting in a concentrated electric field in some regions of the collector layer. It never happens. Therefore, the electric field strength generated in the collector layer is much weaker than in the case of the first conventional HBT, and the electrons traveling in the collector layer are raised by the electric field of an appropriate size without transitioning to the conduction band or valley. Driven by speed.

The collector portion of the HBT of the present invention has a simple structure consisting only of a thin semiconductor layer and a metal electrode, and there is no need to provide a high concentration semiconductor layer to form an ohmic contact between the semiconductor and the electrode.

Furthermore, since the structure has a large base-collector breakdown voltage, it can also be applied to power transistors to which a large bias is applied. As described above, the HBT of the present invention has the characteristics of being easy to manufacture and having high reliability while maintaining the high-speed performance of the second conventional E(BT).

(Example) Examples of the present invention will be described below.

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

On a semi-insulating GaAS substrate 1, by molecular beam epitaxial method, a highly concentrated emitter layer consisting of 500 OA of n''-GaAs to obtain an ohmic contact of the emitter 2.3000 N-Alo, 25 Gao, 7.A
Emitter layer consisting of 3.1000 p+-AlxG
a1. A compositionally graded base layer 4 made of xAs (x=o→0.1). A collector layer 5 made of 2,000 1-GaAs layers is sequentially laminated, and a desired area is formed by the insulating region 1a formed by proton ion implantation. After exposing the base layer 4 and the emitter high concentration layer 2 in the area partitioned into the pattern, a collector electrode 6c made of 5000A tungsten silicide, a base electrode 6b made of 200OA AuMn, and an emitter made of 12000A AuGe/Ni/Au. This is a collector top type HBT structure manufactured by forming an electrode 6e. In this embodiment, a magnesium ion implantation region 4p is provided in the external region in order to prevent current gain from deteriorating due to electron injection from the emitter to the external base.

Figure 2 is a diagram showing the energy band structure of the HBT shown in Figure 1. Figure 2 (a) is when no bias is applied, and Figure 2 (b) is when a bias is applied and the transistor is in an operating state. FIG.

Collector layer 5 and collector electrode 6c are Schottky barrier 8
Therefore, the conduction band bottom 2 does not need to fall close to the collector electrode 6c, and since the collector layer 51 is non-doped, the bending of the band can be almost ignored. As a result, the electric field strength of an appropriate magnitude is distributed almost uniformly within the collector layer 51, and electrons traveling in the collector layer 5 can travel at high speed without transitioning to the conduction band valley 9 (in the figure). 10 position).

FIG. 3 is a diagram showing the energy band structure of the HBT according to the second embodiment of the present invention. In this embodiment, the collector layer 5n is made of n-GaAs instead of 1-GaAs, and the impurity concentration is so low that it is already completely depleted in a thermal equilibrium state with no bias applied. Expressing this in a formula, the thickness of the collector layer is WC, the impurity concentration of the base is NA, the impurity concentration of the collector is ND, the dielectric constant of the semiconductor is 88, the charge element is q, the built-in potential of the base-collector junction is V, Doping is done to meet the conditions of In this example, I x 1016/cm3 was used.

In the first embodiment, since there are no impurity ions in the collector layer, when the collector current density exceeds a certain level, it is limited by the space charge, so it cannot be operated at a large current density. , second
In this embodiment, the presence of impurity ions of opposite sign to the electrons allows the current density threshold at which space charge limitation begins to be increased. In this case, the conduction band is slightly distorted due to the distribution of impurity ions, so the electric field strength distribution is slightly biased, but since the potential drop within the collector layer 5n is small, this does not have a large effect.

4 and 5 are energy band diagrams of the fourth and fifth embodiments. In the embodiments shown in FIGS. 4 and 5, the point where each collector layer 5g or 5s is in Schottky contact with the collector electrode 6c is the first and second
This is a common feature with the embodiments, but in these embodiments, the semiconductor layer constituting the collector layer is the collector electrode 6c.
Another feature is that it is a single or multiple compositionally graded layer such that the energy gap increases as it approaches . By gradually increasing the energy gap of the collector layer in this manner, large changes in the potential of electrons traveling through the collector layer can be further reduced. When an AlxGa1-xAs mixed crystal semiconductor is used for the collector layer, as the aluminum composition increases, the energy difference between the conduction band bottom and the conduction band valley becomes extremely small, creating a state in which electrons easily transition to the conduction band valley. Therefore, in this example, the energy difference between the conduction band bottom and the conduction band valley is larger than that of AlxGa and xAs-based materials.
■nAlxGa1-xA lattice matched to P substrate
An s-mixed crystal semiconductor is used. In Figure 4, 1-In
The aluminum composition is graded from 0 to 0.7 over the entire A1xGa□-xAs collector layer from the base layer contact portion toward the collector electrode. By the way, in the fourth embodiment, since the aluminum composition in the collector layer near the electrode is quite large, electrons are likely to transition to the conduction band and to the valley for the reason mentioned above. Therefore, in the fifth embodiment, the collector layer is It has a stepped structure divided into several semiconductor layers, and each semiconductor layer is made of InGaAs.
InAlo, 2G, which has a relatively small aluminum composition from
This layer has a composition gradient toward ao and 8As.

In this embodiment, tungsten silicide, which has excellent heat resistance, was used as the collector electrode, but if the electrode material forms a Schottky junction, Ti or Au may be used.

Anything is fine, such as pt. In addition, the semiconductor material is AlGaAs/
The material is not limited to the GaAs system, but may also be a lattice mismatch system. The present invention is effective for petrojunction bipolar transistors having base structures not limited to the compositionally graded structure as in this example, but also any base structure such as a hot electron injection type base structure or a diffused electron transport type uniform base structure. Needless to say.

(Effects of the Invention) As described above, according to the present invention, it is possible to realize an HBT in which electrons can travel at high speed in the collector layer using an extremely simple crystal layer structure and which has a high breakdown voltage against collector reverse bias.

Because of its reliable voltage resistance, it is expected to be used in a wide range of applications such as power transistors and logic circuits while taking advantage of its high-speed performance.

[Brief explanation of the drawing]

FIG. 1 is a main sectional view of the HBT of the present invention, and FIGS.
The figure is an energy band structure diagram of an example of the present invention. FIGS. 6 and 7 are energy band diagrams of HBTs each having a known collector layer structure.

Claims (1)

[Claims] 1. In a heterojunction bipolar transistor consisting of the main layers of an emitter layer, a base layer, and a collector layer, the erector layer having a thickness W_C is made of (a) a semiconductor material not doped with impurities, or (b) Base impurity concentration is N_A, collector impurity concentration is N_D,
It is a semiconductor layer made of a semiconductor material doped to satisfy the following conditions: ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ where the dielectric constant of the semiconductor is ε_S, the elementary charge is q, and the built-in potential of the base-collector junction is V_b_i. , and the collector layer is in Schottky type contact with the collector electrode. 2. The collector layer includes at least one semiconductor layer in which the energy bandgap of the semiconductor material constituting the collector layer increases as it approaches the collector electrode from the base-collector junction surface. heterojunction bipolar transistor.