Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Bipolar transistor
Reexamination Certificate
2003-04-07
2004-06-29
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Bipolar transistor
C257S197000, C257S201000
Reexamination Certificate
active
06756615
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-103370, filed on Apr. 5, 2002; the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a heterojunction bipolar transistor and its manufacturing method, and more particularly, it relates to a heterojunction bipolar transistor being capable of a high-speed operation and having a high breakdown voltage and a high current gain and a method of manufacturing it.
In recent years, revolutionary development has accomplished in information technology including the Internet. For this reason, further improvement in the speed and capacity of wireless communications, such as an optical fiber communications system which makes that basis, and a cellular phone which makes that circumference, are required.
A transistor which is capable of high-speed operation is a key device of these high-speed communications. These transistors are made using semiconductors, such as Si (silicon) and GaAs (gallium arsenide). In a field for which more high-speed operation is needed, SiGe (silicon germanium), InP (indium phosphorus), etc. are capturing the spotlight as a next-generation material.
Further, high-power output is also needed for a device used for these optical communications and wireless communications besides high-speed operation. For this reason, improvement in resisting voltage of a transistor is also required. “A double heterojunction bipolar transistor (DHBT)” in which an emitter and a collector has a wider bandgap compared with a base is a hopeful transistor which fills this demand.
In the conventional general (single) heterojunction bipolar transistor (SHBT), base-collectors junction is a homojunction. That is, material with a comparatively small bandgap is used for the collector like the base. For this reason, in order to attain a high-breakdown voltage, it is necessary to make a collector thick enough. On the other hand, in DHBT, material with a wide bandgap is used not only for the emitter but for the collector. For this reason, thickness of the collector can be made thinner than SHBT. As the result, since electron transit time can be shortened, operation becomes possible at higher speed and a higher breakdown voltage.
By the way, it is known that there are two kinds of junction forms called “Type I” and “Type II” in a heterojunction. The “Type I” and “Type II” defined in this specification will be explained hereafter.
FIG. 12
is a band diagram showing a heterojunction of Type I. That is, this figure schematically shows a band structure of a heterojunction in an equilibrium state of two kinds of different semiconductors
12
and
14
. Here, Ec shows a lower end of a conduction band, Ev shows energy of an upper end of a valence band, and Evac shows energy of a vacuum level as a standard of energy. Ec of the semiconductor
12
is located more closely to Evac than Ec of the semiconductor
14
. Further, Ev of the semiconductor
12
is located remoter from Evac than Ev of the semiconductor
14
. Such a heterojunction shall be called a heterojunction of “Type I” in this specification.
On the other hand,
FIG. 13
is a band diagram showing a heterojunction of Type II. In this case, Ec of the semiconductor
22
is located remoter from Evac than Ec of the semiconductor
24
.
Further, Ev of a semiconductor
22
is also located remoter from Evac than Ev of the semiconductor
24
. Such a heterojunction shall be called a heterojunction of “Type II” in this specification.
FIG. 14
is a band diagram of a principal part of DHBT that has been embodied for a trial by the Inventors of the present invention in the course of attempting to make the invention complete. That is, this diagram shows the state where Emitter E, Base B, and Collector C are junctioned.
As shown to
FIG. 14
, in this DHBT, the base-collector heterojunction is of Type I shown in FIG.
12
. As materials of the base and the collector, GaAs/InGaP, InGaAs/InP, etc. are used, for example. Thus, in the case of the DHBT which has the heterojunction of Type I in the base-collector interface, as typically shown in
FIG. 14
, potential barrier &Dgr;Ec exists between the collector and the base. Running of electrons is barred by this potential barrier and collector injection efficiency falls in a high current condition.
FIG. 15
is the so-called “gummel plot” showing the dependability of a collector current to the voltage VBE between the base and the emitter of the transistor. The curve B in this figure shows the characteristic of DHBT which has the band structure shown in FIG.
14
. On the other hand, curve A shows the characteristic of a single-heterojunction bipolar transistor in which the same material is used for a collector and a base.
Since barrier &Dgr;Ec exists in the base-collector interface, it turns out that the collector current saturates at lower VBE in the curve B, than curve A. Thus, if the structure shown in
FIG. 14
is used to obtain a high breakdown-voltage element, it is difficult to obtain a practically sufficient collector current. The outstanding performances, such as high-speed operation, high-level current gain, and high-level linearity can not fully able to be obtained.
As a means for avoiding this problem, it may be considered to connect the conduction bands of base and collector smoothly by making of the composition near the base-collector interface change gradually. Or by providing an intermediate layer containing high-concentration n-type impurities between base and collector, potential barrier width thereof may be decreased, and an electronic running may become easier by a tunneling effect.
However, in order to introduce such a composition inclination or an intermediate layer, it is necessary to perform a precise gas flow control when the semiconductor layer is grown by a Metal Organic Chemical Vapor Deposition (MOCVD) etc.
Consequently, a big burden is placed in equipment and its operation, and it is inferior in respect of manufacturing efficiency.
Further, even if the intermediate layer for obtaining the tunneling effect is introduced, it is difficult to obtain a sufficient current density.
On the other hand, it is considered to use Type II which is shown in
FIG. 13
instead of Type I.
FIG. 16
is a band diagram of DHBT formed by using a heterojunction of Type II. By using the heterojunction of Type II, as shown in
FIG. 16
, the potential barrier between the base B and the collector C is lost. For this reason, unlike the case where heterojunction of Type I is used, decline in collector injection efficiency is eliminated, and DHBT having high collector injection efficiency can be realized.
However, as a result of a detailed examination of the Inventors, it turned out that in DHBT which had the type II heterojunction for emitter-base junction and base-collector junctions, there were some problems explained below.
First, if a junction of Type II is used, while a band barrier between base and collector will be lost, conversely, a band barrier arises between emitter and base, and a saturation value of collector current falls. As the result, since current density per unit area of the transistor decreases, it becomes necessary to enlarge size of the transistor to some extent in order to secure an output level.
When junction of Type II is provided between emitter E and base B, there maybe a problem that mobility of electrons in the base region falls. That is, as a trend of high-speed element development in recent years, in order to obtain low base resistance, impurities are needed to be added in the material of Base B in a high concentration.
When forming DHBT, GaAsSb and InGaAs can be mentioned as a typical material system in which high-concentration impurity doping is possible. However, in these material systems, the alloy scattering effect is severe, and since electrons injected into the base from the emitter experience remarkable scattering, mobility becomes extremely low.
For example, mobil
Nozu Tetsuro
Yoshioka Akira
Kabushiki Kaisha Toshiba
Ngo Ngan V.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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