JPS6381854A - Semiconductor device - Google Patents

Abstract

PURPOSE:To reduce sharply base delay time, and to enable superhigh speed action by a method wherein a collector layer consisting of a single conductive semiconductor, a graded base layer in which a forbidden band gap extends gradually from the collector layer, and an emitter layer in which the forbidden band gap is broader than that of the graded base layer are laminated. CONSTITUTION:A graded base layer 8 in which a forbidden band gap increases grdually from the collector layer 2 side up to an emitter layer 4 is provided. In the figure to express typical band structure ranging over the emitter layer 4, the graded base layer 8 and the collector layer 2, Ec represents the energy difference at the conduction band end between the emitter layer 4 and the graded base layer 8, and Eb represents the difference of the forbidden band gap in the graded base layer 8. Because the graded base layer 8 has grading of composition, the forbidden band gap becomes larger on the emitter side than the collector side. Because the graded base layer 8 thereof is a P-type, the difference of the forbidden band gap thereof is represented as the difference of energy. Base transit time can be shortened, and the reduction of life time of minority carrier can be controlled in spite of high impurity concentration of the base as such.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a semiconductor device capable of high-speed operation.

(Prior Art) One of the active semiconductor devices considered to be capable of high-speed operation is a Peter junction bipolar transistor (HBT) having a wide bandgap emitter (WGE). For example, Asbeck et al.
DM, Technical Digest, page 629, 19
In 1981), a prototype HBT was reported.

This device has the following features: (1) The base resistance can be significantly reduced and the base width can be narrowed without deteriorating the emitter injection efficiency.

(2) Since the impurity concentration in the emitter region can be reduced, the emitter-base capacitance can be reduced.

Because of this advantage, it is more suitable for high-speed operation than ordinary bipolar transistors made only of homojunctions.

FIG. 6 shows a schematic cross-sectional view of a bipolar transistor with a conventional structure. In FIG. 6, 1 is a semiconductor substrate, 2 is a collector layer having one conductivity type and made of a first semiconductor, 3 is a base layer having a conductivity type different from collector layer 2 and is made of a first semiconductor, and 4 is a collector layer made of a first semiconductor. an emitter layer made of a second semiconductor having the same conductivity type as the collector layer 2 and having a wider forbidden band width than the collector layer 2 and the base layer 3; 5, a collector electrode forming ohmic contact with the substrate 1 and the collector layer 2; 6; 7 is a base electrode that forms ohmic contact with the base 3, and 7 is an emitter electrode that forms ohmic contact with the emitter layer 4.

The operation of this conventional structure is as follows: The semiconductor substrate 1 is n''-GaAs with a donor concentration of about I x 1018 cm-3, and the collector layer 2 is n''-GaAs with a donor concentration of I x 1016 cm-3.
n−-GaAs with an acceptor concentration of about I×10110l9 as the base layer 3;
Emitter layer 4 has a donor concentration of 5 x 1017cr
This will be explained using FIG. 7, which shows this band structure, using n A1o3Gao, 7As of about n-''.

FIG. 7 schematically shows a band structure spanning the emitter layer 4, base layer 3, and collector layer 2 shown in FIG.

E in FIG. is the conduction band edge, Ev is the charge band edge, Ef
is the Fermi level, Veb is the emitter-base voltage,
Vbc is the voltage between base and collector.

When a forward bias of Veb is applied between the emitter and the base and a reverse bias of Vbc is applied between the base and collector, electrons are injected from the emitter to the base by diffusion, and most of these electrons diffuse through the base layer and are transferred to the collector. It moves to the side, is accelerated by the strong electric field in the depletion layer between the base and collector, and reaches the collector. Since the ability to inject electrons from the emitter to the base changes depending on Veb, the collector current is controlled by the base voltage. In a bipolar transistor that has only a normal homojunction, when electrons are injected from the emitter to the base, holes are injected from the base to the emitter, reducing emitter injection efficiency (ratio of electron current to emitter current). do. However, in HBT, A1o3Gao7As/G between the emitter and base
Because of the presence of the aAs hetepi interface, there is a barrier of about 50 meV against holes when looking from the base side to the emitter side, and injection of holes from the base to the emitter is suppressed. Therefore, the hole concentration in the base can be increased and the emitter electron concentration can be kept low to some extent without reducing the emitter injection efficiency. As a result, the base resistance is small,
The emitter-base capacitance is small and the base width is narrow, making it suitable for high-speed operation.

(Problem to be solved by the invention) However, although the conventional HBT has the above-mentioned advantages, sufficient speed-up has not been achieved because it still includes elements that hinder speed-up. do not have.

One of the factors that hinders speeding up is the structure of the base. As a result of containing a high concentration of impurities, the minority carrier movement speed decreases due to impurity scattering and the minority carrier lifetime decreases due to an increase in recombination centers. Furthermore, since minority carriers move within the base by diffusion, the base transit time increases as the temperature decreases, and the operating speed at low temperatures is slow.

An object of the present invention is to eliminate the drawbacks of conventional HBTs and to provide a semiconductor device capable of ultra-high-speed operation.

(Means for Solving the Problems) A semiconductor device of the present invention has a collector layer made of a semiconductor having one conductivity type, and a conductivity type different from the collector layer, and a forbidden band width gradually widens from the collector layer side. and an emitter layer made of a semiconductor having the same conductivity type as the collector layer and having a wider forbidden band width than the graded base layer. .

(Function) In the semiconductor device of the present invention, minority carriers injected from the graded emitter layer to the graded base layer are first injected with high energy due to band discontinuity between the emitter layer and the graded base layer. ,
Furthermore, it is accelerated by the internal electric field of the graded base layer and passes through the graded base layer at high speed, making ultra-high-speed operation possible.

(Examples) Hereinafter, the present invention will be described in detail with reference to drawings showing examples.

FIG. 1 is a schematic cross-sectional view showing a first embodiment of the present invention. Components in FIG. 1 with the same numbers as in FIG. 6 are equivalent to those in FIG. 6 and perform the same functions.

8 is a graded base layer whose bandgap width gradually increases from the collector layer 2 side to the emitter layer 4 side. 1st
As an example of each layer in the embodiment, the graded base layer 8 is made of p'' with an acceptor concentration of about 1 x 1019 cm-3.
-AlxGa, , As, the composition of the A1 group is changed linearly from 0 to 0.2 from the collector side to the emitter side, and the rest is the same as the above-mentioned conventional example.

The difference in operation of this first embodiment from the conventional example will be explained using the above-mentioned materials and using FIG. 2 showing the band structure.

FIG. 2 schematically shows a band structure spanning the emitter layer 4, graded base layer 8, and collector layer shown in FIG. ΔEc is the difference in conduction band edge energy between the emitter layer 4 and the graded base layer 8, and ΔEb is the difference in forbidden band width within the graded base layer 8.

Since the graded base layer 8 has composition grading, the forbidden band width is larger on the emitter side than on the collector side. Since the graded base layer 8 is p-type, the difference in the forbidden band appears as a difference in energy in the conduction band. Therefore, for electrons, there is a difference in the forbidden band width.

A corresponding potential difference will exist and the electrons will be accelerated in the base by the internal electric field.

The graded base layer 8 of the above material is p”-GaAs.
If it changes from p+-AIo2Gao to 8As, the potential difference will be about 0.24V. Also,
If the graded base layer thickness is 100OA, it is 24K.
An electric field of V will be applied. Therefore, electrons entering the graded base layer by diffusion from the emitter layer 4 are first accelerated by the conduction band difference ΔEc (approximately 0.07 V in the above material) between the emitter layer 4 and the graded base layer 8. , further accelerated by the electric field in the graded base. Of course, electrons lose energy in the grate base layer due to 7-onone scattering and impurity scattering.

However, since it is accelerated again by the electric field, the average electron speed is the saturation speed (~I x 107cm/5e
c) or more. This state hardly changes even at low temperatures. For example, with a graded base layer thickness of 1oooA, the transit time is less than lpsee, which is shortened to less than half of that of the conventional structure.

Furthermore, since most of the electrons are hot, the probability of capture by a recombination center (at a higher energy position than the conduction band edge) is reduced, and the decrease in minority carrier lifetime is also suppressed.

As described above, according to the structure of the present invention, even though the impurity concentration of the base is high, the base transit time can be shortened and the decrease in minority carrier lifetime can be suppressed.

As a result, it has a high current amplification factor and can operate at ultra high speed.

Next, a manufacturing method of the first embodiment described above will be explained. The crystal growth method is MBE (Molecular
Beam Epitaxy), a donor concentration of I
n-GaAs collector layer 2 of 16 cm, thickness 50
p"- with 0 people and acceptor concentration of 2 x 1019 cm-3
An AlxGa, -xAs (x=0-0.2) graded base layer 8, and an nAlO3Gao, 7As graded base layer 4 having a donor concentration of 5x107 cm-3 and a thickness of 0.5 pm were successively grown. The emitter electrode 7 was formed by depositing and then alloying AuGe/Au on the surface of the emitter layer 4, and the base electrode 6 was formed by depositing AuZu/Au from which the emitter layer at the base electrode portion was removed by etching. The collector electrode 5 was made of In. In the HBT produced by this manufacturing method,
A delay time of 25 ps was obtained per transistor stage.

FIG. 3 is a schematic diagram of the band structure of the second embodiment of the present invention. In Figure 3, items with the same numbers as in Figure 2 are
It is equivalent to a diagram and performs the same function. 9 is provided between the emitter layer 4 and the graded base layer 8, and conduction band edge energy gradually increases from the graded base layer side to the emitter layer side so that notches are not formed between the emitter layer 4 and the graded base layer 8. This is a great emitter layer made of a certain material.

The operation of this second embodiment is almost the same as the first embodiment, but since there is no conduction band edge energy notch between the emitter and base, there is no tunneling current component passing through the notch, and the emitter and base The energy of electrons injected into the graded base layer 8 is approximately constant regardless of the voltage Veb. In the first embodiment, while Veb is low, some of the electrons pass through the notch by tunneling, so
Electrons with energy lower than ΔEo are also injected into the graded base layer 8, and the effect of the present invention is not fully exhibited at low current levels. In contrast, in this embodiment, the effects of the present invention are exhibited regardless of the current level.

The graded emitter layer 9 has a thickness of 500 layers and is A1o from the graded base layer side to the emitter layer side.
n-AlxGa1-xA5 (n=5X 1
01017a'u), and the transmitter layers 4 and 12 are
- An HBT was fabricated using A1o4Gao6As, but in the same manner as in the first example. As a result, a delay time of 25 ps was obtained per transistor stage, and a decrease in current amplification factor at low current levels was suppressed.

In the third and second embodiments of the present invention described below, np
Although only an n-type HBT has been shown, it is clear that the present invention can be similarly applied to a pnp-type HBT in which the conductivity type of the semiconductor is reversed.

FIG. 4 is a schematic band structure diagram of a third embodiment of the present invention, in which the conductivity type of the first embodiment is reversed.

In FIG. 4, the same numbers as in FIG. 2 indicate materials with opposite conductivity types. The delta gun is the difference in band edge energy between the graded base layer 8 and the emitter layer 4. The operation of this embodiment is the same as the first embodiment except that the carriers are holes. A delay time of 30 ps was obtained using the same materials and structure as in the first embodiment with the conductivity type reversed.

FIG. 5 is a schematic band structure diagram of a fourth embodiment of the present invention, in which the conductivity type of the second embodiment is reversed.

A delay time of 30 ps was obtained using the same materials and structure as in the second example in which the conductivity type was reversed, and a decrease in the current amplification factor at a low current level was suppressed.

As described above, the present invention provides npn and pnp type H
It is clear that it can be applied to BT, and the structure may be not only a mesa type but also a Brehner type using ion corporations or regrowth, and the growth order of each layer may be reversed. Furthermore, although only the MBE method was shown for growth of each layer, MOCVD (Metal Organic Chemistry)
Other growth methods such as cal vapor deposition, vapor phase growth, and liquid composition techniques may also be used.

As a semiconductor, only GaAs/AlGaAs system was shown, but GaAs/I using GaAs as well
nGaAsP/InGap system, electron saturation velocity is Ga
InGaAs using InGaAs larger than As
/ InGaAlAs / InAlAs series, InGa
As / InGaAsP / InP system, Ga
III-V such as rb/AlGaSb/AlSb system
Compound semiconductors, elemental semiconductors such as Ge/5iGe/Si, CdTe/CdZnTe/ZnTe
It is clear that the present invention can be applied to II-VI compound semiconductors such as II-VI compound semiconductors and other various semiconductors. Furthermore, although the materials shown above are combinations in which the lattice constants are almost the same, there are also materials with different lattice constants and strains (for example, InGaAs/InAlGaAs/A
The present invention is also applicable to GaAs-based materials.

(Effects of the Invention) The semiconductor device of the present invention significantly reduces the base delay time and enables ultra high-speed operation.

[Brief explanation of the drawing]

FIG. 1 is a schematic Ifi diagram of the first embodiment of the present invention;
2 is a band structure diagram thereof, FIGS. 3 to 5 are band structure diagrams of the second to fourth embodiments, FIG. 6 is a schematic sectional view of a conventional heterojunction bipolar transistor, and FIG. Figure 7 is a diagram of its band structure. 1...Semiconductor substrate 2...Collector layer 3...Base layer 4...Emitter layer 5...Collector electrode 6...Base electrode 7...Emitter electrode 8...Graded base layer 9...Graded emitter layer EC...Conduction band edge Ev...Filling band edge Er,...Fermi level Veb...Emitter-base voltage Vbc"'Base-collector voltage ΔEb... Forbidden band width difference Δ in graded base layer
E0...Conduction band edge difference Fig. 1 Fig. 2 Fig. 3 Fig. 4 21Lz U layer

Claims (3)

[Claims] (1) a collector layer made of a semiconductor having one conductivity type;
a graded base layer made of a semiconductor having a conductivity type different from that of the collector layer and whose forbidden band width gradually widens from the collector layer side; and a graded base layer having the same conductivity type as the collector layer and whose forbidden band width gradually widens from the collector layer side. 1. A semiconductor device characterized by having a structure in which an emitter layer made of a semiconductor having a width larger than that of the emitter layer is laminated. (2) The semiconductor device according to claim (1), wherein the emitter layer is an n-type semiconductor, and the conduction band edge energy of the emitter layer is higher than the conduction band edge energy of the emitter layer side of the graded base layer. (3) The semiconductor device according to claim (1), wherein the emitter layer is a p-type semiconductor, and the filling band edge energy of the emitter layer is lower than the filling band edge energy of the emitter layer side of the graded base layer.