JPH04355928A - Hetero-junction bipolar transistor - Google Patents

Abstract

PURPOSE:To avoid the surface re-coupling on the exposed base emitter junction interface peripheray thereby enabling a HBW to be miniaturized. CONSTITUTION:The title hetero-junction bipolar transistor composed of an emitter layer 3 comprising the first conductivity type the first semiconductor, a base layer 2 comprising the second conductivity type the second semiconductor in contact with the emitter layer 3 and in narrower band gap than that of the first semiconductor and a collector layer 1 comprising the first conductivity type the third semiconductor in contact with the base layer 2 and in wider band gap than that of the second semiconductor is provided with a sulfur single crystal film formed covering the exposed part of the junction interface periphery between the emitter layer 3 and the base layer 2.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heterojunction bipolar transistor (HBT), and more particularly to a high-performance compound semiconductor HBT with a fine emitter.

0002

2. Description of the Related Art With the recent progress in HBTs, it has become necessary to reduce the emitter size in so-called emitter-up type devices in order to achieve higher density. As the device becomes smaller, carrier recombination current at the periphery of the base-emitter junction interface exposed by mesa etching cannot be ignored, and this limits the current amplification factor of HBT. Peripheral passivation is required. Up to now, as a passivation technology for the exposed peripheral part of the bonding interface, Applied Physics
There is a method of forming a film on the surface of an element using Na2S.9H2O as shown in Letters, 52(3) 218 (1988).

0003

[Problems to be Solved by the Invention] However, the above-mentioned surface passivation using Na2S.9H2O
) has high water absorption and properties are sensitive to moisture in the atmosphere,
(2) The Na2S.9H2O thin film itself has ionic conductivity, and (3) its effect lasts only for a short time in air. These problems pose significant obstacles to practical application.

SUMMARY OF THE INVENTION An object of the present invention is to provide an HBT having a passivation structure around the exposed base-emitter junction interface that solves the above-mentioned problems in order to enable miniaturization of the HBT.

[0005]

Means for Solving the Problems The present invention provides an emitter layer made of a first semiconductor of a first conductivity type, and a second semiconductor of a second conductivity type that is in contact with the emitter layer and has a narrower bandgap than the first semiconductor. a base layer consisting of; in contact with the base layer;
a collector layer made of a third semiconductor of a first conductivity type having a wider bandgap than the second semiconductor; The present invention provides a heterojunction bipolar transistor comprising the following.

[0006] The above-mentioned sulfur monolayer film is a film in which only one layer of sulfur molecules or sulfur atoms is formed, preferably one in which only one layer of sulfur atoms is formed. Then, the sulfur single layer film is formed to cover the depletion layer around the exposed junction interface, and preferably the base emitter bias is 0V.
It is preferable that the width is greater than the depletion layer width when the depletion layer region is covered.

[0007]

[Operation] When a sulfur single layer film is formed to cover the exposed periphery of the bonding interface between the emitter layer and the base layer, the sulfur single layer film acts as a passivation film, reduces the number of surface recombination centers, and promotes the surface recombination of carriers. Suppress current. Since this carrier recombination current becomes a problem in the depletion layer region, a single sulfur film covering the depletion layer around the exposed junction interface substantially has a passivation effect. In addition, a multilayer sulfur film, which is made up of multiple layers of molecules or atoms, is unstable and tends to change over time due to the detachment of sulfur atoms, whereas a single-layer sulfur film is stable, does not change over time, and is an insulating material. It doesn't make you feel bad either.

[0008]

【Example】

Example 1 Embodiments of the present invention will be described below with reference to the drawings.

FIGS. 1(a) to 1(c) are diagrams showing the main process for manufacturing an AlGaAs/GaAs npn-type HBT according to an embodiment of the present invention.

First, as shown in FIG. 1(a), n+-GaAs
On one main surface of the substrate 4, an n+ GaAs collector layer 1 doped with 5 x 1015 cm-3 of Si and a 5 x 1013 Be doped layer are formed.
A pGaAs base layer 2 doped with cm-3 and an n-AlGaAs emitter layer 3 doped with 2x1017 cm-3 of Si are grown using an MBE apparatus. Then,
As shown in Figure 1(b), n-A is
An lGaAs emitter layer 3 is formed in a mesa, and then an n-layer contact electrode 5 made of AuGe/Ni/Au is formed on the emitter layer 3 and the back surface of the substrate 4, and an n-layer contact electrode 5 made of AuGe/Ni/Au is formed on the base layer 2 exposed by the mesa formation. After forming the p-layer contact electrode 6 made of AuZn/Au, 4
Contacts are formed by alloying for 2 seconds at 20°C.

Next, it was immersed in a NH4OH/H2O (1:2) solution for 5 seconds and a HNO3/H2O (1:19) solution for 2 seconds, and then immediately immersed in a (NH4)2SX solution containing about 8% excess S. (This serves to remove the natural oxide film on the semiconductor surface.) A sulfur film (not shown) is formed to cover the exposed peripheral edge of the mesa-shaped emitter layer 3 and base layer 2 bonding interface. This sulfur film covers the entire exposed portions of the emitter layer 3 and base layer 2. Subsequently, after diluting the solution by gradually adding pure water, the substrate 4 is taken out. Next, as shown in FIG. 1(c), polyimide 7 is spin-coated on one main surface of the substrate 4 as a water-resistant protective film. After this, each electrode 5 is etched using ordinary photolithography and oxygen plasma.
, 6 are opened. The HBT thus formed is referred to as sample A.

When the semiconductor surface 3 is treated with the (NH4)2SX solution, the surface of the sulfur film, which is initially multilayered, is gradually reduced as described above or exposed to vacuum after the treatment. The sulfur atoms are gradually removed, and the final result is a monoatomic sulfur layer that is stable and shows the least change over time. This means that the bonds of As-S and Ga-S are S-S
This is because the bond is stronger than that of .

Example 2 After forming electrodes in the same manner as above, NH4OH/H2O (1
:2) immerse the mesa-shaped emitter layer 3, base layer 2, and bonding interface in the mesa-shaped emitter layer 3, base layer 2, and bonding interface. A sulfur monoatomic layer film (not shown) is formed on the exposed peripheral portion in the same manner as in Example 1. At this time, in order to form a sulfur monoatomic layer film, the solution concentration may be gradually lowered as described above, or the solution may be exposed to vacuum after treatment. Note that a monomolecular layer film may also be formed depending on the degree of this treatment. Subsequently, the substrate 4 is taken out, and polyimide 7 is spin-coated as a water-resistant protective film on one main surface of the substrate 4, as shown in FIG. 1(c). Thereafter, contact holes to each of the electrodes 5 and 6 are opened by ordinary photolithography and etching using oxygen plasma. The HBT thus formed is referred to as sample B.

(NH4)2SX solution and P2S5
When the solution containing sulfur comes into contact with the surface of a compound semiconductor, it removes the native oxide film on the surface of the compound semiconductor that acts as a surface recombination center and forms a single layer of sulfur. This sulfur single layer film is more stable than a film made of NaS.9H20, but as described above, by further providing a water-resistant protective film, the stability of the element can be further improved.

Comparative Example 1 After forming an electrode in the same manner as above, NH4OH/H2O (1
:2) Immerse the substrate 4 in the solution for 5 seconds and in the HNO3/H2O (1:19) solution for 2 seconds.
Sample C was obtained by spin coating on one main surface of the film and drying it to form a protective film.

Comparative Example 2 After forming an electrode in the same manner as above, NH4OH/H2O (1
:2) Immerse the substrate 4 in the solution for 5 seconds and in the HNO3/H2O (1:19) solution for 2 seconds, immediately wash the substrate 4 with water, and then spin-coat polyimide 7 as a water-resistant protective film on one main surface of the substrate 4. Then, contact holes to each of the electrodes 5 and 6 were opened by ordinary photolithography and etching using oxygen plasma to obtain sample D.

FIG. 2 shows the above samples A, B, C, and D prepared.
After storing in the atmosphere for 10 days, the current amplification factor β of HBT
This figure shows the results of measuring the emitter size dependence of . As is clear from the figure, in Samples C and D, the current amplification factor β decreases rapidly as the emitter size becomes smaller, whereas in Samples A and B, the decrease in the current amplification factor β is very small. By forming a sulfur monolayer film around the junction interface between the emitter layer and the base layer in this way, it is possible to reduce the surface recombination current, thereby maintaining a high current amplification factor even in a finer HBT structure. It becomes possible to do so.

In the above embodiment, AlGaAs/Ga
Although the explanation was made using As-based HBT, InGaAs/I
Other lattice-matched systems such as nAlAs and InGaAs/InP systems, or equilattice-mismatched InGaAs/AlGaAs systems may be used, and the present invention is not limited to the above embodiments.

Furthermore, regarding the structure of the HBT, it is possible to use not only a single hetero bipolar transistor with a large bandgap only for the emitter, but also a double hetero bipolar transistor using a wide bandgap material for the collector as well.

Furthermore, the water-resistant protective film is not limited to polyimide, and other protective films such as SiN and SiO2 may also be used.

Furthermore, in addition to the surface treatment shown in the above embodiment, for example, the HBT shown in FIG. 1(b) may be treated with Na2S.9H2O.
There is also a method in which the HBT in Figure 1(b) is soaked in water and then washed with water to remove Na2S and leave a sulfur monolayer film.
It is also possible to use a method of immersing the film in 2O, washing with water to remove Na2S, and then further performing the treatment of the above embodiment in order to form a more complete sulfur monoatomic layer film.

[0022]

[Effect of the invention] The passivation structure according to the present invention has
It is stable against moisture in the atmosphere, has a long-lasting effect with little change over time in the atmosphere, and has a passivation structure that eliminates the risk of leakage current due to the passivation film itself, so there is no need to sacrifice current amplification characteristics. Without, H
It becomes possible to miniaturize BT. This makes it possible to put high frequency, highly integrated HBTs into practical use.

[Brief explanation of drawings]

FIG. 1: AlGaAs/GaA, which is an embodiment of the present invention.
FIG. 2 is a diagram illustrating the main processes for manufacturing an npn-type HBT.

FIG. 2 is a diagram showing the results of measuring the emitter size dependence of the current amplification factor β of the HBT after being stored in the atmosphere for 10 days.

[Explanation of symbols]

1. ．． ．． Collector layer 2. ．． ．． base layer
3. ．． ．． Emitter layer 4. ．． ．． Substrate 5. ．． ．． N-layer contact electrode 6. ．． ．． p-layer contact electrode
7. ．． ．． Water resistant protective film

Claims (1)

[Claims] 1. An emitter layer made of a first semiconductor of a first conductivity type; a base layer made of a second semiconductor of a second conductivity type and in contact with the emitter layer and having a narrower bandgap than the first semiconductor; a collector layer made of a third semiconductor of a first conductivity type having a wider bandgap than the second semiconductor; and a sulfur single layer formed to cover an exposed peripheral edge of the bonding interface between the emitter layer and the base layer. A heterojunction bipolar transistor comprising: a film;