JP2576573B2 - Bipolar transistor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Overview] Regarding the structure of a bipolar transistor having a heterojunction, an emitter made of one conductivity type Si (silicon) and an opposite conductivity type SixGe _1-x (silicon A base made of a mixed crystal of germanium) and a collector made of one conductivity type Si (silicon), wherein the x value in the SixGe _1-x is gradually increased from 0.5 to 1 from the collector side to the emitter side. The energy band gap is gradually narrowed from the emitter side to the collector side.

[Industrial applications]

The present invention relates to a structure of a bipolar transistor having a heterojunction.

Heterojunction bipolar transistors (HBTs) are attracting attention and are being studied as future high-speed LSIs (large-scale integrated circuits).

[Conventional technology]

By the way, among various heterojunction bipolar transistors, a heterojunction bipolar transistor in which a base made of a mixed crystal of SiGe (silicon germanium) is crystal-grown on a collector made of Si, and further, an emitter is crystal-grown,
Compared to a homo-junction bipolar transistor made of Si (silicon) alone, there is an advantage that the emitter injection efficiency is large even if the emitter is not doped at a high concentration, and the structure is expected to achieve higher speed.

FIG. 4 shows a schematic sectional view of such a conventional heterojunction bipolar transistor, wherein 1 is an n-Si collector region, 2 is a p-Si _0.5 Ge _0.5 base region, 3 is an n-Si emitter region, and 1E Is a collector electrode, 2E is a base electrode, and 3E is an emitter electrode. In this structure, a base region and an emitter region are sequentially grown on the collector region by MBE (Molecular Beam Epitaxy), and the lithography technology is used. It is formed by patterning.

[Problems to be solved by the invention]

However, in the conventional transistor shown in FIG. 4, the base region 2 is a Si _0.5 Ge _0.5 mixed crystal, and the energy band gap Eg is about an intermediate value between 1.1 eV (Eg of Si) and 0.7 (Eg of Ge). There is a disadvantage that a built-in electric field (built-in field) is not formed because the Eg is 0.9 eV and the Eg is a constant value.

Therefore, the present invention proposes a bipolar transistor having a structure in which a built-in electric field is formed in the base region to further increase the speed.

[Means for solving the problem]

Collector its object has its emitter formed of one conductivity type Si, the base made of opposite conductivity type SixGe _1-x mixed crystal, comprising a collector formed of one conductivity type Si, the x value of the SiGe in _1-x This is achieved by a bipolar transistor which is configured such that the energy band gap gradually decreases from 0.5 to 1 from the side toward the emitter side so that the energy band gap gradually decreases from the emitter side toward the collector side.

(Operation)

That is, in the present invention, the x value of the SixGe _1-x mixed crystal in the base region is changed from 0.5 from the collector side to the emitter side from 0.5 to 1 as a graded base, and the energy band gap Eg is changed from the emitter side to the collector side. To narrow. Then, the built-in electric field E becomes E = (1.1 eV−0.9 eV) / qW (where, q; charge amount, W; base width), and the electric field E is once accelerated by the minority carriers injected from the emitter into the base. You. Therefore, the speed can be increased as compared with a conventional transistor in which minority carriers travel only by diffusion.

〔Example〕

 Hereinafter, embodiments will be described in detail with reference to the drawings.

FIG. 1 is a schematic cross-sectional view of a heterojunction bipolar transistor according to the present invention, wherein 1 is an n-Si collector region, 5
Is a p-SixGe _1-x (x = 0.5 to 1) base region, and 3 is an n-Si
The emitter region, 1E is a collector electrode, 2E is a base electrode, and 3E is an emitter electrode.The x value of the base region increases from 0.5 to 1 from the collector region toward the emitter region, and its Eg increases from the emitter side to the collector side. It is as narrow as 1.1 eV to 0.9 eV.

FIG. 2 shows an energy band diagram of this heterojunction bipolar transistor. Ec is a conduction band, Ev is a valence band, and a band gap Eg is between Ec and Ev. At the same time, an inclination is generated from the emitter to the collector, so that the electrons travel faster in this conduction band (valence band).

When the speed is calculated, in the base region of this transistor, the minority carriers injected from the emitter travel by the electric field E, and the base traveling time τ is τ = qW ² /μ(1.1 eV−0.9 eV) (where , Μ; mobility, W; base width).

On the other hand, since the conventional transistor shown in FIG. 4 travels by diffusion, its base transit time τ ′ is τ ′ = W ² / 2D D = μkT / q (where, D; diffusion constant, k; Boltzmann constant T ; Absolute temperature).

Therefore, τ / τ ′ = 2 kT / (1.1 eV−0.9 eV), and
Assuming that T = 0.052 eV (at normal temperature), the base transit time of the transistor according to the present invention is reduced to about one-fourth of the base transit time of the conventional transistor, thereby increasing the speed.

Next, FIGS. 3 (a) to 3 (d) are cross-sectional views in the order of the steps of forming the transistor shown in FIG. 1, and will be described step by step.

First, as shown in FIG. 3 (a), a p-SixGe _1-x (x = 0.5 to 1) base region 5 (film thickness 100 nm) is heterogeneously formed on an n-Si substrate 1 (ρ = 1Ωcm) by MBE. It grows epitaxially. At this time, the X value is gradually changed from 0.5 to 1 by controlling the Si source and the Ge source separately, and a Ga source is added as a dopant to contain about 10 ¹⁸ / cm ³ to form a p-type. .

Next, an n-Si emitter region 3 (about 200 nm in film thickness) is epitaxially grown thereon by the same MBE method as shown in FIG. As the dopant, As or P is used to contain about 10 ¹⁹ / cm ³ .

Next, as shown in FIG. 3C, the base region is exposed by selective etching using lithography to pattern the emitter region 3.

See FIG. 3 (d); Next, the emitter electrode 3E,
A ring-shaped base electrode 2E and a collector electrode 1E on the back surface of the substrate are provided to complete the process. The electrodes are all made of aluminum.

As described above, the transistor structure according to the present invention is formed by growing the mixed crystal composition of the base region with good controllability, and the other forming method is the same as the conventional structure, so that it can be formed relatively easily. .

Although the above description has been made with reference to the row of npn transistors, p
It goes without saying that an np-type transistor can be similarly formed.

〔The invention's effect〕

As is apparent from the above description, the heterojunction bipolar transistor according to the present invention has a structure in which a built-in electric field is formed in the base region to shorten the base transit time of carriers, and an LSI is designed using such a transistor. Then, it can be operated at higher speed.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view of a heterojunction bipolar transistor according to the present invention, FIG. 2 is an energy band diagram of the heterojunction bipolar transistor according to the present invention, and FIGS. 3 (a) to 3 (d) are transistors according to the present invention. FIG. 4 is a schematic cross-sectional view of a conventional heterojunction bipolar transistor. In the figure, 1 is an n-Si collector region or an n-Si substrate, 2 is a p-SiGe base region, 3 is an n-Si emitter region, 5 is a p-SixGe _1-x (x = 0.5 to 1) base The region, 1E indicates a collector electrode, 2E indicates a base electrode, and 3E indicates an emitter electrode.

Claims (1)

(57) [Claims] An emitter made of one conductivity type Si (silicon), a base made of mixed crystal of an opposite conductivity type SixGe _1-x (silicon germanium), and a collector made of one conductivity type Si (silicon); The x value in the SixGe _1-x is gradually increased from 0.5 from the collector side to the emitter side from 0.5 to 1 so that the energy band gap is gradually narrowed from the emitter side to the collector side. Bipolar transistor.