JP2518385B2 - Bipolar transistor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a bipolar transistor having a new structure which is excellent in high speed and can be easily integrated.

[Prior Art] The structural requirements for a bipolar transistor with excellent high speed are that the base width and emitter stripe width are as small as possible, and the external base region is minimized to reduce the base resistance as much as possible. It is to reduce the parasitic base-collector capacitance associated with the external base. In order to avoid punch-through (penetration) between the emitter and the collector, it is required to reduce the base width and gradually increase the impurity concentration in the base region. Ordinary bipolar transistor (hereinafter referred to as homo BT) in which all the emitter, base and collector regions are made of silicon single crystal
In order to secure the emitter injection efficiency, it is indispensable that the impurity concentration in the emitter region is higher than the impurity concentration in the base region, and the following problems are raised as the impurity concentration increases.

The first problem relates to an increase in emitter junction capacitance. That is, as the impurity concentration in both the emitter and base regions increases, the emitter junction capacitance increases and the cut-off frequency ( _FT ) in the low current operating region decreases, resulting in low power consumption, which is essential for high-speed / high-integrated LSI. Make high speed operation difficult. Further, lowering the breakdown voltage of the emitter junction is also a problem.

The second problem is that the increase of the impurity concentration lowers the effective band gap, and therefore the increase of the impurity concentration of the emitter region lowers the emitter injection efficiency, thereby limiting the upper limit of the impurity concentration and thus the lower limit of the base width. To do.

To alleviate the above-mentioned problems faced by homo-BTs, homo-BTs having a polysilicon emitter structure (hereinafter referred to as polysili-emitter BTs) are currently in practical use. In order to avoid the above problems, a heterojunction bipolar transistor (hereinafter referred to as a hetero BT) is being developed.

An example of the polysilicon emitter BT is shown in FIG.
In the figure, 21 is an emitter region made of high impurity concentration n-type polycrystalline silicon, 22 is an emitter region made of high impurity concentration n-type single crystal silicon, 23 is a natural oxide film of silicon, 24
Is a base region made of p-type silicon, 25 is a collector region made of n-type silicon, and 26 is an n-type substrate having a high impurity concentration.

The structural feature of this poly-silicon emitter BT is that the emitter region is an n-type single crystal region 22 formed by shallow diffusion adjacent to the base region 24 and a low resistance n-type polycrystalline silicon (polysilicon) region deposited thereon. That is, an extremely thin (about 1 nm) silicon native oxide film 23 is formed between the single crystal silicon region 22 and the polycrystalline silicon region 21.

The characteristic feature of the poly-silicon-emitter BT is that the natural oxide film 23 effectively forms a large transmission barrier for holes compared to electrons, thus reducing the flow of holes from the base to the emitter. This results in an improvement in emitter injection efficiency, and as a result, enables a current amplification factor 5 to 6 times larger than that of a normal homo BT.

The problem facing the poly-silicon emitter BT is the natural oxide film.
23 in controllability. That is, the natural oxide film is not intentionally formed, but is naturally formed as a result of the n-type single crystal surface being exposed to the air, or by the chemical treatment of the n-type single crystal surface during the deposition of polycrystalline silicon. However, it is difficult to control reproducibly. As shown in FIG. 2B, the natural oxide film has a large potential barrier 27 (about 0.7 eV) not only for the potential barrier 28 for holes but also for electrons.
Therefore, an extremely thin natural oxide film is required to realize the required sufficiently small emitter resistance, which is a cause of difficulty in controlling it. As a result of having potential barriers of approximately the same size for electrons and holes, the effective transmission barrier effect for each depends on that the effective mass of holes is large compared to the effective mass of electrons. The effective transmission barrier ratio of holes is not large enough. Therefore, to obtain a practically large current amplification factor (about 50 or more),
The impurity concentration in the emitter needs to be higher than the impurity concentration in the base, and the problem faced by the homo BT cannot be fundamentally solved.

On the other hand, in the hetero BT, the emitter region is composed of a material having a larger forbidden band width than that of the base region, and the potential barrier against holes formed at this time blocks holes injected from the base to the emitter. As a result, even in a structure in which the impurity concentration in the emitter is smaller than the impurity concentration in the base, a current amplification factor of a large practical level is possible. Hetero BTs therefore have the potential feature of avoiding the challenges facing regular Homo BTs. The main emitter / base heterojunctions currently under study are microcrystalline Si / crystalline Si, amorphous Si / crystalline Si, SiC / Si, Si / Si.
Ge alloy and the like.

The problem with these heterojunctions is that they have excellent characteristics sufficient to realize a practical bipolar transistor and do not have highly reliable characteristics. In other words, microcrystalline Si / crystalline Si and amorphous Si / crystalline Si systems are crystallized by heat treatment at about 450 ° C. or higher, so that the forbidden band width of microcrystalline Si or amorphous Si is reduced and the effect of the heteroemitter is reduced. The SiC / Si system is poor in practical use due to the increase in emitter resistance due to the difficulty of low resistance ohmic contact to SiC, and the Si / Si / Ge system uses Si / Ge alloy in the base region of the transistor. Therefore, it is extremely difficult to put it into practical use as a VLSI process due to the large process restrictions.

An object of the present invention is to provide a bipolar transistor that solves the above problems.

[Means for solving the problem]

The present invention provides an npn-type bipolar transistor,
A silicon nitride film (SiN _x , where 0.2 <x <0.8) is sandwiched in the n-type emitter region near the boundary between the p-type base region and the n-type emitter region or the junction between the emitter and the base. And

In the present invention, the donor concentration in the n-type emitter region between the silicon nitride film and the base region is made lower than the acceptor concentration in the base region.

[Action]

The present invention is remarkable in that the SiN _x / Si system using an amorphous silicon nitride (SiN _x , 0.2 <x <0.8) film with excess silicon is small for electrons in silicon and large for holes. It takes advantage of the fact that it exhibits asymmetric electron barrier properties. That is, in the SiN _x / Si system, an extremely small potential barrier of about 0.1 eV is exhibited for electrons in silicon regardless of x, whereas for holes, from about 0.9 eV at x = 0.2, Based on the finding that there is a significant asymmetry in the potential barrier that increases almost linearly with increasing x and reaches a value of about 1.6 eV at x = 0.8, the SiN _x film is used as an emitter-base junction boundary or emitter junction. By inserting into the adjacent emitter region, the hole current from the base to the emitter in the npn transistor is selectively and effectively blocked.

〔Example〕

FIG. 1 (a) is a diagram showing the structure of an embodiment of a bipolar transistor according to the present invention. FIG. 1 (b) shows an energy band diagram in thermal equilibrium of the structure of FIG. 1 (a).

According to the embodiment of FIG. 1, the emitter region has a high concentration (>
N-type polycrystalline silicon region 1 containing arsenic (As) of 10 ²⁰ cm ^-3 ), silicon nitride film 3 of N / Si ~ 0.5 (x = 0.5) and a thickness of about 30Å, and a relatively low concentration (~ 10 ¹⁸ cm ^-3 ) n containing arsenic
The mold single crystal silicon layer 2 is formed. The base area is
A p-type layer (width ~) containing high concentration (~ 10 ²⁰ cm ^-3 ) of boron (B)
0.1 μm) 4. The collector is similar to the conventional structure, and is composed of an n-type layer 5 adjacent to the base and a low-resistance n-type region 6 which is a high impurity concentration n-type substrate thereunder. A feature of the structure of this embodiment compared to the conventional structure is that the donor concentration in the emitter region 2 adjacent to the emitter junction is higher than the acceptor concentration in the base region 4.

The energy band diagram in the thermal equilibrium of the structure of this embodiment is the first
It is shown in FIG. Reference numerals 7 and 8 denote potential barriers for electrons and holes of the silicon nitride film 3, respectively. The feature compared with the energy band diagram of the conventional structure using the natural oxide film shown in FIG.
This is because the potential barrier 7 for electrons formed by is significantly smaller than the potential barrier 17 of the natural oxide film. x = 0.
In the case of the silicon nitride film (SiN _x ) of 5, the potential barrier heights for electrons and holes are about 0.1 eV and 1.3 eV, respectively.

Next, the electrons and holes are assumed to be transmitted through the potential barrier of the silicon nitride film by the tunnel effect, and the transmission probability thereof is calculated. When the barrier height is φ, the barrier thickness is d, and the effective mass of electrons or holes in the barrier is m, the energy E
The transmission probability T for electrons or holes of Given in. here Is the Planck constant h Is. The subscripts e and h attached to the respective symbols represent the quantities for electrons and holes, respectively. φ _e
＝ 0.1eV, φ _h ＝ 1.3eV, d ＝ 30Å, m _e ＝ m _h ＝ m ₀ (= free electron mass, 9.8E−28gr) as thermal energy (E = 0.025e)
When determining the transmission probability ratio T _h / T _e of the holes with respect to electrons having _{V), T h / T e} 1.6 × 10 -13 , and the transmission probability of holes to electrons is extremely small substantially holes flow Turns out to be blocked. Therefore, even in the structure of this embodiment in which the acceptor concentration in the base is about two orders of magnitude higher than the donor concentration in the emitter, a sufficiently large emitter injection efficiency and thus a sufficiently large current amplification factor are realized.

The silicon nitride thin film used in the structure of the present invention can be easily formed by chemical vapor deposition using the reaction of silane (SiH ₄ ) and ammonia (NH ₃ ). Its composition can be controlled by controlling the ratio of silane and ammonia supplied. Further, the deposition of polycrystalline silicon, the formation of a thin single crystal silicon layer, etc. can be easily realized by applying the currently developed silicon technology such as the silicon crystal growth technology including chemical vapor deposition and molecular beam epitaxy, respectively.

〔The invention's effect〕

As described above, the bipolar transistor according to the present invention can be easily realized by the conventional process technology,
Moreover, the conventional bipolar transistor avoids the problems faced in the pursuit of structural miniaturization in that the impurity concentration in the base can be increased regardless of the impurity concentration in the emitter without impairing the current amplification factor. is there. In particular, the structure in which the impurity concentration in the base is higher than the impurity concentration in the emitter realizes excellent characteristics such as low base resistance, low emitter capacitance, and low temperature operation, which have a substantially heterojunction bipolar transistor. is there. The structure of the present invention has no technical difficulty in high integration, and is ultra-high speed.
It is a useful device for realizing LSI.

[Brief description of drawings]

FIG. 1 (a) is a diagram showing a configuration of a bipolar transistor according to an embodiment of the present invention, FIG. 1 (b) is an energy band diagram in thermal equilibrium of the bipolar transistor of FIG. 1 (a), and FIG. 2 (a). ) Is a diagram showing the structure of a conventional bipolar transistor using a polysilicon emitter, and FIG. 2 (b) is an energy band diagram in thermal equilibrium of the bipolar transistor of FIG. 2 (a). 1 ... High impurity concentration n-type polycrystalline emitter region 2 ... Low impurity concentration n-type single crystal emitter region 3 ... Silicon nitride thin film 4 ... High impurity concentration p-type base region 5 ... N-type collector region 6 ... High impurity concentration n-type substrate 7 ... Silicon nitride film electron potential barrier 8 ... Silicon nitride film hole potential barrier 21 ... High impurity concentration n-type polycrystalline emitter region 22 ... High N-type single-crystal emitter region with impurity concentration 23 ... natural oxide film of silicon 24 ... p-type base region 25 ... n-type collector region 26 ... n-type substrate with high impurity concentration 27 ... against electrons in natural oxide film Potential barrier 28 ... Potential barrier against holes in natural oxide film

Claims (2)

(57) [Claims] 1. An npn-type bipolar transistor,
A silicon nitride film (SiN _x , where 0.2 <x <0.8) is sandwiched in the n-type emitter region near the boundary between the p-type base region and the n-type emitter region or the junction between the emitter and the base. And a bipolar transistor. 2. The bipolar transistor according to claim 1, wherein the donor concentration in the n-type emitter region between the silicon nitride film and the base region is lower than the acceptor concentration in the base region.