JP2758611B2 - Bipolar transistor element - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a homojunction bipolar transistor device using GaAs, and more particularly to a bipolar transistor device having a large current amplification factor and excellent high-frequency characteristics.

[Conventional technology]

Bipolar transistors have higher current driving capability than field-effect transistors (FETs), and thus have attracted attention as circuit elements for operating circuits with large loads at high speed. In particular, a compound semiconductor such as GaAs (gallium / arsenic) has an electron mobility higher than that of Si (silicon), and is therefore advantageous in utilizing the high speed of a bipolar transistor.

Conventionally, in a bipolar transistor using GaAs as a main material, an emitter is formed of Al with respect to a base of GaAs.
Transistors with a heterojunction between the emitter and base using a material with a wide bandgap consisting of three elements of GaAs (aluminum, gallium, arsenic) occupy the mainstream of development. Such heterojunction bipolar transistors are It is already about to be put to practical use. The reason why the heterojunction bipolar transistor has been developed is that this structure enables a high concentration of the base impurity without lowering the emitter injection efficiency, so that the base resistance is reduced and a high-speed device can be realized.

On the other hand, a homojunction bipolar transistor in which the emitter is formed of the same GaAs as the base has not received much attention in the past, but has the following advantages as compared with the heterojunction bipolar transistor.

First, since an n-type layer and a p-type layer can be formed by diffusing or ion-implanting impurities into a commercially available GaAs substrate, a complex epitaxial growth apparatus used when fabricating a heterojunction bipolar transistor is used. It is relatively easy to manufacture without having to do so.

Second, conventional silicon bipolar transistor technology and MESFET technology without using a heterojunction can be easily diverted, which is suitable for the production and mass production of large-scale integrated circuits.

Third, since there is no recombination current in the depletion layer due to a trap level in AlGaAs, stability and reproducibility of device characteristics are excellent.

[Problems to be solved by the invention]

Despite these advantages, homo-junction bipolar transistors made of GaAs are not widely used because the emitter injection efficiency decreases as the impurity concentration of the base increases in order to lower the base resistance. This is because it cannot be obtained.

An object of the present invention is to provide a homojunction bipolar transistor in which the reduction in emitter injection efficiency is improved and a sufficient current gain can be obtained even when the base resistance is lowered.

[Means for solving the problem]

In order to solve the above problems, the present invention provides an n-type conductive Ga
In a bipolar transistor element in which an As layer, a p-type conductive GaAs layer, and an n-type conductive GaAs layer are provided in this order, and an ohmic electrode is connected to each of the layers, the n-type conductive GaAs layer An np junction, which is an interface between the p-type conductive GaAs layer, and the p-type conductive GaAs layer and the n-type conductive GaAs layer.
The pn junction, which is an interface of the As layer, is substantially parallel to the (111) plane of GaAs single crystal constituting the bipolar transistor element or a plane equivalent thereto.

Each of the above layers is formed by epitaxial growth on a semi-insulating or n-type GaAs single crystal substrate having a (111) plane as a surface, or an impurity having n-type and p-type conductivity with respect to GaAs. Is formed by ion implantation or thermal diffusion.

[Action]

Hereinafter, the operation of the configuration of the present invention for solving the above problems will be described.

Cutoff frequency, a figure of merit for bipolar transistors
_ft and the maximum oscillation frequency _fmax are respectively expressed as follows.

Here, in equation (1), τ _E is the emitter charging time, τ _B is the base traveling time of the carrier, τ _C is the collector traveling time,
τ ′ _C is the collector charging time. Also, r _{b in} equation (2)
Is the base resistance, and C _C is the collector capacitance.

tau _B represents the time required for the carriers to diffuse the base region, is an important factor for determining the value of f _t, this value is the base width W, when the diffusion constant of the carriers and D In the case of a uniform impurity base, it is given by the following equation.

τ _B = W ² / 2D Therefore, in order to improve the high frequency characteristics, it is necessary to make the base thinner and shorten the carrier transit time. On the other hand, since the base resistance r _b is increased when thinning the base width W,
Equation (2) shows that the maximum oscillation frequency f _max decreases. As described above, in order to improve the high frequency characteristics of the bipolar transistor, it is necessary to reduce the sheet resistance by increasing the impurity concentration of the base layer while reducing the thickness of the base layer.

While Another characteristic important in the bipolar transistor is a current amplification factor h _FE, the current amplification factor h _FE of the emitter grounding circuit in DC operation is given by the following equation.

_{h FE = γα T M / (} 1-γα T M) (3) However, gamma: an emitter injection efficiency alpha _T: based transportation efficiency M: a collector doubling factor.

Here, since the base transport efficiency alpha _T and collector doubling factor M it can be regarded approximately 1, h _FE becomes γ / (1-γ). Therefore, in the case of an npn transistor, the current amplification factor _hFE is expressed by the following equation.

Where n _B : minority carrier (electron) concentration in the base p _E : minority carrier (hole) concentration in the emitter D _B : minority carrier (electron) diffusion constant in the base D _E : minority carrier (positive) in the emitter Hole) Diffusion constant L _B : Diffusion length of minority carrier (electron) in base L _E : Diffusion length of minority carrier (hole) in emitter.

By the way, there is a relationship of D = kTμ / q (k: Boltzmann constant, T: absolute temperature, q: charge of the carrier) between the diffusion constant D and the mobility μ of the carrier. Then Holds, the current amplification factor h _FE is given by the following equation.

Here, μ _nB : mobility of electrons in the base μ _pE : mobility of holes in the emitter τ _nB : lifetime of electrons in the base τ _pE : lifetime of holes in the emitter.

As is apparent from equation (5), the current amplification factor h _FE is inversely proportional to the square root of the hole movement μ _{pE in} the emitter. On the other hand, the mobility μ is inversely proportional to the effective mass m * of the hole and raised to the 3/2 power. It can be seen that the current amplification factor h _FE increases in proportion to the 3/4 power of the effective mass m * of holes. That is, in order to increase the current amplification factor h _FE can by increasing the effective mass of a hole.

Since the effective mass is related to the second derivative of the energy band in k-space, the same material may have different values depending on the crystal orientation. In particular, in GaAs, according to a report by Hayakawa et al., The effective mass of heavy holes m _hh *
The [111] is different from the azimuth and the [100] direction, the electron mass in free space when the _{m 0, m hh * [111} ] = 0.9m 0 m hh * [100] = 0.34m 0 to the report Have been. (Toshiro Hayakawa et al., Physical Review Letters Vol. 60, No. 4, 349-352
Therefore, heavy holes moving in the [111] direction have low mobility, and the current amplification factor h _FE of the homojunction bipolar transistor increases as can be seen from equation (6). Also,
According to Hayakawa et al., The effective mass of the light hole does not change, and is m _lh * [111] = m _lh * [100] = 0.117m ₀ .

The effective mass m * considering both the heavy holes m _hh * and the light holes m _lh * is given by m * = (m _hh * ^3/2 + m _lh * ^3/2 ) ^2/3. approximately from [111] and [100] respectively orientation effective mass m * is the, m * [111] = 0.93m 0 m * [100] = 0.38m 0 next, m * [111] the m * [100] 2.5 times.
Therefore, it can be seen from the equation (6) that the current amplification factor h _FE increases twice.

As described above, in GaAs, the characteristics of the bipolar transistor can be improved by using the effective mass increase of holes in the [111] direction and using the (111) plane of GaAs.

FIG. 1 compares the current amplification factor of a homojunction bipolar transistor fabricated on a (100) plane GaAs and a homojunction bipolar transistor fabricated on a (111) plane as a function of the base impurity concentration. Here, the calculation is performed taking into account the band gap reduction due to the high concentration doping. As is clear from the figure, the base concentration at which the same current amplification factor can be obtained is about twice as large in the (111) plane, and it can be seen that the orientation enables the base to be highly concentrated.

FIG. 2 shows that the impurity concentration of the emitter and the base is 1 × 10 ¹⁶
This shows the relationship between the current amplification factor h _FE and the base width W when cm ^-3 is set. As shown in the figure, in the transistor manufactured on the (111) plane, even when the base width W is 500 °, the current amplification factor h _FE is as large as 300, and the _hFE decreases due to the surface recombination. Therefore, it is considered that practically sufficient amplification is possible.

Based on the above calculations, the homojunction bipolar transistor fabricated on the (111) plane of GaAs single crystal has a sufficiently large current amplification factor, and is suitable for increasing the base concentration, thinning the layer, and miniaturizing the element. Speedup can be achieved.

Example Next, a specific element structure will be described with reference to examples.

Embodiment 1 FIG. 3 is a diagram showing a schematic cross-sectional structure of a semiconductor device according to a first embodiment of the present invention.

In the figure, 31 is a semi-insulating GaAs (111) substrate, 32 is n
Type GaAs collector / contact layer, 33 is an n-type GaAs collector layer, 34 is a p-type GaAs base layer, 35 is an n-type GaAs emitter layer,
36 is an n-type GaAs cap layer, 37 is an emitter electrode, 38 is a base electrode, and 39 is a collector electrode.

The device of this embodiment is formed on a semi-insulating GaAs (111) substrate.
GaAs is grown by an epitaxial growth method such as molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (OMVPE).
This is an element manufactured by forming a multilayer film of s thin film crystal.

That is, to fabricate the device of the present embodiment, first, a high-concentration (1 × 10 ¹⁸ cm ⁻³ or more) n-type GaAs collector / contact layer 32 is grown on a semi-insulating GaAs substrate 31. Then, an n-type GaAs collector layer 33 with a low concentration (about 1 × 10 ¹⁵ to 1 × 10 ¹⁷ cm ⁻³ ) is grown, and then a high-concentration (1 × 10 ¹⁸ cm ⁻³ or more) p-type GaAs base layer is formed. Grow layer 34, then 1 × 10 ¹⁷ to 1 × 10
An n-type GaAs emitter layer 35 of about ¹⁸ cm ^-3 is grown, and then a high concentration (1 × 10 ¹⁸ cm ^-3 or more) n-type GaAs cap layer 36 is grown. Next, the base layer 34 and the collector contact layer are formed by wet etching or dry etching.
32 exposed, n-type GaAs for emitter and collector
A metal such as AuGeNi that makes ohmic contact
Metals such as AuZnNi and AuCr that make ohmic contact to p-type GaAs are connected to the base as 7, 39
Connect as 38.

In this embodiment, a semi-insulating GaAs substrate is used.
A similar layer configuration or a layer configuration excluding only the collector contact layer 32 may be formed on the As substrate, and in this case, the collector electrode can be taken from the back surface of the substrate.

Embodiment 2 FIG. 4 is a view showing a schematic sectional structure of a semiconductor device according to a second embodiment of the present invention. In the figure, 41 is semi-insulating Ga
As (111) substrate, 42 is an n-type collector layer, 43 is a p-type base layer, 44 is an n-type emitter layer, 45 is an emitter electrode, 46 is a base electrode, and 47 is a collector electrode.

The device according to the present embodiment is configured such that n-type conductive layers 44 and 42 and p-type
This is an element in which the mold conductive layer 43 is formed. That is, first, Si,
An n-type impurity such as Se or S is implanted or diffused at a high concentration, and then a p-type impurity such as Zn, Be, or C is implanted or diffused at a high concentration. Finally, an n-type impurity is again implanted or diffused to form an npn layer, and then ohmic electrodes 45, 46, 43 according to the conductivity of each layer are formed, thereby completing the device.

In the present embodiment, an n-type Ga
It can also be manufactured using an As substrate, and in that case, the substrate can be used as a collector layer, so that the first n-layer forming step can be omitted.

[Explanation of effects]

As described above, the bipolar transistor element according to the present invention has a higher current amplification factor than a conventional homojunction bipolar transistor element formed on a conventional GaAs {100} substrate. The concentration can be increased, and thus the high-frequency characteristics of the transistor can be improved. Furthermore, the device according to the present invention can be manufactured using an epitaxial technique, but in addition, it can be manufactured by an ion implantation method or a thermal diffusion method used in a Si bipolar process and a GaAs MESFET process. have. Therefore, since the manufacturing process is simple, there is an advantage that it is suitable for manufacturing a large-scale integrated circuit.

[Brief description of the drawings]

FIG. 1 shows the current amplification factor h _FE of a conventional homojunction bipolar transistor fabricated on a GaAs (100) plane and a homojunction bipolar transistor fabricated on a (111) plane according to the present invention.
Is a function of the base impurity concentration.
Figure is a graph showing the current amplification factor h _FE of the homojunction bipolar transistor fabricated in manufactured in a conventional GaAs (100) plane was a homojunction bipolar transistor according to the present invention (111) plane as a function of the base width W, the FIG. 3 schematically shows a cross section of the device according to the first embodiment of the present invention. FIG. 3 shows an example of a device constituted by an epitaxial layer. FIG. BRIEF DESCRIPTION OF THE DRAWINGS It is the figure which showed the cross section of the element which is an Example simply, and shows the example of the element | device distributed in the impurity by the ion implantation method or the thermal diffusion method to the GaAs (111) substrate. 31 ... semi-insulating GaAs (111) substrate 32 ... n-type GaAs collector / contact layer 33 ... n-type GaAs collector layer 34 ... p-type GaAs base layer 35 ... n-type GaAs emitter 36 ... n-type GaAs Emitter / cap layer 37 Emitter electrode 38 Base electrode 39 Collector electrode 41 Semi-insulating GaAs (111) substrate 42 N-type collector layer 43 P-type base layer 44 N-type emitter Layer 45 Emitter electrode 46 Base electrode 47 Collector electrode

Claims (1)

(57) [Claims] An n-type conductive GaAs layer; a p-type conductive GaAs layer;
In a bipolar transistor element in which an n-type conductive GaAs layer is provided in this order and an ohmic electrode is connected to each of the layers, an interface between the n-type conductive GaAs layer and the p-type conductive GaAs layer np junction, the p-type conductive GaAs layer and the n-type conductive
A bipolar transistor element, wherein a pn junction which is an interface of the GaAs layer is substantially parallel to a (111) plane of GaAs single crystal constituting the bipolar transistor element or a plane equivalent thereto.