JP2587826B2 - Bipolar transistor and method of manufacturing the same - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bipolar transistor capable of operating at a very high speed and a method for manufacturing the same.

[Conventional technology]

Conventional heterojunction bipolar transistors (hereinafter HBTs)
2 is shown in FIG.

In the drawing, 1 is a semi-insulating GaAs substrate such as Cr-doped, 2 is a first buffer layer made of GaAs containing a high concentration n-type impurity for making ohmic contact with the collector layer, and 9 is a uniform concentration of n.
Layer 3 made of GaAs containing a p-type impurity
A base layer made of GaAs containing n-type impurities, an emitter layer made of AlGaAs containing n-type impurities, a second buffer layer made of GaAs containing high-concentration n-type impurities for making ohmic contact with the emitter layer, Reference numerals 6, 7, and 8 are a collector electrode, a base electrode, and an emitter electrode, respectively.

The region under the emitter layer 4 in the collector layer 9 is called an intrinsic collector region 13, and the other region is an external collector region 14.
That. The region under the emitter layer 4 in the base layer 3 is called an intrinsic base region 15 and the other region is an external base region 16.
That. The external collector region 14 is selectively increased in resistance by proton implantation or the like in order to reduce the base-collector junction capacitance _Cbc . This HBT is connected to the external collector region 1
4 has an advantage that _Cbc is smaller than HBT which does not increase the resistance, so that the base-collector junction capacitance charging time τ _cc is shorter, which is advantageous for speeding up.

However, in this HBT, the impurity concentration of the collector layer 9 before proton implantation is made relatively low in order to facilitate the above-described high resistance.

When C _bc is reduced by the above-described high resistance, the thickness of the collector layer 9 before proton implantation is made relatively thick in order to increase the effect.

As described above, since the collector layer 9 has a low impurity concentration and a large layer thickness, the thickness of the base / collector depletion layer in the intrinsic collector region 13 of the HBT becomes relatively thick, which will be described below. This is a factor that hinders further increase in the speed of the HBT.

FIG. 3 shows the state of energy band and electron conduction at the base-collector junction in the intrinsic region (the intrinsic base region 15 and the intrinsic collector region 13) in the operating state of the HBT. The electrons mainly travel in the conduction band of the Γ valley 17 (the lower end of the energy is indicated by a solid line) or the L valley 18 (the lower end of the energy is indicated by a broken line), but as shown in FIG. Some of the electrons injected from the base region 15 into the intrinsic collector region 13 have high energy, so that when passing through the base-collector depletion layer formed at the base-collector junction, the electrons are higher than the valley 17. L valley with high energy
Drive 18 It is well known that the average parallel velocity of the electrons traveling in the L valley 18 is much lower (for example, about 1/10) than the average traveling velocity of the electrons traveling in the valley 17. Therefore, in such an HBT, when passing through the average base-collector depletion layer, τ _c is strongly limited to the time required for traveling in the L valley 18. This effect is remarkable because the traveling distance of the L valley increases as the width of the base / collector depletion layer becomes larger. Therefore, the HBT shown in the conventional example is disadvantageous in terms of increasing τ _c and is disadvantageous in increasing the speed. have.

[Problems to be solved by the invention]

HBT having a collector layer 9 having such a uniform impurity concentration
A disadvantage of increased tau _c in by methods of thinning or by ii collector layer 9 thickness method of increasing the i collector impurity concentration, thinning the base-collector depletion layer,
Can be improved.

However, according to the method (i), although τ _c can be reduced, the higher the impurity concentration of the collector layer 9 before the implantation of protons or the like, the more the implantation of a large amount of protons or the like is required to cancel the impurities. by injection, it has the disadvantage that selective high resistance of the external collector region 14 is practically difficult, can not be sufficiently reduced C _bc. According to the method ii, τ _c can be reduced and the resistance of the external collector region 14 can be easily increased selectively by proton implantation or the like. However, since the thickness of the external collector region 14 is small, the reduction of C _bc can be _achieved . There is a disadvantage that the effect is small.

As described above, in a conventional HBT having a collector layer having a uniform impurity concentration, C _bc cannot be sufficiently reduced by using any of the methods i and ii, so that if τ _c is reduced, τ _cc cannot be sufficiently reduced. Has disadvantages.

The operating speed of the HBT is represented by a cut-off frequency f _T , where f _T is the base-collector depletion layer transit time τ
It is a function of the sum of _c and the base-collector junction capacitance charging time τ _cc .

Where τ is the emitter-base junction capacitance charging time τ _EE
And a constant determined by the sum of the emitter-base depletion layer transit time tau _E and the base transit time tau _B. Therefore, in order to increase the speed of the HBT, it is necessary to reduce both τ _c and τ _cc , but in the conventional method, τ _c and τ _cc have a trade-off relationship as described above. There is a disadvantage that it is disadvantageous for realizing ultra-high-speed operation.

[Means for solving the problem]

An object of the present invention is to provide a base-collector depletion layer transit time τ
_c is small and base-collector junction capacitance charging time τ
_It is an object of the present invention to provide a bipolar transistor having a small _{cc and} capable of operating at a very high speed, and a method of manufacturing the same.

 FIG. 1 shows an example of a sectional structural view of the HBT of the present invention.

Reference numerals 1 to 8 and 15, 16 are the same as those of the conventional HBT shown in FIG. Reference numeral 10 denotes a collector layer having a distribution such that the impurity concentration is low near the base layer and high on the side opposite to the base layer. Reference numeral 19 denotes a region under the emitter layer 4 in the collector layer 10, which is called an intrinsic collector region, and reference numeral 20 denotes another region, which is called an external collector region. As shown in FIG. 1, in the present invention, the impurity concentration of the collector layer 10 is set to be low near the base layer 3 and high on the side opposite to the base layer 3 and to such an extent that the resistance can be easily increased by ion implantation or the like. By providing the distribution, the base / collector depletion layer generated in the intrinsic region (the intrinsic base region 15 and the intrinsic collector region 19 is made thinner, τ _c is reduced, and the external collector region 20 is reduced.
The main features of the present invention are that the resistance is increased thickly by ion implantation and τ _cc is reduced.

By making both τ _c and τ _cc sufficiently small according to the present invention, it is possible to increase the speed of the bipolar transistor.

(Operation)

Generally, the base impurity concentration of the bipolar transistor is higher than the collector impurity concentration, so that the thickness of the base-collector depletion layer of the intrinsic regions 19 and 15 of the bipolar transistor according to the present invention is determined by the impurity concentration distribution of the collector 10. Therefore, by distributing the impurity concentration of the collector layer 10 spatially higher in the vicinity of the first buffer layer 2 than in the vicinity of the base layer 3, the intrinsic regions 19 and 15 can be formed.
, The base-collector depletion layer can be made sufficiently thin, and as a result, a bipolar transistor having a sufficiently small τ _c can be realized.

Also, by performing ion implantation sufficient to trap majority carriers in the collector layer before ion implantation, the external collector region can be made sufficiently thick and sufficiently high in resistance, and as a result, the base-collector junction capacitance can be increased. Since C _bc can be made sufficiently small, a bipolar transistor having a small τ _cc can be realized.

〔Example〕

FIGS. 4 and 5 show sectional structures of an embodiment of the HBT according to the present invention and an embodiment of the conventional HBT, respectively. Reference numerals 1 to 9 and 13 to 16 in FIGS. 4 and 5 are the same as those in FIGS. 1 and 2. In FIG. 4, reference numeral 11 denotes a first collector layer containing a relatively low concentration of impurities, and reference numeral 12 denotes a second collector layer containing a higher concentration of impurities than the first collector layer.
Region of the first collector layer 11 in FIG. 4 below the emitter layer 4
21 is called an intrinsic first collector region, and the other region 22 is called an external first collector region. Second collector layer of FIG.
The region 23 under the emitter layer 4 is called an intrinsic second collector region, and the other region 24 is called an external second collector region.

Tables 1 and 2 show the material, impurity concentration, and layer thickness of each layer of the present invention and the conventional example to be compared with, respectively.

In FIG. 4, reference numeral 11 denotes a first n-type GaAs containing an impurity having the same concentration (5 × 10 ¹⁶ cm ⁻³ ) as that of the collector layer 9 of the conventional HBT.
The collector layer 12 has a higher concentration (2
This is a second collector layer made of n-type GaAs containing an impurity of (× 10 ¹⁷ cm ⁻³ ). The first collector layer 11 and the second collector layer
The total thickness of the 12 layers is 0.5
μm.

The resistance of the external first collector layer 22 and the external second collector layer 24 is selectively increased by proton implantation.

Next, FIG. 6 shows an embodiment (a cross-sectional structure during the manufacturing process) of an HBT manufacturing method according to the present invention. 1-8,11-12,15
16, 21 to 24 are the same as in FIG. FIG. 6 (a) shows 1,
FIG. 2 is a sectional structural view of a laminate of 2, 12, 11, 3, 4, and 5.

FIG. 6 (b) is a sectional structural view in which the following steps 1 to 3 are performed on the laminate shown in FIG. 6 (a).

FIG. 6 (c) is a cross-sectional structural view showing a further step 4.

FIG. 6 (d) is a cross-sectional structural view (completed view) after further performing step 5, which is the same as FIG.

Step 1: Part of the second buffer layer 5 is masked.

Step 2: Simultaneously satisfy the following expression to selectively increase the resistance of only the first and second collector regions 22 and 24 of the first and second collector layers 11 and 12 which are not masked by the mask. Under the conditions described above, protons are injected through the second buffer layer 5, the emitter layer 4, and the base layer 3.

Where N _C : the maximum value of the majority carrier density per unit volume of the first collector layer 11 and the second collector layer 12 N _B : the majority carrier density per unit volume of the base layer N: the ion implantation amount per unit area E: Ion implantation acceleration voltage α ₁ : type of implanted atoms, types of first and second collector layers 11 and 12 to be implanted, constant determined by N and E α ₂ : type of implanted atoms, type of base layer to be implanted , N, E
Constant determined by A trap density of majority carriers per unit volume generated by ion implantation in a region where the majority carrier density of the first collector layer 11 and the second collector layer 12 is maximum;
α ₁ , E, N function The trap density of majority carriers per unit volume generated by ion implantation in the base layer, and is a function of α ₂ , E, N.

Step 3: Using the mask produced in Step 1 as it is,
The buffer layer 5 and a part of the emitter layer 4 are removed to expose the base layer 3 on the surface.

Step 4: The entire remaining second buffer layer 5 and emitter layer 4 and a part of the external base region 16 are masked, and the unmasked external base region 16, external first collector region 22, and external second collector region 24 are not masked. Is removed to expose the first buffer layer 2 on the surface.

Step 5: On the first buffer layer 2, the base layer 3 and the second buffer layer 5 exposed on the surface, respectively, a collector electrode 6,
The base electrode 7 and the emitter electrode 8 are formed.

In this embodiment, the example in which the step 2 is performed before the step 3 is described, but the order of the step 2 and the step 3 can be changed.

In this embodiment, the impurity concentration of the collector layer is distributed by the first collector layer 11 and the second collector layer 12 each having a uniform impurity concentration. However, even if the impurity concentration of the collector layer is continuously changed. Good. Further, in this embodiment, protons are used for ion implantation, but ions other than protons can be used.

The effect of selectively increasing the resistance of the external first collector region 22 and the external second collector region 24 by proton implantation of the HBT of the present invention shown in FIG. 4 and FIG. Collector layer 11 and second collector layer 12
Pn junction diode having the same structure as that of
FIG. 7 shows the experimental results of the dependence of the junction capacity on the amount of proton injection measured using the method of (m × 300 μm).

The proton injection conditions are an acceleration voltage E = 140 KV and a proton injection amount N = 5 × 10 ¹² to 1 × 10 ¹⁴ / cm ² . Under these injection conditions, n _t1 (α ₁ , E, N), n _t2 (α ₂ , E, N) and N _c (=
Table 3 shows 2 × 10 ¹⁷ cm ⁻³ ) and N _B (= 4 × 10 ¹⁹ cm ⁻³ ).

As is apparent from FIG. 7, the junction capacitance under the implantation conditions and shows the applied voltage dependence as in the case without proton implantation. As shown in Table 3, under these conditions, n _t1 <N _C , and the injection amount is insufficient.
It is considered that the resistance of the second collector layer 12 was not sufficiently increased.

On the other hand, the injection condition or the capacitance in the case does not show the dependency on the applied voltage, and it is considered that the resistance of the first collector layer 11 and the second collector layer 12 is sufficiently increased by the injection. By the way, the reverse bias is -0.5V at the base-collector junction.
The junction capacitance with ₁ applied is 37% of the uninjected junction capacitance
Can be reduced to This is as n _t1 ≧ N _C shown in Table 3, presumably because the proton injection amount is sufficient.

In addition, as a result of measuring the current-voltage characteristics of the HBT manufactured under the same injection conditions as the injection conditions or the diode,
Characteristic differences from unimplanted HBT, e.g. external base region 1
No increase in the sheet resistance of No. 6 or an increase in the contact resistance between the external base region 16 and the base electrode 7 was observed. This is probably because an n _t2 <N _B as shown in Table 3.

Furthermore, under the implantation conditions, the contact resistance between the first buffer layer 2 and the collector electrode 6 did not increase as compared with the unimplanted one. This is presumably because, as shown in Table 1, the first buffer layer 2 had a sufficiently high impurity concentration and the implantation energy was appropriate, so that the resistance was not increased by proton implantation.

By appropriately selecting the ion implantation conditions from the above results, without adversely affecting the electrical characteristics of the HBT,
External first collector region 22 and external second collector region 24
Can be selectively increased in resistance, and the base-collector junction capacitance can be made comparable to that of a conventional HBT in which protons are implanted into a collector having a uniform impurity concentration.

The base of the HBT shown in FIG. 4 of the present invention and FIG.
Code 15,21,23 The intrinsic region (Fig. 4 at the time of applying a reverse bias -0.5 V ₁ to the collector junction, in the FIG. 5 and reference numeral 15
FIGS. 8 (a) and 8 (b) show the energy band diagram of FIG. 13 and the calculation results of the electric field distribution and the impurity distribution in the base / collector depletion layer. 8 (a) and 8 (b) are the same as those in FIGS. 4 and 5, respectively.

FIG. 9 shows an energy band and electron conduction at the base-collector junction in the intrinsic region (15, 21, 23) of the HBT of the present invention. 9 are the same as those in FIGS. 3 and 4.

As is clear from FIGS. 8 and 9, the HBT according to the present invention
Has a base-collector depletion layer thickness of 0.17 μm,
0.0 compared to the HBT base / collector depletion layer thickness of 0.23 μm
6 μm thinner. The average velocity of electrons in the depletion layer is 5 × 1 in the valley
Assuming 0 ⁷ cm / sec, 1 × 5 × 10 ⁶ cm / sec at the L valley,
It is assumed that 0.15 μm immediately after being injected from the intrinsic base region 15 into the base-collector depletion layer travels along the Γ valley, and the remaining depletion layer (0.02 μm in the present invention, 0.08 μm in the conventional example) travels on the L valley. In the HBT according to the present invention, the average electron transit time τ _c of the base / collector depletion layer is 0.7 psec,
With conventional HBTs, this is 1.9 psec, and with the HBT structure of the present invention, τ _c can be reduced to about 1/2 or less of the conventional HBT.

On the other hand, the base / collector depletion layer thickness in the intrinsic region is 0.23μ.
Although the base-collector junction capacitance of the intrinsic region is increased by about 35% by reducing from 0.1 m to 0.17 μm, the ratio of the intrinsic region of the actually manufactured HBT to the total base-collector junction region is small. The total base-collector junction capacitance C _bc hardly increases.

For example, in an HBT having an emitter-base junction area of 1 μm × 1 μm (corresponding to the base-collector junction area of the intrinsic region) and a base-collector junction area (corresponding to the total base-collector junction area) of 3 μm × 3 μm, the total base The increase in the collector junction capacitance _Cbc is about 8%, and therefore, the increase in τ _cc is also about 8%, which is very small when viewed from the whole τ _cc .

As is evident from the present embodiment, the HBT according to the present invention has almost no increase in τ _cc and greatly reduces τ _c alone, as compared with a conventional HBT in which proton injection is performed using a collector having a uniform impurity concentration. It has the advantage that it can be reduced.

Further, in this embodiment, the case where the impurity concentration of the collector is changed in steps has been described.
The same effect can be obtained when the voltage is continuously changed so as to be low near the first buffer layer 2 and high near the first buffer layer 2.

In the present embodiment, protons are used as implanted ions. However, the present invention is not limited to the case where protons are used, but may be oxygen ions, boron ions, or any other ions capable of causing a carrier trap by implantation. It is feasible.

〔The invention's effect〕

As described above, according to the bipolar transistor and the method of manufacturing the same of the present invention, the base region of the intrinsic region
Since the collector depletion layer is sufficiently thick and the external collector region is sufficiently thick and high resistance can be obtained by ion implantation, the base-collector transit time τ _c and the base-collector junction capacitance charging time τ _cc can be reduced at the same time. There is an advantage that a bipolar transistor that can operate at high speed can be realized.

Further, according to the manufacturing method of the present invention, as compared with the conventional manufacturing method, the impurity concentration of the collector layer is set to a concentration that can easily and selectively increase the resistance by proton implantation or the like, and the spatial distribution is reduced. The only difference is that
It has the advantage of a very simple manufacturing method.

[Brief description of the drawings]

FIG. 1 is a sectional structural view of a heterojunction bipolar transistor of the present invention. FIG. 2 is a sectional structural view of a conventional heterojunction bipolar transistor. FIG. 3 is a diagram showing the state of energy band and electron conduction at the base-collector junction in the intrinsic region in the operating state of the conventional heterojunction bipolar transistor. FIG. 4 is a sectional structural view of a heterojunction bipolar transistor according to an embodiment of the present invention. FIG. 5 is a sectional structural view of a heterojunction bipolar transistor according to a conventional embodiment. FIG. 6 is a cross-sectional structure diagram during a manufacturing process of the heterojunction bipolar transistor according to the embodiment of the present invention. FIG. 7 is a diagram for explaining the dependence of the junction capacitance on the amount of proton implantation for the bipolar transistor of the present invention. FIG. 8 is a diagram showing the relationship between the energy band, electric field distribution and impurity concentration at the base-collector junction in the intrinsic region of the present invention and the conventional heterojunction bipolar transistor. FIG. 9 is a diagram showing a state of electron conduction with an energy band at a base-collector junction in an intrinsic region of a heterojunction bipolar transistor of the present invention. 1 ... Semi-insulating GaAs substrate, 2 ... First buffer layer, 3 ...
... a base layer, 4 ... an emitter layer, 5 ... a second buffer layer, 6 ... a collector electrode, 7 ... a base electrode, 8 ... an emitter electrode, 9 ... a collector layer containing a uniform impurity concentration,
10 a collector layer having an impurity concentration distribution, 11 a first collector layer, 12 a second collector layer, 13 an intrinsic collector region of a collector layer having a uniform impurity concentration, 14 a uniform collector concentration External collector area of collector layer, 15 ……
Intrinsic base region, 16 ... External base region, 17 ...
18 ... L valley, 19 ... intrinsic collector region of collector layer having impurity concentration distribution, 20 ... external collector region of collector layer having impurity concentration distribution, 21 ... intrinsic first collector region, 22 ... external first 1 collector region, 23... Intrinsic second collector region, 24... External second collector region.

Continuation of the front page (72) Inventor Tadao Ishibashi 3-1 Morinosato Wakamiya, Atsugi-shi Nippon Telegraph and Telephone Corporation Atsugi Telecommunications Research Institute (56) References JP-A-62-49662 (JP, A) JP-A-61- 182257 (JP, A)

Claims (2)

(57) [Claims] A buffer layer of a first conductivity type having a high concentration for ohmic connection between a collector electrode and a collector layer; a collector layer of a first conductivity type formed on the buffer layer; A bipolar transistor, comprising a base layer of a second conductivity type formed on the collector layer and an emitter layer of a first conductivity type formed on the base layer, wherein at least an intrinsic collector region of the collector layer Has a spatial distribution such that the impurity concentration is lower than the buffer layer, lower near the base layer, and higher on the opposite side of the base layer, and the external collector region of the collector layer A bipolar transistor characterized by selectively having high resistance. 2. A high concentration buffer layer of a first conductivity type for ohmic connection between a collector electrode and a collector layer is formed, and a collector layer of a first conductivity type is formed on the buffer layer. Forming a base layer of the second conductivity type on the collector layer, forming an emitter layer of the first conductivity type on the base layer, comprising the collector layer, the base layer, and the emitter layer; In a manufacturing method of manufacturing a bipolar transistor, the collector layer has a spatial distribution such that an impurity concentration is lower than that of the buffer layer, lower in the vicinity of the base layer, and higher in an opposite side of the base layer. A method of manufacturing a bipolar transistor, wherein the resistance of an external collector region of the collector layer is increased by ion implantation.