JPH11121461A - Hetero junction bipolar transistor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain small base resistance and large maximum oscillation frequency, by constituting a base layer of a hetero junction bipolar transistor of a composition gradient type base layer consisting of In, Ga, As of specific composition ratio, and providing the base layer with impurity concentration gradient. SOLUTION: A base layer of a hetero junction bipolar transistor is constituted of a composition gradient type base layer 2 consisting of In0.x Ga1-x As, and the base layer 2 is provided with impurity concentration gradient. Therefore, it is possible to reduce base resistance by forming a region of high impurity concentration. Since drift electrolysis is generated by using the composition gradient type base layer 2, a base traveling time is reduced. As a result, shielding frequency fT is improved and a maximum oscillation frequency fmax is thereby enlarged, and a base traveling time is also reduced. Therefore, surface recombination in a base layer is reduced and reliability is improved.

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).
The present invention relates to a heterojunction bipolar transistor characterized by a configuration for reducing the base resistance of an InP / InGaAs heterojunction bipolar transistor.

[0002]

2. Description of the Related Art Conventionally, compound semiconductor devices such as heterojunction bipolar transistors using III-V compound semiconductors having high electron mobility such as GaAs and InGaAs have been widely used as high-frequency devices or high-speed switching devices.

A conventional npn emitter-up type InP / InGaAs-based HBT will now be described with reference to FIG. First, an n ⁺ -type In _0.53 Ga _{0.47 is} formed on a semi-insulating InP substrate 31 by metal organic chemical vapor deposition (MOVPE).
As the sub-collector layer 32, i-type In _0.53 Ga _{0. 47} As intrinsic collector layer 33, p ⁺ -type an In _0.53 Ga _0.47 As the base layer 34, n-type InP emitter layer 35, n ⁺ -type InP second
The emitter layer 36 and the n ⁺ -type In _0.53 Ga _0.47 As contact layer 37 are sequentially epitaxially grown.

Next, an emitter electrode 38 made of WSi is used.
With n as the mask⁺Type In_0.53Ga _0.47As contact
Etching the layers 37 to n-type InP emitter layer 35
Forming an emitter mesa⁺Type In_0.53Ga_0.47Asbe
Then, the base layer 39 is exposed.
Utilizing the step disconnection in the emitter electrode 38 and the emitter mesa
To form an emitter electrode 38 in a self-aligned manner.

Then, a base electrode 39, p ⁺ -type In _0.53
Ga _0.47 As base layer 34, i-type In _0.53 Ga _0.47 As
Intrinsic collector layer 33 and n ⁺ -type In _0.53 Ga _0.47 A
A part of the s sub-collector layer 32 is etched to form a base mesa, and then a collector electrode 40 is formed by a lift-off method using a photoresist pattern to complete the basic structure of the HBT.

For such an electronic device, reduction of the parasitic resistance is an essential problem for improving the performance of the device, but an InP / InGaAs-based HB having a high intrinsic speed performance is used.
T becomes more susceptible to parasitic capacitance and resistance.

[0007] For example, the maximum oscillation frequency f _max which is an index representing the operation characteristics of the HBT, the frequency cut off f _T, the base resistance of R _B, when the C _BC and base / collector capacitance, f _max = {f _T / is represented by _{(8πR B · C BC)}} 1/2, as the base resistance R _B is small, and, as a parasitic capacitance base / collector capacitance C _BC is small, to increase the maximum oscillation frequency f _max Can be.

Accordingly, the maximum oscillation frequency to the f _max is increased, it is necessary to reduce the base resistance R _B, but need to be doped to a high impurity concentration of about 1 × 10 ²⁰ cm ^-3 base layer in order that In order to improve the reliability of the operating characteristics, it is necessary to use an impurity having a small diffusion coefficient. Recently, as a p-type impurity for the InGaAs layer, Zn (zinc) having a large diffusion coefficient has been used. The use of C (carbon) has become mainstream.

[0009]

However, the conventional InP
/ InGaAs-based HBT, I
In lattice matched to nP substrate_0.53Ga_0.47Using an As layer
But In_0.53Ga _0.47Carbon dope into As layer
Ping is very difficult, p-type In_0.53Ga_0.47As Ba
There is a problem that it is difficult to lower the resistance of the semiconductor layer.

That is, when the MOVPE method is used, I
As serving as a group V element source constituting the n _0.53 Ga _0.47 As layer
Although carbon is doped together with H ₃ and PH ₃ , the carbon reacts with H (hydrogen) as a carrier gas or H (hydrogen) generated by the decomposition of AsH ₃ and PH ₃ to deactivate the carbon. This is because it is difficult to be taken into the growth layer, and the current technology has settled to a concentration of about 2 × 10 ¹⁹ cm ⁻³ .

However, at such an impurity concentration, the p-type I
The resistance of the n _0.53 Ga _0.47 As base layer was 1000 Ω / □ or more in sheet resistance and the base resistance was considerably high, so that the maximum oscillation frequency f _max could not be increased.

Therefore, the present invention provides a p-type InGaAs
An object of the present invention is to improve element characteristics by increasing the impurity concentration of a base layer and reducing the base resistance.

[0013]

FIG. 1 is an explanatory view of the principle configuration of the present invention. Referring to FIG. 1, means for solving the problems in the present invention will be described. In addition, FIG.
2 is a schematic band diagram of an HBT. FIG. 1 (1) In the present invention, in a heterojunction bipolar transistor, the base layer is composed of a compositionally graded base layer 2 made of In _x Ga _{1 -x} As, and the base layer has an impurity concentration gradient. It is characterized by.

Since the impurity taking-up efficiency depends on the composition ratio of the grown layer, the base layer is composed of the composition-graded base layer 2 made of In _x Ga _{1 -x} As, so that the impurity concentration in the base layer is reduced. And a region having a high impurity concentration can be formed to reduce the base resistance.

Further, by using a base layer 2 of the composition gradient type, cutoff frequency f _T is improved because drift field is reduced is generated base transit time, thereby increasing the maximum oscillation frequency f _max And the base transit time is reduced, so that surface recombination in the base layer is reduced and reliability is improved.

(2) Further, according to the present invention, in a heterojunction bipolar transistor, the base layer is made of In _x Ga _{1 -x}
It is composed of a composition-graded base layer 2 made of As, and is characterized in that carbon is used as a conductivity type determining impurity of the base layer.

When the composition of InGaAs is closer to GaAs, that is, Ga-rich InGaAs has a higher carbon uptake efficiency, the base layer is made of In _x Ga _{1 -x} A.
By using the composition gradient type base layer 2 made of s, a low-resistance base layer can be formed even when carbon having a low incorporation rate is used as the conductivity-type determining impurity.

(3) The present invention is characterized in that in the above (1) or (2), the composition gradient type base layer 2 is a base layer whose composition changes continuously.

(4) The present invention is characterized in that in the above (1) or (2), the composition gradient type base layer 2 is a base layer whose composition changes stepwise.

As described above, the composition gradient type base layer 2 may be a base layer whose composition continuously changes or a base layer whose composition changes stepwise.

(5) According to the present invention, in the above (4), the thickness of each of the plurality of layers constituting the step-like composition gradient structure is such that the non-equilibrium transport by hot electron injection becomes possible. It is characterized by having.

As described above, when the step-like composition gradient structure is adopted, the non-equilibrium transport by hot electron injection becomes possible depending on the thickness of each of the plurality of layers constituting the step-like composition gradient structure. It is desirable to make the layer thickness, that is, a thickness capable of maintaining a hot electron state.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG.
The first embodiment will be described. In addition, FIG.
FIG. 2B is a sectional view of a main part of the HBT, and FIG.
It is a band diagram of. First, the MOVPE method is used on the semi-insulating InP substrate 11 as shown in FIG.
With a thickness of 350 nm and an impurity concentration of 1 × 10¹⁹cm
^-3N⁺Type In_0.53Ga_0.47As subcollector layer 12,
300 nm thick undoped i-type In_0.53Ga_0.47A
s intrinsic collector layer 13, thickness 30 to 200 nm, for example
For example, p doped with C at 50 nm⁺Type In _xGa_1-xA
s graded base layer 14 and a thickness of 50 nm,
3 × 10 impurity concentration¹⁷cm^-3N-type InP emitter layer
15, 25 nm thick and 5 × 10 impurity concentration¹⁸cm^-3
N⁺Type InP second emitter layer 16 and a thickness of 50
nm and the impurity concentration is 1 × 10¹⁹cm^-3N⁺Type In
_0.53Ga_0.47As contact layers 17 are sequentially grown.

In this case, the p ⁺ type In _x Ga _{1 -x} A
The composition change of the s graded base layer 14 is caused by n-type InP
Continuously, the In ratio x on the emitter layer 15 side is 0.01 to 0.52, for example, 0.46, and the In ratio on the i-type In _0.53 Ga _0.47 As intrinsic collector layer 13 side is 0.53. The n-type InP emitter layer 15
The impurity concentration in the vicinity of the side is In _0.53 Ga _0.47 As
In When 3 × 10 ¹⁹ cm ^-3, it is possible to doping of In _0.46 Ga _0.54 As at 3.6 × 10 ¹⁹ cm ^-3.

Next, the emitter electrode 18 made of WSi
With n as the mask⁺Type In_0.53Ga _0.47As contact
Etching layers 17 to n-type InP emitter layer 15
Forming an emitter mesa⁺Type In_xGa_1-xAs Gre
Exposed base layer 14 and then Pt / Ti
Base electrode 19 made of / Pt / Au is connected to emitter electrode 1
8 and emitter using the step break in the mesa
The electrode 18 is formed in a self-aligned manner.

Next, the base electrode 19, p⁺Type In_xG
a_1-xAs graded base layer 14, i-type In_0.53G
a_0.47As intrinsic collector layer 13 and n⁺Type In_0.53
Ga _0.47Etching a part of the As subcollector layer 12
To form a base mesa, and then a photoresist pattern
Ti / Pt / Au by lift-off method using
Forming a collector electrode 20 made of HBT
The basic structure of is completed.

In the first embodiment of the present invention,
Is p as a base layer⁺Type In_xGa _1-xAs Grede
Since the base layer 14 is used, impurities in the base layer
2 × 10¹⁹Doping can be performed as described above.

That is, Ga-rich InGaAs (GaAs)
In s-like InGaAs), stable Ga-C
Since the bond is easily formed, the efficiency of capturing carbon is increased, and therefore, a part of the base layer is made of i-type In _0.53 Ga.
_0.47 be constituted by a layer of As intrinsic collector layer 13 lattice-matched to the composition by the composition of the emitter side to the Ga-rich, enables high concentrations of doping, therefore, possible to reduce the base resistance R _B since it is, the maximum oscillation frequency f is inversely proportional to the square root of the base resistor R _B
max can be increased.

Since the base layer has a graded band gap structure, a drift electric field of about 6 kV / cm is generated in the base layer, and the drift electric field shortens the base transit time. cut-off frequency f _T is improved since that.

As described above, since the maximum oscillation frequency f _max is proportional to the square root of the cut-off frequency f _T , the cut-off frequency f _{max
As T} increases, the maximum oscillation frequency f _max further increases.

Further, as a result of shortening the base transit time by the drift electric field, the surface recombination in the p-type In _x Ga _{1 -x} As graded base layer 14 is reduced, so that the reliability of the device is improved.

Next, referring to FIG. 3, a second embodiment of the present invention will be described.
An embodiment will be described. FIG. 3 (a) shows the essentials of the HBT.
FIG. 3B is a partial cross-sectional view, and FIG.
It is an eargram. 3 (a) and 3 (b) In the HBT according to the second embodiment of the present invention,
Only the configuration of the source layer is different.
Since it is completely the same as the embodiment, only the configuration of the base layer is changed.
explain. P in the second embodiment⁺Type InG
The aAs base layer 21 is formed by a C-doped p-type layer from the emitter side.
⁺Type In_0.46Ga_0.54As layer 22, C-doped p⁺Type I
n _0.5Ga_0.5As layer 23 and C-doped p⁺Type I
n_0.53Ga_0.47It is composed of an As layer 24.

In this case, the impurity concentration of the p ⁺ -type In _0.46 Ga _0.54 As layer 22 is about 3.6 × 10 ¹⁹ cm ⁻³ ,
Further, p ⁺ -type In _0.53 Ga _0.47 As layer 24 impurity concentration becomes about 3.0 × 10 ¹⁹ cm ^-3, further, p ⁺ -type I
The impurity concentration of the n _0.5 Ga _0.5 As layer 23 is intermediate between these.

In this case, the p ⁺ type In _0.46 Ga _0.54
The thickness t ₁ of the As layer 22 is 5 to 30 nm, for example, 20 nm.
and the thickness t ₂ of the p ⁺ -type In _0.5 Ga _0.5 As layer 23 in nm.
Is 5 to 30 nm, for example, 20 nm, and the thickness t ₃ of the p ⁺ -type In _0.53 Ga _0.47 As layer 24 is 5 to 30 n.
m, for example, 10 nm. Each of these individual layer thicknesses t _{1 to} t ₃ is a thickness that enables non-equilibrium transport by hot electron injection, that is, hot electron injection for each electron mean free path. It is desirable to make the thickness such that the base travel time is shortened, and thus the cutoff frequency f _T is improved.

In the second embodiment of the present invention as well, the p ⁺ -type In _0.53 Ga _0.47 As layer 24 lattice-matched to the semi-insulating InP substrate is used as the collector-side layer. Ga-rich p ⁺ type I
Since the n _0.46 Ga _0.54 As layer 22 is used, the carbon concentration can be made 2 × 10 ¹⁹ cm ⁻³ or more.

In this case, the number of the semiconductor layers constituting the p ⁺ -type InGaAs base layer 21 does not need to be three.
It may be composed of two layers, or may be composed of four or more layers. In any case, the forbidden band width may be gradually reduced from the emitter side to the collector side.

Although the embodiments of the present invention have been described above, the present invention is not limited to the configurations of the above embodiments, and the collector layer may be made of InP or InGaAs.
A double hetero junction using sP may be used.
The emitter layer is not limited to InP.
It may be composed of AsP.

In the description of the embodiment of the present invention, a single HBT is described for the sake of simplicity. However, in practice, it is often used in an integrated manner.
In this case, it is necessary to perform semi-insulation by implanting hydrogen ions, that is, protons or oxygen ions, into the periphery of the collector layer for isolation between elements.

[0039]

According to the present invention, InP / InGaAs is used.
Since the base layer of the HBT is composed of a compositionally graded base layer, it can be doped at a high concentration even if carbon having low incorporation efficiency is used as a conductivity-type determining impurity, thereby reducing the base resistance. Therefore, device characteristics can be improved.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a basic configuration of the present invention.

FIG. 2 is an explanatory diagram of the first embodiment of the present invention.

FIG. 3 is an explanatory diagram of a second embodiment of the present invention.

FIG. 4 is an explanatory diagram of a conventional HBT.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 emitter layer 2 composition-graded base layer 3 collector layer 11 semi-insulating InP substrate 12 n ⁺ -type In _0.53 Ga _0.47 As sub-collector layer 13 i-type In _0.53 Ga _0.47 As intrinsic collector layer 14 p ⁺ -type In _x Ga _{1 -x} As graded base layer 15 n-type InP emitter layer 16 n ⁺ -type InP second emitter layer 17 n ⁺ -type In _0.53 Ga _0.47 As contact layer 18 emitter electrode 19 base electrode 20 collector electrode 21 p ⁺ -type InGaAs base layer 22 p ⁺ -type In _0.46 Ga _0.54 As layer 23 p ⁺ -type In _0.5 Ga _0.5 As layer 24 p ⁺ -type In _0.53 Ga _0.47 As layer 31 semi-insulating InP substrate 32 n ⁺ -type In _0.53 Ga _0.47 As sub-collector layer 33 i-type In _0.53 Ga _0.47 As intrinsic collector layer 34 p ⁺ -type In _0.53 Ga _0.47 As the base layer 35 n-type InP emitter layer 36 ⁺ -Type InP second emitter layer 37 n ⁺ -type In _0.53 Ga _0.47 As contact layer 38 emitter electrode 39 base electrode 40 collector electrode

Claims (5)

[Claims] 1. A hetero-junction bipolar transistor, wherein a base layer is composed of a composition-graded base layer of In _0.x Ga _1-x As, and the base layer has an impurity concentration gradient. 2. A heterojunction bipolar transistor comprising a base layer comprising a compositionally graded base layer of _{In.sub.xGa.sub.1 -} xAs and using carbon as a conductivity type determining impurity of the base layer. . 3. The heterojunction bipolar transistor according to claim 1, wherein the composition gradient type base layer is a base layer whose composition changes continuously. 4. The heterojunction bipolar transistor according to claim 1, wherein the composition gradient type base layer is a base layer whose composition changes stepwise. 5. The method according to claim 4, wherein the thickness of each of the plurality of layers constituting the stepwise composition gradient structure is a layer thickness that enables non-equilibrium transport by hot electron injection. Heterojunction bipolar transistor.