JPH08288300A - Heterojunction bipolar transistor - Google Patents

Abstract

PURPOSE: To make it possible to easily form a fine, effective protective construction for emitter.base junction. CONSTITUTION: Doping concentration for spacer layer 4 is made sufficiently lower than that of an emitter layer 3, its film thickness is made thin below 100 angstrom approximately, and its operation is set to be carried out under a depletion state. In addition, this spacer layer 4 acts as a potential barrier for a large number of carriers and a small number of carriers of a base layer 5 and loses most of its potential barrier to less than 100meV for a larger number of carriers of the emitter layer 3.

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 having a high performance by using a compound semiconductor material.

[0002]

2. Description of the Related Art A compound semiconductor HBT (heterojunction bipolar transistor) formed on a GaAs substrate has excellent characteristics in terms of operation speed, high output operation, and degree of freedom in manufacturing. Expected as a high-speed device. Like many other devices, miniaturization of elements for reducing parasitic resistance and parasitic capacitance is a basic method for speeding up this HBT. Among the miniaturization, the technique of bringing the base electrode as close as possible to the emitter mesa is particularly important for reducing the base resistance, and various self-aligned structures have been used for that purpose.

Here, the problem with miniaturization is that the excess base current generated around the emitter-base junction becomes relatively larger than the total base current and the current gain decreases. is there. Therefore, in the normal HBT, a so-called guard ring structure is used in which the periphery of the junction is covered by using a part of the emitter layer, and measures are taken to suppress the leakage current due to surface recombination. However, if the base electrode / emitter mesa spacing becomes narrower than a certain value, the effect of the guard ring is lost. Therefore, it is desired to develop a new emitter / base junction protective structure and material structure that can withstand miniaturization.

In terms of material structure, InGaP having InGaP as an emitter, which is considered to be more stable than AlGaAs, is used.
/ GaAs is drawing attention. It has already been reported as a result of the life test that the life is longer than that of the AlGaAs / GaAs system. However, in manufacturing an HBT using InGaP as an emitter, a more advanced technique is required due to the difference in the chemical properties of the materials of GaAs and InGaP. Although the application of InGaP is advantageous in terms of materials, as long as a general transistor structure is used,
It is difficult to manufacture a fine guard ring structure.

FIG. 3 shows a conventional typical npn type InGa.
The structure of P / GaAs HBT is shown, (a) is a sectional view,
(B) is a band diagram. In the figure, 1
Reference numeral 1 is an emitter cap layer made of GaAs, 12 is an emitter cap layer made of a compositionally graded AlGaAs, and 13 is an emitter cap layer made of AlGaAs. Further, 14 is an I doped with silicon as an n-type impurity.
An emitter layer made of nGaP, a base layer 15 made of GaAs in which carbon is introduced as a p-type impurity, a collector layer 16 made of GaAs in which silicon is introduced as an n-type impurity, and a subcollector layer 17 made of GaAs , 18 is an emitter electrode made of AuGe alloy, 19 is a base electrode made of a laminated metal of Ti and Au, and 20 is A
It is a collector electrode made of a uGe alloy.

In the HBT of FIG. 3, AlGaAs is used for the emitter cap layers 12 and 13, but the emitter layer 14 is InGaP, and the formation of the guard ring makes the emitter layer 14 around the junction thin and uniform. It is performed by processing to the width (Reference 1: F.Ren et al., IEE
E Electron Device Lett. 14, p. 322, 1993). InGaP
Valence band discontinuity at the / GaAs hetero interface is due to AlGa
It is larger than that of As / GaAs and reduces the hole current that is reversely injected from the base into the emitter. That is,
It is possible to prevent a decrease in current gain. This is particularly effective when the element temperature becomes high. In the HBT shown in FIG. 3, since the semiconductor material of the emitter layer 14 is InGaP, a plurality of cap layers are provided.

FIG. 4 shows another conventional example, an npn type I.
The structure of nGaP / GaAsHBT is shown, (a) is sectional drawing, (b) is a band diagram. In the figure, 21 is an emitter layer made of GaAs, and 22 is InG.
aP barrier layer 23, GaAs base layer 24, GaAs collector layer 25, GaA
s subcollector layer, 26 is an emitter electrode, 27
Is a base electrode and 28 is a collector electrode.

The HBT having the structure shown in FIG. 4 is called a tunnel emitter type, and the barrier layer 22 made of InGaP is thinner than a certain thickness so that a sufficient tunnel current can be obtained, and an emitter-base breakdown voltage can be secured. It is necessary to make it thicker than a certain amount. In the reported example, the barrier layer 2
The thickness of 2 is about 200Å (Reference 2: H. Leier et al., Electron. Lett. 29, p. 868, 1993).
In this tunnel emitter type HBT, the barrier ring 22 is not intentionally formed into a guard ring structure, but the barrier layer 22 is left on the entire surface of the base layer 23 to protect the junction.

[0009]

Since the conventional configuration is as described above, there are the following problems.
First, the first problem of the HBT structure shown in FIG.
This is a fine processing for forming the emitter layer 14 made of aP into a guard ring structure. InGaP as a material characteristic
The wet etching is generally poor in controllability. Therefore, dry etching is used for this fine processing. However, in the process of dry etching, the InGaP layer and the GaA layer may be damaged by ion bombardment.
Damage occurs to the s layer. As a result, the recombination center density on the surface / bulk of InGaP or GaAs increases,
It may increase the base current or deteriorate the device life.

The second problem is that even if the guard ring structure is formed without being damaged, the remaining guard ring portion becomes relatively thick due to the restriction of the controllability of processing, and therefore the guard ring portion is formed. This is the point where a current path in which electrons are injected occurs. Here, even if the emitter layer 14 is appropriately thinly formed from the beginning to the extent that it is not necessary to thin the layer, the influence of the damage of the dry etching for forming the guard ring structure remains as described above. Moreover, the current path in the guard ring cannot be completely suppressed. After all, as long as the guard ring structure is to be formed in the n-type InGaP layer whose doping concentration and thickness are designed to function as the emitter, it is difficult to process the guard ring as the junction protective structure. Then, the problem that the damage of dry etching remains is inevitable.

On the other hand, the problem of the tunnel emitter type HBT is that, firstly, the barrier voltage of the barrier layer 22 is relatively thin, and therefore the breakdown voltage of the emitter / base is not sufficient. Secondly, the metal formed on the barrier layer 22 is alloyed through the barrier layer 22 to form the base electrode 27. For this reason,
It is necessary to control the alloy depth, and the base layer 2
The contact resistance with 3 is not necessarily low. On the other hand, if the barrier layer 22 is made thin, there arises a problem that the emitter capacitance increases and the emitter-base breakdown voltage decreases.

The present invention has been made to solve the above problems, and it is an object of the present invention to easily form a fine and effective protective structure for an emitter-base junction.

[0013]

A heterojunction bipolar transistor according to the present invention comprises a collector layer of a first conductivity type, a base layer of a second conductivity type formed on the collector layer, and a first conductivity type formed on the collector layer. An electrode connected to each layer is provided, and an electrode connected to this base layer is formed in an external base region where the base layer extends beyond the region under the emitter layer. ing.
Then, as described below, a spacer layer is formed on the base layer. First of all,
The spacer layer has an impurity concentration equal to or lower than the impurity concentration doped in the emitter layer. Secondly, it acts as a potential barrier against majority and minority carriers in the base layer. Third, the potential barrier for majority carriers in the base layer is higher than the potential barrier for the emitter layer, and acts as a barrier of 100 meV or less for majority carriers in the emitter layer. Fourth, depletion occurs in the operating state of the heterojunction bipolar transistor. Further, this spacer layer is characterized in that its band gap energy is larger toward the emitter layer side. The spacer layer is characterized in that the band gap energy thereof increases stepwise from the base layer side toward the emitter layer side.

[0014]

The spacer layer has a higher potential barrier than the emitter layer for majority carriers in the base layer and hardly acts as a barrier for majority carriers in the emitter layer, so that it does not cause a large change in the operation of the transistor. . Since this spacer layer acts as a potential barrier against majority carrier and minority carrier of the base layer, surface recombination can be suppressed as compared with the case where the base layer is exposed.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, an outline of the invention will be described before describing embodiments of the present invention. In this invention, HB
A feature of the T structure is that a "spacer layer" is newly introduced, which does not function as an emitter layer or a barrier layer of a junction, but works effectively as a surface protection of the base layer. The spacer layer adjacent to the base layer does not directly contribute to the control of the injection current.

An embodiment of the present invention will be described below with reference to the drawings. Example 1. FIG. 1 is a sectional view (a) and a band diagram (b) showing a structure of an npn type AlGaAs / GaAs HBT which is one embodiment of the present invention. In the figure, 1 is an emitter cap layer made of GaAs, and 2 is A.
This is an emitter cap layer made of AlGaAs having a composition gradient in which the composition ratio of 1 gradually increases from the emitter cap layer 1 side. Further, 3 is an emitter layer made of AlGaAs, 4 is a spacer layer made of InGaP, and 5 is GaA.
s is a base layer, 6 is a GaAs collector layer, 7 is a GaAs subcollector layer, 8 is an emitter electrode, 9 is a base electrode, and 10 is a collector electrode.

The spacer layer 4 has a doping concentration sufficiently lower than that of the emitter layer 3 and has a film thickness of 100Å.
The thickness is made less than or equal to a certain level so that it is in a depleted state in an operating state. The spacer layer 4 acts as a potential barrier for the majority carriers and minority carriers of the base layer 5, and has a potential barrier of 100 meV or less for the majority carriers of the emitter layer 3. In the structure of this embodiment, the guard ring structure in which a part of the emitter layer 3 is processed is not formed, and the spacer layer 4 is used as the surface protection layer of the base layer 5.
It was formed over the entire area.

Here, the band energy state of the portion (external base) of the base layer 5 protruding from below the emitter layer 3 is not determined only by the crystal structure or doping structure of this layer. The problem here is the surface charge.
When the surface state density is high, the positive charge (in the case of npn type) accumulated on the level of the surface of the external base serves to lower the band energy of the surface. As a result, a portion (electron channel) having a low potential for electrons is formed on the surface of the external base. These are the leak paths of injected electrons, and cause excess base current.

On the other hand, in InGaP used for the spacer layer 4, the conduction band discontinuity with GaAs is AlG.
It is smaller than the case of aAs / GaAs, and the valence band discontinuity with GaAs is sufficiently larger than that of AlGaAs / GaAs. And the spacer layer 4 is
The surface level density is lower than the base layer 5 made of GaAs and the emitter layer 3 made of AlGaAs. Since the spacer layer 4 acts as a potential barrier against the carriers of the base layer 5, surface recombination can be prevented.

By the way, also in the above-mentioned conventional structure (FIG. 3), the emitter layer 14 made of InGaP is provided on the base layer 15 made of GaAs, and the AlGaA is provided thereon.
The emitter cap layer 13 made of s is formed. That is, GaAs (base layer) -InG
The layer structure of aP-AlGaAs is similar to the structure of the emitter layer 3 (AlGaAs), spacer layer 4 (InGaP), and base layer 5 (GaAs) of this embodiment. However, in the structure shown in FIG. 3, the emitter layer 14 for controlling the electron injection is InGaP, and a neutral region, that is, a depleted region in the operating state remains in a part of this layer, which is the same as that of the above-described embodiment. The structure is completely different.

In the structure shown in FIG. 3, it is intended that the emitter layer 14 made of InGaP functions as an emitter, that is, the potential of the emitter constituting the transistor is defined by this emitter layer 14. Therefore, the existence of the neutral portion, that is, the existence of the carrier is essentially required, which is completely different from the structure of this embodiment. Further, in the conventional tunnel emitter type HBT (FIG. 4) described above, the barrier layer 22
Needs to have a certain thickness or more, and this barrier layer 2
2 controls the injection current.

On the other hand, the spacer layer 4 (FIG. 1) in this embodiment is made thinner and does not directly contribute to the control of the injection current. The role of the spacer layer 4 in the region (external base) protruding from below the emitter layer 3 in this embodiment is mainly to reduce the surface state density of the base layer 5 made of GaAs. Although the spacer layer 4 is thin so as to be depleted in the operating state, the reduction of its surface state density is achieved. Since the spacer layer 4 is thin in this way, the current path due to the “potential saddle point in the guard ring” can be cut off. Then, the band bending on the surface of the base layer 5 due to the finite surface space charge becomes small, and the carrier concentration accumulated in the surface channel of the base layer 5 can be suppressed.

Embodiment 2 FIG. FIG. 2A is a band diagram showing the configuration of the HBT which is another embodiment of the present invention. In the figure, 31 is an emitter layer made of AlGaAs, 32 is a spacer layer made of AlGaInP, and the composition ratio of Al is higher toward the emitter layer 31 side, 33
Is a base layer made of GaAs. Since the spacer layer 32 has a bandgap profile in which the Al composition ratio increases on the side of the emitter layer 31, an accelerating electric field for electrons is generated in this. Therefore, it is possible to prevent a part of the electrons injected from the emitter layer 31 and returned in the base layer 33 due to backscattering from accumulating at the interface between the emitter layer 31 and the spacer layer 32.

Example 3. FIG. 2B shows a third embodiment of the present invention.
3 is a band diagram showing the configuration of an npn-type HBT, which is the example of FIG. In the figure, 41 is AlGaA.
s is an emitter layer made of s, 42 is a spacer layer made of AlGaInP on the side of the emitter layer 41, 43 is a spacer layer made of GaInP, and 44 is a base layer made of GaAs. In this embodiment, the spacer layer 42 on the side of the emitter layer 41 has a larger band gap, and the spacer layer 4
The thicknesses of 2, 43 are sufficiently thin so that a potential notch cannot be formed. Therefore, the emitter layer 4
A part of the electrons injected from 1 and returned in the base layer 44 due to backscattering is partially emitted.
Can be prevented from accumulating at the interface.

In the above embodiment, the npn type A
Although the description has been made on the 1GaAs / GaAsHBT, the invention is not limited to this. For example, InAlAs / InGa
It may be AsHBT or another HBT. It can also be applied to a pnp type HBT.

[0026]

As described above, according to the present invention, the majority barrier and the minority carrier in the base layer act as a potential barrier, and the potential barrier for the majority carrier in the base is higher than that in the emitter layer, and at the same time, the emitter barrier. There is almost no potential barrier for majority carriers in the layer, and a spacer layer that is depleted in the operating state is formed on the base layer. Therefore, the bias condition of the transistor is the normal HBT.
Although it is basically the same as the above, it has the following excellent effects in terms of reduction of excess base current and element processing.

First, the spacer layer is thin so as to be depleted in the operating state, and even if it is thin like this, it acts as a potential barrier against majority carriers and minority carriers of the base layer, and causes surface recombination in the external base. It can be reduced. As described above, since the spacer layer is thin, it is possible to cut off the current path due to the “potential saddle point in the guard ring” that is found in the conventional structure. As a result, this prevents the injected carriers from flowing into the surface channel on the outer base protruding from below the emitter layer, and reduces the excess base current. Further, since the spacer layer is thin, band bending on the surface of the base layer due to finite surface space charge is reduced, and carrier concentration accumulated in the surface channel of the base layer is reduced. This will reduce surface recombination and again reduce excess base current.

Since the guard ring structure is not provided, the processing margin becomes large, and the base electrode can be formed close to the emitter mesa. Therefore, it is possible to reduce the base resistance, reduce the base-collector junction area, and reduce the collector capacitance. The above contributes to high speed and low power consumption of the integrated circuit using the HBT. Even if the spacer layer is present, the emitter layer has the same doping concentration and thickness as the normal H
Since it can be set almost in the same manner as the BT structure, a necessary and sufficient emitter-base breakdown voltage can be obtained.

Further, since the bandgap energy of the spacer layer is made larger toward the emitter side, an accelerating electric field for electrons is generated in this. Therefore, it is possible to prevent a part of the electrons injected from the emitter layer and returned by the backscattering in the base layer from accumulating at the interface between the emitter layer and the spacer layer. In addition, I
Since nAlGaP or InGaAsP is used, the surface state density in the external base can be further reduced, and the above-mentioned effects can be further exerted.

[Brief description of drawings]

FIG. 1 is an npn-type AlG according to one embodiment of the present invention.
It is sectional drawing (a) and band diagram (b) which show the structure of aAs / GaAsHBT.

FIG. 2 is a band diagram showing the configuration of an HBT that is another embodiment of the present invention.

FIG. 3 Conventional typical npn-type InGaP / GaA
The structure of sHBT is shown, (a) is sectional drawing, (b) is a band diagram.

FIG. 4 is another example of conventional npn-type InGaP /
The structure of GaAs HBT is shown, (a) is a sectional view, (b)
Is a band diagram.

[Explanation of symbols]

1 ... Emitter cap layer, 2 ... Emitter cap layer, 3
... emitter layer, 4 ... spacer layer, 5 ... base layer, 6 ... collector layer, 7 ... subcollector layer, 8 ... emitter electrode, 9
... base electrode, 10 ... collector electrode.

Claims (5)

[Claims] 1. A collector layer of a first conductivity type, a base layer of a second conductivity type formed on the collector layer, and a first layer formed on the base layer of the second conductivity type.
An electrode including a laminated structure composed of conductive type emitter layers, electrodes for connecting to the respective layers being provided, and electrodes for connecting the base layer to an external base region extending from a region below the emitter layer. Wherein the spacer layer is formed on the entire surface of the base layer, the spacer layer having an impurity concentration equal to or lower than the impurity concentration doped in the emitter layer, It acts as a potential barrier for minority carriers and majority carriers in the layer, the potential barrier for majority carriers in the base layer is higher than the potential barrier for the emitter layer, and as a barrier of 100 meV or less for majority carriers in the emitter layer. Working, in the operating state of the heterojunction bipolar transistor Heterojunction bipolar transistor, characterized in that it is formed so as depleted. 2. The heterojunction bipolar transistor according to claim 1, wherein the spacer layer has a larger bandgap energy toward the emitter layer side. 3. The heterojunction bipolar transistor according to claim 1, wherein the band gap energy of the spacer layer increases stepwise from the base layer side toward the emitter layer side. Heterojunction bipolar transistor. 4. The heterojunction bipolar transistor according to claim 1, wherein the emitter layer is made of n-type AlGaAs and the spacer layer is made of InAlGaP. Transistor. 5. The heterojunction bipolar transistor according to claim 1, wherein the emitter layer is made of n-type InAlGaAs and the spacer layer is made of InGaAsP. Transistor.