Heterojunction bipolar transistor and method for producing...

Details Heterojunction bipolar transistor and method for producing... Heterojunction bipolar transistor and method for producing...

2000-04-19

2001-08-07

Fourson, George (Department: 2823)

Semiconductor device manufacturing: process

Forming bipolar transistor by formation or alteration of...

Having heterojunction

C257S197000

Reexamination Certificate

active

06271098

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the realization of high performance of a heterojunction bipolar transistor (HBT). More particularly, it relates to the realization of high performance of an HBT having a ballast resistor layer in an emitter layer.
2. Description of the Related Art
FIG. 11
is a sectional view of an HBT based on GaAs/AlGaAs of the conventional structure. An emitter layer consists mainly of an n-AlGaAs layer, a base layer mainly of a p-GaAs layer and a collector mainly of an n-GaAs layer. The emitter layer includes an n-InGaAs contact layer having a narrow band gap and the collector layer includes an n-GaAs contact layer formed therein. In the drawing, the numeral
1
denotes an n
+
-In
0.5
Ga
0.5
As layer,
2
denotes an n
+
-In
x
Ga
1−x
As (x=0 to 0.5) layer,
3
denotes an n
+
-GaAs, layer
4
denotes an n
−
-GaAs ballast resistor layer,
6
denotes an n-Al
y
Ga
1−Y
As layer (y=0 to 0.26),
7
denotes an n-Al
0.26
Ga
0.74
As layer,
8
denotes a non-doped GaAs layer,
9
denotes a p
+
-GaAs base layer,
10
denotes an n
−
-GaAs collector layer,
21
denotes an emitter electrode,
22
denotes a base electrode and
23
denotes a collector electrode.
In such a structure as described above, when the device temperature rises due to the HBT operation, the number of electrons flowing through the device, namely the current in the emitter region, increases leading to instability of the device characteristics.
In order to counter this problem, a ballast resistor layer
4
(carrier concentration: 1×10
16
cm
−3
, film thickness: 2000 Å) which acts as a resistor when the device temperature rises resulting in increased current, is provided between the n
+
-GaAs layer
3
and the n-AlGaAs layer
6
in the emitter region, thereby causing the current to flow through the resistor layer
4
and preventing excessive current from flowing.
Although device characteristics can be prevented from being destabilized, by increasing the device temperature in the HBT which has the ballast resistor layer
4
formed therein, a new problem arises in that the current amplification factor &bgr; (rate of change in collector current I
C
to a minute change in base current I
B
when voltage V
CE
between the collector and the emitter is constant) of the HBT decreases compared to the HBT having a conventional structure.
The present inventors found the following fact as a result of an intensive study. That is, in the HBT shown in
FIG. 11
, since the ballast resistor layer
4
(carrier concentration: 1×10
16
cm
31 3
) is formed on the n-AlGaAs layer
6
(carrier concentration: 5×10
17
cm
31 3
) in the emitter layer, carriers are diffused from the n-AlGaAs layer
6
where the carrier concentration is high to the n
−
-GaAs layer of the ballast resistor layer
4
, resulting in decreased carrier concentration in the n-AlGaAs layer
6
. While such a decrease in the carrier concentration in the n-AlGaAs layer
6
causes the absolute value of the temperature coefficient &phgr;(=&dgr;V
BE
/&dgr;T) of the emitter-base voltage V
BE
to increase under a condition that the emitter current of the HBT is constant and, as a result, causes the current amplification factor &bgr; of the HBT to decrease.
SUMMARY OF THE INVENTION
Thus, the present invention includes providing an HBT structure with a ballast resistor layer in an emitter layer thereof, which can prevent the current amplification factor &bgr; from decreasing, and a method for producing the same
The present inventors have intensively studied. As a result, it has been found that the carrier concentration in the n-AlGaAs layer can be prevented from decreasing by forming an n-GaAs carrier supply layer having a specified carrier concentration between the ballast resistor layer and the n-AlGaAs layer, thereby completing the present invention.
That is, the present invention provides a first conductivity type GaAs collector region; a second conductivity type GaAs base region formed on the first conductivity type GaAs collector region; an emitter region formed on the second conductivity type GaAs base region, including at least a first conductivity type AlGaAs layer and a first conductivity type GaAs ballast resistor layer formed on the first conductivity type AlGaAs layer; and a first conductivity type GaAs carrier supply layer which includes carriers of such a concentration that prevents the carriers from being depleted, and is provided between the first conductivity type AlGaAs layer and the first conductivity type GaAs ballast resistor layer.
By providing the first conductivity type GaAs carrier supply layer including carriers of, such a concentration that prevents the carriers from being depleted between the first conductivity type AlGaAs layer and the first conductivity type GaAs ballast resistor layer, drifting of the carriers from the AlGaAs layer can be prevented by replenishing the carriers which have moved from the AlGaAs layer to the GaAs ballast resistor layer of a lower concentration to balance the Fermi levels of both layers in the related art.
This configuration prevents the carrier concentration of the AlGaAs layer from decreasing even in the HBT having the GaAs ballast resistor layer, thereby making it possible to stabilize the absolute value of &phgr;(&dgr;V
BE
/&dgr;T) of the HBT and prevent the current amplification factor &bgr; from decreasing.
The carrier concentration in the first conductivity GaAs carrier supply layer is preferably higher than the carrier concentration in the first conductivity AlGaAs layer.
By setting the carrier concentration in the GaAs carrier supply layer higher than that of the AlGaAs layer, carriers can be prevented from flowing out of the AlGaAs layer.
The carrier concentration in the GaAs carrier supply layer is preferably from about 1.0 to 5.0×10
18
cm
−
.
The proportion of Al in the first conductivity type AlGaAs layer is preferably 0.20 or more.
The HBT of the present invention preferably further comprises an In
x
Ga
1−x
As (0<×<0.5) emitter contact layer on the emitter region.
The present invention also provides a method for producing an HBT, which comprises the steps of: forming a first conductivity type GaAs collector substrate; laminating a second conductivity type GaAs base region, and an emitter region which includes at least a first conductivity type AlGaAs layer, a first conductivity type GaAs carrier supply layer formed on the first conductivity type AlGaAs layer, and a first conductivity type GaAs ballast resistor layer formed on the first conductivity type GaAs carrier supply layer, on the first conductivity type GaAs collector substrate; forming an emitter electrode on the emitter region; etching the emitter region until the base region is exposed by using the emitter electrode as a mask, thereby forming emitter mesa portions; and providing a base electrode on the base region and a collector electrode on the collector region, respectively.
The step of forming the emitter mesa portions preferably includes a first step of etching the emitter region down to the AlGaAs layer through selective etching by using the AlGaAs layer as the etching stopper layer, and a second etching step of etching the remaining emitter region until the base region is exposed.
The etching step which uses the AlGaAs layer as the etching stopper layer is capable of etching to uniform depth, resulting in uniform depth of the etching mesa which is formed by etching the remaining emitter region to expose the base surface, thereby making it possible to reduce the variations in the characteristics among devices.
The first etching step is preferably selective dry etching employing an etching gas which is a mixture gas that includes halogen gas, oxygen gas and nitrogen gas, the oxygen gas concentration being 3%.
This is so because the use of such an etching gas makes it possible to easily carry out selective etching by the use of AlGaAs as the

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