Method for manufacturing a junction field effect transistor

The present invention provides a method for manufacturing a field effect transistor which overcomes problems occurring in the manufacture of InP material junction field effect transistors. Because the electron saturation velocity is higher than that of silicon or GaAs it is desirable to have a gate length shorter than the mask length as well as to have the source, drain, and gate metals evaporated by the self-aligned method. The present invention provides a method of achieving gate lengths of 1 .mu.m or shorter without requiring an expensive electron beam apparatus or X-ray lithography apparatus.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a junction type field effect transistor 
used in high frequency integrated circuits and optoelectronic integrated 
circuits and more particularly to a homojunction type and heterojunction 
field effect transistor of a self-aligned structure having a short gate 
length. 
2. Information Disclosure Statement 
Generally, in the manufacture of a junction field effect transistor, it has 
been known to that the manufacture of a PN junction below a gate is the 
most important technology. 
The prior art methods for manufacturing the PN junction below the gate 
metal are as follows; 
First, the method of diffusing a P type dopant into a N type channel layer; 
Next, the method of implanting a P type dopant ion into a N type channel 
layer; 
Next, the method of etching an undesired portion after growing an P type 
epitaxial layer on a N type channel layer. 
However, in view of that the length the gate electrode must be short in 
order to obtain large transconductance and high cut-off frequency, there 
are some problems in the above mentioned prior art methods. 
The method of manufacturing JFET by a diffusion and implantation is not 
reproducible since thickness control of a channel layer resulted in P 
dopant diffusion is difficult, and manufacture of JFET having a short gate 
below 1.about.2 .mu.m is difficult because of non-self-aligned structure. 
When the junction forming technology by the P type epitaxial layer growth 
method is used, there is a problem in that the etching control is 
difficult and not reproducible, whereas the short gate length and 
self-aligned structure can be obtained. 
Accordingly, it is an object of the present invention to solve above 
mentioned problems. 
It is a purpose of the present invention to provide a method for 
manufacturing a junction field effect transistor in which the problem 
occuring in the manufacture of the junction field effect transistor of the 
InP type materials having an electron saturation velocity higher than that 
of silicon or GaAs, that is, the problem of the restriction of the gate 
length, is eliminated, further a gate length shorter than the mask length 
is obtained and the source, drain and gate metal are evaporated by the 
self-align method, to thereby obtain a gate length 1 .mu.m or a gate 
length shorter than 1 .mu.m without utilizing an electron beam apparatus 
with high cost or X-ray lithography apparatus. 
SUMMARY OF THE INVENTION 
A method for manufacturing a junction field effect transistor of the 
present invention is defined by the claims with a specific embodiment 
shown in the attached drawings. 
The invention relates to a method for manufacturing a homojunction field 
effect transistor comprising: a first epitaxy growth process for growing a 
N type InP channel layer and GaInAs(P) layer on a semi-insulating 
substrate, a selective etching process of said GaInAs(P) layer for making 
an etching mask of photoresistor by lithography and, thereafter, for 
selectively anisotropically etching only said GaInAs(P) layer by using a 
selective etching solution, a second epitaxy growth process for removing 
entirely said photoresistor pattern and for growing the P type InP layer 
for the PN junction thereon, a gate metal evaporation process for 
evaporating a metal which is in ohmic contact with said P type InP layer, 
to thereby form a gate electrode, a selective etching process of the P 
type InP layer and GaInAs(P) layer for etching selectively said P type InP 
layer by using an etching mask as a gate metal and for forming large 
under-cuts near the both sides of the PN junction by selectively etching 
the GaInAs(P) layer which is exposed on surface after said selective 
etching is completed, a source and drain metal evaporation process for 
forming a source and drain electrode by evaporating the metal which is in 
ohmic contact with the N type InP layer, to thereby obtain a gate length L 
shorter than a length W on the photoresistor pattern mask by selective 
mesa etching and obtain an electrode by the self-align method, a method 
for manufacturing a heterojunction type field effect transistor 
comprising: a first epitaxy growth process for growing a N type GaInAs(P) 
channel layer, an N type InP layer and GaInAs(P) layer on a 
semi-insulating substrate, a selective etching process of said GaInAs(P) 
layer and N type InP layer for making an etching mask of photoresistor by 
lithography and, thereafter, for selectively anisotropically etching said 
GaInAs(P) layer by using a selective etching solution and selectively 
etching said N type InP layer additionally, a second epitaxy growth 
process for removing entirely said photoresistor pattern and for growing 
the P type InP layer for PN junction thereon, a gate metal evaporation 
process for evaporating a metal which is in ohmic contact with said P type 
InP layer, to thereby form a gate electrode, a selective etching process 
of the P type InP layer and GaInAs(P) layer for etching selectively said P 
type InP layer by using an etching mask as a gate metal and for forming 
large under-cuts near both sides of the PN junction by selectively etching 
the GaInAs(P) layer which is exposed on the surface after said selective 
etching is completed, a sorce and drain metal evaporation process for 
forming a source and drain electrode by evaporating the metal which is in 
ohmic contact with the N type InP layer, to thereby obtain a gate length L 
shorter than a length W on the mask determined by the lithography and to 
obtain an electrode by the self-align method.

DETAILED DESCRIPTION 
FIG. 1 illustrates the cross sectional view of a InP type homojunction 
field effect transistor according to the present invention. 
Referring to FIG. 1, a N type InP channel layer 32 and GaInAs(P) layer are 
grown on the semi-insulating InP substrate 31, thereafter, only a 
GaInAs(P) layer is anisotropically selectively etched using a selective 
etching solution, to thereby expose a (111) In layer. 
Next, a highly doped P type InP layer 34 is grown for forming a PN junction 
so that the homojunction type PN junctior is formed below an etching 
portion, to thereby form a gate 35. In FIG. 1, the number 36 shows a drain 
and a source. 
FIG. 2 illustrates the cross-sectional view of a heterojunction field 
effect transistor. 
Referring to FIG. 2, a N type GaInAs(P) channel layer 42 and a N type InP 
43 and GaInAs(P) layer are grown on a semi-insulating InP substrate 41, 
thereafter, the GaInAs(P) layer is anisotropically selectively etched and 
the N type InP layer is selectively etched additionally. 
Next, a highly doped P type InP layer is grown so that a heterojunction 
type PN junction is formed, to thereby form a gate 46. 
It should be noted that the length of the gate is determined as follows; 
EQU L=W-2d/ tan .theta. 
(Here, L=the length of the gate, 
W=the length of the gate on the mask determined by the mask for etching, 
d=the thickness of the GaInAs(P) layer, 
.theta.=the angle of the anisotropic etching) 
The thickness d is determined by an epitaxial growth so that the thickness 
below 0.1 .mu.m can be controlled and if the surface (111) is exposed by 
utilizing the selective etching solution, the etching angle .theta., that 
is, a reproducible angle of 54.44.degree., is obtained. 
Accordingly, though the gate length W on the mask determined by a 
lithography is long, the short length L of the gate is obtained by the 
control of the thickness of the epitaxial layer. 
Thereafter, the gate metal having a length larger than the length W of the 
gate electrode formed on the mask is evaporated by a lift-off process and 
the P type InP layer formed on the porting except for the gate metal is 
selectively etched. Next, the GaInAs(P) layer is selectively etched. 
Thereafter, the metal which is in ohmic contact with the N type InP layer 
is evaporated so that the self-aliged source electrode 47 and drain 
electrode 47 is formed, owing to the under-cut formed below the P type InP 
layer. 
FIG. 3A through 3F illustrate cross-sectional view of the InP homojunction 
field effect transistor in which each manufacturing process is illustrated 
sequentially according to the present invention. 
Here, a first epitaxy growth layer is formed on the semi-insulating InP 
substrate as shown in FIG. 3A. Thereafter, the GaInAs(P) layer is 
selectively anisotropically etched for determining the gate length L as 
shown in FIG. 3B. A second epitaxy growth process is completed on the P 
type InP layer as shown in FIG. 3C, and the gate metal is evaporated 
thereon by a lift-off process as shown in FIG. 3D. A P-type InP layer of 
the surface and a GaInAs(P) layer are selectively etched as shown in FIG. 
3E, next, a source and drain metal of self-aligned structure are 
evaporated by the lift-off process as shown in FIG. 3F. 
The manufacturing sequence of the above mentioned homojunction field effect 
transistor are described as follows. 
In the first epitaxy growth in FIG. 3A, a N type InP channel layer 32 and a 
GaInAs(P) layer 33 are grown on a simi-insulating InP substrate 31 by 
using a liquid phase epitaxy method (LPE) or an organometallic vapour 
phase epitaxy method (OMVPE). 
Here, the thickness of the GaInAs(P) layer 32 determines the length of the 
gate as mentioned above. 
In the selective etching process of the GaInAs(P) layer in FIG. 3B, an 
etching mask of a photoresistor is formed by lithography. Next, only the 
GaInAs(P) layer 33 is selectively anisotropically etched by using a 
selective etching solution. An etching surface is (111) In surface having 
an etching angle about 54.44.degree.. 
In a second epitaxy growth in FIG. 3C, after the photoresistor pattern for 
etching mask is entirely removed, the second epitaxy growth is 
accomplished in order to form a PN junction. 
Here, the doping concentration of the P type InP layer is above 10.sup.18 
cm.sup.-3, the high doping concentration is desirable. 
In the gate metal evaporation process in FIG. 3D, after the second epitaxy 
growth is completed, a gate metal 35 is evaporated by utilizing the 
lift-off method. 
In the selective etching process of the InP layer and GaInAs(P) layer in 
FIG. 3E, the InP layer 34 is selectively etched by using the gate metal 35 
formed in the previous process as the etching mask and thereafter, the 
GaInAs(P) layer 33 exposed on the surface is selectively etched. 
After the etching process is completed, a large under-cut is formed on both 
sides of the PN junction as shown in drawing. 
In the source and the drain metal evaporation process in FIG. 3F, the metal 
which is in N type ohmic contact with the InP, for example, the Au-Ge/Au 
is evaporated by using the lift-off method, to thereby form source and 
drain electrodes 36. 
Here, if the metal is evaporated without distinction of the source, drain 
and gate, each electrode is self-aligned, owing to the under-cut formed in 
the previous process. 
FIG. 4A through 4F illustrate cross-sectional view of the GaInAs(P) 
heterojunction field effect transistor in which each manufacturing process 
is illustrated sequentially according to the present invention. 
Here, a first epitaxy growth is completed on the semi-insulating InP 
substrate as shown in FIG. 4A. An anisotropic etching process of the 
GaInAs(P) layer and selective etching process of the InP layer are 
completed for determining the gate length L in FIG. 4B. A second epitaxy 
growth process of the P type InP layher is completed in FIG. 4C. A gate 
metal is evaporated thereon by a lift-off process in FIG. 4D. An P tpye 
InP layer of the surface and GaInAs(P) are selectively etched in FIG. 4E. 
A source and a drain metals of the self-aligning structure are evaporated 
by a lift-off process FIG. 4F. 
The manufacturing sequence of the above mentioned hetero-junction field 
jeffect transistor is described as follows. 
In the first epitaxy growth in FIG. 4A, a N type GaInAs(P) channel layer 
42, N type InP layer 43 and GaInAs(P) layer 44 are grown on a 
semi-insulating InP substrate 41 by the LPE method or OMVPE method. 
Here, the thickness of the GaInAs(P) layer 44 determines the length of the 
gate as mentioned above. 
In the selective etching process of the GaInAs(P) layer and N type InP 
layer in FIG. 4B, after an etching mask of the photoresist is formed by 
lithography, only GaInAs(P) layer 44 is selectively aniotropically etched 
by using the selective etching solution. 
Next, the N type InP layer 43 is selectively etched. 
Here, the etching surface of the GaInAs(P) layer is (111) In surface having 
an etching angle about 54.44.degree.. 
In a second epitaxy process in FIG. 4C, after the photoresistor pattern for 
etching mask is entirely remove, the second epitaxy growth is accomplished 
in order to form a PN junction. 
Here, the doping concentration of the P type InP layer is above 10.sup.18 
cm.sup.-l, the high doping concentration is desirable. 
In the gate metal evaporation process in FIG. 4D, after the second epitaxy 
growth is completed, a gate metal 46 is evaporated by utilizing the 
lift-off method. 
In the selective etching process of the InP layer and GaInAs(P) layer in 
FIG. 4E, the InP layer 45 is selectively etched by using the gate metal 46 
formed in the previous process as the etching mask and thereafter, the 
GaInAs(P) layer exposed on the surface is selectively etched. 
After the etching process is completed, a large under-cut is formed on both 
sides of the PN junction as shown in drawing. 
In the source and the drain metal evaporation process in FIG. 4F, the metal 
which is in N type ohmic contact with the InP is evaporated by uing the 
lift off method, to thereby form source and drain electrodes 47. 
Here, if the metal is evaporated without distinction of the source, drain 
and gate, each electrode is self-aligned, owing to the under-cut formed in 
the previous process. 
It is noted that the present invention has a plurality of alternatives to 
those examples as described in FIG. 3 and FIG. 4. 
First, if the first channel layer formed on the semi-insulating InP 
substrate in the first epitaxy growth (FIG. 3A and FIG. 4A) is the GaInAs 
layer and the second layer which determines the gate length by using the 
anisotropic selective etching is the InP layer, the P type InP layer 34 in 
FIG. 3C and the P type InP layer 45 in FIG. 4C grown in the second epitaxy 
process can be substitued for the P type GaInAs(P) layer. 
Second, the method for manufacturing the junction field effect transistor 
can be applied to a GaAs structure without variance of the structure. 
That is, a GaAs layer instead of the InP layer can be grown on a 
semi-insulating GaAs substrate and an AlGaAs layer instead of the 
GaInAs(P) can be grown thereon. 
The field effect transistor having a short gate length and manufactured by 
the above mentioned method has a high transconductance and cut-off 
frequency so that the field effect transistor may be applied to the high 
frequency devices and integrated circuits. 
Also, the field effect transistor can be integrated with the GaInAs long 
wavelength photo-detector in a single chip so that can be applied to an 
optical receiver of the optical communication system having a high 
velocity and very large capacity without variance of the structure. 
the junction field effect transistor accordiang to the present invention 
has the following benefits in contrast with the prior art transistor: 
1) As the diffusing process or activation process is not used, widening of 
the gate length owing to the dopant diffusion does not occur. 
2) In the etching process for forming a gate, the gate length W on the mask 
is much longer than the practical gate length L so that the lithography 
process can be easily accomplished. 
3) The manufacture according to the present invention can be simply 
accomplished because of the self-aligned structure. 
4) In the case of the heterojunction type, the Zn diffusion effect in the 
channal layer of the GaInAs(P) is significantly decreased in contrast with 
InP layer, to thereby obtain a rapid PN junction during a crystal growth 
process.