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Патент USA US3085320

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April 16, 1963
Filed Dec. 12, 1958
2 Sheets-Sheet 2
Jig. 6‘.
j {13
Patented Apr. 16, 1953
Richard F. Ruiz, Fishirill, N.Y., assignor to International
Business Machines Corporation, New York, N.Y., a
corporation of New York
Filed Dec. 12, 1958, Ser. No. 779,959
1 Claim. (Cl. 29-253)
FIG. 1 is an elevation of a switching diode illustrating
the invention.
FIG. 2 is a cross section of the crystal wafer after
diffusing impurities into it.
FIG. 3 is a cross section of the body of the diode after
removal of the material not required for the body.
FIG. 4 is a cross section of the body or" the diode after
This invention relates to semiconductor devices and
more particularly to high-speed, high-current switching
diodes and an improved method of making the same.
If a voltage in the reverse direction is suddenly placed
across a semiconductor diode, after the diode has been
ohmic contacts have been applied thereto.
FIG. 5 is 1a view of the body of the diode after etching.
FIG. 6 is curve showing a variation of resistivity in
the diode from one surface to the other.
FIG] is a schematic diagram of the circuit employed
in testing the diode.
Extrinsic conductivity in semiconductors is dividedlinto
conducting a heavy current in the forward direction, it
has been observed that a relatively large reverse current 15 two classes; when electrons are the predominant current
is initially conducted which then decays to the normal
carriers, the class is designated as N type, and when holes
low steady state value. This decay time, known as re
are the predominant current carrier, the class is desig
covery time, limits the rate at which the diode can ‘be
nated P type. Each particular conductivity class is \de—
switched. This diode recovery time is caused by minority
termined solely by the impurities present in the semi
current carriers remaining in the bulk of the semicon
ductor material for a period of time after the heavy for—
conductor crystal structure. Small amounts of impurities
such as arsenic, antimony and phosphorous which ‘are
ward current is removed. Switching diodes, according
classed as group V elements are termed donor impurities
to the prior art, have recovery times of the order of
when added to semiconductor elements of group IV,
tenths of a microsecond with ‘forward currents in the ten
which may be germanium or silicon, since they con
milliamperes range and of the order of tens of micro 25 tribute an excess of free electrons to the crystal structure. 7
seconds with forward currents in the ampere range. Ad
These excess free electrons can become current carriers
ditionally, when a semiconductor diode, which has been
‘and therefore the semiconductor material is converted
reverse-biased, is suddenly switched to its conduction state
to N type conductivity. In a similar manner impurities
an excess of current or overshoot results.
such as gallium and indium of group :III are known as
My invention is a diode having parameters and char 30 acceptor impurities since they provide an excess of free
acteristics which make it ideal for switching applications
holes to the crystal structure, thereby converting the semi
and includes a method of fabricating such a diode.
conductor material to P type conductivity. If both types
The diode of this invention employs a region of ?rst
of impurities are present the conductivity is determined
conductivity type having, essentially, a constant value
by the predominant impurity.
of resistivity and a second region of opposite conductivity
When the semiconductor crystal structure contains two
type having a gradient of resistivity. This graded re
zones of opposite conductivity types contacting in ya com
sistivity region produces an electric ?eld within this region
mon plane or barrier the crystal is useful as an electrical
of the diode and the presenceof this electric ?eld adds
recti?er aliordin-g a relatively low resistance to a ?ow
a drift component to the diffusion component of motion
of current when one polarity of voltage is impressed
of the minority carriers in the region, so that the minority
across ‘the barrier, and a relatively high resistance when
carriers reach the junction more rapidly and those car
the polarity of voltage is reversed. Electrical recti?ers
riers that are stored when the supply voltage is suddenly
constructed in this manner are known as junction type
reversed will more rapidly be swept out of the region.
rectifiers to distinguish them from the well-known point
Additionally, the diode employs a novel geometry which 45 contact type and it is to the junction type recti?er that
minimizes the distance from the PN junction to the semi—
the invention ‘relates.
conductor surfaces at which the external terminals are
In a junction type switching diode the recovery time
applied. These surfaces act as regions where ideally
during which the diode back resistance is varying from
in?nite recombination rates occur and hence can be
a low value to its normal high steady state value, which
considered vas sinks for minority carriers in excess of 50 recovery time occurs when a normally conducting diode
the equilibrium concentration. The diode, ‘according to
the invention, exhibits improved recovery characteristics
minority carriers. During forward conduction, carriers
and freedom from overshoots as well as a low ‘forward
which are majority carriers on one side of the junction
is suddenly reverse-biased, is caused by the storage of
resistance, while maintaining a high back resistance and
are injected into‘the other side where they become minor
minimum capacitance.
55 ity carriers in excess of the amount normally present
An object of the invention is to provide an improved
switching diode.
‘and the conductivity is modulated so that a very low
impedance is achieved. On the reverse bias condition,
Another object of the invention is to provide a semi
‘the very low impedance condition persists until the ex
conductor diode having a minimum recovery time.
cess carriers migrate to the terminals or recross the junc
Yet another object of the invention is to provide a high‘ 60 tion, or else recombine with majority carriers in the bulk
current switching diode.
Still another object of this invention is to provide a \
Referring now to FIG. 1, a novel semiconductor 1 il
switching diode combining the features of low forward
resistance and small minority carrier storage ‘time.
lustrative of my invention is shown having, by way of
example, a P type zone of conductivity 5 and an N type
Another object of the ‘invention ‘is’ to provide a method
zone of conductivity 6 meeting in the common 'PN barrier
of making a high-speed, ‘high-‘current switching diode.
or junction 7. Terminal leads 2 and 3 are connected to the
Other objects of the 'inven‘tion‘will be pointed out in
N and P type conductivity regions by the ohmic contacts
the following description and ‘claim ‘and illustrated in
8 and '4; the ohmic contact 18 being connected to the N
the accompanying drawings, which disclose, ‘by ‘way of
type conductivity region'o by a soldering operation and
example, the principle of the invention and the ‘best mode 70 ohmic contact 4 being connected to the P type conductiv
which has been contemplated of applying ‘that principle.
it'y region 5 by an alloying operation. Additionally the
In the drawings:
ohmic contacts v3 and 4 serve as heat sinks for the semi
conductor diode. The N type region ‘6, as shown, con
tains a gradient of resistivity which varies from a value
which is low at the ohmic contact ‘8 to a higher value
at the PN junction 7, as will be more particularly here
eration produces a thin region 9 in the original conduc
tivity material which contains a relatively low value of
resistivity to form a low resistance surface for the ohmic
The next step in the process is an etching operation to
inafter described. The P type region 5 as shown con
tains a relatively constant value of resistivity which is re
convert the structure shown in FIG. 4 to the structure
shown in FIG. 5. The body of the diode is electrically
etched in a solution of potassium hydroxide, sodium hy
droxide or the ‘like, to remove all the semiconductor ma
duced to a relatively low value of resistivity near the
surface of the ohmic contact 4.
The semiconductor diode of FIG. 1 is preferably made
in the following novel manner which comprises the steps 10 terial except that directly between the alloyed region
and the ohmic contact 8. The ?nal step in the process
of vapor diffusion of a v?rst conductivity type crystal struc
is the attaching of terminal leads 2 and 3 to the respec
tive ohmic contacts 8 and 4.
The semiconductor recti?er fabricated by the above de
to obtain a diode having a maximum electrode surface
process has a variation of resistivity through the
area to junction area. This process results in a diode
body of the device as shown in FIG. 6. As there shown,
having parameters that are nearly ideal for switching ap
beginning at ohmic contact 8, the device has a relatively
plications. A thin ?at water of a ?rs-t conductivity type
low value of resistivity which increases to a relatively
semiconductor material having a resistivity su?iciently
high value at the PN ‘junction 7, then falls to a relative
high to allow the formation of a graded resistivity re
ture, shaping the resultant crystal wafer, supplying ohmic
‘contacts to the crystal wafer, and etching the crystal wafer
gion by means of vapor ‘diffusion is provided by any of 20 ly constant value through the original conductivity type
material thence falling to a relatively low value in the
several processes standard in the art. A portion of this
zone produced by the alloying operation and terminating
?rst conductivity type semiconductor wafer is next con
in ohmic contact ‘4.
verted to a conductivity type opposite that of the original
In order to aid in understanding the invention the fol
semiconductor wafer by means of vapor diffusion. The
technique of vapor diffusion is well known in the art and 25 lowing information on speci?c values and materials is
presented, by way of example, but it should be under
stood that variations in the speci?c values and materials
may be employed by those skilled in the art while main
taining the advantages afforded by the hereinbefore de
impurity of the type opposite to the conductivity of the
semiconductor wafer so that the energy imparted by the 30 scribed process.
A germanium wafer of P type conductivity having a
heat to the impurity atoms causes them to penetrate into
resistivity of 7 ohm-centimeter was maintained at a tem
the crystal structure. This produces a region of opposite
perature of 800 degrees centigrade for 24 hours in an
conductivity in the wafer and provides this region with
arsenic atmosphere. The resulting wafer containing a
a resistivity that is low at the surface and is nearly equal
to the intrinsic resistivity of the semiconductor at the j unc 35 converted region of N type conductivity was cut to pro
ducepellets having N and P type regions meeting in a
tion 7. FIG. 2 shows a cross-sectional view of a wafer
PN junction. The PN iunction was essentially in the
prepared as described above and in which region 5 is of
is accomplished by heating the semiconductor wafer in
a controlled atmosphere at a high temperature in the
presence of a vapor containing a conductivity directing
the original conductivity type surrounded by region 6
of the converted opposite conductivity type.
The next step in the process is to remove from the
wafer the materials not necessary in the formation of
center of the pellets and the error if any resulted in
the PN junction being located slightly towards the N
40 type conductivity surface.
the body of the diode, Referring again to FIG. 2 the
A pellet was next lapped to
produce a die having a thickness of about 0.002 inch,
and'a length, parallel to the PN junction, of about 0.02
inch. Lead solder was employed to attach a copper base
wafer is out along the lines shown as 20—20, 21—21
to the surface of the N type conductivity region. An
and 22-22 and converted to the wafer shown in FIG.
3. In FIG. 3 there is shown a region 5 of P type con 45 impurity dot containing indium was then alloyed to the
surface of the P type region and the alloying operation
ductivity, a region 6 of N type conductivity containing a
controlled so that the depth of penetration of the in
gradient of resistivity which varies from the low value at
dium was kept to a few tenths of one thousandths of an
the surface to a higher value at the junction 7. The re—
inch. After the alloying operation, the die was elec
moval of the material from the wafer shown in FIG. 2 to
50 trically etched in a'5% potassium hydroxide solution to
provide the wafer of FIG. 3 may be performed in any con
remove all the germanium that was not directly under the
ventional manner such as sawing, etching or abrading.
alloyed indium dot. The resulting germanium die had a
The next step in the process is to attach low resistance
diameter of 0.009 inch and a height of 0.0015 inch.
ohmic contacts to the surfaces of the original conductiv
The diode formed by the above-described process had
ity type region and the converted opposite type conduc
the characteristics as shown in the following table:
tivity region. Because the converted conductivity type
0.38 volt at 0.1 ampere.
region has a low value of resistivity on the surface, re
Forward voltage drop "" " 0.54 volt at 1.0 ampere.
sulting from the vapor di?usion process, a soldering op
Reverse breakdown voltage- 125 volts.
eration is sufficient to obtain a low resistance contact.
Through the use of an electrically inert solder such as
60 Reverse current _________ _- 5 microamperes at 20 volts.
Capacity ____________ _..'__. 2 micromicrofa-rads.
lead, the ohmic contact a is attached directly to the sur
face of the converted conductivity type region 6. How
As a means of determining the recovery time of the
ever, since the region of original conductivity material
diode after conducting a forward current of 1 ampere,
5 has a relatively high value of resistivity, an alloying
the circuit shown in vFIG. 7 was used. The components
operation is performed to reduce the surface resistivity
of FIG. 7 had the following values:
of the original conductivity type region. A dot contain
ing conductivity type directing impurities of the same
Resistor 41
type as the original semiconductor material is alloyed to
Resistor 42 _______________ __'_________ __do____
the region 5 for a time and at a temperature whereby
Resistor 45 __________________________ __do____
the depth of penetration of the dot is less than the depth
of the original region 5. As an example, if the original
conductivity was P type, acceptor impurities should be
employed and conversely if the original conductivity was
N type, then donor impurities should be used in the al
loying operation. As shown in FIG. 4, the alloying op 75
Capacitor 43 __________________ __microfarads__ 0.01
Battery 44 ________________________ __volts____
The measured recovery time of the diode when switch
ing from a forward current of 1 ampere to a reverse bias
of =6 vol-ts was 50‘ millimicroseconds. Various other di
odes were constructed by the hereinbefore described proc
ess and exhibited recovery times between 40 and 70 milli
N-type conductivity to a predetermined depth, said
converted surface of N-type conductivity exhibiting
What has been disclosed is a novel combination of
steps producing a novel high-speed, high-current diode
having in combination the following advantages:
Minimum recovery time permitting higher speed of
Freedom of overshoots.
Very low forward resistance reducing the amount of
power dissipated within the diode body thereby obtaining 10
more stable operating characteristics.
high-speed, high-current diode only the major steps in
a P-N junction, said gradient of resistivity in said
region of N-type conductivity varying from a value
which is low at the major surface thereof which is
parallel to said P-N junction to a high value at said
soldering a large area ohmic contact to said entire
major surface of said N~type conductivity region;
alloying a small area ohmic contact to the major sur
face of said P-type conductivity region which is
parallel to said P-N junction, the depth of said al
loyed zone thus formed being less than the depth of
the process have been stressed and the ?ne points in the
technology made necessary by the small sizes being han
said P-type conductivity region, said alloyed ohmic
contact containing sufficient P-type conductivity-de
dled have been omitted since they are familiar to one
skilled in the art.
termining impurities to impart to said alloyed zone
a resistivity much less than the resistivity of said P
While there have been shown and described and pointed
type germanium wafer; and
out the fundamental novel features of the invention as
applied to the preferred embodiment, it will be understood
that var-ions omissions and substitutions ‘and changes in 25
the form and details of the device illustrated and in its
operation may be made by those skilled in the art with
out departing from the spirit of the invention. It is the
intention, therefore, to be limited only as indicated by
the scope of the following claim.
What is claimed is:
A process of making a high speed, high current switch
ing diode comprising, in combination, the steps of:
providing a P-type germanium wafer which exhibits a
relatively high value of resistivity;
ductivity, each of said regions being substantially
equal in thickness and said regions being joined in
P-N junction;
_ Minimum reverse current allowing ‘greater isolation of
Greater forward currents, and,
A high reverse breakdown voltage.
In the above-description of the process of making this
converting all of the external surfaces of said wafer to
a gradient of resistivity;
cutting a die from said water, said die having a region
of P-type conductivity and a region of N-type con
removing by eaching all of the germanium wafer not
directly under said alloyed small area ohmic contact.
References Cited in the ?le of this patent
Kircher ______________ __ July 15,
Fuller _______________ __ Apr. 8,
Koch et a1 _____________ __ July 8,
Pfann _______________ __ July 15,
Beale _______________ __ Aug. 26,
Rutz ________________ -_ Oct. 27, 1959
Loferski ____________ _- Mar. 22, 1960
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