Controllable power semiconductor element with buffer zone and method for the manufacture thereof

Controllable power semiconductor components such as, for example, IGBTs and thyristors are provided, which, compared to known components, have a relatively lightly doped n-buffer zone, a relatively flat p-emitter, and an n-base having a comparatively long charge carrier life expectancy. An advantage is achieved that the controllable power semiconductor component has a temperature-independent tail current, despite a low on-state dc resistance and a high blocking voltage.

BACKGROUND OF THE INVENTION 
The invention is directed to a controllable power semiconductor component 
wherein a cathode-side structure comprises a respective cathode terminal 
and a gate terminal, an n- base zone, an n buffer zone, and a p emitter 
zone are electrically contacted to an anode terminal and are provided in 
sequence. 
A controllable power semiconductor component of this type is known from the 
IEEE paper to the 5th International Symposium on Power Semiconductor 
Devices and IC's having the title "A High Power IGBT Module For Traction 
Motor Drive" by M. Mori et al (pp. 287-291). In controllable power 
semiconductor components such as, for example, IGBT's (insulated gate 
bipolar transistor) and thyristors, an n.sup.+ -buffer zone is often 
provided, for example, between an n.sup.- -base zone and a p-emitter. A 
limitation of the space charge zone is possible as a result thereof and 
leads to a shorter base, which permits a lower on-state dc resistance. In 
order to diminish the latch-up tendency and, thus, in order to raise the 
breakdown voltage, the life expectancy of the charge carriers in the 
n.sup.- -base is shortened, for example by platinum diffusion or electron 
irradiation, as a result whereof, however, what is referred to as a tail 
current that is disadvantageously temperature-dependent arises upon 
shut-off of the controllable power semiconductor component. 
SUMMARY OF THE INVENTION 
It is an object of the invention to specify a controllable power 
semiconductor component having low on-state do resistance in which a tail 
current that is nearly temperature-independent occurs upon shut-off. 
According to the invention, a controllable power semiconductor component 
comprises a cathode-side structure having a cathode terminal and a gate 
terminal. An n.sup.- -base zone, an n buffer zone, and a p emitter zone 
are provided in sequence beginning at the cathode-side structure. An anode 
terminal is electrically contacted to the p-emitter zone. The n buffer 
zone has a thickness between 20 and 80 .mu.m and a doping concentration at 
an anode-side edge of 8.times.10.sup.13 through 5.times.10.sup.14 
cm.sup.-3. The p-emitter zone has a thickness of 400-1000 nm and a doping 
concentration at said anode-side edge of 10.sup.17 through 10.sup.18 
cm.sup.-3. The n.sup.- base zone has a life expectancy of the charge 
carriers that is longer than 10 .mu.sec. 
An advantage obtainable with the invention is particularly that no 
shortening of the life expectancy of the charge carriers, for example, due 
to a platinum diffusion or an electron irradiation, is required, despite 
an existing n-buffer layer, and that the controllable power semiconductor 
components can be manufactured in a technologically simple way for a broad 
range of current and voltage. 
The invention shall be set forth in greater detail below with reference to 
the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a controllable power semiconductor component of the invention 
in the form of an IGBT (insulated gate bipolar transistor), whereby a 
cathode-side structure 4 . . . 6, an n.sup.- -base zone 1, an n-buffer 
zone 2 and a p-emitter zone 3 are provided in sequence. The cathode-side 
structure 4 . . . 6 is comprised therein that at least one n.sup.+ -doped 
zone 5 is separated from the n.sup.- -base zone 1 by a p.sup.+ -doped zone 
6, and the p.sup.+ -doped zone 6 is contacted to a cathode terminal K and 
covers at least one gate electrode which is connected to a gate terminal G 
and a part of the n.sup.+ -doped zone 5, a part of the p.sup.+ -doped zone 
6 and a part of the n.sup.- -base zone 1. The gate electrode is separated 
from these regions of the n.sup.- -base zone by an insulating layer 4. The 
p-emitter zone 3 is electrically conductively connected to an anode 
terminal A. 
The dimensions and doping concentrations with respect to the cathode-side 
structure correspond to those of traditional IGBTs. 
The thickness of the n.sup.- -base zone 1 can, as usual, be selected at 
approximately 100 .mu.m per kV, and the doping concentration typically 
lies between 8.times.10.sup.12 and 10.sup.14 cm.sup.-3. Given a typical 
blocking voltage value of 3 kV, the length of the n.sup.- -base and, thus, 
the substrate thickness as well essentially lies at approximately 300 
.mu.m. 
In the IGBT of the invention, the n-buffer zone 2 comprises a thickness of 
approximately 20-80 .mu.m and has a doping concentration of 
8.times.10.sup.13 through 5.times.10.sup.14 cm.sup.-3 at the anode-side 
edge. By comparison to known components, the p-emitter 3 is implemented 
comparatively flat, has a thickness of 400-1000 nm (typically 600 nm), and 
comprises a doping concentration of 10.sup.17 through 10.sup.18 cm.sup.-3 
at the anode-side edge. 
In any case, the life expectancy of the charge carriers of the n.sup.- 
-base zone 1 is longer than 10 .mu.sec and comprises typical values of 80 
.mu.sec since no additional recombination centers are provided. The 
increase in the life expectancy of the charge carriers in this case has 
hardly any effect on the charge carrier density, since this is already at 
a high level. 
The doping of the n-buffer zone 2 is thereby selected so low that it has 
only an extremely slight influence on the injection behavior of the flat 
p-emitter 3. The dopant quantity of the emitter 3 is selected so slight 
that practically no charge carrier recombination occurs in the emitter 3, 
but rather in the metal contact. Consequently, the threshold voltage 
between the layers 2 and 3--by contrast to known power semiconductor 
components--can be selected temperature-independent, and the life 
expectancy of the charge carriers can be selected comparatively long, as a 
result whereof the semiconductor component of the invention is far less 
sensitive to temperature fluctuations and the tail current is practically 
independent of the temperature. The doping of the n-buffer zone 2, 
however, is adequate in order to avoid what is referred to as a 
break-through of the space charge zone up to the p-emitter, as a result 
whereof the ohmic losses in the n.sup.- -base are low, even given 
comparatively high blocking voltages since the base length can be shorter 
due to the buffer layer 2. 
FIG. 2 shows an inventive power semiconductor component in the form of a 
thyristor that differs from the inventive power semiconductor component 
shown in FIG. 1 on the basis of the cathode-side structure 7, 8. The 
cathode-side structure 7, 8 is composed of a p-doped zone that is 
connected to the gate terminal and into which an n.sup.+ -doped region 7 
is introduced, the n.sup.+ -doped region 7 being electrically contacted to 
an electrode that is electrically conductively connected to the cathode 
terminal K. 
In order to explain the method of the invention for manufacturing a 
controllable power semiconductor component of the invention, FIG. 3 shows 
an intermediate product that is composed, in sequence, of an n.sup.- 
-doped zone 1', an n-doped zone 2', and of a carrier layer 9. As a rule, 
the layers 1', 2', and 9 are composed of silicon, whereby the carrier 
layer 9 can be undoped, or can comprise an arbitrary doping. Typically, 
the layers 1' and 2' are approximately 300 .mu.m thick together and the 
carrier layer is likewise approximately 300 .mu.m thick. 
Since wafers having diameters of a type that are standard for power 
semiconductor components can only be poorly processed given this 
thickness, the n-buffer layer 2', together with the carrier layer 9 
(support wafer), are connected to the contact surface 10 between the two 
layers on the basis of what is referred to as direct wafer bonding. 
Further particulars with respect thereto may be derived, for example, from 
the Japanese Journal of Applied Physics, Vol. 27, No. 12, December, 1988, 
pages L2364-L2366. 
In the manufacture of the controllable power semiconductor component of the 
invention, a wafer composed of an n.sup.- -doped silicon substrate 1' is 
employed as an initial material. The n-buffer layer 2' is produced either 
by epitaxial growth or by drive-in of, for example, phosphorous atoms into 
the n.sup.- -substrate. As shown in FIG. 2, this is followed by the 
joining of the wafer composed of the n.sup.- -doped silicon substrate and 
the further wafer 9 that serves as a carrier substrate. The joined wafers 
1' and 9 now comprise an adequate thickness and can thus be supplied to a 
further process step for producing the respective cathode-side structure. 
The production of the respective cathode-side structure occurs in a known 
way, for example, by diffusion. Since the further wafer serves only as a 
carrier substrate, it is removed by grinding after the cathode-side 
structure has been produced. For improving the surface properties, the 
grinding process can potentially be followed by an etching step. In 
conclusion, the p-emitter zone 3 is produced by implantation from the 
ground and a potentially etched surface, whereby the implantation occurs 
in a known way. 
FIG. 4 shows a diagram of the chronological curve of the load current I in 
a time interval between 0 and 5 .mu.sec after shut-off. The current curves 
11 . . . 14 are shown there and wherein: curve 11 corresponds to a 
comparable, conventional power semiconductor component at a temperature of 
T=300 K.; curve 12 corresponds to the comparable, conventional power 
semiconductor component at a temperature T=400 K.; curve 13 corresponds to 
a power semiconductor component of the invention at the temperature of 
T=300 K.; and curve 14 corresponds to the power semiconductor component of 
the invention at a temperature of T=400 K. 
It becomes clear that the curves 13 and 14 decay to the value 0 
significantly faster than the curves 11 and 12, and that the curves 13 and 
14, by contrast to the curves 11 and 12, are largely identical, i.e. 
temperature-independent. A significantly slower drop in current occurs in 
curve 12, and even the beginning of the drop occurs later than in curve 
11. 
Although various minor changes and modifications might be proposed by those 
skilled in the art, it will be understood that I wish to include within 
the scope of the patent warranted hereon all such changes and 
modifications as reasonably come within my contribution to the art.