Active solid-state devices (e.g. – transistors – solid-state diode – Regenerative type switching device – Bidirectional rectifier with control electrode
Reexamination Certificate
2001-01-02
2004-08-10
Nadav, Ori (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Regenerative type switching device
Bidirectional rectifier with control electrode
C257S156000, C257S329000, C257S617000
Reexamination Certificate
active
06774407
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device carrying out switching control on a current flowing in the thickness direction of the semiconductor device. In particular, the present invention relates to a semiconductor device with a suppressed increase in turned-on resistance and an improved turn-off response.
2. Description of the Prior Art
Traditionally, a so-called isolated-gate bipolar transistor (IGBT), a transistor made by integrating bipolar and field-effect transistors into a single body, is used in applications where a high input impedance and a low output impedance are required.
FIG. 19
is a diagram showing the general structure of an IGBT. As shown in the figure, an ordinary IGBT has an n epitaxial layer
202
on a p
+
silicon (Si) substrate
201
. In addition, the IGBT also includes an n
+
source region
206
, a p body region
207
and a p
+
body region
209
on the surface of the n epitaxial layer
202
. A portion of the n epitaxial layer
202
other than the p body
207
is called an n drift region
202
d
. On the surface of the n epitaxial layer
202
, there is a gate electrode
204
which is insulated from the n epitaxial layer
202
by a gate insulation film
203
, and insulation films
205
and
208
. The gate electrode
204
veils a portion of the n drift region
202
d
, a portion of the p body region
207
and a portion of the n
+
source region
206
on the surface of the n epitaxial layer
202
. In addition, on the front-surface side of the IGBT, there is provided a source electrode
210
which is conductive with respect to the n
+
source region
206
and the p
+
body region
209
. On the back-surface side of the IGBT, on the other hand, there is provided a drain electrode
211
which is conductive with respect to the p
+
substrate
201
.
In the structure of the IGBT described above, the gate electrode
204
, the n
+
source region
206
, the p body region
207
and the n drift region
202
d
constitute a field-effect transistor. To be more specific, the p body region
207
is the channel region and the n drift region
202
d
is the drain region. On the other hand, the p
+
body region
209
, the n drift region
202
d
and the p
+
substrate
201
form a bipolar (pnp) transistor. To put it in detail, the p
+
body region
209
is the collector, the n drift region
202
d
which also serves as the drain region of the field-effect transistor is the base of the bipolar transistor and the p
+
substrate
201
is the emitter.
The main function of the IGBT with the configuration described above is to switch a current flowing from the drain electrode
211
to the source electrode
210
by controlling the voltage applied to the gate electrode
204
. To put it in detail, with no voltage applied to the gate electrode
204
, even if a voltage is applied to the IGBT so that the drain electrode
211
is set at a potential higher than the source electrode
210
, no current flows from the drain electrode
211
to the source electrode
210
because the applied voltage is in a reverse direction with respect to a pn junction between the p body region
207
and the n drift region
202
d
as well as a pn junction between the p
+
body region
209
and the n drift region
202
d
. If a positive voltage is applied to the gate
204
(with respect to the source electrode
210
), however, an n channel is created on the surface of the p body region
207
, putting the field-effect transistor in a turned-on state. In this state, electrons flow from the n
+
source region
206
to the n drift region
202
d
by way of the n channel. Accordingly, the concentration of carriers (electrons in this case) in the n drift region
202
d
increases, reducing the resistance thereof. As a result, a diode formed by the n drift region
202
d
and the p
+
substrate
201
conducts, causing holes to be injected from the p
+
substrate
201
to the n drift region
202
d
. For this reason, the bipolar transistor is turned on, flowing a current from the drain electrode
211
to the source electrode
210
in the thickness (transversal) direction.
Here, when the positive voltage applied to the gate electrode
204
is cut off, the IGBT returns to the turned-off state. In the preceding turned-on state, however, the n drift region
202
d
was filled with both electrons and holes each at a high concentration. Thus, even when the positive voltage applied to the gate electrode
204
is removed, cutting off the injection of electrons from the n
+
source region
206
, the concentration of carriers in the n drift region
202
d
does not decrease immediately. As a result, the transient characteristic of the IGBT at a switch-off operation time indicates that the current does not decrease in magnitude immediately right after the switch-off operation as shown by a dashed line in a graph of FIG.
21
. In this way, there is raised a problem of a long turning-off time and a conventional technology has been proposed to solve the problem, that is, to improve the turn-off characteristic of the IGBT.
Basically, according to a proposed means for shortening the turning-off time, a region for distributing recombination centers such as heavy-metal atoms and lattice defects at a high concentration is provided in the IGBT. Such recombination centers cause carriers to mutually extinguish each other so that the concentration of carriers causing the problem described above can be reduced at an early time. According to a technology disclosed in Japanese Published Unexamined Patent Application No. Sho 64-19771, for example, protons are radiated from the back-surface side of the IGBT (or the side of the p
+
substrate
201
in the case of the IGBT shown
FIG. 19
) to distribute lattice defects over a narrow range in close proximity to the p
+
substrate
201
inside the n drift region
202
d
. Refer to a distribution of concentration of lattice defects and impurities in a conventional semiconductor device shown in FIG.
20
.
In the case of an IGBT with lattice defects distributed in a narrow range as is the case with the IGBT disclosed in Japanese Published Unexamined Patent Application No. Sho 64-19771, however, the reduction in turning-off time is extremely inadequate. This is because, in regions outside the narrow range, the reduction of the carrier concentration is slow. As a result, in the last portion of the turn-off characteristic, the convergence of the current is late as indicated by a solid line of a graph shown in FIG.
21
. In addition, there is raised a problem that, since the lattice-defect distribution region is narrow, the location of the region may vary from device to device due to variations caused by manufacturing processes, greatly affecting the characteristics of the semiconductor device. This problem will arise even if the lattice-defect distribution region is provided in the p
+
substrate
201
. It should be noted that, instead of radiating ions such as protons as described above, an electron beam can be radiated to distribute lattice defects widely over the entire semiconductor device so as to adequately reduce the turning-off time as is disclosed in Japanese Published Unexamined Patent Application No. Hei 3-272184. In this case, however, lattice defects are also distributed in the portions serving as the field-effect transistor, causing the turned-on resistance to increase.
SUMMARY OF THE INVENTION
The present invention addresses the problems described above; it is thus an object of the present invention to provide a semiconductor device that has an adequately shortened turning-off time without an accompanying increase in turned-on resistance.
In order to achieve the object described above, according to an aspect of the present invention, there is provided a semiconductor device comprising:
a switching element provided on a surface of a semiconductor layer;
a substrate at another surface of the semiconductor layer;
a portion of the semiconducto
Nadav Ori
Pillsbury & Winthrop LLP
Toyota Jidosha & Kabushiki Kaisha
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