Active solid-state devices (e.g. – transistors – solid-state diode – Regenerative type switching device – Having only two terminals and no control electrode – e.g.,...
Reexamination Certificate
2002-02-25
2004-09-14
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Regenerative type switching device
Having only two terminals and no control electrode , e.g.,...
C257S480000, C257S486000
Reexamination Certificate
active
06791121
ABSTRACT:
BACKGROUND
The present invention relates to semiconductor devices such as diodes.
FIG. 25
is a cross sectional view of a conventional pin diode. Referring now to
FIG. 25
, the conventional pin diode includes a first n-type semiconductor layer working as an n-type cathode layer
55
with low specific resistance, and a very resistive second n-type semiconductor layer grown epitaxially on the first n-type semiconductor layer. The surface of the second n-type semiconductor layer is mirror finished, a thermal oxide film is coated on the mirror finished surface of the second n-type semiconductor layer, the thermal oxide film is patterned, and a p-type anode layer
51
is formed in the surface portion of the second n-type semiconductor layer. The portion of the second n-type semiconductor layer, where no p-type anode layer
51
is formed, works as an n-type drift layer
58
. Thus, an epitaxial substrate
200
is obtained. A heavy metal such as platinum is diffused for controlling the carrier lifetime. An anode electrode
56
and a cathode electrode
57
are formed by metallizing the surface of p-type anode layer
51
and the other surface (back surface) of n-type cathode layer
55
.
Although not illustrated, an FZ substrate (bulk substrate) and such another type of substrate are used in substitution for epitaxial substrate
200
described above, and the constituent semiconductor layers are formed by ion implantation and by subsequent thermal drive. In this case, a p-type anode layer is formed in one surface portion of an n-type semiconductor substrate by diffusion, and an n-type cathode layer is formed in another surface portion of the n-type semiconductor substrate by ion implantation and by subsequent thermal derive. The portion of the n-type semiconductor substrate, where neither p-type anode layer nor n-type cathode layer is formed, works as an n-type drift layer.
After diffusing a heavy metal such as platinum for controlling the carrier lifetime, an anode electrode and a cathode electrode are formed on the p-type anode layer
51
and n-type cathode layer
55
, respectively, by metalization.
When the conventional pin diode used widely in these days switches from the ON-state to the OFF-state, a high transient current, the so-called reverse recovery current, flows in the opposite direction. The electrical loss, that is the product of the high reverse recovery current and the reverse recovery voltage, is high. It is very necessary to reduce the reverse recovery loss and to increase the switching speed of the diodes.
During reverse recovery, the electrical duties such as the applied voltage, the current and the losses are heavier than those in the steady state. Increase of the steady state current or increase of the voltage in the reverse blocking state causes heavy electrical duties, further causing breakdown of the diode sometimes. For obtaining a very reliable diode for electric power use, it is very necessary to improve the reverse recovery withstanding capability so that the diode may endure the heavy electrical duties.
To improve the reverse recovery characteristics and the reverse recovery withstanding capability, control of the minority carrier lifetime using heavy metal diffusion or electron beam irradiation is employed widely in these days. By shortening the minority carrier lifetime, the total carrier concentration in the steady state is reduced, the concentration of the carriers swept out during reverse recovery by the expanding space charge region is reduced, the reverse recovery time is shortened, the peak reverse recovery current is reduced, and the reverse recovery charge amount is reduced so that the reverse recovery loss may be reduced.
By reducing the hole concentration, the strength of the electric field caused during reverse recovery by the holes flowing through the space charge region is relaxed, and the duties caused during the reverse recovery are reduced so that the reverse recovery withstanding capability may be improved and the diode may not be broken down.
It is also important to provide the diodes with soft recovery characteristics. For the sake of environmental safety, it has been required to reduce the electromagnetic noises caused from power electronic instruments and apparatuses. One method that meets the demand described above makes the reverse recovery current of the diode behave softly to prevent the reverse recovery current and the reverse recovery voltage from oscillating so that the electromagnetic noises caused by the oscillation of the reverse recovery current or the reverse recovery voltage may be reduced.
One means for providing the diode with soft recovery characteristics is a soft recovery structure that suppresses the efficiency of minority carrier injection from the anode side. Typical diodes having the soft recovery structure include a merged pin/Schottky diode (MPS) disclosed in B. J. Baliga, “The Pinch Rectifier”, IEEE Electron. Dev. Lett., ED-5, p. 194, 1984 and a soft and fast recovery diode (SFD) disclosed in M. Mort, et. al., “A Novel Soft and Fast Recovery Diode (SFD) with Thin P-layer Formed by Al—Si Electrode”, Proceedings of ISPSD '91, pp. 113-117, 1991.
As described in M. Nemoto, et. al., “An Advanced FWD Design Concept with Superior Soft Reverse Recovery Characteristics”, Proceedings of ISPSD 2000, pp. 119-122, 2000, there exists a tradeoff relation between the soft recovery and the fast and low-loss reverse recovery.
To provide the diode with soft recovery characteristics, the total amount of the carriers accumulated in the drift layer in the ON-state of the diode is increased so that the amount of the minority carriers accumulated on the cathode side may increase. As the amount of the minority carriers accumulated on the cathode side increases, many of the minority carriers may remain on the cathode side while the space charge region is expanding from the anode side to the cathode side at the time of reverse recovery. As the number of the minority carriers remaining on the cathode side while the space charge region is expanding from the anode side to the cathode side increases, the reduction rate of the reverse recovery current dir/dt, the so-called reverse-recovery-current reduction-rate, is reduced.
However, when too many carriers are accumulated in the drift layer in the ON-state of the diode, the reverse recovery loss increases and it takes a long time until the reverse recovery ends; that is the reverse recovery time is elongated. On the other hand, a fast and low-loss diode is obtained by controlling the carrier lifetime, which introduces a lifetime killer uniformly into the drift layer, or by thinning the drift layer to reduce the amount of carriers accumulated in the drift layer in the ON-state of the diode. However, as the amount of carriers accumulated in the drift layer is reduced, the amount of the minority carriers accumulated on the cathode side is also reduced, causing the so-called snappy and hard recovery, in which the reverse-recovery-current reduction-rate dir/dt is large. During the snappy and hard recovery, the reverse recovery voltage and the reverse recovery current sometimes oscillate.
Soft recovery is realized by the low-injection-type diodes, such as the MPS and the SFD disclosed in the Bagalia and Mort references. However, lowering of the breakdown voltage and increase of the leakage current under the applied reverse bias voltage are caused more often in the low-injection-type diodes than in the pin diode having the drift layer of the same thickness due to the Schottky junction or the lightly doped anode layer.
Local lifetime control conducted by irradiating a particle ray of a light ion such as proton and helium causes high manufacturing costs, since the cost of irradiation per a wafer is high. If one tries to reduce the tradeoff relation between the fast and low-loss reverse recover and the soft recovery by employing the low-injection-type diode having the MPS structure or the SFD structure or by thinning the drift layer and by employing the local lifetime control, a space for accumul
Naito Tatsuya
Nemoto Michio
Nishiura Akira
LandOfFree
Semiconductor device and method of manufacturing the same does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Semiconductor device and method of manufacturing the same, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Semiconductor device and method of manufacturing the same will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3246016