Semiconductor element having a p-zone on the anode side and an a

Details Semiconductor element having a p-zone on the anode side and an a Semiconductor element having a p-zone on the anode side and an a

1988-06-10

1991-11-05

Hille, Rolf

357 20, 357 12, 357 46, H01L 2974, H01L 2906, H01L 2988, H01L 2702

Patent

active

050634283

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The invention relates to a semiconductor element having a weakly doped continuous interior p-zone which is provided on its surface with an ohmic contact layer and which is adjacent to a weakly doped n-base zone adjacent to which is arranged a highly doped n.sup.+ -zone.
Power rectifier diodes have a p.sup.+ nn.sup.+ -zone structure, i.e. they are composed of two highly doped zones of p or n-type conductivity which lie adjacent to the surface of the semiconductor disc and of a weakly doped zone, generally of n-type conductivity, disposed therebetween. In addition to a sufficiently low forward voltage and a sufficiently low reverse current, fast rectifier diodes must meet the requirement that, during commutation from the forward state, the reverse current peak and its holding time must be small and, in particular, after the reverse current maximum the reverse current must drop gradually (soft recovery behavior). A known means to realize this to a certain extent resides in the reduction of charge carrier lifetime by means of recombination centers, such as gold, platinum or by means of defects produced by electron, gamma or proton irradiation.
One drawback of this method is that the reverse current increases in proportion to the recombination center density and may become too high at the particularly effective recombination levels near the center of the band gap. This effect is very annoying at higher temperatures in connection with gold which, due to its otherwise favorable characteristics, is employed most frequently; it limits the permissible gold concentration and thus the degree of improvement of the recovery behavior.
Moreover, under consideration of the forward voltage, the recombination center density must not be selected too high, i.e. the charge carrier lifetime must not be selected too short. If the ratio of base thickness/.sqroot.D.tau. (where D is the ambipolar diffusion constant, .tau. the charge carrier lifetime at high injection) becomes greater than 2, the forward voltage increases exponentially with N.sub.Rec .about.1/.tau. and becomes too high. With .tau., the stored charge, which is important for the recovery behavior, is also limited downwardly. An additional limitation results from the fact that, in order to avoid too much compensation, the recombination center concentration must clearly lie below the common conductivity doping of the n-base zone which, in turn, is determined by the desired blocking behavior. If this condition is not met, the application of a current pulse will initially produce dynamic voltage peaks which are too high and the forward recovery time--after which the forward voltage approaches the stationary value to within 10% of the initial excess--becomes too long. This also results in a lower limit for the charge carrier lifetime to be set, below which it must not fall. It has been found that this limitation is more drastic in many cases than that set by the stationary forward behavior.
The local distribution of the recombination centers is generally substantially predetermined by the respective technology, i.e. cannot simply be selected on the basis of optimum rectifier behavior. Only with the proton irradiation method can very favorable center distributions be realized. However, proton irradiation is an expensive procedure which has so far not been used in the manufacture of semiconductors. The other irradiation methods also have the drawback that it is generally not possible to us them to set the charge carrier lifetime in the manufacturing plants themselves.
To improve the recovery behavior of rectifier diodes, European Patent No. 0,090,722 discloses a semiconductor structure in which an n-zone of average doping concentration which lies in a range between 10.sup.14 and 10.sup.16 /cm.sup.3 is disposed between the weakly doped n-base zone and the highly doped n.sup.+ -region. A considerable amount of engineering expenditures are required for the production of this intermediate zone since this zone must be produced in the starting silicon b

REFERENCES:
patent: 4117507 (1978-09-01), Pacor
patent: 4146906 (1979-03-01), Miyata et al.
patent: 4587547 (1986-05-01), Amemiya et al.
"High Speed Low-Loss P-N Diode Having a Channel Structure", Shimizu et al., EEE Transactions on Electron Devices, vol. ED-31, No. 9, Sep. 1984.
"Schnelle Diode", Neues Aus der Technik, vol. 15, #3, Jun. 15, 1981, p. 1.

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