1. Field of the Invention
The present invention relates to a semiconductor device, in particular, to a semiconductor device having high electrostatic discharge (ESD) immunity.
2. Description of the Related Art
An ESD element is indispensable in maintaining reliability of an integrated circuit (IC), though it is not directly responsible for a function of the IC. The ESD element refers to an electrostatic discharge element and serves to release static electricity in order to avoid destruction of the IC by the static electricity.
Accordingly, it is an essential condition that the ESD element itself is not thermally destroyed by the static electricity and is able to protect an internal circuit by dissipating electric charges quickly before the static electricity enters the internal circuit. In order to satisfy this condition, the ESD element is required to have properties of high driving ability and suppressing local heat generation. A generally employed method is to increase a sectional area of the ESD element at a point through which current flows, resulting in inevitable increase in the size of the ESD element. It is thus important to obtain the above-mentioned properties within an ESD element of a small size.
Further, in order not to disrupt the normal function of the IC, the ESD element is required to have a breakdown voltage that is not smaller than the absolute maximum rating of the IC. Especially in a case of a high withstanding voltage IC, the above-mentioned local heat generation problem becomes more serious since the ESD protection element must dissipate electrostatic charges while being applied with a voltage that is not smaller than the absolute maximum rating.
JP 2004-335634 A discloses a structure for preventing melting and destruction of an electrode due to local heat generation of an ESD element. FIG. 2 shows a sectional view of a conceptually illustrated structure. When positive static electricity is injected, for example, to an input pad (PAD), the static electricity flows through a collector electrode 7 to an n+ collector layer 2 and into an N-well electric field relaxation layer 23 for improving withstanding voltage. However, with the N-well electric field relaxation layer 23 having high resistance, electric charges tend to accumulate in the n+ collector layer 2, and the electric field becomes strong to locally generate heat around a boundary (“heat generation area” encircled in FIG. 2) between the n+ collector layer 2 and the N-well electric field relaxation layer 23 on a path having a small distance from the n+ collector layer 2 to an n+ emitter layer 6. The generated heat is conducted to a collector contact region 1 to melt the collector electrode 7. In order to solve this problem, the prior are attempts to prevent the melting and destruction of the collector electrode 7 by extending the distance b from the above-mentioned heat generation area to the collector contact region 1.
Since the method disclosed in JP 2004-335634 A is not, however, for suppressing local heat generation itself, melting and destruction of the silicon where the heat is generated may occur. Further, extension of the distance b not only increases the size, but also increases resistance of the n+ collector layer 2 and reduces the driving ability of the ESD element itself to increase a possible risk of exposing the internal circuit to a static electricity. A long base length of the ESD element is needed to increase the driving ability, further giving an increase in the size.
As described above, in order to obtain sufficient properties without increasing the size of the ESD element, it is important to suppress the local heat generation itself in the ESD element.