The present invention relates to semiconductor devices and in particular to semiconductor devices including a voltage regulator element.
A semiconductor device often includes a voltage regulator element called a clamp diode. The voltage regulator element is to obtain a desired voltage from a voltage boosted by a booster element provided inside the semiconductor device and is also known as a Zener diode. The voltage regulator element can provide a desired voltage by making use of a breakdown phenomenon in the reverse direction of a pn junction which is created between a semiconductor substrate or a well and a doped layer formed on the substrate or the well.
FIG. 10 is a sectional view showing an exemplary structure of a conventional semiconductor device a portion of which is formed with a clamp diode (for example, Japanese Unexamined Patent Publication No. 2002-141517). Referring to FIG. 10, this structure will now be described.
As shown in FIG. 10, the clamp diode is basically composed of an n-type well 1 formed in a semiconductor substrate and a heavily p-doped layer 5 formed in the surface of the n-type well 1. The heavily p-doped layer 5 is surrounded and insulatively isolated with an isolation insulating film 2. A region surrounded with the isolation insulating film 2 is referred hereinafter to as an active region 3. Below the isolation insulating film 2, an n-doped layer 4 as a channel stopper is introduced. The heavily p-doped layer 5 is spaced apart from the isolation insulating film 2 and the n-doped layer 4 by a distance LA.
A thin oxide film 7 is formed on the surface of the active region 3. The thin oxide film 7 is provided with a contact hole 10, which exposes the heavily p-doped layer 5. A portion of the heavily p-doped layer 5 exposed in the contact hole 10 is connected to an aluminum interconnect 11 for an electrode. On the thin oxide film 7, an electrode 8 of a single-layered structure made of polycrystalline silicon is formed which terminates at a position apart from the heavily p-doped layer 5 by an offset region LB. An interlayer insulating film 9 is formed between the electrode 8 and the electrode 11. The electrode 8 is connected to an aluminum interconnect 12 for an electrode through contact holes 10.
In the case of the clamp diode shown in FIG. 10, with the electrode 8 made floating, a negative voltage is applied to the heavily p-doped layer 5 through the aluminum interconnect 11 until the breakdown voltage in the reverse direction applied to the pn junction between the heavily p-doped layer 5 and the n-type well 1 reaches 10 to 20 V, for example, thereby providing a desired constant voltage. In the above structure, provision of the distance LA and the offset region LB makes it possible to suppress fluctuations in the breakdown value of clamp voltage of the clamp diode with passage of time.
The conventional technique described above attains the clamp diode of which the fluctuations in the clamp breakdown voltage with passage of time are suppressed. However, the problem the clamp diode carries is not limited to such fluctuations. Therefore, there is demand for the development of a clamp diode with fewer defects, that is, the development of a clamp diode with more outstanding properties, and it is desirable that the demand be met.