1. Field of the Invention
The present invention relates to photodiodes and, more particularly, to a photodiode with improved photoresponse behavior.
2. Description of the Related Art
Conventional imaging circuits rely on photodiodes to convert a pixel of light energy into an electrical charge that represents the intensity of the light energy. In general, the light energy varies the electrical charge in a manner which is proportional to the photon absorption rate.
FIG. 1 shows a cross-sectional diagram that illustrates a conventional p+/n photodiode 10. As shown in FIG. 1, photodiode 10 includes an n-type substrate 12, and a p+ region 14 which is formed in substrate 12.
In operation, p+ region 14 is initially reverse-biased with respect to n-type substrate 12, and then floated. Under these conditions, light energy, in the form of photons, strikes photodiode 10, thereby creating a number of electron-hole pairs in substrate 12 and p+ region 14. As shown in FIG. 1, the holes formed in n-type substrate 12 diffuse to the p-n junction where they are swept to p+ region 14 under the influence of the electric field at the junction, while the electrons are attracted to the positive voltage applied to n-type substrate 12.
Similarly, the holes formed in p+ region 14 remain in region 14, while the electrons formed in p+ region 14 diffuse to the p-n junction where they are swept to n-type substrate 12. Thus, with the addition of each photogenerated hole in p+ region 14, the voltage on p+ region 14 is correspondingly increased. As a result, photodiode 10 varies the voltage on p+ region 14 in a manner which is proportional to the photon absorption rate.
One of the major disadvantages of photodiode 10 is that photodiode 10 is susceptible to thermally-generated, as well as other, sources of noise. For example, holes originating from thermally-generated electron-hole pairs formed in substrate 12 diffuse up from n-type substrate 12 into p+ region 14 where each additional hole erroneously represents another photon.
One technique for limiting the effect of noise is to use a p+/n-well photodiode. FIG. 2 shows a cross-sectional diagram that illustrates a conventional p+/n-well photodiode 20. As shown in FIG. 2, photodiode 20 includes an n-well 24 which is formed in a p-type substrate 22, and a p+ region 26 which is formed in n-well 24.
In operation, n-well 24 is reverse-biased with respect to p-type substrate 22 by applying a negative voltage to substrate 22, and a positive voltage to n-well 24. In addition, p+ region 26 is initially reverse-biased with respect to n-well 24, and then floated.
Under these conditions, the holes formed in n-well 24 diffuse to the p-n junction where they are swept to p+ region 26 under the influence of the electric field, while the electrons are attracted to the positive voltage applied to n-well 24. Similarly, the holes formed in p+ region 26 remain in region 26, while the electrons formed in p+ region 26 diffuse to the p-n junction where they are swept to n-well 24 and then collected by the positive voltage applied to n-well 24. Thus, as with photodiode 10, the addition of each photogenerated hole in the p+ region correspondingly increases the voltage on the p+ region.
The principal advantage of photodiode 20 is that by maintaining a reverse-bias across the well-to-substrate junction, holes from thermally-generated as well as other noise sources originating in substrate 22 are prevented from diffusing up from substrate 22 into p+ region 26 by the p-n junction.
Instead, the holes in substrate 22 are attracted to the negative voltage applied to substrate 22, while the electrons in substrate 22 from these electron-hole pairs diffuse to the p-n junction where they are swept to n-well 24 and then collected by the positive voltage applied to n-well 24. Thus, a p+/n-well photodiode significantly reduces the level of noise.
One major problem with photodiode 20, however, is that photodiode 20 has a relatively poor quantum efficiency. As further shown in FIG. 2, in addition to diffusing to p+ region 26, photogenerated holes formed in n-well 24 can also diffuse to substrate 22 where these holes, and the photo-information they represent, are lost. In a typical CMOS process, about one-half of the photogenerated holes formed in n-well 24 are lost to substrate 22.
Thus, there is a need for a structure which increases the quantum efficiency of photodiode 20 while at the same time maintaining the noise resistance of photodiode 20.