MOSFET Light Detecting Device Having a Schottky Junction

The present invention relates to a device for detecting light in which a semiconductor element is used in a similar manner to that used for detecting infrared rays.

Conventionally, such a detecting device has employed a Schottky silicon type, silicon impurity doped type, or a compound semiconductor type structure, or the like. Each of these devices has certain disadvantages.

The Schottky silicon type, if used as a photocell, provides a uniform sensitivity and the production thereof is simple due to its use of a silicon substrate. However, the device has a narrow sensing range for the incident light and the signal output thereof is low. The silicon impurity doped type has a wide sensing range and the signal output thereof is high, but the production of the substrate thereof is more difficult than for the silicon Schottky type. The compound semiconductor type has a wide sensing range, but the production of the crystal substrate thereof is also more difficult than for the Schottky silicon type. Moreover, the leakage current thereof is very large.

A structure such as a silicoon Schottky type, in comparison with the other structure, is most suitable for use in a monolithic circuit such as a "CCD" (charged-couple device) in a two-dimensional array because conventional silicon LSI techniques are applicable for the fabrication thereof. However, the greatest disadvantage of this device is a lower sensitivity for detecting infrared rays.

Conventional structures of the Schottky silicon type, for example, are constructed by evaporating a metal such as platinum silicide or gold onto a surface of P-type silicon substrate having a specific resistance of about 10 .OMEGA.-cm. Schottky junctions formed between the metal and the P-type silicon substrate can be used for detecting infrared rays.

Usually, the barrier height between Pt-Si and P-type silicon is 0.275 eV, and that between Au and P-type is 0.25 eV. The detectable wavelength, based on quantum mechanics, calculated from these barrier values is 4.47 .mu.m in the case of Pt-Si and 4.92 .mu.m in the case of Au. Therefore, the ratio of detecting sensitivity D is usually about 10.sup.9 cm Hz1/2W.sup.-1, that is, this value is 1 to 2 orders of magnitude lower than that obtainable with other more desirable methods.

A primary object of the present invention is to provide an improved structure for a device for detecting light.

Overcoming the above noted defects, the invention provides a novel device for detecting light including a predetermined conductivity type semiconductor substrate having a surface region, a source region and a drain region of a conductivity type opposite that of the surface region, an oxidation resistant layer formed on the surface region, a metal or a metal silicide layer formed on the oxidation resistant layer between the source region and the drain region, and a polysilicon layer used as a gate electrode formed on the metal or metal silicide layer.

The single accompanying FIGURE shows a cross-sectional view of a device for detecting light according to the present invention.

A preferred embodiment according to the present invention will be described with reference to the accompanying drawing.

In the FIGURE, numeral 1 designates a P-type silicon substrate forming a main semiconductor body, 2 and 3N.sup.+ impurity regions, 4 a silicon dioxide layer, 5 a platinum silicide layer, 6 a polysilicon layer. This structure is easily produced with conventional well-known techniques. N.sup.+ impurity regions 2 and 3 are formed on the P-type silicon substrate 1 using a silicon dioxide mask. These two regions become a source region and drain region, respectively, for a "MOS" transistor, as will become more clear from the following description.

Further, a silicon dioxide layer 4 is formed thereon by annealing or the "CVD" method. Following that, a platinum layer is formed thereon by sputtering. Next, a polysilicon layer 6 doped with boron is formed thereon by a plasma CVD method. Preferably, the impurity density of the P-type polysilicon layer is less than 10.sup.17 atoms/cm.sup.3.

Following this, the device thus formed is annealed in a nitrogen atmosphere at a temperature of 600.degree. C. During annealing, the polysilicon reacts with the platinum forming the Pt-Si layer 5 and the polysilicon layer 6 by reaction at the interface. Then a Schottky junction is formed between the layers 5 and 6. Further, the portion of the Pt-Si layer 5 and the polysilicon layer 6 other than a gate region of a "MOS" transistor is etched using a conventional photolithographic method. A cross-sectional view of the device thus constructed is shown in the accompanying drawing. Next, between the source region 2 and the drain region 3, the gate electrode 6 is formed as a metal lead for connection, again using a conventional method.

The operation of the device thus constructed will be described.

The Schottky diode formed by the Pt-Si layer 5 and the P-type polysilicon layer 6 has relatively ideal characteristics, the same as that of a single crystal silicon. In order to further increase the sensitivity of the device, it is operated at a temperature of about 77.degree. K. At the same time, the mobility of carriers in the "MOS" transistor thus formed is increased to a desired level.

In operation, the substrate 1 and the source 2 are grounded and the drain 3 is connected through a load resistor to a positive voltage source. Thus, the Schottky junction formed between the layers 5 and 6 is reverse biased. Therefore, the Pt-Si layer 5 holds a positive charge so that the "MOS" transistor is in the "on" state. Thereby, a channel current flows, the magnitude of which is determined by the load resistance and the channel resistance.

In this state, upon irradiation with infrared rays on the upper surface of the P-type polysilicon layer 6, the positive charge held in the Pt-Si layer 5 is discharged into the P-type polysilicon layer 6. Therefore, the positive voltage of the Pt-Si layer 5 is shifted in a negative direction. This potential change reduces the channel current. Specifically, a potential change .DELTA.V.sub.G of the Pt-Si layer 5 causes a channel current change .DELTA.I.sub.D, This amplification factor corresponds to the transconductance of the "MOS" transistor. The amplification factor is regulated by the channel length, channel width, and silicon dioxide layer thickness. Furthermore, it is possible to vary this value as desired. Accordingly, with the device thus constructed, an electric charge generated by irradiation with infrared rays is converted into a current change. Moreover, the amount of this current change can be set as desired.

According to the present invention, the sensitivity of the device is effectively increased over that of a conventional Schottky type detector for infrared rays due to the amplication action of the transistor.

In the embodiment described, the present invention is described with reference to a "MOS" transistor. However, an oxidation-resistant layer of other than silicon dioxide is also applicable with the present invention. Also, metals such as gold and palladium are applicable in place of Pt-Si.

Moreover, according to the present invention, it is possible to detect light having wavelengths other than infrared.