JPH0549152B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a solid-state imaging device, in which a circuit element having a charge scanning function and a source region for accumulating signal charges generated by incident light are formed on a semiconductor substrate. It is an object of the present invention to provide a solid-state imaging device in which only the excess charge in the source region is reliably discharged to the semiconductor substrate side, and there is no charge overflow or afterimage in the scanning circuit section.

Conventionally, there are various types of solid-state imaging boards with a charge scanning function, but circuit elements with a self-scanning function such as CCDs or BBDs have been proposed as ones with a charge transfer function with particularly low noise ( (See Japanese Patent Application Laid-Open No. 61-95721).

However, in the above configuration, if the amount of charge handled by the charge transfer stage consisting of a CCD or BBD is smaller than the amount of charge generated by the incident light and accumulated in the source region, the excess charge generated by the strong incident light Charge overflow occurred in the scanning circuit section, and the voltage applied to the photoelectric conversion section was limited, causing burn-in and flickering.

The present invention provides a source region for accumulating signal charges on a semiconductor substrate, forms a diode with the substrate and the source region, and when a signal charge larger than the amount of charge handled by a circuit element having a charge transfer function is generated in this diode, , by putting the diode in a forward bias state while preventing the charge in the source region from being read into the circuit elements, the excess charge generated in the photoelectric conversion section flows out to the semiconductor substrate under the source region, thereby reducing the transfer of unnecessary signals due to overflow. The object of the present invention is to provide a solid-state imaging device that does not generate false signals due to leakage to other parts, and does not cause afterimages.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A solid-state imaging device according to an embodiment of the invention will be described below with reference to the drawings. The scanning circuit will be explained using a charge transfer type which has less noise. As a charge transfer type
Although there is a difference in whether charge is stored in a depletion layer or an impurity region, CCDs and BBDs operate essentially the same. Therefore, this will be explained in the future on BBD.

Figure 1 shows a unit of a solid-state imaging device in which a BBD formed on a Si substrate, a transfer stage thereof as a drain, a gate electrode and a source region, and a photoconductor and electrodes formed on these regions. be.

An n ⁺ type region 11 serving as a source region is formed in a p-type semiconductor substrate 10, and a diode is provided. 12 is
It is a potential well in the n ⁺ type region and becomes a charge transfer circuit element. Reference numeral 13 denotes a gate electrode, which has a portion overlapping with the n ⁺ type region 11. 14 is an insulating film between the semiconductor substrate 10 and the gate electrode 13;
This is the gate oxide film. 15 is an insulating layer for electrically separating the first electrode 16 from the semiconductor substrate 10 and the gate electrode 13. A first electrode 16 is an electrode of a diode electrically connected to the n ⁺ type region 11 and also serves as an electrode of the hole blocking layer 17 . 18 is (Zn _1-x Cd _x Te) _1-y
(In ₂ Te ₃ ) _y (O < x < 1, O < y < 0.3). A transparent electrode (second electrode) 19 is formed on the photoconductor, and the transparent electrode 19 side The incident light 20 is irradiated.

First, the optical information reading operation when the present invention is not used will be explained.

In the configuration shown in FIG. 1, a drive pulse as shown in FIG. 2a is applied to the gate electrode 13. At time t _T , electrode 16 is set at (V _CH -V _T ) as shown in FIG. 2b. Here, V _T is composed of n ⁺ regions 11 and 12 and gate electrode 13.
This is the threshold voltage of a MOS field effect transistor (FET). If there is now incident light 21, electron-hole pairs are generated in the photoconductor 18, which reach the electrodes 16 and 19, respectively, and the potential of the electrode 16 where signal charges are accumulated in the source region 10 decreases. Furthermore, when V _CH is applied to gate electrode 13 at time t ₂ , signal charges are transferred from n ⁺ region 11 to n ⁺ region 12 . As a result, the potential of n ⁺ region 11 rises again to (V _CH -V _T ). The signal charge transferred to the n ⁺ region 12 is then transferred to the output section by the transfer pulse V _O shown in FIG. 2a.

The above is the optical information reading operation when the present invention is not used, and the voltage applied to the photoconductor is when the second electrode 19 is kept at the ground potential (V _CH -V _T ).

However, if the transfer circuit is blocked and the amount of charge handled by the charge transfer stage, which is composed of a CCD or BBD, is less than the maximum amount of charge generated by the photoconductor, which is the photoelectric conversion section, the only option is to reduce the voltage applied to the photoconductor. Otherwise, burn-in, frizz, etc. will occur. On the other hand, if a sufficient voltage is applied to the photoconductor, the signal cannot be read in a single reading operation, and the generated excess charge (overflow charge) leaks into the transfer stage, causing unnecessary signals to be output. An afterimage also occurs.

Next, the operation of the present invention as an example of draining excess charge will be described. FIG. 3 shows a method of applying a pulse to the second electrode 19 as an example of a method for forward biasing a diode formed by a semiconductor substrate and a source region and draining excess charge to the substrate without causing unnecessary charge loading. and a method using control pulses to the gate electrode. time
If an excess charge (generated charge greater than the transferred charge amount) is generated by the incident light at T ₁ and the diode potential becomes below V _C , the external voltage applied to the second electrode 19, that is, the negative pulse (-V _C '), the diode potential (the potential of the source region 11 is
It is applied to the substrate and is pulled down by V _C to a negative value, resulting in a forward bias state, and the charge flows to the substrate until it reaches the same potential as the substrate (ground potential).

At this time, a control pulse 30 shown in FIG. 3a is applied to the gate electrode 13. This pulse 3
0 has a polarity opposite to that of the transfer and read pulses, and naturally the barrier potential of the signal charge reading portion located directly below the gate electrode 13 is higher than the barrier potential of the other portion of the substrate, that is, directly below the source region 11. With such means, excess charges flowing into the substrate 10 in a forward bias state are not read into the region 12 of the transfer stage, and no unnecessary pseudo signals are output. After such outflow, the pulse of the second electrode 19
Returning to OV, the diode potential is set to V _C ,
After that, a read pulse transfers the signal charge corresponding to the potential of {(V _CH −V _T )−T _C } to the region 12 at the transfer stage.
is loaded into. When the diode potential is equal to or higher than V _C at time T ₁ , that is, when the signal charge is equal to or less than the transferable charge amount, the pulse of the second electrode does not reduce the potential to below OV, and the diode does not become forward biased. Therefore, the diode potential is V _C
In the above case, the optical information is proportional to the incident light intensity, but
When V _C or less, that is, when the incident light intensity is stronger, the optical information remains constant at a certain value (white clip) by draining the excess charge to the substrate without causing unnecessary reading in the transfer stage. becomes. The above is an example of the optical information reading operation when the present invention is used.

The above is a photoconductor (Zn _1-x Cd _x Te) _1-y
Although the case where (In ₂ Te ₃ ) _y is used has been described, the photoconductor may be made of other materials such as amorphous silicon. moreover,
A diode including a semiconductor substrate and a source region may be used as the photoelectric conversion section. Furthermore, similar effects can be obtained by using an insulator instead of the photoconductor.

As described above, the solid-state imaging device of the present invention has a mechanism that can forward bias the diode, which is composed of a semiconductor substrate and a source region that accumulates signal charges, and has By applying a voltage of opposite polarity to the read pulse, the barrier potential of the signal charge reading section can be increased.If the present invention is used, the amount of charge handled by the charge transfer stage consisting of a CCD or BBD can be increased. Even if the amount of charge is smaller than the maximum amount generated by the photoelectric conversion section, the excess charge can reliably flow to the substrate without overflowing to the charge transfer stage, and unnecessary spurious signals will not be output due to excess charge. It also prevents afterimages, makes it possible to output signal charges that are faithful to incident light, and greatly contributes to the realization of high-performance solid-state imaging devices.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional structural diagram of one unit of a solid-state imaging device in which a photoconductor is provided on a circuit element having a charge transfer function, and Fig. 2 is a diagram for explaining the operation of the solid-state imaging device shown in Fig. 1. A waveform diagram showing the time relationship of clock pulses and the potential relationship of the photoconductor, and FIG. 3 is an operation when a diode composed of a semiconductor substrate and a source region is put into a forward bias state in a solid-state imaging device according to an embodiment of the present invention. FIG. 2 is a potential waveform diagram for explaining. DESCRIPTION OF SYMBOLS 10...P-type semiconductor substrate, 11...N ⁺ type region which becomes a source region, 12...N ⁺ type region for transfer which becomes a potential well, 13...Gate electrode, 14...
Insulator film, 15... Insulator layer.

Claims (1)

[Claims] 1. A source region formed on a semiconductor substrate having one conductivity type and having the other conductivity type and accumulating signal charges generated by incident light to form a diode;
A circuit element that transfers the signal charge formed in a portion of the substrate other than the source region, a MOSFET that applies a read pulse to the gate electrode to read the signal charge in the source region into the circuit element, and one electrode. has a photoconductor electrically connected to the source region, puts the diode in a forward bias state, drains excess charge to the semiconductor substrate, and supplies the gate electrode of the MOSFET with a photoconductor other than during the signal charge loading period. A solid-state imaging device characterized in that a voltage having a polarity opposite to that of the read pulse is applied to. 2. A source region formed on a semiconductor substrate having one conductivity type and having the other conductivity type and accumulating signal charges generated by incident light to form a diode;
A circuit element that transfers the signal charge formed in a portion of the substrate other than the source region, a MOSFET that applies a read pulse to the gate electrode to read the signal charge in the source region into the circuit element, and one electrode. has a photoconductor electrically connected to the source region, and when more charge than the amount of charge handled by the circuit element is accumulated in the source region,
The diode is forward-biased to drain excess charge to the semiconductor substrate, and the diode is forward-biased.
A solid-state imaging device characterized in that a voltage having a polarity opposite to the read pulse is applied to a gate electrode of a MOSFET during a period other than a signal charge read period.