JPS6342170A - Signal-charge detecting circuit of charge transfer device - Google Patents

Abstract

PURPOSE:To eliminate reset noises even if thermal noises are present in a reset transistor, by setting the concentration, the junction depth and the drain voltage of a floating diffused layer so that the floating diffused layer can be completely depleted. CONSTITUTION:In this circuit, the following parts are included: a floating diffused layer 11; a reset transistor 3, which absorbs all the signal charges in the diffused layer 11; and a source follower circuit, which detects the potential change in the floating diffused layer 11. The concentration of the floating diffused layer 11 and a reset drain voltage VRD are set so that the signal charges in the floating diffused layer 11 can be completely absorbed and the floating diffused layer 11 is completely depleted when the reset transistor 3 is conducted. Namely, the floating diffused layer 11 is not an ordinary high concentration N<+> diffused layer but a low concentration N<-> diffused layer. Therefore, the depletion layer is expanded to the side of the floating diffused layer 11 at a P-N junction between a P-type substrate 10 and the floating diffused layer 11. Thus reset noises are reduced, and a high S/N ratio is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a signal charge detection circuit for a charge transfer device, and particularly to increasing the S/N thereof.

[Conventional technology]

Charge transfer devices using CCD (charge coupled device) elements are being actively used.

For example, semiconductor imaging devices and the like are already significantly encroaching on the field of use of conventional electronic imaging tubes.

However, since the signal charge amount of the charge transfer device is extremely small, the quality of the detection circuit function depends on the 7t!% charge transfer device. It greatly influences the characteristics of the child device.

Figure 3 shows a charge detection circuit (FDA) using a floating diffusion layer, which is one of the signal charge detection methods in a conventional charge transfer device.
). The CCD is of a buried channel type, and has an N-type well region 8t'' on a P-type substrate 10, and a P ion-implanted layer 9 serving as a barrier region within the CCD.
At the final stage of the OD, a composite capacitor iC consisting of the gate capacitance and wiring capacitance of the floating diffusion layer 1 and the MOS transistor 2 is charged, and the entire potential change is output. This will be explained using a potential diagram corresponding to the figure. At time tx (FIG. 4(a)), a high level is applied to the gate of the reset transistor 3 to make the reset transistor 3 conductive, and the potential VFD of the floating diffusion layer 1 is increased.
It is assumed to be the same as the potential VRD of the t-reset drain. Next, after applying a low level to the gate of the reset transistor 3 to shut off the reset transistor 3, a voltage is applied to the potential well under the CCD transfer gate 6 immediately before the gate OG5 at time tz (in FIG. 4(b)). The accumulated signal charge Q is caused to flow into the floating diffusion layer 1. The potential change of the floating diffusion layer due to the inflow of this signal charge is Δ■FD
The source follower circuit consisting of the ``MOS'' transistor 2 and the resistor 7 erupts.Here, [the problem to be solved by the invention] As mentioned above, the conventional FDA is Therefore, the potentials of the floating diffusion layer l immediately before shutting off the reset transistor 3 have high frequency components.
Each time the floating diffusion layer is interrupted, the potential VFD of the floating diffusion layer fluctuates by this noise component.

This is what is called reset noise.
Specifically, the amount of fluctuation is proportional to kTC (where k is Portsma's constant and T is absolute temperature).

This reset noise essentially exists in FDA, and in the past, noise was reduced by reducing the capacitance C in the design, but even now the number of electrons (11! What fluctuations exist in 50?
It is no exaggeration to say that the S/N ratio when the signal charge rate is small is determined entirely by the reset noise.

[First means for solving the problem] The charge detection circuit of the present invention includes a floating diffusion layer, a reset transistor that drains all the signal charges in the diffusion layer,
Change in the floating diffusion M? In a signal charge detection circuit of a charge transfer device including a source follower circuit for detection, signal charges in a floating diffusion layer? Is it possible for the floating diffusion layer to be completely depleted when the reset transistor is turned on in order to be able to completely deplete it? The concentration of the floating diffusion layer and the reset drain voltage VRD are adjusted so that the rl function is achieved.
It is characterized by setting the following.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a longitudinal sectional view of one embodiment of the present invention.

The difference from the conventional embodiment is that the floating diffusion layer 11 is not the usual high concentration N+ diffusion layer but a low concentration N- diffusion layer. By setting the floating diffusion layer 11 to N- in this way, the P type between the P type substrate 10 and the floating diffusion layer 11 is reduced.
In the −N junction, the depletion layer also extends to the floating diffusion layer 11 side. Therefore, m degrees of the floating diffusion layer 11 and its depth XJ,
Depending on the bias voltage, the floating diffusion layer 11 can be completely depleted. To be specific,
ND is placed in the floating diffusion layer 11 as a type of donor, NAiP
The acceptor wave of the type substrate 10, the junction between the floating diffusion layer 11 and the P type substrate 10 is step-junctioned, and a reverse bias voltage tV is applied.
t and the diffusion potential is φ, it can be expressed as. Here, 851 is the relative dielectric constant of the Si substrate.

g@ is the vacuum electric force, q is the charge amount of the electron, and k is the Boltzmann constant.

If you want to increase the specific value, N1=10” 5crn
-3゜Xj=LO#m, ND=3X10 Cm
Then, X(IXIO-')2 = 9.03V = 0.603V. Therefore, if a reverse bias voltage of &4 is applied, the floating diffusion layer 11 is completely depleted.

After complete depletion, the expansion of the depletion layer stops and the potential of the floating diffusion layer 11 does not change below 9.03V.

For example, by using the N-1 floating diffusion layer 11 which can be completely depleted at 9.03 V,
Reset noise? Using the potential diagram in Figure 2 (al work (b)), we will explain what can be done without any problems.
(Figure 2(a) shows the gate of reset transistor 3)
By applying a high level K, the reset transistor 3 is made conductive, and all the carriers in the floating diffusion layer 11 are taken out. At this time, the floating diffusion layer 11 is completely depleted at 9.03 V, so no further change in potential occurs.

This potential of 9.03 V due to complete depletion is sufficiently deep compared to the channel potential under the reset transistor 3 of 9.03 V even if there is thermal noise in the reset transistor 3, and the potential of the reset drain 4 vILD
If the voltage is sufficiently high, the floating diffusion Ni 111 is always set to 9.03 V at every reset, regardless of the thermal noise of the reset transistor j13.

That is, the potential of the floating diffusion layer 11 does not fluctuate.

In this state without fluctuation, at time tz (bl), the signal charge flows into the entire floating diffusion layer 11, and its potential changes iM(J
The reset noise detected by the source follower circuit of the S transistor 2 and the load resistor 7 can be completely eliminated even if thermal noise is present in the reset transistor 3.

〔Effect of the invention〕

As explained above, the invention is possible by setting the shape, junction depth, and drain voltage of the floating diffusion layer so that it can be completely depleted, even if there is thermal noise in the reset transistor. Reset noise? It has the effect of completely eliminating it, and has a great effect on the high S/H of CCD.

[Brief explanation of the drawing]

Figure 1 is a sectional view showing one embodiment of the present invention, Figure 2 (a)
, (bl is a potential diagram to explain its operation, FIG. 3 is a cross-sectional view of a conventional signal charge detection circuit part, and FIG. 4 (al, (b) is a bottom diagram to explain its operation. 1...Floating diffusion layer, 2...MOS transistor, 3...Reset transistor, 4...
...Reset drain, 5...Output gate, 6...Transfer gate, 7...Load resistance, 8...N-well, 9 ...P+
In-coupler layer, 10... substrate. Child 2 figure

Claims (1)

[Claims] In a signal charge detection circuit of a charge transfer device including a floating diffusion layer, a reset transistor for extracting signal charges in the diffusion layer, and a source follower circuit for detecting potential changes in the floating diffusion layer, the floating diffusion layer A signal charge detection circuit for a charge transfer device, wherein an impurity concentration of is set to a value such that the reset transistor is completely depleted when turned on.