JP3211291B2 - Thin film transistor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film transistor having a back gate electrode.

[0002]

2. Description of the Related Art In a thin film transistor, it is desirable that a higher on / off ratio is desirable. Therefore, in order to reduce only off current while keeping on current,
Some include a back gate electrode. FIG. 3 shows an example of such a conventional thin film transistor.
In this thin film transistor, a channel region 2 of a semiconductor thin film 1 made of polysilicon containing an impurity of one conductivity type is used.
A source region 3 and a drain region 4 containing impurities of the same conductivity type at a high concentration are formed on both sides of the semiconductor thin film 1.
A gate electrode 6 is provided on one surface of a channel region 2 of the semiconductor thin film 1 with a gate insulating film 5 interposed therebetween.
A back gate electrode 8 is provided on the other surface of the source region 3 and the drain region 4 with a back gate insulating film 7 interposed therebetween.

Next, the operation of the thin film transistor will be described with reference to FIG.
When a negative gate voltage V _GS having a sufficiently large absolute value is applied to the gate electrode 6, the number of holes in the channel region 2 increases, so that the drain voltage V _DS is applied between the drain region 4 and the source region 3. When the voltage is applied, a large drain current I _SD, that is, an on current flows between the source region 3 and the drain region 4 to turn on. When the gate voltage V _GS is increased from this state to the positive side, the number of holes in the channel region 2 decreases, and therefore, the drain current I _SD decreases under the same level of the drain voltage V _DS , and the gate voltage V _GS = 0 Near the minimum value (off current), and the transistor is turned off. At this time, when the back gate voltage V _BS is applied to the back gate electrode 8, the ON current hardly changes and only the OFF current decreases under the same level of the drain voltage V _DS . In this case, the off-state current decreases as the back gate voltage V _BS increases. Therefore, only the off current can be reduced while the on current remains unchanged.

[0004]

However, in such a conventional thin film transistor, when the transistor is turned off, the back gate voltage _VBS is continuously applied to the back gate electrode, so that the power consumption increases. was there. An object of the present invention is to provide a thin film transistor that can reduce power consumption.

[0005]

According to the present invention, there is provided a semiconductor having a source region and a drain region in which impurities of the same conductivity type are diffused at a high concentration on both sides of a channel region in which impurities of one conductivity type are diffused. thin film and said gate electrode provided via a gate insulating film on one surface of the channel region of the semiconductor thin film, the other surface of the semiconductor thin film, the channel region,
A back gate electrode provided in a region corresponding to the entire source region and drain region via a ferroelectric film, wherein the ferroelectric film includes
The back surface over the entire lower surface of the source region and the drain region.
A predetermined voltage applied between the gate electrode and the gate electrode
Caused by voltage and maintained after application of the voltage is stopped
It has a certain polarity of polarization .

[0006]

According to the present invention, in the case of a p-channel, when a voltage in a predetermined direction is applied between the gate electrode and the back gate electrode, the ferroelectric film is polarized, and the surface facing the semiconductor thin film is shifted. +, The off-state of the thin film transistor
Current is reduced, and the polarization is maintained even when the application of the voltage between the gate electrode and the back gate electrode is stopped. When the surface side opposite to is +, the same effect as when a + voltage is constantly applied to the back gate electrode is obtained, and thus the power consumption can be reduced.

[0007]

FIG. 1 shows an essential part of a field effect type thin film transistor according to an embodiment of the present invention. In this thin film transistor, a back gate electrode 12 made of aluminum or the like is provided at a predetermined position on an upper surface of an insulating substrate 11 made of glass or ceramic. On the upper surface of the insulating substrate 11 including the back gate electrode 12, a ferroelectric film 13 made of a ceramic material such as barium titanate or an organic material such as polyvinylidene fluoride or ethylene trifluoride is provided. On the upper surface of the ferroelectric film 13 corresponding to the back gate electrode 12, a semiconductor thin film 14 made of polysilicon is provided. The semiconductor thin film 14 is made of n-type or p-type polysilicon and has a channel region 1
On both sides of 5, a source region 16 and a drain region 17 in which impurities of the same conductivity type as the channel region 15 are diffused at a high concentration are formed. An active layer of p-plus p-channel is formed. A gate insulating film 18 made of silicon nitride, silicon oxide, or the like is provided on the upper surface of the channel region 15 of the semiconductor thin film 14, and a gate electrode 19 made of aluminum or the like is provided on the upper surface of the gate insulating film 18.

Next, the operation of the thin film transistor will be described by taking a p-channel field effect thin film transistor as an example. First, a voltage between the gate electrode 19 and the back gate electrode 12 in a direction in which the gate electrode 19 side is-and the back gate electrode 12 side is +, which is a voltage sufficient to cause polarization in the ferroelectric film 13. When a gate-back gate voltage is applied, the ferroelectric film 13 is polarized so that the surface of the ferroelectric film 13 facing the semiconductor thin film 13 becomes +. This polarization is maintained even after the application of the gate-back gate voltage is stopped. Note that, when the temperature is increased, the gate-back gate voltage required to cause polarization of the ferroelectric film 13 can be reduced.

Then, when the application of the gate-back gate voltage is stopped and a negative gate voltage V _GS having a sufficiently large absolute value is applied between the gate electrode 19 and the source region 16, the channel region 15 is applied. When the drain voltage _VDS is applied between the drain region 17 and the source region 16, the number of holes near the interface with the gate insulating film 18 increases. The current I _SD, that is, the ON current flows, and the transistor is turned ON.
When the gate voltage V _GS is increased from this state to the positive side, the number of holes in the channel region 15 decreases, and therefore, the drain current I _SD decreases under the same level of the drain voltage V _DS ,
In the vicinity of the gate voltage V _GS = 0, it becomes a minimum (off current) and turns off. At this time, since the ferroelectric film 13 is maintained in a polarized state so that the surface facing the semiconductor thin film 13 becomes +, the back gate electrode 12
The same effect is obtained as if the voltage of
Therefore, under the same level of the drain voltage _VDS , the on-current hardly changes, and only the off-current decreases.

Thus, in this thin film transistor,
If the ferroelectric film 13 is once polarized and its surface facing the semiconductor thin film 13 is defined as +, the back gate electrode 1
The same effect as when a positive voltage is constantly applied to 2 can be obtained, and thus the power consumption can be reduced.

In the above embodiment, the case where only the ferroelectric film 13 is provided between the back gate electrode 12 and the semiconductor thin film 14 has been described, but the present invention is not limited to this. For example, as shown in FIG.
The back gate insulating film 21 may be provided between the semiconductor thin film 3 and the semiconductor thin film 14. In this case, the same effect as in the above embodiment can be obtained, and the back gate insulating film 21
The electric field due to the polarization of the ferroelectric film 13 acting on the semiconductor thin film 14 can be controlled by changing the film thickness of the semiconductor thin film 14. Further, in the above-described embodiment, the case of the p-channel field-effect thin film transistor has been described as the operation principle. Should be made to wake up.

[0012]

As described above, according to the present invention, a voltage in a predetermined direction is applied to the ferroelectric film to cause polarization once, and the surface of the ferroelectric film facing the semiconductor thin film is set to + or- . Reduces the off-current of the thin film transistor
And the polarization is applied to the ferroelectric film.
Is maintained even after stopping, the same effect as when + or-voltage is constantly applied to the back gate electrode can be obtained, and therefore, power consumption can be reduced.

[Brief description of the drawings]

FIG. 1 is a sectional view of a main part of a thin film transistor according to an embodiment of the present invention.

FIG. 2 is a sectional view of a main part of a thin film transistor according to another embodiment of the present invention.

FIG. 3 is a schematic configuration diagram of an example of a conventional thin film transistor.

FIG. 4 is an operation characteristic diagram of a conventional thin film transistor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Insulating substrate 12 Back gate electrode 13 Ferroelectric film 14 Semiconductor thin film 15 Channel region 16 Source region 17 Drain region 18 Gate insulating film 19 Gate electrode 21 Back gate insulating film

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01L 29/786 H01L 21/336

Claims (2)

(57) [Claims] 1. A semiconductor thin film having a source region and a drain region in which impurities of the same conductivity type are diffused at a high concentration on both sides of a channel region in which impurities of one conductivity type are diffused, and a channel of the semiconductor thin film. A gate electrode provided on one surface of the region with a gate insulating film interposed therebetween, and a back electrode provided on the other surface of the semiconductor thin film via a ferroelectric film in a region corresponding to the entire channel region, source region and drain region. Ri Na and includes a gate electrode, the ferroelectric film, the channel region, Oyo source region
The back gate electrode and the entire lower surface of the drain region
And a predetermined voltage applied between the gate electrodes.
Of the predetermined polarity, which is maintained even after the application of the voltage is stopped.
A thin film transistor and said Rukoto which have a pole. 2. The thin film transistor according to claim 1, wherein a back gate insulating film is provided between the other surface of the channel region, the source region, and the drain region of the semiconductor thin film and the ferroelectric film.