Electronic apparatus

Abstract

To provide a portable electronic apparatus that can operate even when a small power is supplied through non-contact charging of an electromagnetic induction system.SOLUTION: A portable electronic apparatus comprises: a reflection type liquid crystal display that includes a transistor using an oxide semiconductor; a power source part that includes a rechargeable battery chargeable through non-contact charging; and a signal processing part including a non-volatile semiconductor storage device. In the portable electronic apparatus, a power stored in the rechargeable battery is used for the reflection type liquid crystal display and the signal processing part.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The technical field relates to portable electronic devices that use a contactless charging system. [Background technology]

[0002] Electronic book devices that display digitized book data on a display and enable reading Portable electronic devices are becoming more and more popular. These devices are designed to be carried around and used. It is designed to be powered by a built-in battery. These devices are suitably powered using an external power supply.

[0003] Patent Document 1 proposes an electronic book device that is powered by an AC adapter.

[0004] In Patent Document 2, a display unit, a console unit for inputting operations from the outside, and a wireless signal The antenna section for transmitting and receiving signals, and the console section receives the signal input and transmits it through the antenna section. A controller section for controlling the signal to be received and a radio signal received by the antenna section and a battery unit that converts the power into electricity and stores the converted power as electricity for driving the display unit. A device has been proposed. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2007-147871 A [Patent Document 2] JP 2008-181108 A Summary of the Invention [Problem to be solved by the invention]

[0006] In order to supply power from an AC adapter, a metal terminal or other contact must be provided on the portable electronic device. Therefore, the contacts must be durable. It is necessary to take measures against leakage current caused by moisture, etc.

[0007] In addition, even if the power supplied by the electromagnetic induction type non-contact charging is small, the operation of portable electronic devices is possible. To achieve this, it is necessary to reduce the power consumption of portable electronic devices.

[0008] In one aspect of the present invention, a device that operates even with a small amount of power supplied by electromagnetic induction type non-contact charging is provided. The object of the present invention is to provide a portable electronic device that can [Means for solving the problem]

[0009] One embodiment of the present invention is a reflective liquid crystal display having a transistor including an oxide semiconductor. a power supply unit having a rechargeable battery that can be charged by non-contact charging; and a signal processing unit for performing the same.

[0010] Alternatively, one embodiment of the present invention is a semiconductor memory device including a first transistor including a first oxide semiconductor. A projection type liquid crystal display, a power supply unit having a rechargeable battery that can be charged by non-contact charging, and a non-volatile the non-volatile semiconductor memory device includes a second transistor and a third transistor The second transistor has a first terminal connected to the first wiring and a capacitance element. the first terminal is electrically connected to the second wiring, and the gate is electrically connected to the third transistor. A third transistor is electrically connected to the first terminal of the transistor and one electrode of the capacitance element. The second terminal of the transistor is electrically connected to the third wiring, and the gate is electrically connected to the fourth wiring. the other electrode of the capacitor is electrically connected to a fifth wiring. It is a portable electronic device.

[0011] The third transistor may be a transistor including a second oxide semiconductor. The second oxide semiconductor may be the same as or different from the first oxide semiconductor.

[0012] The power stored in the rechargeable battery may be used to power the reflective liquid crystal display and the signal processing unit. .

[0013] The power supply unit may include a solar cell.

[0014] The oxide semiconductor is intrinsic or substantially intrinsic, and Off-state current is 100aA / μm or less (a is 10 -18 ), preferably 1 aA / μm or less, more preferably 1zA / μm or less (z is 10 -21 (representing) In this specification, the term "intrinsic" refers to a carrier concentration of 1×10 12 / cm 3 "Substantially intrinsic" refers to the state of a semiconductor in which the carrier concentration is less than 1 × 1 0 12 / cm 3 More than 1×10 14 / cm 3 This refers to the state of a semiconductor being less than Effect of the Invention

[0015] Since it is possible to charge without contact, there is no need to provide contacts such as metal terminals on the portable electronic device. In addition, it can operate even with a small amount of power supplied by non-contact charging using electromagnetic induction. A portable electronic device can be provided. [Brief description of the drawings]

[0016] [Figure 1] Block diagram of a portable electronic device [Diagram 2] A diagram showing an example of the configuration of a portable electronic device and a charger [Diagram 3] FIG. 1 is a diagram showing an example of a circuit configuration of a nonvolatile semiconductor memory device; [Figure 4] FIG. 1 is a schematic diagram showing an example of a display panel of a liquid crystal display device and a diagram showing an example of a method for driving the display panel. [Diagram 5] Block diagram of a portable electronic device [Figure 6] Diagram showing the connection relationship between the antenna, solar cell, and rechargeable battery [Figure 7] FIG. 1 is a diagram showing an example of the configuration of a portable electronic device; [Figure 8] FIG. 1 illustrates an example of a transistor structure. [Figure 9] 1 illustrates an example of a method for manufacturing a transistor. [Figure 10] Graph showing the electrical characteristics of a transistor DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, the embodiments of the present invention will be described with reference to the drawings. The embodiments of the present invention are not limited to the following description and may be modified without departing from the spirit and scope of the present invention. It will be readily understood by those skilled in the art that various modifications may be made to the details. The invention should not be construed as being limited to the description of the embodiments shown below.

[0018] (Embodiment 1) In the present embodiment, with reference to Figs. 1 to 4(C), a handset of a portable electronic device according to the present embodiment will be described. This section explains the configuration of the hardware.

[0019] FIG. 1 is a block diagram of a portable electronic device according to the present embodiment. The device is composed of a sprayer 10, a power supply unit 20, and a signal processing unit 30. The portable electronic device 1 is embodied as an electronic book device (also called an "electronic book terminal device") as an example. He explains.

[0020] (Power supply section 20) The power supply unit 20 includes an antenna 21, a rectifier circuit 22, a rechargeable battery 23, and a DC-DC converter 2 It has 4.

[0021] (Antenna 21) The antenna 21 is a power receiving coil for contactless charging.

[0022] FIG. 2 shows an example of the configuration of a portable electronic device and a charger. As shown in FIG. The device 1 is equipped with an antenna 21, and the charger 40 is equipped with a power transmission coil 41. The power is transmitted between the two devices without contact by electromagnetic induction between the coils, without using metal terminals. conduct.

[0023] Non-contact charging does not require contacts such as metal terminals for charging between the portable electronic device 1 and the charger 40. Therefore, there is no risk of contact failure, short circuit, moisture, etc. This makes it less likely for leakage current and other problems to occur.

[0024] (rectifier circuit 22) The power received by the antenna 21 is rectified by the rectifier circuit 22 and charged in the rechargeable battery 23. do.

[0025] (Rechargeable battery 23) The rechargeable battery 23 can be used repeatedly as a battery by storing electricity through charging. The rechargeable battery 23 is, for example, a lithium ion battery or a lithium ion charger. In addition, the rechargeable battery 23 is provided with a control circuit for preventing overcharging and overdischarging. Circuit included.

[0026] (DC-DC converter 24) The power stored in the rechargeable battery 23 is supplied to the display 10 via a DC-DC converter 24. and the signal processor 30, and is used as a power source for these.

[0027] (Signal processing unit 30) The signal processing unit 30 includes an antenna 31, a down converter 32, a signal processing circuit 33, and an NVM3. 4 (Non-Volatile Memory) and display controller Equipped with a Trolla 35.

[0028] (Antenna 31) The antenna 31 receives electronic book data selected by the user of the portable electronic device 1 (hereinafter, “ The download start request for the electronic book data is sent to the server, and the server responds to the request. The electronic book data transmitted from the electronic book server is received.

[0029] (Down converter 32, signal processing circuit 33) The electronic book data received by the antenna 31 is down-converted by the down-converter 32. The baseband signal is then processed by the signal processing circuit 33. will be done.

[0030] (NVM34) The electronic book data received by the antenna 31 is larger than the amount of information that can fit on one screen of the display. The received data is stored in NVM34.

[0031] Here, referring to FIG. 3, a circuit configuration of a nonvolatile semiconductor memory device applicable to the NVM 34 will be described. An example will be described.

[0032] The nonvolatile semiconductor memory device shown in FIG. It is composed of element 73.

[0033] A transistor using a material other than an oxide semiconductor is suitable for the transistor 71. Examples of materials other than nitride semiconductors include single crystal silicon and crystalline silicon. Transistors using materials other than oxide semiconductors can operate at high speed; This enables high-speed data read operations from the volatile semiconductor memory device.

[0034] On the other hand, a transistor including an oxide semiconductor is suitable for the transistor 72. A transistor using a semiconductor has a feature that the off-state current is extremely small. Therefore, data stored in the nonvolatile semiconductor memory device can be retained for a long period of time. Therefore, refresh operations are unnecessary or infrequent, and non-volatile semiconductor The power consumption of the storage device can be reduced.

[0035] The gate of the transistor 71 is connected to either the source or drain of the transistor 72. The source is connected to wiring 50 (source line), and the drain is connected to wiring 51 (bit line). The other of the source and the drain of the transistor 72 is connected to the wiring 52 (the first signal The gate is connected to a wiring 53 (a second signal line). One of the electrodes of element 73 is connected to the gate of transistor 71 and the source of transistor 72. One of the drains is connected to the wiring 54 (word line), and the other is connected to the wiring 54 (word line).

[0036] Next, a data write operation to the nonvolatile semiconductor memory device shown in FIG. 3 will be described.

[0037] First, a voltage is applied to the gate of the transistor 72 connected to the wiring 53, and the transistor 7 As a result, the wiring 52 and the transistor 71 are brought into electrical conduction, and the transistor 7 Data is written by applying a voltage to the gate of the first transistor and the capacitance element 73. do.

[0038] Thereafter, transistor 72 is turned off, so that the gate of transistor 71 The applied voltage is maintained, and the data is stored. At this time, the transistor 71 If a voltage that turns on the transistor 71 is applied to the gate, the transistor 71 will remain in the on state for a long time. On the other hand, a voltage is applied to the gate of transistor 71 to make it non-conductive. If a voltage is applied, transistor 71 will remain non-conductive for a long period of time.

[0039] Next, a data read operation from the nonvolatile semiconductor memory device shown in FIG. 3 will be described.

[0040] When the transistor 71 is maintained in a conductive or non-conductive state, the wiring 50 A constant voltage is applied to the line 52 and a read voltage is applied to the line 54. As a result, the transistor When the transistor 71 is in a conductive state, the voltage of the wiring 51 changes. When the wire 50 and the wire 51 are in a conductive state, the voltage of the wire 51 does not change. By comparing the voltage with 1, the data stored in this non-volatile semiconductor memory device can be read out.

[0041] The data rewrite operation to the nonvolatile semiconductor memory device shown in FIG. 3 is the same as the data write operation. It seems that.

[0042] (Display Controller 35) The display controller 35 transmits data for the content to be displayed on the display 10. .

[0043] Since most e-book data is still images, data rewrite speed is not very important. Therefore, the down converter 32, the signal processing circuit 33, the NVM 34 and the digital High-speed operation of the display controller 35 is not required. Therefore, these circuits are Therefore, the power supplied by the electromagnetic induction type non-contact charging is small. Even if the power is turned off, the mobile electronic device 1 can still operate.

[0044] (Display 10) The display 10 displays the contents of the electronic book data. In addition, displays that do not emit light themselves, such as reflective liquid crystal displays and electrophoretic displays, By using the above, the power consumption of the display 10 can be set to 10 mW or less. Therefore, even if the power supplied by the electromagnetic induction type non-contact charging is small, Therefore, the portable electronic device 1 can operate.

[0045] Here, referring to FIG. 4(A) and FIG. 4(B), a circuit configuration applicable to the display will be described. An example will be described.

[0046] FIG. 4(A) is a schematic diagram showing an example of a display panel of a liquid crystal display device. 0, a pixel section 61, a gate signal line 62, a gate signal line driving circuit 62D, and a data signal line 63 , a data signal line driving circuit 63D, a pixel 64, a common electrode 65, a capacitance line 66 and a terminal portion 6 It has 7.

[0047] FIG. 4B is a diagram showing pixel 64 extracted from FIG. 4A. The display device includes a transistor 75 using an oxide semiconductor, a liquid crystal element 76, and a storage capacitor 77. There are.

[0048] Next, an example of a method for driving a display panel will be described with reference to FIG.

[0049] First, in order to write the image signal BK / W to the pixel, the transistor 75 is made conductive. A period T1 during which a voltage based on an image signal is supplied to the pixel electrode of the liquid crystal element 76 (hereinafter referred to as “write In the writing period T1, the driving circuit control signal is The signal is supplied to the drive circuitry of the display 10 and the display controller 35. The road is working.

[0050] After the writing period T1, the pixel electrode of the liquid crystal element 76 is supplied with a voltage V pix occurs. After that, the transistor 75 is turned off, so that a voltage is applied to the pixel electrode of the liquid crystal element 76. V pix is retained.

[0051] Next, a voltage V pix The retention period T2 (hereinafter, In the "2" (hereinafter referred to as "3"), the image signal BK / W is not written. No signal is provided to the driver circuitry and display controller 35 of the display 10; These circuits are inoperative.

[0052] The length of the hold period T2 is determined by the off-current I 75 and flows through the liquid crystal element 76 current I 76 To prevent screen flicker caused by these current fluctuations, Therefore, a periodic refresh operation is required to rewrite the screen.

[0053] Here, the off-state current I 75 is extremely small. Therefore, during the holding period T2, the current I 76 Therefore, The screen is refreshed 60 times per second, but this is reduced to about 1 / 1000 of that. This becomes possible.

[0054] As described above, during the hold period T2, the drive circuit and the display control circuit of the display 10 Therefore, the operation of the display 10 and the display controller 35 can be stopped. The power consumption of the play controller 35 can also be reduced to about 1 / 1000. do.

[0055] Next, an oxide semiconductor used in a transistor will be described.

[0056] The oxide semiconductor used in the transistor is a semiconductor that is free of hydrogen, moisture, a hydroxyl group, or water, which are donors. After intentionally removing impurities such as oxides (also called hydrogen compounds), By supplying oxygen, which is also reduced during the removal process, high purification and electrical It is i-type (intrinsic) in order to suppress fluctuations in the electrical characteristics of the transistor. be.

[0057] By removing hydrogen from the oxide semiconductor as much as possible, the carrier density in the oxide semiconductor is 1×10 14 / cm 3 Less than 1 x 10 12 / cm 3 Less than 1× 10 10 / cm 3 It will be less than.

[0058] Oxide semiconductors, which are wide-gap semiconductors, have low minority carrier density and Therefore, in a transistor using an oxide semiconductor, Therefore, a neutral current is unlikely to occur, and therefore an off-current is unlikely to flow.

[0059] In addition, in a transistor using an oxide semiconductor, which is a wide-gap semiconductor, Ionization and avalanche breakdown are unlikely to occur. Transistors are resistant to hot carrier degradation. Main causes of hot carrier degradation In the case of avalanche breakdown, the number of carriers increases, and the carriers accelerated to high speed are transported to the gate insulator. It is injected into the velum.

[0060] In this specification, the off-state current is the current at room temperature in the range of -20 V to -5 V. An n-channel transistor with a positive threshold voltage Vth when a given gate voltage is applied. This refers to the current that flows between the source and drain of a transistor. Room temperature is between 15°C and 25°C. Refers to the lower temperature.

[0061] The transistor using an oxide semiconductor disclosed in this specification has a channel width of 1 The current value per μm is 100 aA / μm or less, preferably 1 aA / μm or less, and more preferably It is preferably 10zA / μm or less.

[0062] As described above, a highly purified and electrically i-type (intrinsic) oxide semiconductor is used. This makes it possible to provide a transistor with an extremely small off-state current. G) and the results of measuring the off-current characteristics are described below.

[0063] The fabricated TEG has a transistor with L / W=3μm / 50μm (film thickness d is 30nm). By connecting 200 transistors in parallel, a transistor with L / W=3μm / 10000μm can be created. In addition, W is the channel width, and L is the channel length.

[0064] FIG. 10 is a graph showing the electrical characteristics (log(Id)-Vg) of a transistor provided in a TEG. In FIG. 10, the horizontal axis represents the gate voltage Vg [V], and the vertical axis represents the drain current I The substrate temperature is room temperature, and the source-drain voltage Vd is 1 V (graph is dashed) or 10V (graph is solid). -The gate voltage Vg is changed from -20V to +20V, and the drain current Id changes Measured.

[0065] As shown in FIG. 10, a transistor with a channel width W of 10000 μm has a Vd of 1 V and At both 10V and 10V, the off-state current is 1×10 -13 It is below A. This is because Measuring instrument (semiconductor parameter analyzer, Agilent 4156C; Agilent This off-current value is below the resolution (100 fA) of the MOSFET (manufactured by TDK). This is equivalent to 10aA / μm.

[0066] (Embodiment 2) In this embodiment, with reference to Figs. 5 to 7, the hardware of the portable electronic device according to the first embodiment will be described. This section describes the hardware configuration of the portable electronic device, which is different from the software configuration.

[0067] FIG. 5 is a block diagram of a portable electronic device according to the present embodiment. The portable electronic device 1 shown in FIG. 1 has a power supply unit 20 to which a solar cell 25 is added. By providing such a configuration, when the portable electronic device 2 is exposed to sunlight or illumination, The rechargeable battery 23 can be charged by the solar cell 25 .

[0068] FIG. 6 shows the connection relationship between the antenna, the solar cell, and the rechargeable battery. A reverse current prevention diode is provided between the solar cell 25 and the rechargeable battery 23, and between the solar cell 25 and the rechargeable battery 23. It is being done.

[0069] An example of the configuration of a portable electronic device equipped with a solar cell will be described with reference to FIG.

[0070] The portable electronic device 2 shown in FIG. 7 is in an open state and displays electronic book data on the display 10. Therefore, the solar cell 25 is also exposed on the outer surface of the mobile electronic device 2 when the mobile electronic device 2 is opened. If the display 10 is provided so as to be exposed to the outside, the electronic book data can be displayed on the display 10 while the solar panel is turned on. This allows charging using the battery 25.

[0071] In particular, the display 10 may be a reflective liquid crystal display or an electrophoretic display. When using a display that does not emit light by itself, the usage environment is such that charging by solar cells 25 is required. It is suitable because it is possible.

[0072] (Embodiment 3) In this embodiment, referring to FIG. 8(A) to FIG. 9(E), an intrinsic or substantially intrinsic oxide An example of a structure of a transistor using a semiconductor and an example of a manufacturing method thereof will be described. .

[0073] 8A and 8B are diagrams showing an example of a planar structure and a cross-sectional structure of a transistor. FIG. 8(A) is a plan view of a transistor having a top gate structure. FIG. 8(B) is a plan view of a transistor having a top gate structure. 8(A) is a cross-sectional view of the portion indicated by the line C1-C2 in FIG.

[0074] The transistor 410 includes an insulating layer 407, an oxide semiconductor layer 412, a first The semiconductor device includes an electrode 415a, a second electrode 415b, a gate insulating layer 402, and a gate electrode 411. The first electrode 415a is connected to a wiring layer 414a, and the second electrode 415b is connected to a wiring Layers 414b are in contact with each other.

[0075] The transistor 410 shown in FIGS. 8A and 8B has a single-gate structure. However, the structure of the transistor is not limited to this. For example, a multi-gate A transistor having the structure may be applied.

[0076] Next, with reference to FIGS. 9A to 9E, a process for manufacturing the transistor 410 will be described. explain.

[0077] First, an insulating layer 407 serving as a base film is formed on a substrate 400. The insulating layer 407 is The insulating layer 407 may be formed while removing residual moisture. This is to ensure that no objects or other items are included.

[0078] Next, an oxide semiconductor layer is formed over the insulating layer 407 by a sputtering method. Before the deposition of the compound semiconductor layer, the substrate 400 on which the insulating layer 407 is formed may be preheated. This is to prevent hydrogen, moisture, and a hydroxyl group from being contained in the oxide semiconductor layer as much as possible. By preheating, impurities such as hydrogen and moisture adsorbed on the substrate 400 are desorbed and exhausted.

[0079] As a target for forming an oxide semiconductor layer, a metal oxide having zinc oxide as a main component is used. For example, a target having a composition ratio of In2O3:Ga2O3:Z A target with nO=1:1:1, i.e., In:Ga:Zn=1:1:0.5, is used. In addition, In:Ga:Zn=1:1:1 or In:Ga:Zn= A target having a composition ratio of 1:1:2 may also be used.

[0080] Others: In-Sn-Ga-Zn-O metal oxides, In-Sn-Zn-O metal oxides , In-Al-Zn-O metal oxides, Sn-Ga-Zn-O metal oxides, Al-Ga -Zn-O metal oxides, Sn-Al-Zn-O metal oxides, In-Zn-O metal acids compound, Sn-Zn-O metal oxide, Al-Zn-O metal oxide, Zn-Mg-O metal oxide Metal oxides, Sn-Mg-O metal oxides, In-Mg-O metal oxides, In-O metal oxides Metal oxide targets such as oxide, Sn-O metal oxide, Zn-O metal oxide, etc. It can be used.

[0081] In addition, the oxide semiconductor layer is InMO3(ZnO) m (m>0) Thin films with conductors can also be used, where M is Ga, Al, Mn, or Co. For example, M is Ga, Ga, and Al. , Ga and Mn, or Ga and Co.

[0082] The oxide semiconductor layer thus formed is an island-shaped oxide semiconductor layer formed by a first photolithography process. After that, hydrogen, water, and the like are removed from the oxide semiconductor layer 412 (see FIG. 9A). In order to remove hydroxyl groups and the like, the substrate 400 on which the oxide semiconductor layer 412 is formed is subjected to an electric field. The oxide semiconductor layer 412 is dehydrated or heated in a furnace. or dehydrogenation effects.

[0083] The temperature of this heat treatment is 400° C. or higher and 750° C. or lower, preferably 400° C. or higher to prevent distortion of the substrate. The heating temperature should be less than 100° C. The atmosphere in this heating process should be free of water, hydrogen, and the like.

[0084] After this heat treatment, the mixture is continuously cooled in an oxygen atmosphere or an atmosphere containing nitrogen and oxygen (e.g., nitrogen The heat treatment is preferably performed in a volume ratio of nitrogen to oxygen of 4:1. This is to repair the element deficiency.

[0085] In FIG. 9B, a first electrode 415a and a second electrode 415b are formed over the insulating layer 407 and the oxide semiconductor layer 412. The first electrode 415a is a source electrode and the second electrode 415b is formed. The second electrode 415b functions as one of the source and drain electrodes. It serves as the other electrode.

[0086] In FIG. 9C, the insulating layer 407, the oxide semiconductor layer 412, the first electrode 415a, and the second The gate insulating layer 402 is formed over the electrode 415b. It is preferable that the film formation atmosphere does not contain hydrogen.

[0087] In FIG. 9D, a part of the gate insulating layer 402 is removed to form a first electrode 415a. and an opening 421b reaching the second electrode 415b are formed. This indicates the state.

[0088] In FIG. 9E, a gate electrode is formed on the gate insulating layer 402, the opening 421a, and the opening 421b. 4 shows a state in which a pole 411, a first wiring layer 414a and a second wiring layer 414b have been formed.

[0089] As described above, a transistor using an intrinsic or substantially intrinsic oxide semiconductor is manufactured. It is possible. [Explanation of symbols]

[0090] 1. Portable electronic devices 2. Portable electronic devices 10. Display 20 Power supply section 21 Antenna 22 Rectifier circuit 23 Rechargeable batteries 24 DC-DC converter 25 Solar Cells 30 Signal Processing Section 31 Antenna 32 Down Converter 33 Signal Processing Circuit 34 NVM 35 Display Controller 40 charger 41 Transmission coil 50 Wiring 51 Wiring 52 Wiring 53 Wiring 54 Wiring 60 Display Panel 61 Pixel section 62 Gate signal line 62D Gate signal line driver circuit 63 Data signal line 63D Data signal line driver circuit 64 pixels 65 Common electrode 66 Capacitive Line 67 Terminal section 71 Transistor 72 Transistor 73 Capacitive element 75 Transistors 76 Liquid crystal element 77 Holding capacity 400 Substrates 402 Gate insulating layer 407 Insulating Layer 410 Transistor 411 Gate electrode 412 Oxide semiconductor layer 414a wiring layer 414b wiring layer 415a electrode 415b electrode 421a aperture 421b Aperture

Claims

[Claim 1] A first antenna; A second antenna; and Memory, A battery capable of storing electricity; The first antenna has a function of receiving power in a wireless manner, The power received by the first antenna is charged into the storage battery; the second antenna has a function of receiving data; the memory includes a first transistor and a second transistor; a gate of the first transistor is electrically connected to a source or a drain of the second transistor; The first transistor has a channel formed in silicon; the second transistor has a channel formed in an oxide semiconductor.

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