Patent Publication Number: US-2023154931-A1

Title: Displays With Silicon and Semiconducting Oxide Thin-Film Transistors

Description:
This application is a continuation of patent application Ser. No. 17/502,909, filed Oct. 15, 2021, which is hereby incorporated by reference herein in its entirety and which is a continuation of patent application Ser. No. 17/224,305, filed Apr. 7, 2021, now U.S. Pat. No. 11,177,291, which is hereby incorporated by reference herein in its entirety and which is a continuation of patent application Ser. No. 16,917,601, filed Jun. 30, 2020, now U.S. Pat. No. 10,998,344, which is hereby incorporated by reference herein in its entirety and which is a continuation of patent application Ser. No. 16/678,599, filed Nov. 8, 2019, now U.S. Pat. No. 10,741,588, which is hereby incorporated by reference herein in its entirety and which is continuation of patent application Ser. No. 16/125,973, filed Sep. 10, 2018, now U.S. Pat. No. 10,707,237, which is hereby incorporated by reference herein in its entirety and which is continuation of patent application Ser. No. 15/727,475, filed Oct. 6, 2017, now U.S. Pat. No. 10,096,622, which is hereby incorporated by reference herein in its entirety and which is a continuation of patent application Ser. No. 14/249,716, filed Apr. 10, 2014, now U.S. Pat. No. 9,818,765, which is hereby incorporated by reference herein in its entirety and which is a continuation-in-part of patent application Ser. No. 14/228,070, filed Mar. 27, 2014, now U.S. Pat. No. 9,564,478, which is hereby incorporated by reference herein in its entirety and which claims the benefit of U.S. provisional patent application No. 61/869,937, filed Aug. 26, 2013, which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     This relates generally to electronic devices and, more particularly, to electronic devices with displays that have thin-film transistors. 
     Electronic devices often include displays. For example, cellular telephones and portable computers include displays for presenting information to users. 
     Displays such as liquid crystal displays are formed from multiple layers. A liquid crystal display may, for example, have upper and lower polarizer layers, a color filter layer that contains an array of color filter elements, a thin-film transistor layer that includes thin-film transistors and display pixel electrodes, and a layer of liquid crystal material interposed between the color filter layer and the thin-film transistor layer. Each display pixel typically includes a thin-film transistor for controlling application of a signal to display pixel electrode structures in the display pixel. 
     Displays such as organic light-emitting diode displays have an array of display pixels based on light-emitting diodes. In this type of display, each display pixel includes a light-emitting diode and thin-film transistors for controlling application of a signal to the light-emitting diode. 
     Thin-film display driver circuitry is often included in displays. For example, gate driver circuitry and demultiplexer circuitry on a display may be formed from thin-film transistors. 
     If care is not taken, thin-film transistor circuitry in the display pixels and display driver circuitry of a display may exhibit non-uniformity, excessive leakage currents, insufficient drive strengths, poor area efficiency, hysteresis, and other issues. It would therefore be desirable to be able to provide improved electronic device displays. 
     SUMMARY 
     An electronic device may be provided with a display. The display may have an array of display pixels on a substrate. The display pixels may be organic light-emitting diode display pixels or display pixels in a liquid crystal display. 
     In an organic light-emitting diode display, hybrid thin-film transistor structures may be formed that include semiconducting oxide thin-film transistors, silicon thin-film transistors, and capacitor structures. The capacitor structures may overlap the semiconducting oxide thin-film transistors. Capacitor structures may also be formed from multiple overlapping electrode layers formed from source-drain metal layers, a polysilicon layer, and a gate metal layer may be used. 
     Organic light-emitting diode display pixels may have combinations of oxide and silicon transistors. Transistors such as drive transistors that are coupled to light-emitting diodes may be formed from oxide transistor structures and switching transistors may be formed from silicon transistor structures. 
     In a liquid crystal display, display driver circuitry may include silicon thin-film transistor circuitry and display pixels may be based on oxide thin-film transistors. A single layer or two different layers of gate metal may be used in forming silicon transistor gates and oxide transistor gates. A silicon transistor may have a gate that overlaps a floating gate structure. Oxide transistors may be incorporated into display driver circuitry. 
     Display driver circuitry may be configured to expose silicon transistor circuitry to lower voltage swings than oxide transistor circuitry in an array of display pixels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram of an illustrative display such as an organic light-emitting diode display having an array of organic light-emitting diode display pixels or a liquid crystal display having an array of display pixels in accordance with an embodiment. 
         FIG.  2    is a diagram of an illustrative organic light-emitting diode display pixel of the type that may be used in an organic light-emitting diode with semiconducting oxide thin-film transistors and silicon thin-film transistors in accordance with an embodiment. 
         FIG.  3    is a cross-sectional side view of illustrative thin-film transistor structures in accordance with an embodiment. 
         FIG.  4    is a side view of additional illustrative thin-film transistor structures in accordance with an embodiment. 
         FIG.  5    is a diagram of an illustrative organic light-emitting diode display pixel of the type that may include oxide and silicon thin-film transistors in accordance with an embodiment. 
         FIGS.  6 ,  7 , and  8    are cross-sectional side views of illustrative thin-film transistor circuitry in a liquid crystal display in accordance with an embodiment. 
         FIG.  9    is a diagram of an illustrative complementary metal-oxide-semiconductor transistor inverter of the type that may be formed from a hybrid silicon-oxide transistor structure in accordance with an embodiment. 
         FIG.  10    is a cross-sectional side view of an illustrative thin-film transistor structure of the type that may be used to form a hybrid complementary metal-oxide-semiconductor transistor inverter in accordance with an embodiment. 
         FIG.  11    is a circuit diagram of gate driver circuitry in thin-film display driver circuitry in accordance with an embodiment. 
         FIG.  12    is a diagram of a level shifter of the type that may be used in the gate driver circuitry of  FIG.  11    within display driver circuitry on a display in accordance with an embodiment. 
         FIG.  13    is a circuit diagram of an illustrative circuit that may be used to prevent transistors within display driver circuitry on a display from experiencing excessive voltages in accordance with an embodiment. 
         FIG.  14    is a cross-sectional side view of illustrative thin-film transistor circuitry in a liquid crystal display in accordance with an embodiment. 
         FIG.  15    is a cross-sectional side view of illustrative thin-film transistor circuitry that includes a top gate semiconducting oxide transistor in a liquid crystal display in accordance with an embodiment. 
         FIG.  16    is a cross-sectional side view of illustrative thin-film transistor circuitry that includes a top gate semiconducting oxide transistor with a light shield in a liquid crystal display in accordance with an embodiment. 
         FIG.  17    is a cross-sectional side view of illustrative thin-film transistor circuitry that includes a top gate semiconducting oxide transistor in a liquid crystal display in accordance with an embodiment. 
         FIG.  18    is a cross-sectional side view of illustrative thin-film transistor circuitry that includes a top gate semiconducting oxide transistor in an organic light-emitting diode display in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     A display in an electronic device may be provided with driver circuitry for displaying images on an array of display pixels. An illustrative display is shown in  FIG.  1   . As shown in  FIG.  1   , display  14  may have one or more layers such as substrate  24 . Layers such as substrate  24  may be formed from planar rectangular layers of material such as planar glass layers. Display  14  may have an array of display pixels  22  for displaying images for a user. The array of display pixels  22  may be formed from rows and columns of display pixel structures on substrate  24 . There may be any suitable number of rows and columns in the array of display pixels  22  (e.g., ten or more, one hundred or more, or one thousand or more). 
     Display driver circuitry such as display driver integrated circuit  16  may be coupled to conductive paths such as metal traces on substrate  24  using solder or conductive adhesive. Display driver integrated circuit  16  (sometimes referred to as a timing controller chip) may contain communications circuitry for communicating with system control circuitry over path  25 . Path  25  may be formed from traces on a flexible printed circuit or other cable. The control circuitry may be located on a main logic board in an electronic device such as a cellular telephone, computer, set-top box, media player, portable electronic device, or other electronic equipment in which display  14  is being used. During operation, the control circuitry may supply display driver integrated circuit  16  with information on images to be displayed on display  14 . To display the images on display pixels  22 , display driver integrated circuit  16  may supply corresponding image data to data lines D while issuing clock signals and other control signals to supporting thin-film transistor display driver circuitry such as gate driver circuitry  18  and demultiplexing circuitry  20 . 
     Gate driver circuitry  18  may be formed on substrate  24  (e.g., on the left and right edges of display  14 , on only a single edge of display  14 , or elsewhere in display  14 ). Demultiplexer circuitry  20  may be used to demultiplex data signals from display driver integrated circuit  16  onto a plurality of corresponding data lines D. With this illustrative arrangement of  FIG.  1   , data lines D run vertically through display  14 . Each data line D is associated with a respective column of display pixels  22 . Gate lines G run horizontally through display  14 . Each gate line G is associated with a respective row of display pixels  22 . Gate driver circuitry  18  may be located on the left side of display  14 , on the right side of display  14 , or on both the right and left sides of display  14 , as shown in  FIG.  1   . 
     Gate driver circuitry  18  may assert gate signals (sometimes referred to as scan signals) on the gate lines Gin display  14 . For example, gate driver circuitry  18  may receive clock signals and other control signals from display driver integrated circuit  16  and may, in response to the received signals, assert a gate signal on gate lines G in sequence, starting with the gate line signal G in the first row of display pixels  22 . As each gate line is asserted, the corresponding display pixels in the row in which the gate line is asserted will display the display data appearing on the data lines D. 
     Display driver circuitry such as demultiplexer circuitry  20  and gate line driver circuitry  18  may be formed from thin-film transistors on substrate  24 . Thin-film transistors may also be used in forming circuitry in display pixels  22 . To enhance display performance, thin-film transistor structures in display  14  may be used that satisfy desired criteria such as leakage current, switching speed, drive strength, uniformity, etc. The thin-film transistors in display  14  may, in general, be formed using any suitable type of thin-film transistor technology (e.g., silicon-based, semiconducting-oxide-based, etc.). 
     With one suitable arrangement, which is sometimes described herein as an example, the channel region (active region) in some thin-film transistors on display  14  is formed from silicon (e.g., silicon such as polysilicon deposited using a low temperature process, sometimes referred to as LTPS or low-temperature polysilicon) and the channel region in other thin-film transistors on display  14  is formed from a semiconducting oxide material (e.g., amorphous indium gallium zinc oxide, sometimes referred to as IGZO). If desired, other types of semiconductors may be used in forming the thin-film transistors such as amorphous silicon, semiconducting oxides other than IGZO, etc. In a hybrid display configuration of this type, silicon transistors (e.g., LTPS transistors) may be used where attributes such as switching speed and good drive current are desired (e.g., for gate drivers in liquid crystal diode displays or in portions of an organic light-emitting diode display pixel where switching speed is a consideration), whereas oxide transistors (e.g., IGZO transistors) may be used where low leakage current is desired (e.g., in liquid crystal diode display pixels and display driver circuitry) or where high pixel-to-pixel uniformity is desired (e.g., in an array of organic light-emitting diode display pixels). Other considerations may also be taken into account (e.g., considerations related to power consumption, real estate consumption, hysteresis, etc.). 
     Oxide transistors such as IGZO thin-film transistors are generally n-channel devices (i.e., NMOS transistors). Silicon transistors can be fabricated using p-channel or n-channel designs (i.e., LTPS devices may be either PMOS or NMOS). Combinations of these thin-film transistor structures can provide optimum performance. 
     In an organic light-emitting diode display, each display pixel contains a respective organic light-emitting diode. A schematic diagram of an illustrative organic light-emitting diode display pixel  22 - 1  is shown in  FIG.  2   . As shown in  FIG.  2   , display pixel  22 - 1  may include light-emitting diode  26 . A positive power supply voltage ELVDD may be supplied to positive power supply terminal  34  and a ground power supply voltage ELVSS may be supplied to ground power supply terminal  36 . The state of drive transistor  28  controls the amount of current flowing through diode  26  and therefore the amount of emitted light  40  from display pixel  22 - 1 . 
     To ensure that transistor  28  is held in a desired state between successive frames of data, display pixel  22 - 1  may include a storage capacitor such as storage capacitor Cst. The voltage on storage capacitor Cst is applied to the gate of transistor  28  at node A to control transistor  28 . Data can be loaded into storage capacitor Cst using one or more switching transistors such as switching transistor  30 . When switching transistor  30  is off, data line D is isolated from storage capacitor Cst and the gate voltage on terminal A is equal to the data value stored in storage capacitor Cst (i.e., the data value from the previous frame of display data being displayed on display  14 ). When gate line G (sometimes referred to as a scan line) in the row associated with display pixel  22 - 1  is asserted, switching transistor  30  will be turned on and a new data signal on data line D will be loaded into storage capacitor Cst. The new signal on capacitor Cst is applied to the gate of transistor  28  at node A, thereby adjusting the state of transistor  28  and adjusting the corresponding amount of light  40  that is emitted by light-emitting diode  26 . 
     Organic light-emitting diode display pixels such as pixel  22 - 1  of  FIG.  2    may use thin-film transistor structures of the type shown in  FIG.  3   . In this type of structure, two different types of semiconductor are used. As shown in  FIG.  3   , circuitry  72  may include display pixel structures such as light-emitting diode cathode terminal  42  and light-emitting diode anode terminal  44 . Organic light-emitting diode emissive material  47  may be interposed between cathode  42  and anode  44 . Dielectric layer  46  may serve to define the layout of the display pixel and may sometimes be referred to as a pixel definition layer. Planarization layer  50  may be formed on top of thin-film transistor structures  52 . Thin-film transistor structures  52  may be formed on buffer layer  54  on substrate  24 . 
     Thin-film transistor structures  52  may include silicon transistor  58 . Transistor  58  may be an LTPS transistor formed using a “top gate” design and may serve as a switching transistor in an organic light-emitting diode display pixel (see, e.g., transistor  30  in pixel  22 - 1  of  FIG.  2   ). Transistor  58  may have a polysilicon channel  62  that is covered by gate insulator layer  64  (e.g., a layer of silicon oxide). Gate  66  may be formed from patterned metal (e.g., molybdenum, as an example). Gate  66  may be covered by a layer of interlayer dielectric (e.g., silicon nitride layer  68  and silicon oxide layer  70 ). Source-drain contacts  74  and  76  may contact opposing sides of polysilicon layer  62  to form the silicon thin-film transistor  58 . 
     Thin-film transistor structures  52  may also include thin-film transistor and capacitor structures  60 . Structures  60  may include a storage capacitor (i.e., storage capacitor Cst of  FIG.  2   ) and an oxide thin-film transistor structure. The storage capacitor may have a first terminal (sometimes referred to as a plate, electrode, or electrode layer) that is formed from polysilicon layer  62 ′ (patterned as part of the same layer as layer  62 ). Gate insulator layer  64 ′, which may be an extended portion of gate insulator layer  64 , may cover terminal  62 ′. The capacitor may have a second terminal formed from metal layer  66 ′. Metal layer  66 ′ may be patterned from the same metal layer that is used in forming gate  66  of transistor  58 . Dielectric layers  68  and  70  may cover metal layer  66 ′. The thin-film transistor in structures  60  may be a “bottom gate” oxide transistor. Layer  66 ′, which serves as the second terminal of capacitor Cst (i.e., node A of  FIG.  2   ) may also serve as the gate of the oxide transistor. The oxide transistor may serve as drive transistor  28  of  FIG.  2   . The “gate insulator” of the oxide transistor may be formed from the layer of interlayer dielectric (i.e., layers  68  and  70 ). The channel semiconductor of the oxide transistor may be formed from oxide layer  80  (e.g., IGZO). Oxide layer  80  may overlap polysilicon capacitor electrode layer  62 ′ (i.e., the oxide transistor may overlap the capacitor), thereby saving space. Source-drain terminals  82  and  84  may be formed from metal contacting opposing ends of semiconducting oxide layer  80 . 
     Transistors such as LTPS transistors and oxide transistors may be formed with different layouts. For example, LTPS transistors tend to have high carrier mobilities. As a result, LTPS transistors may have relatively long gate lengths L and relatively short gate widths to ensure appropriately low ratios of W/L to compensate for the relatively high mobility of these transistors. This may cause LTPS transistors to be relatively inefficient for pixel layout. Oxide transistors may be constructed with W/L ratios with smaller aspect ratios (e.g., 4/4 for oxide relative to 3/30 for LTPS). Due to these layout efficiency considerations, it may be preferred to use oxide transistors as the drive transistors in display pixels  22 - 1 . The relatively fast switching speed provided by LTPS transistor may make it preferable to use LTPS transistors for switching transistors such as transistor  30  of  FIG.  2   . 
     In display pixels with more transistors (e.g., three or more, four or more, five or more, six or more, seven or more, or eight or more), the selection of which transistors are implemented using LTPS technology and which transistors are implemented using oxide technology may be made so as to balance transistor performance considerations between the two types of transistors. 
     When implementing driving transistors, LTPS transistors tend to exhibit larger size (longer channel length) than oxide transistors, tend to exhibit larger dark currents than oxide transistors, and may exhibit poorer uniformity than oxide transistors. LTPS driving transistors may also exhibit more hysteresis than oxide driving transistors. As a result of these factors, it may often be advantageous to form driving transistors in an organic light-emitting diode display pixel from oxide transistors. The oxide driving transistors may exhibit low leakage current and minimal hysteresis. 
     When implementing switching transistors, LTPS transistors may be smaller than oxide transistors, may exhibit smaller amounts of parasitic capacitance than oxide transistors, and may exhibit lower power consumption than oxide transistors. As a result of factors such as these, it may often be advantageous to form switching transistors in an organic light-emitting diode display pixel from LTPS transistors. The LTPS switching transistors may exhibit high switching speed and low parasitic capacitance. 
     An illustrative hybrid thin-film transistor structure that may be used in implementing both LTPS and oxide transistors in a single organic light-emitting diode display pixel (e.g., to implement a circuit such as display pixel circuit  22 - 1  of  FIG.  2   ) is shown in  FIG.  4   . Hybrid thin-film transistor structures  114  of  FIG.  4    include silicon thin-film transistor  108 , capacitor (Cst)  110 , and oxide transistor  112 . Silicon transistor  108  is formed from polysilicon layer  90 . Gate insulator layer  92  covers polysilicon layer  90 . A layer of gate metal is patterned on top of gate insulator layer  92  to form gate  94 , capacitor electrode  96 , and gate electrode  98 . A layer of interlayer dielectric material such as silicon nitride layer  116  and silicon oxide layer  118  may cover the patterned gate metal structures. Source-drain contacts  100  and  94  for silicon transistor  108  may contact (i.e., may be shorted to) polysilicon layer  90  in the vicinity of channel region  106 . Gate  94  of transistor  108  may serve as an implant mask to allow low-density drain implants to be formed in polysilicon layer  90  in regions  104  adjacent to polysilicon channel region  106  of transistor  108 . 
     Source-drains  100  and  102  of silicon transistor  108 , capacitor electrode  120 , and source-drains  122  and  124  of oxide transistor  112  may be formed from patterned portions of a common metal layer on interlayer dielectric  116  and  118 . 
     Capacitor  110  may have a first terminal formed from metal electrode  120  and from portion  126  of polysilicon layer  90 . Capacitor  110  may have a second terminal formed from metal electrode  96 . 
     Oxide transistor  112  may have a semiconductor oxide layer such as an IGZO layer  128 , source-drain contacts  122  and  124 , and gate  98 . Gate  98  is separated from semiconductor oxide  128 , which serves as the channel region for transistor  112  by dielectric  116  and  118 . Dielectric  116  and  118  therefore serves as the gate insulator for oxide transistor  112 . 
       FIG.  5    is a circuit diagram of another illustrative organic light-emitting diode pixel circuit that may be used in display  14 . Pixel  22 - 2  includes driver transistor  28  for driving current into light-emitting diode  26 . Storage capacitor Cst is used to store signals on the gate of transistor  28  between frames. Sensing line SENSING used to implement a compensation scheme to adjust for pixel-to-pixel variations in transistor performance. Gate lines SCAN and SCAN 2  are used in applying control signals to switching transistors  30 - 1  and  30 - 2 . 
     To optimize performance in display pixel  22 - 2 , it may be desirable to use hybrid structures of the type shown in  FIGS.  3  and  4    or other configurations for forming silicon and/or oxide thin-film transistors and capacitors. For example, it may be desirable to form drive transistor  28  from an oxide transistor (e.g., an NMOS oxide transistor), while forming switching transistors such as transistors  30 - 1  and  30 - 2  from silicon transistors or from a mixture of silicon (NMOS and/or PMOS) and oxide (NMOS) transistors. 
     With a first illustrative configuration, transistor  30 - 1  is an oxide transistor, transistor  30 - 2  is an oxide transistor, and transistor  28  is an oxide transistor. With a second illustrative configuration, transistor  30 - 1  is a silicon transistor, transistor  30 - 2  is a silicon transistor, and transistor  28  is an oxide transistor. A hybrid transistor structure such as the structure of  FIG.  3    or the structure of  FIG.  4    may be used in this scenario (e.g., to implement transistors  30 - 1  and  28  and capacitor Cst). With an illustrative third configuration, transistor  30 - 1  is a silicon transistor, transistor  30 - 2  is an oxide transistor, and transistor  28  is an oxide transistor. As with the second illustrative configuration, a hybrid transistor structure such as the structure of  FIG.  3    or the structure of  FIG.  4    may be used to implement transistors  30 - 1  and  28  and capacitor Cst. 
     If desired, display  14  may be a liquid crystal display. In this type of scenario, each pixel of display  14  may contain an electrode structure for applying an electric field to an associated portion of a liquid crystal layer in the display, a capacitor for storing charge on the electrode between frames of image data, and a thin-film transistor for controlling the application of the electric field to the electrodes. With one suitable arrangement, gate driver circuitry  18  and demultiplexer circuitry  20  ( FIG.  1   ) in the liquid crystal display may be formed from silicon transistors and the thin-film transistors in display pixels  22  may be formed from oxide transistors. The silicon transistors have high mobility channel regions and are well suited for fast switching speeds and high drive currents while operating at low voltages and low power. The oxide thin-film transistors in display pixels  22  exhibit low leakage currents. 
     Thin-film transistor structures of the type that may be used in forming a liquid crystal display with both silicon and oxide transistors are shown in  FIG.  6   . As shown in  FIG.  6   , thin-film transistor structures  242  may include silicon thin-film transistor structures  216  (e.g., for forming parts of peripheral circuits such as display driver circuitry  18  and demultiplexer circuitry  20 ) and oxide thin-film transistor structures  240  (e.g., for forming display pixels  22  in a liquid crystal display having a layout of the type shown by display  14  of  FIG.  1   ). 
     Structures  216  and  240  may be formed on buffer layer  202  on substrate  24 . Polysilicon layer  204  may be deposited on buffer  202 . Gate insulator layer  206  may be formed on polysilicon layer  204 . A common layer of metal may be patterned to form metal structures  218 ,  220 , and  228 . Structure  218  may serve as the gate for a silicon transistor that includes source-drain contacts  212  and  214  and a channel formed from polysilicon  204 . Metal structure  228  may serve as a gate for an oxide transistor formed from semiconducting oxide layer  224  (e.g., IGZO) and source-drain terminals  222  and  226 . Metal structure  228  may also serve as a light shield that helps block backlight in display  14  from reaching oxide layer  224 , so no separate light shielding structures need be incorporated in structures  240 . Interlayer dielectric such as silicon nitride layer  208  and  210  may cover gate  218  in structure  216  and may serve as a gate insulator for gate  228  in the oxide transistor of structures  240 . 
     Metal  230  contacts source-drain  226  of the display pixel thin-film oxide transistor that is formed from oxide layer  224 . Metal  230  may be supported by organic layer  232 . On the surface of organic layer  232 , metal  230  may form an electrode with multiple fingers. Dielectric layer  236  may isolate electrode  230  from common electrode (Vcom)  234 . During operation, electric fields are produced between electrode  230  and electrode  234 . These fields pass through the liquid crystal material in the display. If desired, display  14  may incorporate capacitive touch sensors that are formed from portions of Vcom electrode  234 . In this type of configuration, optional metal lines such as line  238  may be used to help reduce the resistance of the material used in forming electrode  234  (which may be, for example, a somewhat resistive conducting material such as indium tin oxide). 
     The thickness of layers  208  and  210  may be about 6000 angstroms. This relatively large thickness may help minimize capacitance between gate  218  and nearby metal structures such as source-drain  214 , but may limit switching speeds in the oxide transistor. To address this concern, a design of the type used by structures  242 ′ in  FIG.  7    may be used. With the  FIG.  7    arrangement, an additional semiconductor fabrication mask may be used to create a gate for the oxide transistor that is formed from a separate layer of metal from the metal layer used in forming gate  218 . With this approach, only a single 3000 angstrom dielectric layer  210 ′ (formed, e.g., from sublayers of silicon nitride and silicon oxide) is used to separate oxide transistor gate  228 ′ from oxide layer  224 , so oxide transistor switching speed may be enhanced. The arrangement of structures  242 ′ of  FIG.  7    allows gate  218  and gate  228 ′ to be formed from different metals. For example, gate  218  may be formed from a refractory metal such as Mo to accommodate the elevated temperatures associated with activating the silicon transistor, whereas gate  228 ′ may be formed from a lower resistance metal such as copper. 
     In some applications, the handling of high drive voltages (gate-to-source and drain) may need to be considered. Transistor structures  242 ″ of  FIG.  8    may be used in scenarios in which it is desired to handle relatively large (e.g., 20 volt) swings on the silicon transistor gate. In this situation, gate insulator layer  206  may be insufficiently thin to withstand damage from a 20 volt signal. For example, gate insulator  206  may be about 800 angstroms thick, which may not be sufficiently thick to reliably handle 20 volt drive voltages. To ensure that gate insulator layer  206  is not overly stressed, gate structure  218  may be converted into a floating (electrically isolated) metal structure and an additional metal layer (i.e., part of the same metal layer that is patterned to form gate  228 ′ of oxide transistor  240 ) may be used in forming silicon transistor gate  218 ′. Floating gate  218  may be retained to serve as a mask for low density drain (LDD) implants made into the source and drain contact portions of polysilicon layer  204 , even though floating gate  218  is not driven with control signals during operation of silicon transistor  216 . 
     In a hybrid silicon/oxide liquid crystal display, it is not necessary to form display driver circuitry such as gate driver circuitry  18  and demultiplexer circuitry  20  from silicon transistors. If desired, some of this display driver circuitry may be formed from oxide transistors. For example, low drive current CMOS-type circuits in the peripheral circuitry of display  14  such as illustrative CMOS inverter  300  of  FIG.  9    may include oxide transistors. It may be challenging to form PMOS oxide transistors, so circuits such as inverter  300  may, if desired, be formed using an NMOS oxide transistor and a PMOS silicon transistor (as an example). 
     Hybrid oxide-silicon thin-film transistor structures such as illustrative thin-film transistor structures  302  of  FIG.  10    may be used in forming CMOS-type circuitry in display driver circuitry such as gate driver circuitry  18  and demultiplexer circuitry  20 . As shown in  FIG.  10   , structures  302  may have a polysilicon layer  308  that is formed on substrate  24 . P-channel active area  310  may be formed under gate  312 . Gate insulator layer  306  (e.g., silicon oxide) may separate gate  312  from silicon channel region  310  in silicon layer  308 . Dielectric layer  302  (e.g., sublayers of silicon oxide and silicon nitride) may cover gate  312 . Dielectric layer  306  may separate gate  312  from overlapping oxide layer  312 . Oxide layer  312  may be a semiconducting oxide such as IGZO material. Gate  312  may be formed from a first patterned metal layer. A second patterned metal layer may be used in forming output terminal  322 , source terminal  316 , and drain terminal  318 . Passivation layer  320  may cover terminals  316  and  312 . Gate  312  may be formed from materials such as molybdenum, molybdenum tungsten, tungsten, or other metals. Metal for forming structures such as metal structures  322 ,  316 , and  318  may be formed from metal such as aluminum, molybdenum, etc. 
     With the arrangement of  FIG.  10   , gate  314  serves as a common (shared) gate for two transistors. In particular, gate  314  (see, e.g., terminal Vin of  FIG.  9   ) serves as both a gate for a PMOS silicon transistor (transistor TP of  FIG.  9   ) that is formed from silicon layer  308  and as a gate for an NMOS oxide transistor (transistor TN of  FIG.  9   ) that is formed from oxide layer  312 . Oxide layer  312  is located above gate  314  and silicon layer  310  is located below gate  314 . The shared gate arrangement of  FIG.  10    allows a CMOS inverter of the type shown in  FIG.  9    to be implemented compactly. 
       FIG.  11    shows illustrative gate driver circuitry  18  that may be used on a liquid crystal display. Circuitry  18  may use signals with a relatively small voltage swing (e.g., a 15 volt or 16 volt swing) for silicon transistors while producing gate signals G with a larger voltage swing (e.g., a 20 volt swing or more) to ensure satisfactory operation of oxide thin-film transistors in display pixels  22  that are being driven by the gate signals. 
     As shown in  FIG.  11   , circuitry  18  may have a shift register formed from a series of linked SR latches  400  or other register circuits. Each row of circuitry in  FIG.  11    is associated with a separate row of display pixels  22  in a liquid crystal display and provides a respective gate signal G for that row of display pixels. During operation, trigger signal TRIGGER may be applied to the latch in the first row of the shift register in circuitry  18  while a clock signal LOAD CLOCK is being applied to the shift register. The trigger signal causes a cascading signal to ripple down through the shift register. In response, each latch  400  asserts its output OUT in sequence. Each row of gate driver circuitry  18  has a respective level shifter  404  and buffer  404  that receive output signal OUT. 
     Output signal OUT ranges from a high voltage of 15 V (or other suitable voltage) to 0 volts (or other suitable voltage). The 15 volt swing that is associated with this type of configuration can be tolerated by silicon thin-film transistors in latches  400 , whereas larger voltage swings such as 20 volt swings might overly stress the silicon thin-film transistors. Level shifter  402  shifts the 15 volt to 0 volt signal OUT from latch  400  so that the output on path  406  from level shifter  402  ranges from 5 volts to −11 volts (i.e., a swing of 16 volts that can be tolerated by the silicon transistors in level shifter  402 ). Buffer  404  receives the 15 volt to 0 volt signal OUT from latch  400  as input signal IN_H and receives the 5 volt to −11 volt signal as input signal IN_L. Buffer  404  preferably contains silicon thin-film transistors. The design of buffer  404  allows buffer  404  to produce an output signal (gate line signal G) with a large voltage swing (e.g., 15 volts to −11 volts) of the type that is appropriate for controlling oxide transistors in the array of display pixels  22  on the liquid crystal display. 
       FIG.  12    is a circuit diagram of an illustrative circuit of the type that may be used to implement level shifter  402 . Signals from output OUT of latch  400  may be received at input  410  of level shifter  402  and corresponding level-shifted output signals (signals IN_L) for buffer  404  may be provided at output  412  of level shifter  402 . Other level shifter designs may be used for level shifter  402  if desired. The configuration of  FIG.  12    is merely illustrative. Silicon thin-film transistors may be used in forming level shifter  402 . 
     Circuitry  404  of  FIG.  13    is an example of a design that may be used in implementing buffer  404  of  FIG.  11   . With this design, signals IN_H and IN_L are identical square wave pulses with different respective voltage swings. Signal IN_H ranges from 15 to 0 volts. Signal IN_L ranges from 5 to −11 volts. Corresponding output signal (gate line signal) G in this example is a square wave pulse that ranges from 15 volts to −11 volts and therefore has a swing of more than 20 volts. 
     Ground voltage GND is applied to the gates of transistors T 2  and T 3 . This limits that maximum voltage experienced by the transistors of circuit  414  to less than about 16 volts, even though the output swing of circuit  414  is more than 20 volts. The ground voltage GND on the gates of transistors T 2  and T 3  causes these transistors to turn off to protect transistors T 1  and T 4  whenever excessive source terminal voltage swing is detected. Consider, as an example, transistors T 1  and T 2 . Transistor T 2  may be characterized by a threshold voltage Vth. If the source S of transistor T 1  starts to fall below voltage GND-Vth, transistor T 2  will turn off and isolate transistor T 1 . Transistors T 3  and T 4  operate in the same way. Using this arrangement, none of the transistors in buffer  414  is exposed to excessive voltage swings, allowing transistors T 1 , T 2 , T 3 , and T 4  to be formed from silicon thin-film transistors. 
     If desired, other circuit configurations may be used to allow gate driver circuitry  18  to operate in an environment in which gate line signal G has a large voltage swing to accommodate oxide transistors in display pixels  22 . As an example, a subset of the level shifter transistors and a subset of the output buffer transistors may be implemented using oxide thin-film transistor structures in addition to using silicon thin-film transistor structures. 
       FIG.  14    is a cross-sectional side view of additional thin-film transistor circuitry of the type that may be used in a liquid crystal display. As shown in  FIG.  14   , thin-film transistor structures  242  may include silicon thin-film transistor structures  216  (e.g., for forming parts of peripheral circuits such as display driver circuitry  18  and demultiplexer circuitry  20 ) and oxide thin-film transistor structures  240  (e.g., for forming display pixels  22  in a liquid crystal display having a layout of the type shown by display  14  of  FIG.  1   ). 
     Structures  216  and  240  may be formed on buffer layer  202  on substrate  24 . Polysilicon layer  204  may be deposited on buffer  202 . Gate insulator layer  206  may be formed on polysilicon layer  204 . A common layer of metal may be patterned to form metal structures  218 ,  220 , and  228 . Structure  218  may serve as the gate for a silicon transistor that includes source-drain contacts  212  and  214  and a channel formed from polysilicon  204 . Metal structure  228  may serve as a gate for an oxide transistor formed from semiconducting oxide layer  224  (e.g., IGZO) and source-drain terminals  222  and  226 . Metal structure  228  may also serve as a light shield that helps block backlight in display  14  from reaching oxide layer  224 , so no separate light shielding structures need be incorporated in structures  240 . Interlayer dielectric such as silicon nitride layer  208  and  210  may cover gate  218  in structure  216  and may serve as a gate insulator for gate  228  in the oxide transistor of structures  240 . 
     Metal structures  218 ,  220 , and  228  and routing lines such as interconnect line  502  may be formed from a first metal layer (sometimes referred to as an M1 layer). Metal  222  and  226 , which form source-drain contacts for the oxide transistor of structures  240 , and routing lines such as interconnect line  500  may be formed from a second metal layer (sometimes referred to as an SD 1  layer). Metal structures  212 ,  214 , and routing lines such as interconnect line  506  may be formed from a third metal layer (sometimes referred to as an SD 2  layer). Dielectric layers  232 B may separate the second metal layer from the third metal layer. Dielectric layer  232 A may separate the third metal layer from metal structures such as metal layer  234 . 
     Metal  230  contacts metal layer  504  and is thereby coupled to source-drain  226  of the display pixel thin-film oxide transistor that is formed from oxide layer  224 . Metal  230  may be supported by organic layer  232 B. On the surface of organic layer  232 B, metal  230  may form an electrode with multiple fingers. Dielectric layer  236  may isolate electrode  230  from common electrode (Vcom)  234 . During operation, electric fields are produced between electrode  230  and electrode  234 . These fields pass through the liquid crystal material in the display. If desired, display  14  may incorporate capacitive touch sensors that are formed from portions of Vcom electrode  234 . In this type of configuration, optional metal lines such as line  238  may be used to help reduce the resistance of the material used in forming electrode  234  (which may be, for example, a somewhat resistive conducting material such as indium tin oxide). 
     Capacitive coupling between the routing lines in display  14  can lead to switching losses. As an example, source-drain structure  222  may be coupled to the data line in display  14 . The voltage on this line switches relative to Vcom (electrode  234 ) and can lead to power losses. The presence of dielectric layers  232 A and  232 B can help reduce capacitive coupling between the data line and Vcom electrode and thereby reduce power losses. The presence of these dielectric layers can also reduce capacitive coupling between routing lines in display  14  (e.g., capacitive coupling between routing lines and other structures of the first and second metal layers, the first and third metal layers, etc.). Layers  232 A and  232 B may be formed from low-dielectric-constant organic dielectric or other dielectric material. As an example, layers  232 A and  232 B may be acrylic polymers, other polymers, dielectrics of the type sometimes referred to as spin-on-glass, (e.g., spin-on-glass polymers deposited via slit coating tools, etc.), siloxane-based materials, etc. 
       FIG.  15    is a cross-sectional side view of illustrative thin-film transistor circuitry for a liquid crystal display that includes a top gate semiconducting oxide transistor. As shown in  FIG.  15   , thin-film transistor structures  242  may include silicon thin-film transistor structures  216  and semiconducting oxide thin-film transistor structures  240 . Silicon thin-film transistor structures  216  may be used in peripheral circuits such as display driver circuitry  18  and demultiplexer circuitry  20  and/or may be used in forming circuits for display pixels  22  in a liquid crystal display. Semiconducting oxide thin-film transistor structures  240  may be used in peripheral circuits such as display driver circuitry  18  and demultiplexer circuitry  20  and/or may be used in forming circuits for display pixels  22  in a liquid crystal display. Transistors such as silicon (polysilicon) transistor  216  may be n-channel or p-channel devices. Transistors such as semiconducting oxide transistor  240  may be n-channel or p-channel devices. 
     Structures  216  and  240  may be formed on buffer layer  202  on substrate  24 . Buffer layer  202  may be formed from a dielectric such as an inorganic dielectric. Buffer layer  202  may help prevent ions in substrate  24  from migrating into structures  216  and  240 . 
     Polysilicon layer  204  may be deposited on buffer  202 . Gate insulator layer  206  may be formed on polysilicon layer  204 . Gate insulator layer  206  may be formed from a dielectric such as silicon oxide (e.g., a 100 nm silicon oxide layer). A common layer of metal may be patterned to form metal structures  218 ,  220 , and  228 . Structure  218  may serve as the gate for a silicon transistor that includes source-drain contacts  212  and  214  and a channel formed from polysilicon  204 . Metal structure  228  may serve as a gate for a top gate oxide transistor (i.e., a semiconducting oxide transistor) formed from semiconducting oxide layer  224  (e.g., IGZO) and source-drain terminals  222  and  226 . One or more layers of interlayer dielectric (ILD) may cover metal structures  218 ,  220 , and  228 . For example, a first dielectric layer such as layer  208  and a second dielectric layer such as layer  210  may cover metal structures  218 ,  220 , and  228 . Layer  208  may be a silicon nitride layer and layer  210  may be a silicon oxide layer (as examples). Because there is no lateral overlap between gate  228  and source-drain electrodes  222  and  226 , parasitic capacitance between gate  228  and source-drain structures  222  and  226  may be minimized. Moreover, layers  208  and  210  of the oxide transistor of  FIG.  15    may be thicker than layers  208  and  210  in bottom-gate oxide transistor of  FIG.  14   , thereby further reducing parasitic capacitances. 
     Metal structures  218 ,  220 , and  228  may be formed from a first metal layer (sometimes referred to as an M1 layer). Metal  222  and  226 , which form source-drain contacts for the oxide transistor of structures  240  and metal  212  and  214 , which form source-drain contacts for the silicon transistor of structures  216  may be formed from a second metal layer (sometimes referred to as an SD 1  layer or M2 layer). Metal structures such as metal line  238  may be formed from a third metal layer (sometimes referred to as an M3 layer). Dielectric  232  (e.g., an organic dielectric layer such as a polymer layer) may separate the second metal layer from the third metal layer. 
     Metal  230  contacts source-drain  226  of the display pixel thin-film oxide transistor that is formed from oxide layer  224 . Metal  230  may be supported by organic layer  232 . On the surface of organic layer  232 , metal  230  may form an electrode with multiple fingers (e.g., a pixel electrode for a display pixel in the display). Dielectric layer  236  may isolate electrode  230  from common electrode (Vcom)  234 . During operation, electric fields are produced between electrode  230  and electrode  234 . These fields pass through the liquid crystal material in the display that is formed on top of the structures of  FIG.  15   . If desired, display  14  may incorporate capacitive touch sensors that are formed from portions of Vcom electrode  234 . In this type of configuration, optional metal lines such as line  238  may be used to help reduce the resistance of the material used in forming electrode  234  (which may be, for example, a somewhat resistive conducting material such as indium tin oxide). 
     As shown in  FIG.  16   , an optional light shielding structure such as light shield  520  can be formed under semiconducting-oxide transistor  240  or elsewhere in the display. Light shield  520  may be formed from an opaque material such as a metal, an oxidized metal, a dark polymer, or other light blocking materials. The presence of light shield  520  can help prevent stray light from disrupting the operation of semiconducting oxide transistor structures  240  or other overlapping structures. 
     In the example of  FIG.  17   , dielectric layer  232  of  FIG.  15    has been divided into two dielectric layers  232 A and  232 B. Layer  232 A may overlap the source-drain electrodes of transistors  216  and  240 . Layer  232 B may be interposed between the source-drain electrodes and other metal structures formed form the source-drain metal layer and layers  208  and  210 . As described in connection with  FIG.  14   , this type of two-layer approach can reduce capacitive coupling between the metal structures of devices  216  and  240 . A cross-sectional side view of illustrative thin-film transistor circuitry that includes a top gate semiconducting oxide transistor in an organic light-emitting diode display is shown in  FIG.  18   . As shown in  FIG.  18   , circuitry  72  may include display pixel structures such as light-emitting diode cathode terminal  42  and light-emitting diode anode terminal  44 . Organic light-emitting diode emissive material  47  may be interposed between cathode  42  and anode  44 . Pixel definition layer  46  may be a dielectric layer  46  that serves to define the layout of the display pixel. Layer  46  may be formed from a polymer such as a black polymer to help block stray light. 
     Planarization layer  50  may be formed on top of thin-film transistor structures  52 . Thin-film transistor structures  52  may be formed on buffer layer  54  on substrate  24 . Substrate  24  may be formed from metal, glass, polymer, other materials, or combinations of these materials. Buffer layer  54  may be formed from an inorganic dielectric layer that helps prevent ions in substrate  24  from disrupting the operation of structures  52 . Optional functional layer  522  may be interposed between buffer layer  54  and substrate  24 . Functional layer  522  may be a stress relief layer, a light-blocking layer, a layer used in forming components such as capacitors (e.g., capacitor electrodes for pixel circuits and/or peripheral circuits), etc. 
     Thin-film transistor structures  52  may include silicon transistor  58 . Transistor  58  may be an LTPS transistor formed using a top gate design and may serve as a switching transistor in an organic light-emitting diode display pixel (see, e.g., transistor  30  in pixel  22 - 1  of  FIG.  2   ). Transistor  58  may also be used in peripheral circuits (e.g., driver circuitry  18  and demultiplexer circuitry  20 ). 
     Transistor  58  may have a polysilicon channel  62  that is covered by gate insulator layer  64  (e.g., a layer of silicon oxide having a thickness of 100 nm or other suitable thickness). Gate  66  may be formed from patterned metal (e.g., molybdenum, as an example). Gate  66  may be covered by a layer of interlayer dielectric (e.g., silicon nitride layer  68  and silicon oxide layer  70 ). Source-drain contacts  74  and  76  may contact opposing sides of polysilicon layer  62  to form silicon thin-film transistor  58 . 
     Dielectric layer  526  may cover source-drain structures  74  and  76 . Optional metal layer  524  may be formed on layer  526  and may, if desired, contact underlying metal structures though vias (see, e.g., vias  528 ). Structure  66  may be formed in a first (“M1”) metal layer. Source-drain electrodes  74  and  76  may be formed in a second metal layer. Metal layer  524  may be formed as part of a third (“M3”) metal layer. Layer  524  may overlap portions of transistor  58  and/or transistor  60  and may be used for forming capacitors or signal interconnect lines (i.e., routing). Layer  524  may be overlapped by emissive material layer  47  and may form a light-blocking structures that prevent stray light from emissive material  47  from reaching underlying transistor structures, etc. 
     Thin-film transistor structures such as semiconducting-oxide thin-film transistor structures  60  and silicon thin-film transistor structures  58  may be used in forming part of a pixel circuit in an organic light-emitting diode display and/or may be used in forming part of peripheral circuitry  18  and  20 . Thin-film transistor  60  of  FIG.  18    may be a top gate semiconducting-oxide transistor. The gate insulator layer  64 , which serves as the gate insulator for silicon transistor  58 , also serves as the gate insulator for oxide transistor  60 . 
     Metal gate  532  forms the gate of oxide transistor  60 . The channel semiconductor of the oxide transistor may be formed from semiconducting oxide layer  128  (e.g., IGZO). Source-drain terminals  534  and  536  may be formed from metal contacting opposing ends of semiconducting oxide layer  128 . Metal structures  530  and  538  may be used for routing and may be formed from the same layer of metal that is pattered to form gates  66  and  532 . Structures such as source-drain structures  534  and  536  may be formed from the same layer of metal that is used in forming source-drain structures  74  and  76 . 
     The foregoing is merely illustrative and various modifications can be made by those skilled in the art without departing from the scope and spirit of the described embodiments. The foregoing embodiments may be implemented individually or in any combination.