US20040092109A1 - Semiconductor device and method for making the device having an electrically modulated conduction channel - Google Patents
Semiconductor device and method for making the device having an electrically modulated conduction channel Download PDFInfo
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- US20040092109A1 US20040092109A1 US10/697,012 US69701203A US2004092109A1 US 20040092109 A1 US20040092109 A1 US 20040092109A1 US 69701203 A US69701203 A US 69701203A US 2004092109 A1 US2004092109 A1 US 2004092109A1
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
- H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
- H01L29/86—Types of semiconductor device ; Multistep manufacturing processes therefor controllable only by variation of the electric current supplied, or only the electric potential applied, to one or more of the electrodes carrying the current to be rectified, amplified, oscillated or switched
- H01L29/8605—Resistors with PN junctions
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
- H01L21/70—Manufacture or treatment of devices consisting of a plurality of solid state components formed in or on a common substrate or of parts thereof; Manufacture of integrated circuit devices or of parts thereof
- H01L21/77—Manufacture or treatment of devices consisting of a plurality of solid state components or integrated circuits formed in, or on, a common substrate
- H01L21/78—Manufacture or treatment of devices consisting of a plurality of solid state components or integrated circuits formed in, or on, a common substrate with subsequent division of the substrate into plural individual devices
- H01L21/82—Manufacture or treatment of devices consisting of a plurality of solid state components or integrated circuits formed in, or on, a common substrate with subsequent division of the substrate into plural individual devices to produce devices, e.g. integrated circuits, each consisting of a plurality of components
- H01L21/822—Manufacture or treatment of devices consisting of a plurality of solid state components or integrated circuits formed in, or on, a common substrate with subsequent division of the substrate into plural individual devices to produce devices, e.g. integrated circuits, each consisting of a plurality of components the substrate being a semiconductor, using silicon technology
- H01L21/8232—Field-effect technology
- H01L21/8234—MIS technology, i.e. integration processes of field effect transistors of the conductor-insulator-semiconductor type
- H01L21/823412—MIS technology, i.e. integration processes of field effect transistors of the conductor-insulator-semiconductor type with a particular manufacturing method of the channel structures, e.g. channel implants, halo or pocket implants, or channel materials
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
- H01L21/70—Manufacture or treatment of devices consisting of a plurality of solid state components formed in or on a common substrate or of parts thereof; Manufacture of integrated circuit devices or of parts thereof
- H01L21/77—Manufacture or treatment of devices consisting of a plurality of solid state components or integrated circuits formed in, or on, a common substrate
- H01L21/78—Manufacture or treatment of devices consisting of a plurality of solid state components or integrated circuits formed in, or on, a common substrate with subsequent division of the substrate into plural individual devices
- H01L21/82—Manufacture or treatment of devices consisting of a plurality of solid state components or integrated circuits formed in, or on, a common substrate with subsequent division of the substrate into plural individual devices to produce devices, e.g. integrated circuits, each consisting of a plurality of components
- H01L21/822—Manufacture or treatment of devices consisting of a plurality of solid state components or integrated circuits formed in, or on, a common substrate with subsequent division of the substrate into plural individual devices to produce devices, e.g. integrated circuits, each consisting of a plurality of components the substrate being a semiconductor, using silicon technology
- H01L21/8232—Field-effect technology
- H01L21/8234—MIS technology, i.e. integration processes of field effect transistors of the conductor-insulator-semiconductor type
- H01L21/823481—MIS technology, i.e. integration processes of field effect transistors of the conductor-insulator-semiconductor type isolation region manufacturing related aspects, e.g. to avoid interaction of isolation region with adjacent structure
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
- H01L27/02—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having at least one potential-jump barrier or surface barrier; including integrated passive circuit elements with at least one potential-jump barrier or surface barrier
- H01L27/04—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having at least one potential-jump barrier or surface barrier; including integrated passive circuit elements with at least one potential-jump barrier or surface barrier the substrate being a semiconductor body
- H01L27/08—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components specially adapted for rectifying, oscillating, amplifying or switching and having at least one potential-jump barrier or surface barrier; including integrated passive circuit elements with at least one potential-jump barrier or surface barrier the substrate being a semiconductor body including only semiconductor components of a single kind
- H01L27/0802—Resistors only
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. PN junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
- H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
- H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
- H01L29/76—Unipolar devices, e.g. field effect transistors
- H01L29/772—Field effect transistors
- H01L29/78—Field effect transistors with field effect produced by an insulated gate
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M1/00—Analogue/digital conversion; Digital/analogue conversion
- H03M1/06—Continuously compensating for, or preventing, undesired influence of physical parameters
- H03M1/0602—Continuously compensating for, or preventing, undesired influence of physical parameters of deviations from the desired transfer characteristic
- H03M1/0604—Continuously compensating for, or preventing, undesired influence of physical parameters of deviations from the desired transfer characteristic at one point, i.e. by adjusting a single reference value, e.g. bias or gain error
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M1/00—Analogue/digital conversion; Digital/analogue conversion
- H03M1/66—Digital/analogue converters
Definitions
- the present invention relates to the MOS semiconductor art. Specifically, a semiconductor device having an electrically controllable channel width is described which may be used in applications where matching the electrical parameters of devices on a single integrated circuit is desirable.
- Integrated circuits having MOS derived components have manufacturing tolerances which vary with position on the device, and transistors located on one portion of a semiconductor substrate may have different gain characteristics than those in other locations on the substrate. Variations in the device characteristics across the integrated circuit result from random and systematic variations in manufacturing processes.
- the process variations include photolithographic image size variations, etch image size variations, ion implant dopant level variations and the thickness of deposited or grown films. These variations become increasingly significant as the area of the devices decreases.
- Some electronic circuit applications require that the transistors have an essentially matched gain.
- One such application is a digital to analog converter, wherein a ladder network is provided generating a plurality of reference currents from a constant reference current distributed throughout the integrated circuit.
- a designer will provide for a single source of current as the reference value, and the value of the reference current is reproduced in locations throughout the integrated circuit. Variations in transistor gain across the integrated circuit will result in random and systematic variations in reference currents which are replicated throughout the device.
- the present invention has been derived to provide semiconductor devices, such as a transistor or a resistor, having electrical properties which can be matched to other similar devices located on the substrate.
- the present invention provides for a semiconductor device having an electrically modulated conduction channel.
- the device is located within a trench structure formed in a substrate of an MOS integrated circuit, and a diffusion region within the trench structure is electrically modulated by applying a voltage between the trench and substrate.
- the device located within the trench structure may be an FET transistor, having a gate deposited over the diffusion region.
- the channel width below the gate can therefore be modulated by applying an electrical potential between the trench structure and the substrate, producing a change in the transistor gain. Accordingly, the gain of the device may be effectively set by varying the voltage potential between the trench structure and the substrate to match the gain of other transistors on the substrate.
- a pair of transistors having similar characteristics may be formed within the trench. If a known gain is established in one of the transistors, by controlling the trench to substrate potential, so that a known current flows therethrough, the gain on the other transistor is commonly controlled by the same potential between the trench and substrate, and the other transistor can be used to generate a reference current. Different pairs of transistors located in other trenches on the substrate may be set to the same gain, if the known current is set in one of the transistors. The remaining transistor of each pair of transistors in a trench may be advantageously used to generate the same reference current if their gate connections are commonly connected.
- the trench structure may include multiple diffusion regions which serve as resistor controlled from a common control voltage.
- each diffusion region is surrounded by a single trench structure which controls the diffusion region.
- FIG. 1A is a plan view of a channel modulated FET transistor in accordance with the preferred embodiment
- FIG. 1B illustrates a section view of the transistor of FIG. 1A
- FIG. 1C illustrates the effect of applying positive potential between the trench and substrate
- FIG. 1D illustrates the effect of applying a negative potential between the trench and the substrate
- FIG. 2 illustrates an application for controlling the gain of one of a pair of transistor devices in accordance with the preferred embodiment
- FIG. 3 is a schematic illustration of the circuit of FIG. 2;
- FIG. 4 illustrates another embodiment of the invention for matching the value of resistors at different locations on the substrate
- FIG. 5 is a top view of one of the resistor banks of the device of FIG. 4;
- FIG. 6A is a section view of the device of FIG. 5 along lines A-A;
- FIG. 6B is a second section view along lines B-B of the resistor bank of FIG. 5;
- FIG. 7 represents a prior art digital-to-analog converter which may be improved utilizing one embodiment of the invention
- FIG. 8 illustrates an improved digital-to-analog converter which converts a voltage output to a current output having a high linearity
- FIG. 9 illustrates an improved digital-to-analog converter having current sources which are channel modulated to improve the linearity.
- FIGS. 1A through 1D A transistor having an electrically modulated channel in accordance with the present invention is shown more particularly in FIGS. 1A through 1D.
- a trench structure 12 is shown formed in an integrated circuit substrate.
- the trench has a depth D of approximately 0.3 to 0.4 ⁇ .
- a connection 21 is formed on the trench, to which a voltage may be applied, and a similar connection 22 is provided on the substrate 25 .
- a diffusion region 11 is formed in the substrate, surrounded by the trench, and a gate 18 is formed over the diffusion region 11 . Source and drain regions are provided on either side of the gate 18 to form a transistor.
- the trench is filled with a polysilicon material 26 and includes a thin oxide layer along the inner and outer side walls.
- the electric field created between the trench and substrate modulates the channel width as shown in FIGS. 1C and 1D.
- FIG. 1D when a negative voltage potential is applied between the trench contact 21 and substrate 25 , a thin P + layer forms at the trench body interface, effectively narrowing the width of the gate, and the channel underneath the gate.
- the narrowing width does not effect the threshold for the transistor, if the transistor width is above a predetermined factor.
- Control over channel width in turn, provides a direct control over the gain (g m ) of the device.
- FIG. 2 illustrates a basic building block for providing transistors having matched gains in different portions of the integrated circuit substrate.
- Q 2 ′ and Q 3 ′, as well as Q 1 ′, are located on a different portion of the integrated circuit substrate than Q 1 , Q 2 and Q 3 .
- both circuits are capable of generating the identical current through Q 3 and Q 3 ′ by establishing the identical gain for Q 3 and Q 3 ′ and a common gate to source voltage, even though the transistors may not be matched at the time of manufacture.
- the circuit of FIG. 3 takes advantage of the fact that because Q 1 and Q 1 ′ occupy a large area of the substrate, their gain characteristics (g m ) are more closely matched, as the degree of mismatched generally proportional to: 1 A
- A is the area of the transistor on the substrate.
- Q 1 and Q 1 ′ have gates connected to the same reference voltage 42 , which may be Vdd/2, and identical currents are established through Q 1 ′,Q 1 and the serial connection of Q 2 ′ and Q 2 .
- the trench containing Q 2 and Q 3 is modulated with the drain voltage of Q 1 , so that Q 2 assumes a gain value sufficient to support a current determined by the reference voltage 42 on the gate connection of Q 1 and Q 1 ′. While Q 2 and Q 2 ′ may not be matched, their gains will be set to be the same in order to derive the same source-drain current through Q 1 ′, Q 2 ′, and Q 1 and Q 2 .
- FIG. 2 shows the general configuration where the two transistors, Q 2 and Q 3 , are included within the diffusion area 11 .
- Each of the transistors includes a gate 18 , 180 , as well as a source drain region 17 - 19 within the diffusion area 11 .
- the current source 35 carrying the reference current connects to the drain of Q 3 .
- the source of Q 2 and Q 3 is connected to the common ground 32 .
- Each of the nFETs Q 2 and Q 3 are controlled by a current from Q 1 , which is a pFET within an N well 36 .
- the drain of Q 1 is connected to the drain of Q 2 , through terminal 33 , as well as to the trench 12 .
- the trench is effectively a capacitor, coupling the same electrostatic charge across a conduction channel, thereby setting the same gain for Q 2 and Q 3 .
- the illustrated circuit of FIG. 3, duplicating the device is Q 3 ′, Q 2 ′ and Q 1 ′, permits the identical current of current source 35 to be established in RL.
- Q 1 ′ has a large area equal to the area of Q 1 , it has substantially the same gain as Q 1 , and the same current is established for Q 1 ′.
- This sets the gain of Q 2 ′ to be the same as Q 2 by changing the trench voltage to compensate for process variation.
- Q 2 ′ proximity to Q 3 ′ it establishes the same gain for Q 3 ′ which has a gate to source voltage the same as Q 3 . Accordingly, RL must carry the same drain source current as current source 35 provides.
- the foregoing circuit permits the duplication of a reference current 35 in numerous places in the integrated circuit by establishing a large area transistor such as Q 1 and Q 1 ′ in each location which will have the same gain, and carry the same current, given the fact that larger area transistors are already matched. Connecting one of the two transistors within a trench in series with one of the larger transistors, so that the series connected transistors carry the same current, will establish the same gain for the remaining transistor in the trench. The current through each of the drain source circuit of the remaining transistor in each trench will be the same when the gates of these transistors are connected together.
- FIG. 4 illustrates a circuit which matches the resistance values in one location of the substrate to resistance values at another location on the substrate. Due to the aforesaid process variations during the manufacture of the integrated circuit, the resistor values may not be well matched.
- the implementation according to FIG. 4 permits a trimming of the value by changing an electrical potential associated with resistors on one portion of the substrate with respect to an electrical potential of another portion of the substrate containing another set of resistors.
- FIG. 4 a bank of resistors 50 - 54 are shown which are located on a different portion of the circuit substrate than resistors 65 - 69 .
- Each bank of resistors 50 - 54 , 65 - 69 are located within a trench 56 , 71 of a substrate.
- the trench walls which enclose each of the resistor banks are connected via a respective connection 63 , 70 to the output of the first and second differential amplifiers 48 , 49 .
- Differential amplifiers 48 and 49 provide a potential for modulating the width of the resistors of a respective resistor bank.
- a control voltage may be derived for modulating the width of each of the resistors of a resistor bank.
- the resistance of R 1 is set equal to the resistance R 2 and R 3 is set equal to R 4 , by virtue of the fact that differential amplifiers 48 and 49 will assume output values which produce 0 volts between the inverting and non-inverting inputs of amplifiers 48 and 49 .
- the potential applied to the trench walls produces an electrostatic field between the walls of trenches 56 , 69 and the substrate 72 , effectively modifying the width and resistance value of the resistors within a trench.
- FIGS. 5, 6A and 6 B show in greater detail the construction of a given trench 56 and the individual resistors within the trench.
- the resistors comprise a diffusion layer 59 , shown as doped N+, formed within a well of polysilicon 60 deposited within the trench 56 .
- the diffusion 59 is connected to contacts 61 and 64 , formed through an oxide layer 60 .
- Contacts 61 and 64 provide for connections to the resistor.
- each of the resistors 50 - 54 are separated from the trench by a thin oxide layer shown as 50 a for resistor 50 . Accordingly, a potential applied between terminal 63 and the substrate 72 provides an electrostatic field for modulating the width of each resistor of a bank. Thus, the two banks of resistors have matched pairs of resistors at different locations on the substrate. As shown in FIG. 6B, changes in the trench voltage deplete the N+ region near the trench walls, increasing the resistance of each resistor.
- isolation trenches which are in use in semiconductor manufacturing processes. Formation of shallow isolation trenches throughout the substrate permit the creation of actively controlled components such as the foregoing. In those instances where the components are not to be controlled, the trench walls may be tied to the same potential as the substrate.
- a transistor having an electrically modulated channel is useful in digital-to-analog converters.
- a prior art digital-to-analog converter (DAC) is shown more particularly in FIG. 7.
- the device of FIG. 7 includes a digital decoder 74 , which creates pairs of control signals for each digit of an input signal.
- the pairs of control signals control current flow in one or the other transistor of a current branch.
- Each current branch includes a current source, operated in saturation at a value which is weighted according to the order of an applied digit, which supplies current through first or second transistors and converts the bit into a current supplied to output 88 .
- the lowest order bit controls transistors 80 , 81 to supply current from current source transistor 75 .
- the accuracy of the device depends in part on the ability to generate a known and precise current through each branch from current sources 75 , 76 and 77 .
- this is realized by having transistors 75 , 76 and 77 quite wide, making their width (w) proportional to the value of current needed in the respective branch.
- w width
- the transistors 75 , 76 and 77 produce a variation in currents which reduce the linearity of the output.
- the transistor having an electrically modulated channel may be useful in digital-to-analog to converter circuits for reducing the spread in current source gains, and the resulting decrease in DAC linearity, linearity being a measure of the output value with certainty, given a known digital input as determined across the digital input range.
- FIG. 8 A digital signal is decoded in a digital decoder 74 and used to derive an output voltage.
- the devices 90 , 91 and 92 comprise a low power voltage DAC.
- the low power voltage DAC known to those skilled in the art, creates an output voltage which in some applications needs to be converted to a current.
- the lower power voltage DAC charges a capacitor bank 91 to a voltage proportional to the input digital signal.
- a charge stored on the capacitor bank 91 is integrated in an integrator 92 , and the value is sampled and held in the sample/hold circuit 93 while charge/integration takes place.
- the low power voltage DAC utilizes less chip area than the device of FIG. 7, even when the voltage to current converter using a transistor having a modulated channel width is employed.
- the voltage output from the low power voltage DAC 90 modulates the width of transistors 75 a and 80 a .
- An output node 88 a provides the current which is proportional to the voltage applied to the trench connection 94 .
- the result is a current output which, with proper centering of the trench bias, provides a current which oscillates around the current value set by the DC voltage component on trench contact 94 .
- the voltage to current converter 90 provides a current midpoint, reducing thermal and electromigration as well as an improvement in power consumption over the prior art device in FIG. 7.
- the disadvantages of the circuit of FIG. 8, however, include its inability to produce a 0 current at its output, and a reduced high frequency capability due to the fact that the system relies upon the charge and discharge of a capacitor bank 91 .
- FIG. 9 represents a more advantageous application of a trench modulated transistor in accordance with the present invention in a DAC.
- each of the current sources includes a transistor 75 , 76 and 77 , having its own trench bias voltage.
- the current through each of the branches may be advantageously trimmed by controlling the trench bias produced by bias network 100 .
- the bias voltage may be controlled using laser trimming, or through a fuse blow or other known techniques for changing the value of a bias network connected to a source of bias voltage or using a biasing circuit such as shown in FIGS. 2 and 3.
Abstract
Description
- The present invention relates to the MOS semiconductor art. Specifically, a semiconductor device having an electrically controllable channel width is described which may be used in applications where matching the electrical parameters of devices on a single integrated circuit is desirable.
- Integrated circuits having MOS derived components have manufacturing tolerances which vary with position on the device, and transistors located on one portion of a semiconductor substrate may have different gain characteristics than those in other locations on the substrate. Variations in the device characteristics across the integrated circuit result from random and systematic variations in manufacturing processes. The process variations include photolithographic image size variations, etch image size variations, ion implant dopant level variations and the thickness of deposited or grown films. These variations become increasingly significant as the area of the devices decreases.
- Some electronic circuit applications require that the transistors have an essentially matched gain. One such application is a digital to analog converter, wherein a ladder network is provided generating a plurality of reference currents from a constant reference current distributed throughout the integrated circuit. A designer will provide for a single source of current as the reference value, and the value of the reference current is reproduced in locations throughout the integrated circuit. Variations in transistor gain across the integrated circuit will result in random and systematic variations in reference currents which are replicated throughout the device.
- The present invention has been derived to provide semiconductor devices, such as a transistor or a resistor, having electrical properties which can be matched to other similar devices located on the substrate.
- The present invention provides for a semiconductor device having an electrically modulated conduction channel. The device is located within a trench structure formed in a substrate of an MOS integrated circuit, and a diffusion region within the trench structure is electrically modulated by applying a voltage between the trench and substrate.
- In accordance with one embodiment of the invention, the device located within the trench structure may be an FET transistor, having a gate deposited over the diffusion region. The channel width below the gate can therefore be modulated by applying an electrical potential between the trench structure and the substrate, producing a change in the transistor gain. Accordingly, the gain of the device may be effectively set by varying the voltage potential between the trench structure and the substrate to match the gain of other transistors on the substrate.
- In accordance with the invention, a pair of transistors having similar characteristics may be formed within the trench. If a known gain is established in one of the transistors, by controlling the trench to substrate potential, so that a known current flows therethrough, the gain on the other transistor is commonly controlled by the same potential between the trench and substrate, and the other transistor can be used to generate a reference current. Different pairs of transistors located in other trenches on the substrate may be set to the same gain, if the known current is set in one of the transistors. The remaining transistor of each pair of transistors in a trench may be advantageously used to generate the same reference current if their gate connections are commonly connected.
- In yet another embodiment of the invention, the trench structure may include multiple diffusion regions which serve as resistor controlled from a common control voltage.
- In still another embodiment of the invention, each diffusion region is surrounded by a single trench structure which controls the diffusion region.
- FIG. 1A is a plan view of a channel modulated FET transistor in accordance with the preferred embodiment;
- FIG. 1B illustrates a section view of the transistor of FIG. 1A;
- FIG. 1C illustrates the effect of applying positive potential between the trench and substrate;
- FIG. 1D illustrates the effect of applying a negative potential between the trench and the substrate;
- FIG. 2 illustrates an application for controlling the gain of one of a pair of transistor devices in accordance with the preferred embodiment;
- FIG. 3 is a schematic illustration of the circuit of FIG. 2;
- FIG. 4 illustrates another embodiment of the invention for matching the value of resistors at different locations on the substrate;
- FIG. 5 is a top view of one of the resistor banks of the device of FIG. 4;
- FIG. 6A is a section view of the device of FIG. 5 along lines A-A;
- FIG. 6B is a second section view along lines B-B of the resistor bank of FIG. 5;
- FIG. 7 represents a prior art digital-to-analog converter which may be improved utilizing one embodiment of the invention;
- FIG. 8 illustrates an improved digital-to-analog converter which converts a voltage output to a current output having a high linearity; and
- FIG. 9 illustrates an improved digital-to-analog converter having current sources which are channel modulated to improve the linearity.
- A transistor having an electrically modulated channel in accordance with the present invention is shown more particularly in FIGS. 1A through 1D. Referring now to FIGS. 1A through 1D, a
trench structure 12 is shown formed in an integrated circuit substrate. The trench has a depth D of approximately 0.3 to 0.4μ. Aconnection 21 is formed on the trench, to which a voltage may be applied, and asimilar connection 22 is provided on thesubstrate 25. Adiffusion region 11 is formed in the substrate, surrounded by the trench, and agate 18 is formed over thediffusion region 11. Source and drain regions are provided on either side of thegate 18 to form a transistor. - As illustrated in FIGS.1B-1D, the trench is filled with a
polysilicon material 26 and includes a thin oxide layer along the inner and outer side walls. The electric field created between the trench and substrate modulates the channel width as shown in FIGS. 1C and 1D. In response to the application of a positive potential betweenterminals trench contact 21 andsubstrate 25, a thin P+ layer forms at the trench body interface, effectively narrowing the width of the gate, and the channel underneath the gate. - The narrowing width does not effect the threshold for the transistor, if the transistor width is above a predetermined factor. Control over channel width, in turn, provides a direct control over the gain (gm) of the device.
- An application for the electrically modulated transistor which provides a controlled reference current at various points within the integrated circuit is shown in FIGS. 2 and 3. FIG. 2 illustrates a basic building block for providing transistors having matched gains in different portions of the integrated circuit substrate. Q2′ and Q3′, as well as Q1′, are located on a different portion of the integrated circuit substrate than Q1, Q2 and Q3. However, both circuits are capable of generating the identical current through Q3 and Q3′ by establishing the identical gain for Q3 and Q3′ and a common gate to source voltage, even though the transistors may not be matched at the time of manufacture.
-
- where A is the area of the transistor on the substrate. Q1 and Q1′ have gates connected to the
same reference voltage 42, which may be Vdd/2, and identical currents are established through Q1′,Q1 and the serial connection of Q2′ and Q2. The trench containing Q2 and Q3 is modulated with the drain voltage of Q1, so that Q2 assumes a gain value sufficient to support a current determined by thereference voltage 42 on the gate connection of Q1 and Q1′. While Q2 and Q2′ may not be matched, their gains will be set to be the same in order to derive the same source-drain current through Q1′, Q2′, and Q1 and Q2. Process variations between Q2 and Q3 are minimal, even though the devices have a much smaller area on the substrate than Q1. Once the gain of Q2 is fixed, the gain of Q3 which because of its proximity to Q2 is substantially the same as Q2, is also fixed. - FIG. 2 shows the general configuration where the two transistors, Q2 and Q3, are included within the
diffusion area 11. Each of the transistors includes agate diffusion area 11. Thecurrent source 35 carrying the reference current connects to the drain of Q3. The source of Q2 and Q3 is connected to the common ground 32. Each of the nFETs Q2 and Q3 are controlled by a current from Q1, which is a pFET within an N well 36. The drain of Q1 is connected to the drain of Q2, throughterminal 33, as well as to thetrench 12. Current flowing through Q1 is established by the reference voltage connected toterminal 42, and the gain of Q1 establishes a voltage on the trench which in turn sets the gain of Q2 sufficient to carry the current of Q1. As illustrated in FIG. 3, the trench is effectively a capacitor, coupling the same electrostatic charge across a conduction channel, thereby setting the same gain for Q2 and Q3. - The illustrated circuit of FIG. 3, duplicating the device is Q3′, Q2′ and Q1′, permits the identical current of
current source 35 to be established in RL. As Q1′ has a large area equal to the area of Q1, it has substantially the same gain as Q1, and the same current is established for Q1′. This, in turn, sets the gain of Q2′ to be the same as Q2 by changing the trench voltage to compensate for process variation. Because of Q2′ proximity to Q3′, it establishes the same gain for Q3′ which has a gate to source voltage the same as Q3. Accordingly, RL must carry the same drain source current ascurrent source 35 provides. - It is also possible to include more than two transistors in a trench which will permit other circuits to be implemented which need transistors having matching gains.
- The foregoing circuit permits the duplication of a reference current35 in numerous places in the integrated circuit by establishing a large area transistor such as Q1 and Q1′ in each location which will have the same gain, and carry the same current, given the fact that larger area transistors are already matched. Connecting one of the two transistors within a trench in series with one of the larger transistors, so that the series connected transistors carry the same current, will establish the same gain for the remaining transistor in the trench. The current through each of the drain source circuit of the remaining transistor in each trench will be the same when the gates of these transistors are connected together.
- The foregoing principles may also be applied to control devices which are not field effect transistors. For instance, FIG. 4 illustrates a circuit which matches the resistance values in one location of the substrate to resistance values at another location on the substrate. Due to the aforesaid process variations during the manufacture of the integrated circuit, the resistor values may not be well matched. The implementation according to FIG. 4 permits a trimming of the value by changing an electrical potential associated with resistors on one portion of the substrate with respect to an electrical potential of another portion of the substrate containing another set of resistors.
- Referring now specifically to FIG. 4, a bank of resistors50-54 are shown which are located on a different portion of the circuit substrate than resistors 65-69. Each bank of resistors 50-54, 65-69 are located within a
trench respective connection differential amplifiers 48, 49.Differential amplifiers 48 and 49 provide a potential for modulating the width of the resistors of a respective resistor bank. Thus, where the resistors of those banks are to be matched with respect to each other, i.e.,resistor 50=69, 56=68, 51=67, . . . a control voltage may be derived for modulating the width of each of the resistors of a resistor bank. - In the circuit of FIG. 4 the resistance of R1 is set equal to the resistance R2 and R3 is set equal to R4, by virtue of the fact that
differential amplifiers 48 and 49 will assume output values which produce 0 volts between the inverting and non-inverting inputs ofamplifiers 48 and 49. The potential applied to the trench walls produces an electrostatic field between the walls oftrenches substrate 72, effectively modifying the width and resistance value of the resistors within a trench. - FIGS. 5, 6A and6B show in greater detail the construction of a given
trench 56 and the individual resistors within the trench. Referring now to FIGS. 5, 6A and 6B, the resistors comprise adiffusion layer 59, shown as doped N+, formed within a well ofpolysilicon 60 deposited within thetrench 56. Thediffusion 59 is connected tocontacts oxide layer 60.Contacts - The trench is filled with polysilicon, and each of the resistors50-54 are separated from the trench by a thin oxide layer shown as 50 a for
resistor 50. Accordingly, a potential applied betweenterminal 63 and thesubstrate 72 provides an electrostatic field for modulating the width of each resistor of a bank. Thus, the two banks of resistors have matched pairs of resistors at different locations on the substrate. As shown in FIG. 6B, changes in the trench voltage deplete the N+ region near the trench walls, increasing the resistance of each resistor. - The foregoing embodiments utilize isolation trenches which are in use in semiconductor manufacturing processes. Formation of shallow isolation trenches throughout the substrate permit the creation of actively controlled components such as the foregoing. In those instances where the components are not to be controlled, the trench walls may be tied to the same potential as the substrate.
- The process of forming such shallow isolation trenches are conventional, and well known. Such devices are currently used as capacitors, and for other applications where isolation of one component from another is necessary.
- A transistor having an electrically modulated channel is useful in digital-to-analog converters. A prior art digital-to-analog converter (DAC) is shown more particularly in FIG. 7. The device of FIG. 7 includes a
digital decoder 74, which creates pairs of control signals for each digit of an input signal. The pairs of control signals control current flow in one or the other transistor of a current branch. Each current branch includes a current source, operated in saturation at a value which is weighted according to the order of an applied digit, which supplies current through first or second transistors and converts the bit into a current supplied tooutput 88. The lowest order bit controlstransistors current source transistor 75. Higher order bits are represented by the current flow in the currentbranch including source 76,transistors current source 77, andtransistors current source transistors common gates 89. Depending upon the digital value and its complement, current is either directed to theoutput 88 or to ground. Shunting current which is not directed to theoutput 88 to ground provides a reduction in unwanted transients, when switching current to theoutput 88 through one of thetransistors - The accuracy of the device depends in part on the ability to generate a known and precise current through each branch from
current sources transistors transistors - The transistor having an electrically modulated channel may be useful in digital-to-analog to converter circuits for reducing the spread in current source gains, and the resulting decrease in DAC linearity, linearity being a measure of the output value with certainty, given a known digital input as determined across the digital input range.
- One approach of using the device in accordance with the invention is shown in FIG. 8. A digital signal is decoded in a
digital decoder 74 and used to derive an output voltage. Thedevices capacitor bank 91 to a voltage proportional to the input digital signal. A charge stored on thecapacitor bank 91 is integrated in anintegrator 92, and the value is sampled and held in the sample/hold circuit 93 while charge/integration takes place. - The low power voltage DAC utilizes less chip area than the device of FIG. 7, even when the voltage to current converter using a transistor having a modulated channel width is employed. The voltage output from the low
power voltage DAC 90 modulates the width of transistors 75 a and 80 a. An output node 88 a provides the current which is proportional to the voltage applied to thetrench connection 94. - The result is a current output which, with proper centering of the trench bias, provides a current which oscillates around the current value set by the DC voltage component on
trench contact 94. The voltage tocurrent converter 90 provides a current midpoint, reducing thermal and electromigration as well as an improvement in power consumption over the prior art device in FIG. 7. The disadvantages of the circuit of FIG. 8, however, include its inability to produce a 0 current at its output, and a reduced high frequency capability due to the fact that the system relies upon the charge and discharge of acapacitor bank 91. - FIG. 9 represents a more advantageous application of a trench modulated transistor in accordance with the present invention in a DAC. As shown in FIG. 9, each of the current sources includes a
transistor transistors bias network 100. The bias voltage may be controlled using laser trimming, or through a fuse blow or other known techniques for changing the value of a bias network connected to a source of bias voltage or using a biasing circuit such as shown in FIGS. 2 and 3. - The foregoing description of the invention illustrates and describes the present invention. Additionally, the disclosure shows and describes only the preferred embodiments of the invention, but as aforementioned, it is to be understood that the invention is capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein, commensurate with the above teachings, and/or the skill or knowledge of the relevant art. The embodiments described hereinabove are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with the various modifications required by the particular applications or uses of the invention. Accordingly, the description is not intended to limit the invention to the form disclosed herein. Also, it is intended that the appended claims be construed to include alternative embodiments.
Claims (27)
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US10/697,012 US20040092109A1 (en) | 1999-12-20 | 2003-10-31 | Semiconductor device and method for making the device having an electrically modulated conduction channel |
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US09/467,537 US6683345B1 (en) | 1999-12-20 | 1999-12-20 | Semiconductor device and method for making the device having an electrically modulated conduction channel |
US10/697,012 US20040092109A1 (en) | 1999-12-20 | 2003-10-31 | Semiconductor device and method for making the device having an electrically modulated conduction channel |
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US09/467,537 Division US6683345B1 (en) | 1999-12-20 | 1999-12-20 | Semiconductor device and method for making the device having an electrically modulated conduction channel |
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US09/467,537 Expired - Lifetime US6683345B1 (en) | 1999-12-20 | 1999-12-20 | Semiconductor device and method for making the device having an electrically modulated conduction channel |
US10/697,012 Abandoned US20040092109A1 (en) | 1999-12-20 | 2003-10-31 | Semiconductor device and method for making the device having an electrically modulated conduction channel |
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US20100244137A1 (en) * | 2005-09-15 | 2010-09-30 | Renesas Technology Corp. | Semiconductor device and a method of manufacturing the same |
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US7560761B2 (en) * | 2006-01-09 | 2009-07-14 | International Business Machines Corporation | Semiconductor structure including trench capacitor and trench resistor |
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US6683345B1 (en) | 2004-01-27 |
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