Process variable transmitter with cryogenic temperature sensor

Abstract

A process variable transmitter for sensing cryogenic temperatures in an industrial process includes a cryogenic temperature sensor configured to be thermally coupled to the industrial process. The cryogenic temperature sensor has a resistance that varies in response to a change in cryogenic temperature, and the industrial process is at the cryogenic temperature. A resistance measurement circuit is electrically coupled to the cryogenic temperature sensor and measures a sensor resistance within a resistance range and responsively provides a temperature dependent output based on the measured resistance. Transmitter output circuitry is coupled to the measurement circuitry to transmit information related to the cryogenic temperature to a remote location. The cryogenic temperature sensor includes a polysilicon sensor containing a dopant such that the cryogenic temperature sensor has a resistance that varies over a cryogenic temperature range that is within the sensor resistance range of the measurement circuit.

Description

Background Art

[0001] The present invention relates to process variable transmitters for use in process control and monitoring systems. More particularly, the present invention relates to process variable transmitters having cryogenic temperature sensors.

[0002] Process industries employ process variable transmitters to monitor process variables associated with substances such as solids, slurries, liquids, steam, and gases in chemical, pulp, petroleum, pharmaceutical, food, and other food processing plants. Process variables include pressure, temperature, flow, level, turbidity, density, concentration, chemical composition, and other characteristics. Process fluid temperature transmitters provide an output related to the sensed temperature of the process substance. The temperature transmitter output can be transmitted to a control room through a process control loop, or the output can be transmitted to another process device so that the process can be monitored and controlled. To monitor the process fluid temperature, the transmitter includes a sensor, such as a resistance temperature device (RTD).

[0003] RTDs change resistance in response to changes in temperature. By measuring the resistance of the RTD, the temperature can be calculated. This resistance measurement is usually achieved by passing a known current through the RTD and measuring the associated voltage developed across the RTD.

[0004] A microprocessor-based transmitter includes a sensor, an analog-to-digital converter for converting the output from the sensor into a digital format, a microprocessor for compensating the digitized output, and output circuitry for transmitting the compensated output. Typically, this transmission is done over a process control loop, such as a 4mA to 20mA current loop.

[0005] Temperature is measured by converting the sensor output (resistance versus voltage) into an output that indicates the temperature of the sensor. However, in order for a process variable transmitter to obtain the desired measurement, the transmitter's circuitry must be properly calibrated. For example, when operating at cryogenic temperatures, a typical RTD sensor exhibits only a small change in resistance in response to temperature changes. Therefore, it is difficult to accurately measure small temperature changes at low temperatures. Summary of the invention

[0006] A process variable transmitter for sensing cryogenic temperatures in an industrial process includes a cryogenic temperature sensor configured to be thermally coupled to the industrial process. The cryogenic temperature sensor has a resistance that changes in response to changes in the cryogenic temperature, and the industrial process is at the cryogenic temperature. A resistance measurement circuit is electrically coupled to the cryogenic temperature sensor and measures the sensor resistance within a resistance range and responsively provides an output related to the temperature based on the measured resistance. A transmitter output circuit is coupled to the measurement circuit to send information related to the cryogenic temperature to a remote location. The cryogenic temperature sensor includes a polysilicon sensor that includes a dopant so that the cryogenic temperature sensor has a resistance that changes within a cryogenic temperature range that is within the sensor resistance range of the measurement circuit.

[0007] This summary is provided to introduce a series of concepts in a simplified form, which are further described in the following detailed description. This summary is not intended to identify the key features or essential features of the claimed subject matter, nor is it intended to be used to help determine the scope of the claimed subject matter. The claimed subject matter is not limited to specific implementations that solve any or all of the shortcomings mentioned in the background technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 is a diagram of the environment of a process fluid temperature transmitter.

[0009] Figure 2 is a diagrammatic view of a process fluid temperature transmitter.

[0010] Figure 3 is a system block diagram of a process fluid temperature transmitter.

[0011] Figure 4 is a more detailed block diagram of a process fluid temperature transmitter.

[0012] Figure 5 It is a graph of resistance versus temperature for different temperature sensors.

[0013] Figure 6 is a plan view of a cryogenic temperature sensor according to one embodiment.

[0014] Figure 7 is a graph of resistance versus temperature for a cryogenic temperature sensor.

[0015] Fig. 8A and Figure 8B They are encapsulated in protective layers. Figure 6 Plan and side views of a cryogenic temperature sensor.

[0016] Fig. 9is a side cross-sectional view of a temperature sensor package including a cryogenic temperature sensor. DETAILED DESCRIPTION

[0017] In many industrial processes, progress is being made in adopting alternative energy sources that do not produce CO2. Hydrogen is one possible alternative fuel that is being used. Hydrogen can be stored as a compressed gas or as a cryogenic liquid. The temperature of liquid hydrogen is about 20K. With the construction of storage tanks and liquid hydrogen production facilities, there is a need for industrial process temperature transmitters with cryogenic temperature sensors that can accurately measure. In addition, it is desirable that such sensors can be retrofitted so that they can be used without significant modifications to existing process temperature transmitters.

[0018] Figure 1 and Figure 2 An environment for a process fluid temperature transmitter according to an embodiment of the present invention is illustrated. Figure 1 A process fluid control system 10 including a process fluid temperature transmitter 12 is shown. Figure 2 A process control system 10 is illustrated that includes a process fluid temperature transmitter 12 that is electrically coupled to a control room 14 (modeled as a voltage source and a resistor) via a process control loop 16. Transmitter 12 is mounted on and coupled to a process fluid container, such as a pipe 18. Transmitter 12 monitors the temperature of a process fluid in process pipe 18 and sends the temperature information to control room 14 via loop 16.

[0019] Figure 3 is a system block diagram of process fluid transmitter 12 according to an embodiment of the present invention. Process fluid temperature transmitter 12 includes loop communicator 20, a plurality of terminals 22, a current source 24, a voltage measurement device 26, and a controller 28. Figure 3 Also shown is an RTD (resistance temperature device) 30. As discussed herein, the RTD 30 can be a conventional RTD made of platinum, copper and nickel, or it can be a polycrystalline cryogenic temperature sensor in accordance with the present invention.

[0020] The loop communicator 20 can be coupled to the process control loop 16 (shown in dashed lines) and is adapted to communicate through the process control loop 16. The process control loop 16 is any physical configuration capable of transmitting signals related to process information. For example, the process control loop 16 can be a two-wire 4mA to 20mA process control loop. A process variable transmitter coupled to such a process control loop controls the amount of current flowing through the loop so that the current corresponds to a process variable. Some process variable transmitters can operate at such low energy levels that they can receive all the required electrical power from the 4mA to 20mA process control loop.

[0021] The loop communicator 20 may include a 4mA to 20mA communication segment for analog communications. For digital signals, the communicator 20 may include a high-speed addressable remote sensor Communication segment, FOUNDATION TM Fieldbus communication segment or any other suitable segment, such as a Profibus segment, a controller area network (CAN) segment, a DeviceNet segment, or a Lonworks segment. Additionally, the segments may be included together. For example, a 4mA to 20mA segment may be included with a The segments are combined to provide both analog and digital communications. Thus, the loop communicator 20 is adapted to communicate bidirectionally over the process control loop 16 in a known manner according to one or more selected protocols.

[0022] The process control loop 16 may include any operable number of conductors. For example, the process control loop 16 may be a two-conductor, three-conductor, or four-conductor process control loop. The conductors themselves may be electrical wires or fiber optic media. Furthermore, if a wireless process control loop is to be used (such as an infrared (IR) loop or a radio frequency (RF) loop), the conductors may be omitted and the adaptation of the loop communicator 20 to such wireless communication may be effected in a known manner. For example, a wireless communication loop compliant with IEC 62591 may be used. Such wireless communication is provided.

[0023] Figure 3 is shown coupled to a resistive temperature device such as an RTD 30 (in Figure 3 A pair of terminals 22 are shown in dashed lines. Figure 3 A pair of terminals 22 are shown, but any suitable number of terminals may be used to couple to temperature sensor 30. Sensor 30 may be separate from transmitter 12 (as shown) or included within transmitter 12.

[0024] Current source 24 is operatively coupled to terminal 22 and adapted to cause a measurement current to flow through a plurality of terminals. Flowing a known measurement current through an unknown resistor results in an associated voltage drop across the resistor, which voltage drop is indicative of the resistor. Current source 24 is shown in block form and may be any current source that provides a known current output. Thus, current source 24 may include an unknown current source adapted to cause its current to flow through a known resistor so that the current may be known. Alternatively, current source 24 may be a known semiconductor current source or any other suitable current source.

[0025] Voltage measuring device 26 is coupled to terminal 22 and is adapted to provide a voltage value indicative of the voltage across terminal 22. Voltage measuring device 26 may be an analog-to-digital converter or any other suitable device capable of measuring the voltage across terminal 22. If such a measurement is performed while current source 24 causes a measurement current to flow through sensor 30, the measured voltage will be related to the resistance of sensor 30 and, therefore, to the process fluid temperature.

[0026] Controller 28 is coupled to voltage measurement device 26 and loop communicator 20. Controller 28 is adapted to provide a process temperature output value to loop communicator 20 for communication via process control loop 16. The process temperature output value is related to the measured resistance of temperature sensor 30. Controller 28 may be a programmable gate array, a microprocessor, or any other suitable device.

[0027] Figure 4 is a more detailed block diagram of a temperature transmitter 12 connected to measure temperature with a temperature sensor 30. In this embodiment, the transmitter 12 includes a terminal block 44 having terminals 1 through 4 for coupling to the temperature sensor 30. The transmitter 12 includes a multiplexer 40 controlled by a microprocessor (controller) 28, which is coupled to the control loop 16 through an input / output (I / O) circuit (loop communicator) 20. The multiplexer 40 multiplexes the appropriate set of analog signals (including the signals from terminals 1 through 4) to the positive and negative inputs of a differential amplifier 46, which is connected to a high accuracy A / D converter 48. A memory 50 stores instructions and information for the microprocessor 28, which operates at a speed determined by a clock 52. The multiplexer 40 selectively connects the input pairs to the positive and negative inputs of the differential amplifier 46. The reference resistor R REF 58 is coupled to the multiplexer 40 and connected in series with the sensor 30. Figure 4 The use of two connections for current source and two connections for voltage measurement, as illustrated in FIG, is called a Kelvin connection and helps reduce measurement errors due to wiring resistance. However, a two-wire connection such as Figure 3 exemplified in .

[0028] In operation, transmitter 12 measures the temperature of sensor 30 and sends a representation of the temperature through control loop 16. Transmitter 12 calculates the principal value of the temperature of sensor 30 using the following equation.

[0029]

[0030] in:

[0031] R REFNOMis the nominal measured resistance (in ohms) of the reference resistor 58 and / or is stored in the memory 50;

[0032] V RINPUT is the voltage drop across the input; and

[0033] V RREF For cross-R REF Voltage drop

[0034] The current source 24 provides a current I through the sensor 30 S Reference resistor 58 is provided via MUX 40 (via terminals 1 and 4). Figure 4 In the configuration of FIG. 1 , the microprocessor 28 uses the MUX 40 to measure the voltage drop (V RINPUT ) and the voltage drop across resistor 58 (V RREF ). REFNOM The constant is calculated and retrieved from memory 50. In a four-wire resistance measurement such as this, the voltage drop across the connections to terminals 2 and 3 is largely eliminated since substantially all of the current flows between terminals 1 and 4 and has little effect on the accuracy of the measurement. R is converted to 输入 Converted to temperature units. The current source 24 and voltage measurement device provide a resistance measurement circuit that is configured to measure resistance within a sensor resistance range tailored for a standard RTD (resistance temperature device) temperature range. Typical metals used in such RTDs include platinum, copper, and nickel.

[0035] A problem faced by many in the liquid hydrogen industry is that the combination of temperature sensor and transmitter does not work at cryogenic temperatures, such as 20 K. Typical RTD sensors made of copper, platinum or copper are not accurate at such temperatures. Other types of sensors require modifications to the circuitry of existing process temperature transmitters.

[0036] The silicon diode is one of the most common temperature sensors used at cryogenic temperatures. The voltage output of a silicon diode increases as the temperature decreases. Unfortunately, at 20K, the voltage output of a silicon diode temperature sensor is too high for many temperature transmitters.

[0037] Many positive temperature coefficient resistance temperature sensors (PTC RTDs) used for cryogenic temperatures (such as PT100 industrial PRTs and rhodium-iron RTDs) lose sensitivity at low temperatures, resulting in low resolution. Negative temperature coefficient resistance temperature (NTC RTD) sensors (such as germanium RTDs) do not have this problem. However, in order to be used with a temperature transmitter 12, the RTD must also have a resistance value within the measurement capability of the transmitter for a particular temperature range of interest. For example, 0 ohms to 2000 ohms is a typical resistance measurement range for a temperature transmitter. This is problematic for NTC RTDs, whose resistance increases exponentially as temperature decreases. In addition, packaging of cryogenic RTD elements is typically done using metal cans or ceramic packages, both of which are expensive because they must be done on a single sensor, rather than at the wafer level for mass production of the sensor itself.

[0038] Polysilicon is a commonly used material in MEM (Micro-Electro-Mechanical Systems) sensors. They are cost-effective and can be manufactured in large quantities using wafer-level processing. In MEM pressure sensors, the piezoresistive properties of polysilicon can be used to create a Wheatstone bridge circuit fabricated on a crystalline silicon diaphragm. As the crystalline silicon diaphragm deforms due to pressure, the output of the Wheatstone bridge increases. The Wheatstone bridge output is measured by the transmitter electronics.

[0039] Several electrical properties of polysilicon can be controlled by modifying the doping level. Three electrical properties of polysilicon that vary with both doping level and temperature are resistivity, TCR (temperature coefficient of resistivity), and gauge factor (a measure of piezoresistive properties). These properties are known to vary with temperature and doping level. (See " DeterminationofElectricalProperti esofn-Typeandp-TypePolvcrvstalline SiliconThinFilmsasSensorMaterials" (Sensors and Materials, Vol. 18, No. 8 (2006), pp. 433-444). )

[0040] With the present invention, polysilicon resistors are fabricated on a single crystal silicon substrate with doping levels and dimensions optimized to provide high sensitivity to temperature and minimal sensitivity to mounting and packaging effects at cryogenic temperatures between 3K and 50K. However, the temperature range can be extended to 298K, and in some embodiments, to 473K. This is beyond the range of typical cryogenic temperature sensors. Furthermore, the doping allows the sensor to have a resistance range at its operating temperature that is within the operating range of the circuitry of transmitter 12, as determined by a typical RTD-based temperature sensor. For example, the resistance range can be between 0 ohms and 2000 ohms.

[0041] The design of a polysilicon temperature sensor requires simultaneous optimization of the size of the resistors, the resistor network, and the doping level of the polysilicon. In one configuration, the doping material is phosphorus. In another configuration, the dopant is selected from a dopant group consisting of arsenic, antimony, bismuth, and lithium. In another example, the dopant is selected from a dopant group consisting of boron, aluminum, gallium, and indium. The sheet resistivity of the polysilicon can be monitored, and the resistivity varies inversely with the dopant level. For example, a series of possible polysilicon temperature sensors can be manufactured to have a sheet resistivity between 15Ω / sq and 3,500Ω / sq. In this example, a sheet resistance of 15Ω / sq is associated with an extremely high concentration of phosphorus dopants, while 3,500Ω / sq is associated with an extremely low concentration of phosphorus dopants.

[0042] A low sheet resistance of 15Ω / sq can result in a temperature sensor with a positive temperature coefficient, while a sheet resistance of 3,500Ω / sq can provide a negative temperature coefficient device. Varying the sheet resistance between these values ​​(by changing the amount of phosphorus added to the polysilicon) can provide a temperature coefficient anywhere within this range.

[0043] The polysilicon temperature sensor is arranged as a Wheatstone circuit with a bridge resistance that is temperature dependent on the sensor. The temperature coefficient of resistance of the sensor can be controlled by adjusting the sheet resistance, so that the overall resistance of the sensor is changed by adjusting the sheet resistance. To compare sensor performance, the ratio of the resistance at temperature (R) to the resistance in an ice bath (Ro) is used.

[0044] Figure 5 The resistance ratio of an example doped polysilicon sensor compared to other available RTD sensors for cryogenic applications is shown. Note that the slope of the line is equivalent to the sensitivity of the sensor. Since the sheet resistivity of polysilicon can be adjusted between 15Ω / sq and 3500Ω / sq, Figure 5 The grey triangles in the figure outline the range of possible sensors that can be made using polysilicon. The sensitivity of polysilicon sensors can range from low sensitivity (for 15Q / sq polysilicon) to more sensitive than a typical RTD sensor.

[0045] The Ro values ​​of these individual sensors are too high to be used with the temperature transmitter 12. In order to lower the Ro value, a new resistor network is provided.

[0046] The design of the sensor 30 is as follows Figure 6, and uses the 3500 ohm / sq polysilicon discussed above. Sensor 30 includes a polysilicon resistive layer 62 1 micron thick and 1 mm wide, and a metal conductor 64 made of, for example, aluminum or chromium. Conductor 64 connects the individual polysilicon resistors 62 together in parallel and to electrical connection pads 66. These components are deposited on a silicon substrate 68. To reduce the overall resistance to be compatible with temperature transmitter 12, sensor 30 uses 20 polysilicon resistors connected in parallel, resulting in an Ro value of 4 ohms, and a resistance of 2,000 ohms at 18K. Figure 7 A resistance versus temperature curve for the sensor is shown. Thus, the configuration provides a cryogenic temperature sensor that uses a doped polysilicon sensor so that its resistance varies over a cryogenic temperature range that is within the sensor resistance measurement range of the resistance measurement circuit of transmitter 12.

[0047] A temperature transmitter using a polysilicon temperature sensor is provided, which operates at a temperature from the temperature of liquid helium to 470K, by modifying the sheet resistance of polysilicon, the thickness of the polysilicon layer, and the number of parallel polysilicon resistance elements.

[0048] The polysilicon temperature sensor design can be encapsulated by adding a thick photoresist (such as polyimide or SU-8) layer 72 to protect the sensor element. Windows can be opened in the thick photoresist for soldering two leads 70 to pads 66, which in turn can be soldered to leads in the temperature sensor package. This Fig. 8A and Figure 8B Shown in.

[0049] Applying a thick photoresist at the wafer level provides an inexpensive protective layer for the temperature sensors 30 , leaving only sensor leads 70 attached to each individual sensor.

[0050] Traditional temperature sensor package designs perform well, but place a ceramic layer between the sensor element and the process that needs to be temperature measured. The relatively low thermal conductivity of ceramics results in a long response time for the temperature sensor. Fig. 9 8 is a cross-sectional view of a temperature sensor enclosure 80 in which a metal probe housing 82 carries a niobium support 84 having a niobium protrusion 86 welded to the housing 82. The housing 82 may comprise, for example, stainless steel. The sensor 30 is mounted to the support 84 using a substrate 88 formed of single crystal sapphire or alumina. Other example materials for the support 84 include Kovar and Invar.

[0051] exist Fig. 9In the configuration, the strain gauge material (polysilicon or tantalum) is deposited on a single crystal sapphire or alumina substrate 88. Alumina or sapphire can be diffusion bonded to niobium by a process called hot pressing. This bonding method produces a high-strength, hermetic bond between the bonded materials. Although the process is typically performed at a temperature close to 1100°C, the mechanical stress generated by the bonding process is very small. This is due to the almost identical coefficient of thermal expansion (CTE) between the two materials. This solid-state bonding of the sapphire or alumina substrate 88 solves the mechanical creep challenges faced by eutectic solder and achieves a higher temperature range. Additionally, the solid-state bond forms an efficient thermal conduction path between the sensor substrate 88 and the mounting material, thereby improving the time response performance. In addition, since single crystal sapphire or alumina is an electrical insulator, the temperature sensor is electrically isolated from the stainless steel probe housing 82. The substrate 88 provides an intermediate material layer and can be configured to have a thermal expansion coefficient similar to that of the silicon substrate of the sensor 30. For example, the attachment can be performed by brazing or eutectic solder. The support 84 can also be configured as such an intermediate layer. In another configuration, a silicon substrate may be bonded to a dielectric material such as glass by, for example, frit bonding. Anodic bonding may also be used to bond a borosilicate glass dielectric material to a silicon substrate. Alumina ceramics may also provide a dielectric substrate.

[0052] The niobium support 84 is attached to the temperature probe housing 82. One efficient method of doing this is by projection welding. This method of joining forms a strong connection between the sensor subassembly and the probe housing 82 while also achieving efficient heat transfer.

[0053] The embodiments of the present disclosure are described more fully below with reference to the accompanying drawings. Elements identified with the same or similar reference numerals represent the same or similar elements. In order to simplify the description, some elements may not be shown in every figure.

[0054] Various embodiments of the present disclosure may be embodied in many different forms and should not be construed as limited to the specific embodiments set forth herein. On the contrary, these embodiments are provided so that the present disclosure will be thorough and complete and will fully convey the scope of the present disclosure to those skilled in the art. As used herein, low temperature refers to temperatures below 150°C. The present invention provides a temperature sensor designed for cryogenic applications that is lower in cost than a silicon diode and has a customizable temperature response, unlike many typical RTDs. The temperature sensor set forth herein may also be optimized to work with commonly used temperature transmitters.

Claims

1. A process variable transmitter for sensing cryogenic temperatures in an industrial process, the process variable transmitter comprising: a cryogenic temperature sensor configured to be thermally coupled to an industrial process, the cryogenic temperature sensor having a resistance that changes in response to changes in a cryogenic temperature, wherein the industrial process is at the cryogenic temperature; a resistance measurement circuit electrically coupled to the cryogenic temperature sensor, the resistance measurement circuit configured to measure a sensor resistance within a sensor resistance range and responsively provide a temperature-related output based on the measured resistance; and a transmitter output circuit coupled to the measurement circuit, the transmitter output circuit configured to transmit information related to the cryogenic temperature to a remote location; Wherein the cryogenic temperature sensor comprises a polysilicon sensor containing dopants such that the cryogenic temperature sensor has a resistance that varies within a cryogenic temperature range that is within the sensor resistance range of the measurement circuit.

2. The process variable transmitter of claim 1 wherein the cryogenic temperature sensor comprises a doped polysilicon film carried on a single crystal silicon substrate.

3. The process variable transmitter of claim 1 wherein the dopant comprises phosphorus.

4. The process variable transmitter of claim 1 wherein a dopant level in the polysilicon is selected to provide a desired resistance response within the cryogenic temperature range.

5. The process variable transmitter of claim 4, wherein the sensor resistance range is for an RTD (resistance temperature detector) selected from the group consisting of a platinum RTD, a nickel RTD, and a copper RTD.

6. The process variable transmitter of claim 1 wherein the cryogenic temperature sensor comprises a polysilicon film protected by a polymer layer.

7. The process variable transmitter of claim 6, wherein the polymer layer comprises polyimide.

8. The process variable transmitter of claim 6, wherein the polymer layer comprises SU-8.

9. The process variable transmitter of claim 1 including electrical leads coupled to the cryogenic temperature sensor.

10. The process variable transmitter of claim 2 wherein the silicon substrate is attached to an intermediate material having a similar coefficient of thermal expansion. The process variable transmitter of claim 10 , wherein the intermediate material is attached to a metal housing.

12. The process variable transmitter of claim 10, wherein the silicon substrate is attached to the intermediate material by brazing.

13. The process variable transmitter of claim 10, wherein the silicon substrate is attached to the intermediate material by eutectic solder.

14. The process variable transmitter of claim 11, wherein the intermediate material is joined to the metal housing by brazing.

15. The process variable transmitter of claim 11, wherein the intermediate material is joined to the metal housing by projection welding.

16. The process variable transmitter of claim 10, wherein the intermediate material is selected from the group consisting of Kovar, Invar, and Niobium.

17. The process variable transmitter of claim 2 wherein the silicon substrate is bonded to a dielectric material.

18. The process variable transmitter of claim 17, wherein the dielectric material comprises glass.

19. The process variable transmitter of claim 17, wherein the silicon substrate and the dielectric material are bonded by frit bonding.

20. The process variable transmitter of claim 17, wherein the silicon substrate and the dielectric material are bonded by anodic bonding.

21. The process variable transmitter of claim 17, wherein the dielectric material is bonded to the intermediate material.

22. The process variable transmitter of claim 1 wherein the cryogenic temperature sensor is deposited on a dielectric substrate.

23. The process variable transmitter of claim 22, wherein the dielectric substrate material comprises alumina ceramic.

24. The process variable transmitter of claim 22, wherein the dielectric substrate is diffusion bonded to an intermediate material.

25. The process variable transmitter of claim 1 wherein the dopant is selected from the group of dopants consisting of arsenic, antimony, bismuth, and lithium.

26. The process variable transmitter of claim 1 wherein the dopant is selected from the group of dopants consisting of boron, aluminum, gallium, and indium.

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