The present invention relates, in general, to electronics, and more particularly, to semiconductors, structures thereof, and methods of forming semiconductor devices.
Light emitting diodes (LEDs) are gaining acceptance as a light source in a variety of applications that previously used incandescent light sources. In the past, complex circuits such as series-pass voltage regulators or switching voltage regulators or switching current regulators were used to provide a power source for operating the LEDs. Some examples of such power sources are disclosed in U.S. Pat. No. 6,285,139 and United States patent publication number 2007/0024259. These previous power sources contained many elements which resulted in a high cost for using an LED as a light source. In addition, many of these power sources did not provide a stable current to the LEDs as the value of the ambient temperature changed thereby causing undesirable variation in the intensity of the emitted light.
Accordingly, it is desirable to have a lower cost circuit and method that controls a current, and a circuit and method that provides a more stable current due to temperature changes.
For simplicity and clarity of the illustration, elements in the figures are not necessarily to scale, and the same reference numbers in different figures denote the same elements. Additionally, descriptions and details of well-known steps and elements are omitted for simplicity of the description. As used herein current carrying electrode means an element of a device that carries current through the device such as a source or a drain of an MOS transistor or an emitter or a collector of a bipolar transistor or a cathode or anode of a diode, and a control electrode means an element of the device that controls current through the device such as a gate of an MOS transistor or a base of a bipolar transistor. Although the devices are explained herein as certain N-channel or P-Channel devices, or certain N-type or P-type doped regions, a person of ordinary skill in the art will appreciate that complementary devices are also possible in accordance with the present invention. It will be appreciated by those skilled in the art that the words during, while, and when as used herein relating to circuit operation are not exact terms that mean an action takes place instantly upon an initiating action but that there may be some small but reasonable delay, such as a propagation delay, between the reaction that is initiated by the initial action. The use of the word approximately or substantially means that a value of an element has a parameter that is expected to be very close to a stated value or position. However, as is well known in the art there are always minor variances that prevent the values or positions from being exactly as stated. It is well established in the art that variances of up to at least ten per cent (10%) (and up to twenty per cent (20%) for semiconductor doping concentrations) are reasonable variances from the ideal goal of exactly as described. For clarity of the drawings, doped regions of device structures are illustrated as having generally straight line edges and precise angular corners.
As will be seen further hereinafter, transistor 24 and the active semiconductor device are configured so that transistor 24 can receive and conduct a current that also flows through the active semiconductor device to a common node 27 of source 20. Additionally, source 20 is configured to use temperature induced changes in the value of a voltage across the active semiconductor device to adjust the Vgs of transistor 24. For the embodiment illustrated in
Battery 11 provides power for operating LEDs 13-15 and source 20. The voltage from battery 11 forms current 17 which flows through LEDs 13-15 to source 20. Current 17 flows through transistor 24 and diode 26 to common node 27, then through terminal 22 back to battery 11. Current 17 flowing through diode 26 causes a voltage drop across diode 26 that is equal to the forward voltage of diode 26. In the preferred embodiment, the value of current 17 are selected to operate diode 26 at a point in the voltage-current (V-I) characteristic curve of diode 26 that is no less than the knee of the V-I characteristics. Additionally, the value of current 17 are selected so that transistor 24 is operating in the saturation region of the V-I characteristic curve for transistor 24.
For a given constant voltage from battery 11 and a given value of current 17, it is important to keep the value of current 17 substantially constant as temperature changes in order to keep the intensity of the light emitted by LEDs 13-15 substantially constant. The temperature increase may result from a change in the ambient environment, such as an automobile taillight that is exposed to direct sunlight that heats system 10, or it may result from heat from the operation of the LEDs or of source 20. The increased temperature of source 20 increases the internal resistance of transistor 24 thereby causing a reduction of the current that is conducted by transistor 24. The increase in temperature of diode 26 decreases the value of the voltage drop across diode 26 thereby lowering the value of the voltage applied to the source of transistor 24 (making the source closer to the voltage of node 27). Lowering the voltage applied to the source increases the Vgs (makes Vgs less negative and closer to zero) by the same absolute value as the absolute value of the change in the forward voltage drop across diode 26. The increased Vgs causes transistor 24 to conduct more current thereby minimizing the variation in the value of current 17 due to the increased temperature change. Those skilled in the art will understand that the threshold voltage of transistor 24 may vary some in response to the temperature change, but the threshold variation is much smaller than the change n the voltage across diode 26, therefore, the threshold voltage can be considered to be substantially constant. For the preferred embodiment of transistor 24 as a JFET, the less negative Vgs or increased Vgs also decreases the pinch-off, thereby reducing the resistance of the JFET, and allowing more current to flow through the channel of the JFET. As a result, the current flow through transistor 24 and source 20 remains substantially constant as the temperature increases. For the embodiment of transistor 24 as an N-channel depletion mode MOSFET, the increased Vgs causes the channel of transistor 24 to conduct more current. For example, in one embodiment diode 26 had a fifty volt (50V) reverse breakdown and transistor 24 was a JFET with the value of current 17 set to approximately thirty milli-amperes (30 mA) at twenty five (25) degrees Celsius. As the temperature increased from twenty five to one hundred twenty five (25-125) degrees Celsius, the forward voltage decreased about 0.1 to 0.2 volts which caused a corresponding 0.1 to 0.2 volt increase in the Vgs of a JFET transistor. The Vgs increase also increased the value of current 17 approximately one to three milli-amperes which represents an approximately three to ten percent 3%-10%) current compensation.
Those skilled in the art will appreciate that a decrease in temperature would decrease the internal resistance of transistor 24 thereby causing an increase in the amount of current that could be conducted by transistor 24 (for a constant Vgs). The decreased temperature of diode 26 increases the voltage drop across diode 26 thereby increasing the voltage on the source of transistor 24. Increasing the value of the voltage on the source of transistor 24 decreases the Vgs (makes Vgs more negative) which causes transistor 24 to conduct less current. As a result, the current flow through transistor 24 and source 20 remains substantially constant as the temperature decreases. As a result, the current flow through transistor 24 and source 20 remains substantially constant as the temperature decreases.
Consequently, it can be seen that the current flow through transistor 24 and source 20 remains substantially constant as the temperature increases and decreases. Typically, the value of current 17 varies at a rate of only about 0.03 to 0.08 mA/degree Centigrade depending on the size and design of the diode 26 for a temperature of about minus forty to plus one hundred twenty five (−40 to 125) degrees Centigrade for current 17 at about thirty milli-Amperes (30 mA.). For comparison, typical prior art devices have a rate of change of over 0.17 mA/degree Centigrade which usually is several times larger than the change of source 20.
An alternate embodiment of forming source 20 forms transistor 23 and diode 26 to block current flow from terminal 22 to terminal 21 thereby limiting current 17 to flow in only one direction through source 20 and light source 12. This could provide an additional advantage of preventing reverse current flow through system 10. The alternate embodiment is similar to the embodiment of
If the value of the voltage from battery 11 increases (at a given temperature), such as if battery 11 is charged, the value of current 17 would begin to increase. Because of the sharp knee of diode 26, one skilled in the art normally would expect that the change in voltage would cause the value of current 17 to increase. However, it has been found that source 20 also minimizes variations in the value of current 17 as the voltage from battery 11 increases and decreases. Because diode 26 has a sharp knee, the change of the input voltage has substantially no effect on the voltage drop across diode 26, thus, the Vgs of transistor 24 remains substantially constant. Therefore, for a set temperature value, source 20 provides the unexpected result of controlling the current through source 20 to remain substantially constant as the input voltage changes.
In one example embodiment, system 10 included three (3) serial LEDs 13-15 with each having a nominal forward voltage of about one and one-half volts to four volts (1.5V to 4.0V). Also, transistor 24 had a pinch-off voltage of approximately minus three volts (−3V), source 20 conducted a current of approximately five hundred milli-amperes (500 mA.) at room temperature and the knee of diode 26 occurred at a forward voltage of approximately 0.75 volts. The operation of system 10 was compared to a system using transistor 24 connected to a resistor instead of diode 26 such as illustrated in
As can be seen from Table 1, source 20 has the unexpected result of also minimizing the variations of current 17 due to changes in the value of the voltage used for operating source 20 and system 10 (at a given value of temperature). Furthermore, source 20 also has a lower total current consumption resulting in lower power dissipation. Table 1 indicates that, at a given temperature, source 20 controls the variation of current 17 to be no greater than about five per cent (5%) as the voltage doubles. Those skilled in the art will understand that if the value of the voltage from battery 11 decreases, then the value of current 17 would also decrease in a manner similar to that described for the increase of current 17.
It is believed that the variation in light intensity emitted by LEDs 13-15 due to temperature variations is greater than the light intensity variation due to variations of the operating voltage, thus, it is believed that minimizing the variation of current 17 over a range of temperatures, for a given value of voltage from battery 11, is important.
Substrate 70 may also include other circuits that are not shown in
Those skilled in the art will appreciate that source 20, or any of sources 30, 35, 45, 50, may be formed on an integrated circuit that includes a variety of other semiconductor elements. In such an embodiment, terminal 22 may be formed on the first surface of substrate 70. For example, terminal 22 may be formed by a connection to conductor 89 to form the electrical connection to the cathode of diode 26 wherein conductor 89 is not necessarily connected to region 74.
In view of all of the above, it is evident that a novel device and method is disclosed. Included, among other features, is forming a depletion mode FET and an active semiconductor device to control a current as the temperature increases. The configuration more accurately controls the value of the current for temperature variations than prior devices. The configuration also does not require extra circuits to apply a positive gate bias in order to form the current thereby eliminating the cost of the extra gate biasing circuitry. The positive gate bias of prior devices also requires a higher operating voltage in order to generate the positive gate bias, therefore, the instant novel device can operate from a lower voltage thereby providing a power saving advantage. In addition, the extra gate bias circuitry also consumes power, thus, the instant novel device provides another power saving advantage. It has also been found that the configuration has the unexpected result of more accurately controlling the current as the applied voltage varies than prior devices.
From the above descriptions, hose skilled in the art will understand that the previously described advantage are obtained from an embodiment of sources 20, 30, 35, and 50 that includes: first and second terminals; a first depletion mode transistor having a control electrode connected to the second terminal, a first current carrying electrode connected to the first terminal, and a second current carrying electrode; and a diode having an anode connected to the second current carrying electrode of the first depletion mode transistor, and a cathode connected to the second terminal.
Those skilled in the art will understand from the previous explanations that the previously described advantages are obtained from a method of forming sources 20, 30, 35, and 50 that includes: coupling a first FET to conduct a current from a first current carrying electrode of the first FET through the first FET; and coupling a semiconductor device that is one of a diode or a depletion mode MOSFET in series with a second current carrying electrode of the first FET wherein the current flows through a common node that is connected to a gate of the first FET and coupled to the active semiconductor device and wherein the gate is not connected to any other node.
Those skilled in the art will understand that the previously described advantages are obtained from a method of forming sources 20, 30, 35, 45, and 50 that includes: coupling a first current carrying electrode of a first FET to receive a current to conduct through the first FET; coupling an active semiconductor device that is one of a diode or a depletion mode MOSFET between a second current carrying electrode of the first FET and a common node of the current source wherein a voltage across the active semiconductor device varies from changes in temperature; and configuring the current source to use changes in a voltage across the active semiconductor device to adjust a gate-to-source voltage of the first FET.
Those skilled in the art will appreciate that a method of forming sources 20, 30, 35, 45, and 50 includes: providing a substrate of a first conductivity type and having first and second surfaces; forming a first doped region having a second conductivity type on the first surface of the substrate; forming a second doped region having the second conductivity type on the first surface of the substrate and spaced apart from the first doped region; forming a region of the first conductivity type between the first and second doped regions; forming third and fourth doped regions of the second conductivity type on the first surface and within the first doped region as respective source and drain regions of a depletion mode transistor; forming a fifth doped region having the first conductivity type on the first surface and within the first doped region wherein the fifth doped region is spaced apart from and between the third and fourth doped regions; forming a sixth doped region having the first conductivity type on the first surface and within the second doped region; forming a seventh doped region having the second conductivity type on the first surface and within the sixth doped region; and forming a first conductor to electrically couple the third doped region to the sixth doped region.
While the subject matter of the invention is described with specific preferred embodiments, it is evident that many alternatives and variations will be apparent to those skilled in the semiconductor arts. More specifically the subject matter of the invention has been described for an N-channel JFET but those skilled in the art realize that other field effect transistors (FETs) including a P-channel JFET, an N-channel depletion mode MOSFET, or a P-channel depletion mode MOSFET may also be used instead of the N-channel JFET. Additionally, a resistor may be inserted in series with the active semiconductor device to provide additional control of the current for variations of the applied voltage. Although the temperature compensated current sources are described as controlling a current through an LED, those skilled in the art will appreciate that the temperature compensated current sources can also be used for applications that require a temperature compensated current. Additionally, the word “connected” is used throughout for clarity of the description, however, it is intended to have the same meaning as the word “coupled”. Accordingly, “connected” should be interpreted as including either a direct connection or an indirect connection.