Abstract:
A gallium arsenide (GaAs) bandgap circuit includes a plurality of stacked GaAs transistors being connected as Schottky diodes which, together with an amplifier, provide a constant reference voltage being independent of a power supply voltage of the circuit.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a bandgap voltage reference circuit, and more particularly, to a bandgap voltage reference circuit used in a gallium arsenide (GaAs) based semiconductor chip. 
     BACKGROUND OF THE INVENTION 
     Generally, “bandgap” is a term used in physics and its related semiconductor technology. In physics, when the distance between two atoms approaches the equilibrium interatomic spacing of a diamond lattice, energy level splits into two bands. The two bands are separated by a region which designates energies that the electrons in a solid, such as a type of semiconductor material, cannot possess. The region is referred to a forbidden gap, or a bandgap, for this type of semiconductor material. Any change of thermal energy, electron or photon energy may affect the width of a bandgap. For example, any increase in temperature, electron or photon energy will tend to narrow a bandgap, and similarly, any decrease in temperature, electron or photon energy will tend to widen the bandgap. In addition, depending on the types of semiconductor materials, a bandgap can be wide for one type of material but narrow for another type. For example, silicon generally has a much wider bandgap than gallium arsenide (GaAs). 
     Many semiconductor devices, such as diodes, bipolar transistors, BiCOMs Field Effect Transistors (FETs) etc., have used bandgap characteristics of a particular semiconductor material, such as silicon. In these devices, such as a diode, a positive electrical charge can narrow the bandgap, and a negative electrical charge can widen the bandgap. In a certain operating region of a device or a circuit containing a plurality of these semiconductor devices, a bandgap can be wide enough such that a voltage at one point of the circuit is stable independent of an applied power supply. The stable voltage at that point is often used as a voltage reference, and a circuit used for designing such a stable reference voltage is often referred to as a bandgap voltage reference circuit. 
     In silicon bipolar, BiCOMs related technologies, a bandgap circuit employing bipolar transistors has been used to provide stable reference voltages for many years in semiconductor industry. 
     In recent years, gallium arsenide (GaAs) based semiconductor chips have become more and more utilized in semiconductor industry. In such GaAs chips, where bipolar transistors are not an option, it is generally difficult to design a bandgap circuit to provide a reference voltage which is independent of a power supply voltage of the circuit. 
     Based on the physics characteristics of the semiconductor materials described above, it is generally known that a bandgap voltage reference circuit (or in short, “a bandgap circuit”) can be built from the exponential relation between the voltage and the current in an emitter junction of a bipolar transistor. It is also known that a Field Effect Transistor (FET) GaAs-based transistor exhibits a square-law relation between the voltage and the current. As a result, FET GaAs-based transistors generally do not meet requirements to build a bandgap reference circuit. 
     FIG. 1 illustrates a conventional bandgap circuit built from bipolar transistors Q 1 -Q 4 . The reference introducing this type of conventional bandgap circuit can be made to an article authored by A. P. Brokaw, published in IEEE Journal of Solid State Circuits, Vol. SC-9, pp. 388-393, December 1974, entitled “A Simple Three-Terminal IC Bandgap Reference”. In this conventional bandgap circuit, the relation of the currents (I) and resistors (R) are as follows: 
     I 0 =I 1 ; 
     I 2 =I 3 =I 4 ; 
     R 1 =R 2 =R 3 ; and 
     R 5 =R 6 . 
     In addition, an amplifier, AMP, has two inputs that are connected to nodes n 1  and n 2 , respectively. With appropriate values of the resistors, the amplifier, and the bipolar transistors, the bandgap circuit makes use of the fixed voltage difference between the base and the emitter of the bipolar transistors, which operate at different current densities, to produce a stable output voltage Vout at node n 0 , i.e. the bandgap circuit output Vout or Vn 0  is independent of a power supply voltage Vdd of the amplifier. Thus, the stable Vn 0  is used as a reference voltage. 
     Accordingly, there is a need for a bandgap circuit built from FET GaAs-based transistors to provide a stable reference voltage which is independent of a power supply voltage of the circuit. 
     SUMMARY OF THE INVENTION 
     The present invention relates generally to a bandgap voltage reference circuit, and more particularly, to a bandgap voltage reference circuit used in a gallium arsenide (GaAs) based semiconductor chip. 
     The present invention provides a GaAs circuit which uses stacked FETs in which the source and drain of each FET are connected together to form a first terminal and the gate forms a second terminal (an FET configured in this manner being referred to herein as a “Schottky diode”) and an amplifier in an arrangement to provide a stable voltage reference independent of a power supply voltage of the circuit. 
     In one embodiment of the present invention, the GaAs circuit includes a plurality of GaAs FETs arranged as Schottky diodes being connected to a plurality of resistors, wherein the GaAs circuit is arranged such that a voltage output is independent of a voltage input of the circuit. 
     One aspect of the present invention is that the gate of each Schottky diode, which is a metal, forms a Schottky junction with the source and the drain of the Schottky diode. The gate is the anode, and the source and the drain are tied together to form the cathode. Each Schottky diode exhibits an exponential relation between the current flowing through the Schottky diode and the voltage across the Schottky diode. 
     Another aspect of the present invention is that the Schottky diodes are stacked in branches which are electrically connected in parallel. In one embodiment, Schottky diodes are stacked in a first branch and a second branch. The first and second branches are electrically connected in parallel to each other between a voltage output node and ground. In the first branch, a first resistor is electrically connected between the voltage output node and a first node. In the second branch, a second resistor is electrically connected between the voltage output node and a second node. An amplifier has a first input electrically connected to the first node, a second input electrically connected to the second node, and an output electrically connected to the voltage output node. The first branch includes a plurality of sub-branches, such as four sub-branches, of Schottky diodes electrically connected in parallel between the first node and the ground. In each sub-branch, a first Schottky diode is electrically connected to the first node at one end and to a sub-first node at the other end, a second Schottky diode is electrically connected to the sub-first node at one end and to a sub-second node at the other end, and a third resistor is electrically connected to the sub-second node at one end and to the ground at the other end. In the second branch, a first Schottky diode is electrically connected to the second node at one end and to a third node at the other end, and a second Schottky diode is electrically connected to the third node at one end and to the ground at the other end. 
     A further aspect of the present invention is that the current in the first branch and the current in the second branch are the same, and the currents in each sub-branch are the same. Accordingly, the current in each sub-branch is one-fourth (¼) of the current in the second branch. In one embodiment, the first and second resistors have the same resistance, the Schottky diodes of each branch and sub-branch are the same, and the resistors of each sub-branch have the same resistance. 
     An additional aspect of the invention is that the gain of the amplifier and the values of the Schottky diodes and the resistors can be selected such that the voltage output is stable. 
     One advantage of the present invention is that it provides a stable reference voltage in a GaAs-based semiconductor chip. Another advantage of the present invention is that the values of the circuit elements can be selected to provide different stable reference voltages to meet different voltage needs. The stable reference voltages are independent of the input voltage of the circuit, such as the power supply voltage of the amplifier. 
     The solution proposed by the present invention can be used in many industries, such as telecommunication industry, etc. For example, the present invention can be used by makers of Radio Frequency (RF) wireless systems, cell phones, or chips for optical link systems which use GaAs chips. 
     These and other features and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description and corresponding drawings. As will be realized, the invention is capable of modification without departing from the invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
     FIG. 1 is a circuit diagram of a conventional bandgap circuit built from bipolar transistors; 
     FIG. 2 is a circuit diagram of a bandgap circuit built from FET gallium arsenide (GaAs) transistors in accordance with the present invention; 
     FIG. 3 is a plot of the input-output voltages of the bandgap circuit built from FET GaAs transistors shown in FIG. 2; 
     FIG. 4 is a circuit diagram of a second bandgap circuit built from FET GaAs transistors in accordance with the present invention; and 
     FIG. 5 is a plot of the input-output voltages of the second bandgap circuit built from FET GaAs transistors shown in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates generally to a bandgap voltage reference circuit, and more particularly, to a bandgap voltage reference circuit used in a gallium arsenide (GaAs) based semiconductor chip. 
     In the following description of the exemplary embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration the specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized as structural changes may be made without departing from the scope of the present invention. 
     Referring to FIG. 2, a gallium arsenide (GaAs) bandgap circuit  40  is shown. An amplifier AMP  41  has a first input which is electrically connected to node n 1 , a second input which is electrically connected to node n 2 , and an output which is electrically connected to an output node n 0  of the circuit  40 . A first resistor R 1  is electrically connected between node n 1  and node n 0 . A second resistor R 2  is electrically connected between node n 2  and node n 0 . The first and second resistors R 1 , R 2  are stacked in first and second branches, respectively. Two Schottky diodes N 5  and N 12  are stacked with the second resistor R 2  in series in the second branch. Schottky diode N 5  is electrically connected between node n 2  and node n 21 , and Schottky diode N 12  is electrically connected between node n 21  and ground. 
     In the first branch, between node n 1  and ground, it has four sub-branches. In a first sub-branch, two Schottky diodes N 1 ,N 8  and a resistor R 3  are stacked in series, wherein Schottky diode N 1  is electrically connected between node n 1  and node n 3 ; Schottky diode N 8  is electrically connected between node n 3  and n 4 ; and the resistor R 3  is electrically connected between node n 4  and ground. In a second sub-branch, two Schottky diodes N 2 ,N 9  and a resistor R 4  are stacked in series, wherein Schottky diode N 2  is electrically connected between node n 1  and node n 5 ; Schottky diode N 9  is electrically connected between node n 5  and n 6 ; and the resistor R 4  is electrically connected between node n 6  and ground. In a third sub-branch, two Schottky diodes N 3 ,N 10  and a resistor R 5  are stacked in series, wherein Schottky diode N 3  is electrically connected between node n 1  and node n 7 ; Schottky diode N 10  is electrically connected between node n 7  and n 8 ; and the resistor R 5  is electrically connected between node n 8  and ground. In a fourth sub-branch, two Schottky diodes N 4 ,N 11  and a resistor R 6  are stacked in series, wherein Schottky diode N 4  is electrically connected between node n 1  and node n 9 ; Schottky diode N 11  is electrically connected between node n 9  and n 10 ; and the resistor R 6  is electrically connected between node n 10  and ground. 
     The amplifier  41  is a differential-input, single-ended-output amplifier with a gain of 10 or higher, and preferably 20 or higher, for greater accuracy. The loop containing the amplifier, R 1 , and R 2  is balanced when the voltages at nodes n 1  and n 2  are equal, i.e. when resistors R 1  and R 2  are conducting equal currents. The amplifier  41  is able to adjust the voltage at node n 0  until the currents flowing through the resistors R 1 ,R 2  are equal. The amplifier  41  can be a micro-chip circuit which is used to be integrated into the entire chip. It will be appreciated that the amplifier  41  can be any types of operational amplifiers with suitable gains, without departing from the principles of the present invention. 
     The Schottky diodes, N 1 -N 4  and N 8 -N 11 , are electrically connected to the resistors R 3 -R 6 , respectively. Schottky diodes N 1 -N 4  and N 8 -N 11  operate at ¼ the current density of Schottky diodes N 5  and N 12 . Accordingly, each of the diodes N 1 -N 4  and N 8 -N 11  has a smaller forward voltage drop, for example, about 36 mV smaller than the forward voltage drop of Schottky diode N 5  or N 12 . Since the two diodes in each diode pair N 1 N 8 , N 2 N 9 , N 3 N 10 , or N 4 N 11  are connected in series, the total difference in voltage drops is the sum of the two differences, for example, totally 72 mV. At a certain operating current, the voltage drop across resistors R 3 -R 6  is also 72 mV, respectively. Accordingly, the resistors R 3 -R 6  allow that the voltage at node n 1  is equal to the voltage at node n 2 . It will be appreciated that other arrangements of the Schottky diodes can be used within the scope of the present invention. For example, two or three, or more than four sub-branches, e.g. ten sub-branches, can be used. In such cases, the value of the forward voltage drop may vary accordingly. 
     FIG. 3 shows an input-output voltage diagram of the exemplary bandgap circuit  40  illustrated in FIG.  2 . In this case, the input voltage of the bandgap circuit is the power supply voltage of the amplifier, i.e. Vdd. The output voltage of the bandgap circuit is the voltage at the node n 0 . As Vdd increases, the output voltage at node n 0  increases first and then remains approximately 2.328 volts. At that point on, the output voltage at node n 0  is independent of the power supply voltage Vdd, for example, the power supply voltage over 4.0 volts, such as between 4.0 volts and 6.0 volts. Accordingly, the GaAs bandgap circuit of FIG. 2 is applicable for a power supply voltage around 5.0 volts, whereby a reference voltage about 2.3 volts can be obtained. 
     In the preferred embodiment, the values of R 1  and R 2  are selected to be about 10,000 ohms, and the values of R 3 , R 4 , R 5 , and R 6  are selected to be about 1,000 ohms. 
     FIG. 4 illustrates another bandgap circuit  42  which is applicable for a power supply voltage around 3.3 volts. An amplifier AMP  43  is a differential-input, single-ended-output amplifier with a gain of 10 or higher, and preferably 20 or higher, for greater accuracy. Similar to the amplifier in FIG. 2, in FIG. 4, the loop containing the amplifier  43 , R 1 , and R 2  is balanced when the voltages at nodes is n 1  and n 2  are equal, i.e. when resistors R 1  and R 2  are conducting equal currents. The amplifier  43  is able to adjust the voltage at node n 0  until the currents flowing through the resistors R 1 ,R 2  are equal. The amplifier  43  can be a micro-chip circuit which is used to be integrated into the entire chip. It will be appreciated that the amplifier  43  can be any types of operational amplifiers with suitable gains, without departing from the principles of the present invention. 
     FIG. 5 shows an input-output voltage diagram of the exemplary bandgap circuit  42  illustrated in FIG.  4 . In this case, the input voltage of the bandgap circuit is the power supply voltage of the amplifier  43 , i.e. Vdd. The output voltage of the bandgap circuit is the voltage at the node n 0 . As Vdd increases, the output voltage at node n 0  increases first and then remains approximately 1.7858 volts. At this point on, the output voltage at node n 0  is independent of the power supply voltage Vdd, for example, the power supply voltage over 3.1 volts, such as between 3.1 and 4.0 volts. Accordingly, the GaAs bandgap circuit of FIG. 4 is applicable for a power supply voltage around 3.3 volts, whereby a reference voltage about 1.8 volts can be obtained. 
     In the preferred embodiment shown in FIG. 4, the values of the resistors R 1  and R 2  are selected to be about 5,000 ohms, and the values of the resistors R 3 , R 4 , R 5 , and R 6  are selected to be about 4,000 ohms. The arrangement of the Schottky diodes and the resistors in FIG. 4 are the same as those arranged in FIG.  2 . It will be appreciated that other types of arrangements of the Schottky diodes and the resistors can be used in FIGS. 2 and 4 within the principles of the present invention. For example, the resistors in the sub-branches can be placed between node n 1  and the first Schottky diode or between the first and second Schottky diodes. It will also be appreciated that more than one resistor can be used in each sub-branch. 
     Further, it will be appreciated that the numbers of Schottky diodes can be varied without departing from the principles of the invention. For example, in the second branch and the sub-branches, one Schottky diode is used. In this case, a lower power supply voltage is used, and a lower reference voltage, i.e. a reference voltage lower than 1.8 volts, can be obtained. Another example would be that in the second branch and the sub-branches, more than two Schottky diodes connected in series are used. In this case, a higher power supply voltage is used, and a higher reference voltage, i.e. a voltage higher than 2.3, can be obtained. 
     Furthermore, it will be appreciated that the other types of GaAs-based circuit elements can be used without departing from the principles of the present invention. For example, a GaAs-based circuit element which exhibits an exponential relation between a current flowing through the circuit element and a voltage across the circuit element can be used within the scope of the present invention. 
     The present invention has been described in its presently contemplated best mode, and it is clear that it is susceptible to various modifications, modes of operation and embodiments, all within the ability and skill of those skilled in the art and without the exercise of further inventive activity. Further, while the invention has been described in connection with what is presently considered the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.