DIGITAL CIRCUIT USING MESFETS

Abstract: Two MESFETS (12, 16) with the drain of one connected to the source of the other are driven in complementary fashion by a single inverter using a third MESFET (14), in response to digital signals input (20) to the inverter. Means (29) for selectively disconnecting the power supply (V DP ) from the inverter to place the circuit in a low power, standby mode is provided.

Description: This invention relates to digital circuits including metal-semiconductor field-effect transistors (MESFET). Various schemes are known for driving two FETS in complementary fashion. In US-A-4 347 447 two metal-oxide semiconductor FETS (MOSFETS) are driven in complementary fashion by an inverter (see transistor 16, Figure 1 therein). In US-A-4 363 978 two enhancement mode MOSFETS serve to separately invert logic signals and separately drive two enhancement mode MOSFETS in complementary fashion. In US-A-4 380 709 depletion mode MOSFETS are driven in complementary fashion by separate inverters, each of the inverters employing a depletion mode MOSFET. Additionally, the circuits disclosed in these patents each include means for placing the circuits in a third or standby state. In US-A-4 347 447 a disable signal is provided by using two depletion mode MOSFETS. When the circuit is disabled, the output pin is left floating and a high impedance will be present at the logic output pin. In US-A-4 363 978 a "float" mode exists in which the output circuit will neither supply current to the output load or switch the output load to ground, and therefore other drivers can control the logic state of the output load. This patent provides float and non-float signals through separate enhancement mode MOSFETS. In US-A 4 380 709 a disable circuit employs four MOSFETS (including depletion and enhancement types) and leaves the output floating when activated. For many low power, high speed digital operations, MESFETS offer advantages over MOSFET circuits. This is particularly true when the substrate is GaAs instead of silicon due to the high mobility of electrons in GaAs. A digital electrical circuit using metal-semiconductor field effect transistors MESFETs as described in the pre-characterizing portion of claim 1 is known from an article "High Speed GaAs Integrated Circuits" published in Proceedings of the IEEE, Volume 70, No. 1, January 1982, pages 35 - 45, where Figure 3b shows and the article describes a pseudo-complimentary buffered inverter gate having a resistor load connected between the gate and drain electrodes of the second MESFET. It is the object of the present invention to disclose a simple MESFET circuit for driving two MESFETS in complementary fashion. Likewise, an output buffer providing a low power standby state in addition to the simple MESFET circuit is desired. The low power tristate function is an important system feature not previously disclosed for GaAs buffer circuits. These and other objects are achieved by the invention as characterized in claim 1. Preferred details and embodiments of the invention are described in the dependent claims. Providing voltage level shifting means in digital circuits is known from an article "Silicon MESFET Digital Circuit Techniques" published in IEEE Journal of Solid-State Circuits, Volume SC-16 (1981), October, No. 5, pgs. 578 to 584, where Figures 4a and 4b show circuits in all-depletion (AD) technique. The use of Schottky diodes for level shifting is described in a further article "A Gallium Arsenide SDFL Gate Array with On-Chip RAM", published IEEE Journal of Solid-State Circuits, Volume SC-19, No. 1, February 1984, pgs. 10 to 21. In the circuit according to the invention the first MESFET serves as an inverter with the logic input being implied to its gate and the logic output signal being taken from its drain. This drain of the first MESFET is connected to the gate of the second MESFET, and the logic input signal to the gate of the first MESFET is also directly supplied to the gate of the third MESFET. The logic output of the entire circuit is taken at the connection between the source of the second MESFET and the drain of the third MESFET. The logic input signal applied to the gates of the first and second MESFETs drives the second and third MESFETs in complementary fashion. The fourth MESFET selectively disconnects the electrical load from the second voltage supply. This provides a third or standby state of the circuit where the output floats and the circuit is at high impedance. Preferably, depletion mode MESFETS are employed as the first, second and third MESFETS and GaAs is the substrate material. If depletion mode MESFETS are used, the output from the third MESFET/inverter, will be level shifted to accommodate the negative threshold voltages of the depletion mode MESFETS. Further improvements and details are described in the subclaims. The invention will now be described with reference to the attached drawings, in which FIGURE 1 is the basic MESFET driver circuit of the present invention using depletion mode MESFETS; FIGURE 2 is a tri-state output buffer circuit for digital integrated circuits using depletion mode MESFETS and incorporating the circuit of Figure 1; FIGURE 3 is an alternative means for disabling the circuit of Figure 2 using an enhancement mode MESFET; FIGURE 4 is an enhancement mode MESFET version of the circuit of Figure 2; FIGURE 5 is an enhancement mode MESFET version of the circuit of Figure 1. MESFET as used herein refers to the commonly recognized structure of a metallic gate (or gates) overlying a solid substrate between ohmic source and drain.contacts. The substrate is selected to provide a flow of majority carriers between the source and drain contacts upon the application of a sufficient potential difference between the contacts. The flow of majority carriers is controlled by the potential difference between the gate and the substrate. Doped regions of a first conductivity type are provided beneath each of the source and drain contacts. The channel region beneath the metallic gate is a doped region of either a first or second conductivity type. If the channel region is doped with the first conductivity type, providing a negative bias voltage on the gate relative to the source produces a depletion mode device, and providing a positive biasing voltage on the gate relative to the source produces an enhancement mode device. If the channel region is doped with the second conductivity type and the second conductivity type is P-type, a positive gate to source potential difference produces an enhancement device, whereas if the second conductivity type is N-type a negative gate to source potential difference produces an enhancement device. Circuit 10 of Figure 1 includes first 14, second 12 and third 16 MESFETS, an electrical load 18, an input 20, a logic output 22, and a voltage reference point (e.g., ground). The drain of MESFET 12 is directly electrically connected to a first voltage supply (V DD1 ), and the source of MESFET 12 is directly electrically connected to the drain of MESFET 16 and logic output node 22. The source of MESFET 16 is directly electrically connected to ground. The drain of MESFET 14 is directly electrically connected to electrical load 18, which in turn is directly electrically connected to a second voltage supply (V DD2 ). The source of MESFET 14 is directly electrically connected to ground. The drain of MESFET 14 is also electrically connected to the gate of MESFET 12, either directly or through a voltage level shifter 24 connected between the drain of MESFET 14 and the gate of MESFET 12. Input 20 is directly electrically connected to the gates of MESFETS 14 and 16. The first and second voltage supplies may be the same supply depending on one's desires and design considerations, but the supplies are shown separately to depict the most general case. If MESFETS 12, 14 and 16 are depletion mode MESFETS, voltage level shifter 24 is generally employed to reduce "low" logic voltage values at the drain of MESFET 14 to below the threshold voltage of MESFET 12. This insures that MESFET 12 will be off when the drain voltage of MESFET 14 is low. In operation, circuit 10 receives either a high or low logic voltage signal at input 20. If the logic voltage signal applied to input 20 is high, MESFETS 14 and 16 will be turned on. The drain of MESFET 14 will be pulled near to ground, i.e., a low logic value. The low voltage at the drain of MESFET 14 will (after being shifted by voltage level shifter 24) turn MESFET 12 off. With MESFET 12 off and MESFET 16 on, logic output node 22 is pulled towards ground thereby producing a low logic output value at node 22 and for circuit 10. If the logic voltage signal applied to input 20 is low, MESFETS 14 and 16 will be turned off. The voltage at the drain of MESFET 14 will be pulled towards V DD2 , i.e., a high logic value. The high value at the drain of MESFET 14 will in turn (and after being shifted by voltage level shifter 24) turn MESFET 12 on. With MESFET 12 on and MESFET 14 off, logic output node 22 is pulled towards V DD1 thereby producing a high logic output value at node 22 and for circuit 10. From the above description of the operation of circuit 10, it is seen that MESFET 14 and load 18 serve as an inverter which is employed to drive MESFETS 12 and 16 in complementary fashion. Circuit 10 is expanded in circuit 26 of Figure 2. Corresponding components in circuits 10 and 26 are like numbered for clarity. In addition to the components of circuit 10, circuit 26 employs voltage level shifting means 24 as the first of two voltage level shifting means and separates voltage level shifting means 24 into a first resistance means 24a and a first current source 24b. Circuit 26 further includes MESFET 28, disabling means 29 (disabling means 29 is preferably MESFET 30), a second resistance means 32 and a second current source 34. Resistance means 32 and current source 34 comprise a second voltage level shifting means 35. First voltage level shifting means 24 has an input 36 and output 38, and second voltage level shifting means 35 has an input 40 and output 42. Load 18 has a first terminating region (i.e., the same node as input 36) and a second terminating region 44. Circuit 26 also includes a logic input 46 which is directly electrically connected to the gate of MESFET 28 and an enable input 48 which is directly electrically connected to the gate of MESFET 30. The drain of MESFET 30 is directly electrically connected to V DD2 , and the source of MESFET 30 is directly electrically connected to the drain of MESFET 28. The source of MESFET 28 is directly electrically connected to input 40 of the second voltage level shifting means 35. The output 42 of the second voltage level shifting means is directly electrically connected to current source 34 and to input node 20. The first terminating region 36 of the first voltage level shifting means 24 is directly electrically connected to the drain of MESFET 14. The second terminating region 38 of the first voltage level shifting means 24 is directly electrically connected to current source 24b and to the gate of MESFET 12. As stated above, disable means 29 is preferably MESFET 30. For convenience MESFET 30 is a depletion mode MESFET, load 18 is a depletion mode MESFET 50 with source and gate shorted. Further for convenience, current sources 24b and 34 are MESFETS 52 and 54, respectively, with their sources and gates shorted and the source of each of MESFETS 52 and 54 is directly electrically connected to a third voltage supply V SS which is negative relative to ground. It is further convenient to provide a Schottky diode 56 as first resistance means 24a and three series connected Schottky diodes 58, 60 and 62 as second resistance means 32. In operation, corresponding portions of circuits 10 and 26 function in the same manner. It should be noted that the current source 24b is selected so that diode 56 is always forward biased. Similarly, current source 34 keeps the series of diodes 58, 60 and 62 always forward biased. If the logic enable signal applied to input 48 is low, disable means 29 disconnects circuit 26 from voltage supply V DD2 (i.e., MESFET 30 is off). When MESFET 30 is off, circuit 26 is placed in a third standby mode. In the standby mode, logic output node 22 floats and circuit impedance is very high. Logic signals applied to input 46 will not affect node 22 when the enable signal is low. The standby mode is a particularly valuable feature which allows circuit 26 to consume as little as one tenth of the power consumed as when circuit 26 is enabled. If the logic enable signal is high, disable means 29 connects V DD2 to the drain of MESFET 28 and to load 18. In the enabled mode, logic signals applied to input 46 are either high or low. If the logic signal applied at input 46 is high, MESFET 28 is turned on pulling the source of MESFET 28 high which in turn (as described above for circuit 10) results in MESFET 16 being turned on and MESFET 12 being turned off. Logic output node 22 goes low in this case. If the logic signal applied at input 46 is low when circuit 26 is in the enabled mode, MESFET 28 is turned off dropping the drain of MESFET 28 low. This in turn (after the low voltage at the drain of MESFET 28 is further appropriately reduced by second voltage level shifting means 35) results in MESFET 16 being turned off and MESFET 12 being turned on pulling logic output node 22 high. Conveniently, diodes 56, 58, 60 and 62 are all the same. This means that the voltage levels of the outputs from the source of MESFET 28 and the drain of MESFET 14 are shifted by different amounts. This difference is needed in circuit 26 because the threshold voltages of MESFETS 12 and 16 are different due to the extra voltage drop to ground which MESFET 12 experiences (as compared to MESFET 16) as a result of leakage through MESFET 16 when MESFET 16 is off. Simulations of the performance of circuit 26 with V DD1 at 5 volts, V DD2 at 2.5 volts and V SS at -1.5 volts have yielded 0.9mw average power consumption in the standby mode and 10mw average power consumption in the enable mode. Figure 3 depicts an alternative disabling means 29 wherein an enhancement mode MESFET is employed instead of a depletion mode MESFET as MESFET 30. The alternative of Figure 3 could be used in circuit 26 or in the enhancement mode version of circuit 26 (i.e., circuit 64) shown in Figure 4. Note that corresponding components of circuits 26 and 64 are numbered the same. Circuit 64 employs only one depletion mode MESFET (i.e., 50), and does not require MESFETS 28 and 54 or diodes 58, 60 and 62, which served to shift the logic input voltage level in circuit 26. An enhancement mode MESFET version of circuit 10 is shown as circuit 66 in Figure 5. Again corresponding components of circuits 10 and 66 are like numbered. Note that in circuit 64 and 66 the voltage level shifting means are deleted because the output voltages need not be compensated for negative threshold voltages when enhancements mode MESFETS are being switched.

Claims: A digital electrical circuit using metal-semiconductor field effect transistors (MESFETs) having an input (20, 46) receiving a digital electrical signal of either a first or second value, a logic output node (22) and a voltage reference point (ground), and being adapted to be connected to a supply voltage (V DD2 ; V DD1 ), comprising: a) a first MESFET (14) for inverting said digital electrical signal applied to said input (20) to produce an inverted digital electrical signal, said first MESFET (14) having a first source electrode, a first drain electrode and a first gate electrode, wherein said first gate electrode is electrically connected to said input (20, 46), said first drain electrode is electrically connected to a voltage supply (V DD2 ), said first source electrode is directly electrically connected to said voltage reference point (ground) and said inverted digital electrical signal is generated at said first drain electrode; b) a second MESFET (12) having a second source electrode, second drain electrode and a second gate electrode, wherein said second drain electrode is directly electrically connected to a voltage supply (V DD1 ); c) a third MESFET (16) having a third source electrode, third drain electrode and a third gate electrode wherein said digital electrical signal applied to said input (20, 46) is also applied to said third gate electrode, said third source electrode is directly electrically connected to said voltage reference point (ground) and said third drain electrode is directly electrically connected to said second source electrode at said logic output node (22); and wherein d) application of said digital electrical signal of said first value ("0") to said input (20, 46) will turn off said first (14) and third (16) MESFETs, and application of said digital electrical signal of said second value ("1") to said input (20) will turn on said first (14) and third (16) MESFETs; characterized by e) said first drain electrode being connected to a first voltage supply (V DD2 ) and said second drain electrode being connected to a second voltage supply (V DD1 ); f) voltage level shifting means (24) for shifting the voltage level of said inverted digital signal to a lower voltage inverted digital signal, said lower voltage inverted digital signal being applied to said second gate and being sufficient to turn on said second MESFET (12) when said digital electrical signal of said first value ("0") is applied to said input (20, 46), and said lower voltage inverted digital signal being sufficient to turn off said second MESFET (12) when said digital electrical signal of said second value ("1") is applied to said input; and g) a fourth MESFET (30) having fourth source, fourth drain and fourth gate electrodes, wherein said fourth drain electrode is directly electrically connected to said first voltage supply (V DD2 ), said fourth gate electrode is directly electrically connected to an enable input (48) and said fourth source electrode is electrically connected to said first drain electrode. The circuit of claim 1, characterized in that the inverting means comprises an electrical load (18) having first (36) and second (44) terminating regions; said first drain electrode is directly electrically connected to said first terminating region (36) and said second terminating region (44) is directly electrically connected to said first voltage supply (V DD2 ). The circuit of claim 2, characterized by a) the first voltage level shifting means (24a) having a first input node (36) and a first output node (38); b) the first terminating region (36) being directly electrically connected to said first input node (36); c) a logic input (46) and the enable input (48); d) second voltage level shifting means (32) having a second input node (40) and a second output node (42); e) first (34) and second (24b) current sources, wherein said first current source (34) is directly electrically connected to said second output node (42) and said second current source (24b) is directly electrically connected to said first output node (38); f) said fourth electrode being directly electrically connected to said second terminating region (44); g) a fifth MESFET (28) having fifth source, fifth drain and fifth gate electrodes, wherein the fifth drain electrode is directly electrically connected to said second terminating region (44) and to the fourth source electrode, said fifth gate electrode is directly electrically connected to said logic input (46), and said fifth source electrode is directly electrically connected to said second input node (40); h) said first gate electrode being directly electrically connected to said first current source (34) and to said second output node (42); i) said second gate electrode being directly electrically connected to said first output node (38) and to said second current source (24b), and said fourth source electrode being directly electrically connected to said logic output node (22); and j) said third gate electrode being directly electrically connected to said second output node (42) and to said first gate electrode (Fig. 2). The circuit of claim 3, characterized in that said second voltage level shifting (32) means comprises first (58), second (60) and third (62) Schottky diodes, each diode having an anode and a cathode, said first, second and third diodes being connected in series, with the anode of said first diode (58) being said second input node (40) and the cathode of said third diode (62) being said second output node (42). The circuit of claim 4, characterized in that said first voltage level shifting means (24a) comprises a fourth diode (56) with an anode and a cathode, wherein the anode of said fourth diode is said first input node (36) and the cathode of said fourth diode is said first output node (38). A circuit according to claim 3, 4 or 5, being adapted to be connected to a third voltage supply (-V SS ), characterized in that said first current source (34) includes a sixth MESFET (54) having a sixth source, a sixth drain and a sixth gate electrode, wherein said sixth source electrode is directly electrically connected to said third voltage supply (V SS ) and to said sixth gate electrode, and said sixth drain electrode is directly electrically connected to said second output node (42), to said first gate electrode and said third gate electrode. The circuit of claim 6, characterized in that said second current source (24b) includes a seventh MESFET (52) having seventh source, seventh drain, and seventh gate electrodes, wherein said seventh source electrode is directly electrically connected to said third voltage supply (-V SS ) and to said seventh gate electrode, and said seventh drain electrode is directly electrically connected to said first output node (38). The circuit of claim 7, characterized in that said electrical load (18) is an eighth MESFET (50) having eighth source, eighth drain and eighth gate electrodes, wherein said eighth drain electrode is said second terminating region (44), said eighth source electrode is said first terminating region (36) and is directly electrically connected to said eighth gate electrode. The circuit according to one of the claims 1 to 8, characterized in that said MESFETS are depletion mode MESFETS (Fig. 1,2). The circuit according to one of the claims 1 to 8, characterized in that said MESFETS are enhancement mode MESFETS (Fig. 3,4,5). A circuit according to one of the preceding claims, characterized in that said first (V DD2 ) and second (V DD1 ) voltage supplies are the same voltage supply.

Inventors: NELSON, RODERICK D., VU, THO T.
Applicants: HONEYWELL INC.
Application year: 1985
Main classification: H03K19
IPC codes: H03K19