Switch mode power amplifier using MIS-HEMT with field plate extension

[0001] This application is related to co-pending application Ser. No. 11/132,619, assigned to the present assignee, which is incorporated herein by reference for all purposes. This application is related to the following co-pending applications: U.S. Patent Publication No. US20050051796A1, entitled “WIDE BANDGAP TRANSISTOR DEVICES WITH FIELD PLATES”; U.S. Patent Publication No. US20050051800A1, entitled “CASCODE AMPLIFIER STRUCTURES INCLUDING WIDE BAND GAP FIELD EFFECT TRANSISTOR WITH FIELD PLATE”; U.S. patent application Ser. No. 10/958,970, filed Oct. 4, 2004, entitled “WIDE BAND GAP FIELD EFFECT TRANSISTORS WITH FIELD PLATES”; U.S. patent application Ser. No. 10/976,422, filed Oct. 29, 2004, entitled “WIDE BAND GAP FIELD EFFECT TRANSISTOR WITH DUAL FIELD PLATES”; U.S. patent application Ser. No. 10/958,945, filed Oct. 4, 2004, entitled “WIDE BAND GAP FIELD EFFECT TRANSISTORS WITH SOURCE CONNECTED FIELD PLATES”; and U.S. patent application Ser. No. 11/078,265, filed Mar. 11, 2005, entitled “WIDE BAND GAP FIELD EFFECT TRANSISTORS WITH GATE-SOURCE FIELD PLATES”, all of which are incorporated herein by reference for all purposes.

[0002] This invention relates generally to power amplifiers, and more particularly the invention relates to switch-mode power amplifiers and transistors useful therein.

[0003] Switch mode power amplifiers have attracted a significant amount of interest for use in applications requiring highly efficient power amplification of high frequency signals. Examples of applications of such devices include power amplifiers for wireless communications systems, satellite communications systems, and advanced radar systems. In particular, high power, high frequency power amplifiers are needed for digital communication systems such as 3G and 4G PCS systems, WiFi, WiMax and digital video broadcast systems. For applications requiring high output power, the power amplifier accounts for a significant portion of the overall power consumed by the system. Thus, it is desirable to maximize the efficiency of the power amplifier circuit in a communication system.

[0004] Co-pending application Ser. No. 11/132,619, supra, discloses a single-stage switch mode amplifier circuit which includes an active device switch transistor configured to operate in either an ON state or an OFF state depending on the signal level of an input signal. The switch transistor has an output connected to a load network which filters the signal output from the switch transistor to provide a narrow-bandwidth output signal to a load impedance. Energy rejected by the load network is stored in a switch capacitor which continues to drive the output signal while the switch transistor is in the OFF state. Drain voltage to the switch transistor is provided through a drain inductor which prevents instantaneous changes in source current. In some embodiments, the amplifier operates in Class E mode.

[0005] In some embodiments, a switch mode amplifier circuit includes an input matching stage, an active stage and an output matching stage. The active stage includes an active device switch transistor in parallel with a switch capacitor. The switch transistor has an output connected to a load network and a load impedance. The output of the device, which comprises the voltage across the load impedance, is supplied to the output matching stage, which transforms the output impedance of the active stage to the desired output impedance of the circuit. In other embodiments, multiple active transistor stages and matching networks may be used to provide additional amplifier gain (e.g. 2-stage amplifiers, etc.)

[0006] The switch transistor can comprise a wide bandgap MESFET transistor capable of sustaining high drain voltages and/or high current levels while operating at frequencies in excess of 1.0 GHz. In some embodiments, the switch transistor comprises a gallium nitride (GaN) based high electron mobility transistor (HEMT). In some embodiments, the switch transistor comprises a GaN based HEMT having a total gate periphery of about 3.6 mm. In some embodiments, the switch transistor comprises a GaN MESFET. In other embodiments, the switch transistor comprises a different wide bandgap high frequency transistor, such as a SiC MESFET, SiC LDMOS, SiC bipolar transistor, or GaN MOSFET device.

[0007] Switch mode operation of field effect transistors in an amplifier requires robust operation at microwave frequencies under high compression. In practice, this is difficult to realize due to the very large forward currents that flow from gate to source under high input drive as needed for switch mode operation.

[0008] The invention is directed to a field effect transistor which can be used in a switch mode power amplifier with more robust operation under high compression.

[0009] More particularly, the transistor is a high electron mobility transistor (HEMT) which includes a gate dielectric to limit forward conduction from gate to source under high input drive and suppress gate leakage during high-voltage, high-temperature operation.

[0010] Further, a gate field plate extension can be provided to shape the peak electric field with minimum impact on added gate capacitance. Two or more dielectric layers can be employed under the field plate and provide a thicker dielectric to minimize impact on gate capacitance.

[0011] In accordance with a feature of the invention, etch selectivity between the two different insulators can be employed in fabricating the gate electrode.

[0012] The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawings.

[0017] Referring now to FIG. 1, a single-stage switch mode power amplifier circuit model 10 in accordance with an embodiment of the invention is illustrated. Amplifier 10 includes a metal-insulator-semiconductor transistor 12 comprising a wide bandgap transistor which functions as an on/off switch. The use of an insulator separating the gate from the semiconductor body limits forward conduction from gate to source under high stress drive, in some embodiments. The transistor 12 comprises a GaN HEMT. Transistor 12 may alternatively comprise a different wide bandgap high-frequency transistor, such as a SiC MESFET, GaN MESFET, SiC LDMOS, SiC bipolar transistor, or GaN MOSHFET device.

[0018] An input voltage signal vi is applied to the gate of transistor 12, which controls the state of the transistor 12. The input voltage signal vi is biased close to the pinch-off voltage of the transistor 12. The drain of the transistor 12 is coupled to an output node S, and the source of transistor 12 is coupled to ground. A supply voltage VDD is coupled to output node S via an inductor LDS. The voltage at output node S is applied to a series resonant circuit 14 which comprises an inductor L_o and a capacitor C_o. In some applications, the series resonant circuit 14 may be a bandpass circuit tuned to pass a narrow range of frequencies centered on the desired output frequency f_o of the amplifier circuit 10. In other applications such as radar applications, the series resonant circuit may be tuned to pass a broader range of frequencies. At the output frequency, the transistor output is presented with a load equal to R+jX, where X is the reactance of the resonant circuit seen at the output.

[0019] When the transistor 12 is in the on state (i.e. the transistor is saturated), the device acts as a short circuit to ground, pulling the voltage at node S to zero. Current through the inductor L_DS then increases linearly. When the transistor is turned off, the current through L_DS is steered into the drain-source capacitance C_DS, causing the voltage at node S to rise until it reaches a maximum, at which point the voltage at node S begins to decrease as the drain-source capacitance C_DS begins to source current back to the load. The resonant circuit 14 is tuned such that in steady state, the voltage at node S returns to approximately zero before the transistor is turned on again.

[0020] The resonant circuit 14 ideally passes only the fundamental frequency of the voltage at node S. The input voltage vi may carry modulated frequency or phase information that is present in the amplified output signal.

[0021] As illustrated in FIG. 2, an amplifier circuit 20 may include a Class E amplifier 10 having an input 10A and an output 10B. An input matching network 22 is coupled to the input 10A and an output matching network 24 is coupled to the output 10B of the amplifier 10. The input matching network 22 matches the impedance seen by the input signal v_i to the input impedance of the amplifier 10, while the output matching network 24 transforms the output impedance of the amplifier 10 to a desired output impedance, e.g. 50 ohms.

[0022] Referring now to FIGS. 3 and 4, two embodiments of field effect transistors in accordance with the invention are illustrated in cross section.

[0023] In FIG. 3, the transistor includes a structure which can be similar to the structure described in application Ser. No. 11/132,619, supra. For example. substrate 30 can be silicon carbide, buffer or nucleation layer 32 can be AlGaN or GaN, channel layer 34 can be InAlGaN, GaN or AlGaN, and barrier layer 36 can be a group III nitride. As described in application Ser. No. 11/132,619, buffer layer 32 on the substrate 30 provides an appropriate crystalline transition between the substrate 30 and the remainder of the device. Buffer layer 32 may include one or more layers of InAlGaN. In particular embodiments, buffer layer 32 may include GaN, AlN or AlGaN. Silicon carbide has a much closer crystal lattice match to Group III nitrides-than does sapphire (Al₂O₃), which is a very common substrate material for Group III nitride devices. The closer lattice match may result in Group III nitride films of higher quality than those generally available on sapphire. Silicon carbide also has a very high thermal conductivity so that the total output power of Group III nitride devices on silicon carbide is, typically, not as limited by thermal dissipation of the substrate as in the case of the same devices formed on sapphire. Also, the availability of semi-insulating silicon carbide substrates may provide for device isolation and reduced parasitic capacitance. Exemplary HEMT structures are illustrated in U.S. Pat. Nos. 6,316,793, 6,586,781 6,548,333 5,192,987 and 5,296,395 and U.S. Published Patent Application Nos. 2002/0167023 and 2003/0020092 each of which is incorporated by reference as though fully set forth herein.

[0024] Although semi-insulating silicon carbide is the preferred substrate material, embodiments of the present invention may utilize any suitable substrate, such as sapphire, aluminum nitride, aluminum gallium nitride, gallium nitride, silicon, GaAs, LGO, ZnO, LAO, InP and the like. In addition, the substrate may be conductive, semi-insulating or highly resistive. In embodiments comprising a MMIC, it is desirable to use a semi-insulating or highly resistive substrate. In some embodiments, an appropriate buffer layer also may be formed.

[0025] Provided on barrier layer 36 is a first dielectric layer 38 with a second dielectric layer 40 on dielectric layer 38. Contact holes are etched through layers 38,40 for a source contact 42 and a drain contact 44. A preferential etchant can be used to etch only dielectric 40 with a gate contact 46 formed on gate dielectric 38. For example, dielectric 38 can be silicon oxide and dielectric 40 can be silicon nitride. Alternatively, the two dielectric layers can be the same material which is deposited or formed at different times in the process to form two layers. Other known barrier layer materials can be employed, also.

[0026] In accordance with a feature of the invention, gate contact 46 can be extended over dielectric layer 40 towards the drain, as shown at 46′ to form a field plate extension. The field plate extension of the gate electrode towards the drain over the thicker dielectric can be designed to shape the peak electric field with minimum impact on increased gate capacitance. The use of a field plate in other applications is known.

[0027] FIG. 4 illustrates in cross-section another embodiment of a field effect transistor in accordance with the invention. Here, the substrate 30 and layers 32-36 can be the same as in FIG. 3. However, in this embodiment three dielectric layers are employed including layers 38 and 40 as in FIG. 3 along with a third dielectric layer 48. Here the gate contact opening is etched through both dielectric layers 38,40 and then the third dielectric is deposited in the gate opening. Gate metallization is then deposited on the stacked dielectric to form gate electrode 46, which can have a shorter gate length, L_G, and lower capacitance than other structures.

[0028] In FIGS. 3 and 4, dielectric 38 has a thickness d1, dielectric 40 has a thickness d2, and dielectric 48 has a thickness, d3. The thicknesses, d1, d2, and d3 are optimized to reliably support V_GD, maintain frequency response, and minimize C_gd and C_gs. Gate length, L_G, is tuned for the operational frequency of interest. In FIG. 4 dielectrics 38 and 40 can be SiO₂ and SiN or the same material, as in FIG. 3, and dielectric layer 48 can be SiO₂. For maximum benefit, the top and bottom dielectrics should be of higher bandgap than the middle dielectric.

[0029] Novel use of this device in a switch-mode amplifier enables reliable operation because it avoids large detrimental forward gate current during the part of the cycle that the transistor is on.

[0030] Published U.S. 2003/0020092A1 discloses metal contacts and insulating gate structures that can be employed in practicing the invention. Thus, while the invention has been described with reference to specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications and applications may occur to those skilled in the art without departing from the true scope and spirit of the invention as defined by the appended claims.