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  Silicon nitride passivation with ammonia plasma pretreament for improving reliability of algan/gan hemtsUSPTO Application #: 20080105880Title: Silicon nitride passivation with ammonia plasma pretreament for improving reliability of algan/gan hemts Abstract: This invention pertains to an electronic device and to a method for making it. The device is a heterojunction transistor, particularly a high electron mobility transistor, characterized by presence of a 2 DEG channel. Transistors of this invention contain an AlGaN barrier and a GaN buffer, with the channel disposed, when present, at the interface of the barrier and the buffer. Surface treated with ammonia plasma resembles untreated surface. The method pertains to treatment of the device with ammonia plasma prior to passivation to extend reliability of the device beyond a period of time on the order of 300 hours of operation, the device typically being a 2 DEG AlGaN/GaN high electron mobility transistor with essentially no gate lag and with essentially no rf power output degradation. (end of abstract) Agent: Naval Research Laboratory Associate Counsel (patents) - Washington, DC, US Inventors: Andrew P. Edwards, Steven C. Binari, Jeffrey A. Mittereder USPTO Applicaton #: 20080105880 - Class: 257076000 (USPTO) Related Patent Categories: Active Solid-state Devices (e.g., Transistors, Solid-state Diodes), Specified Wide Band Gap (1.5ev) Semiconductor Material Other Than Gaasp Or Gaalas The Patent Description & Claims data below is from USPTO Patent Application 20080105880. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a divisional application of U.S. application Ser. No. 11/311,592, filed Dec. 9, 2005. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] This invention pertains to improving reliability of heterojunction transistors, particularly high electron mobility transistors, with an ammonia plasma pretreatment prior to passivation. [0004] 2. Description of Related Art [0005] Prior methods which have been used to prepare processed AlGaN/GaN high electron mobility transistors (HEMTs) for reliable operation in the past have included using no passivation at all, or direct deposition of a variety of electrically insulating materials intended to passivate surface states. These material films can be deposited by such processes as plasma enhanced chemical vapor deposition. Silicon nitride (SiN) is one of the most commonly used surface passivating films for AlGaN/GaN HEMTs and can be deposited onto the device by the aforementioned method. However, silicon nitride passivation, while an improvement over no passivation at all, still results in a decrease in performance of the device after dc and rf bias stress. This is a significant limitation and disadvantage in the reliability of AlGaN/GaN HEMT electronic devices. [0006] AlGaN/GaN high electron mobility transistors have shown exceptional microwave power output densities, with a recently reported continuous wave power density of 30 W/mm and 50% power added efficiency at a frequency of 8 GHz, In addition, a 36-mm gate-width GaN HEMT has been demonstrated with a total power output of 150 W and a power added efficiency of 54%. However, device reliability remains a major concern for III-N HEMTs. In AlGaN/GaN HEMTs, degradation of the dc, transient, and microwave characteristics are often seen after relatively short periods of normal device operation. Although reliability is improving, microwave power output typically degrades by more than 1 dB in less than 1000 hours of operation. OBJECTS AND BRIEF SUMMARY OF THE INVENTION [0007] It is an object of this invention to pretreat an AlGaN/GaN heterojunction field effect transistor with ammonia plasma prior to passivation in order to improve reliability thereof. [0008] Another object of this invention is to pretreat an AlGaN/GaN transistor with a low-power ammonia plasma prior to silicon nitride deposition for high power applications. [0009] Another object of this invention is to improve reliability of an AlGaN/Gan transistor adapted for use in radars and communication equipment. [0010] Another object of this invention is to improve reliability of AlGaN/GaN HEMTS characterized by the presence of 2 DEG channel. [0011] Another object of this invention is prevention of drain current collapse caused by electron traps. [0012] Another object of this invention relates to reduction of surface and bulk traps in high power and high electron mobility AlGaN/GaN heterojunction transistors that can potentially operate at voltages exceeding 100 volts and with electron mobility in excess of 1000 cm.sup.2/v sec. [0013] These and other objects of this invention can be attained by pretreatment of a 2 DEG AlGaN/GaN heterojunction transistor with a low-power ammonia plasma prior to passivation. BRIEF DESCRIPTION OF THE DRAWINGS [0014] FIG. 1 is a cross-sectional schematic representation of a high mobility transistor with AlGaN, GaN and doped GaN layers deposited on a silicon carbide substrate. [0015] FIGS. 2 (a) and (b) show induced current collapse of unpassivated HEMT in (a) before stress and after stress and in (b) of passivated HEMT devices with silicon nitride only and with silicon nitride passivation, stressed for 60 hours and 176 hours, respectively. [0016] FIG. 3 is a plot of normalized Drain Current versus Time, showing drain current response to pulsed gate voltage, in (a) with no passivation; in (b) for the same device after 64-hour stress; in (c) silicon nitride passivated device after 80-hour stress; and in (d) a device pretreated with ammonia plasma and passivated with silicon nitride, after 176-hour stress. [0017] FIG. 4 is a plot of power _P.sub.out versus Time showing rf output degradation of (a) passivated device with silicon nitride only and (b) pre-treated with ammonia plasma and coated or pre-treated with silicon nitride wherein stress conditions were 20v, 200 mA/mm operated at 2 GHz at 1 dB compression of the gain. DETAILED DESCRIPTION OF THE INVENTION [0018] The purpose of this invention, in a preferred embodiment, is to improve the reliability of aluminum gallium nitride/gallium nitride (AlGaN/GaN) high electron mobility transistors by incorporating an ammonia (NH.sub.3) plasma pre-treatment prior to silicon nitride (SiN) passivation of the heterojunction transistors after all other processing has been completed. [0019] This invention pertains to an electronic device and to a method for making it. The device is a heterojunction transistor, particularly a high electron mobility transistor characterized by the presence of a 2 DEG channel. Transistors of this invention contain an AlGaN barrier and a GaN buffer, with the channel disposed, when present, at the interface of the barrier and the buffer. The method pertains to treatment of the device with ammonia plasma prior to passivation to extend reliability of the device beyond a period of time on the order of 300 hours of operation, the device typically being a 2 DEG AlGaN/GaN high electron mobility transistor with essentially no gate lag and with essentially no rf power output degradation. Continue reading... Full patent description for Silicon nitride passivation with ammonia plasma pretreament for improving reliability of algan/gan hemts Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Silicon nitride passivation with ammonia plasma pretreament for improving reliability of algan/gan hemts patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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