Dongmin Liu

Columbus State University, Columbus, Georgia, United States

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Publications (13)18.35 Total impact

  • Dongmin Liu, Jaesun Lee, W. Lu
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    ABSTRACT: The effects of post gate annealing on microwave noise performance of AlGaN/GaN HEMTs on SiC substrate were investigated. The results show that post Schottky gate annealing under an optimized condition, 400°C for 10min in N2 ambient, can increase the current driving capability, reduce gate leakage current, and improve the microwave noise performance. Specifically, the maximum extrinsic transconductance increased from 223mS/mm to 233mS/mm. The 1mA/mm gate leakage current at Vgs=−30V reduced to 4nA/mm. Before annealing, the device exhibited a minimum noise figure of 0.99dB at 4GHz. It decreased to 0.63dB and the associated gain increased from 13.2dB to 17.4dB at the mean time. The change is even more significant under high current operation. At 4GHz, the 4.90dB NFmin at Ids of 670mA/mm decreased more than 3dB to 1.12dB. Based on a simple noise model, the improvement of the microwave noise performance is attributed to the significant decrease of the gate leakage current.
    Solid-State Electronics 01/2007; 51(1):90-93. · 1.48 Impact Factor
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    ABSTRACT: The dependence of direct current and microwave performance of InGaAs/InAsP composite channel HEMTs on gate length is presented experimentally. Composite channel HEMTs with gate length from 1.13 μm to 0.15 μm were fabricated. Device characterization results showed the extrinsic transconductance increased from 498 mS/mm for 1.13 μm devices to 889 mS/mm for 0.15 μm gate devices, while the unity current gain cutoff frequency increased from 24 GHz to 190 GHz. A simple delay time analysis is employed to extract the effective carrier velocity (veff) of the composite channel. The veff is determined to be 1.9 × 107 cm/s. To our knowledge, this is the first systematic study on gate length scaling effect of composite channel HEMTs.
    Solid-State Electronics 01/2007; · 1.48 Impact Factor
  • Hyeongnam Kim, Jaesun Lee, Dongmin Liu, Wu Lu
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    ABSTRACT: Gate leakage/breakdown mechanism in unpassivated AlGaN / GaN high electron mobility transistors (HEMTs) is investigated by performing temperature-dependent pulsed current–voltage (I–V) and current transient measurements of AlGaN / GaN HEMTs without and with annealing after Schottky gate formation. After post-gate annealing, the devices exhibited significantly smaller gate leakage current and higher breakdown voltage even without any gate dielectrics or passivation layer. The temperature-dependent current transient measurements show that the current dispersion in the unannealed HEMTs is attributed to traps with an emission time constant (t<sub>E</sub>) of ∼0.5 μ s at 295 K and an activation energy of ∼38 meV . On the contrary, the 20-min annealed devices have traps with t<sub>E</sub> of 21.6 μ s at 295 K and an activation energy of ∼0.31 eV . The results suggest that the post-gate annealing removes shallow traps, and creates or activates deeper traps. We propose that the breakdown and gate leakage current is mainly due to the emission current from shallow traps in unpassivated AlGaN / GaN HEMTs. The breakdown voltage improvement after the post-gate annealing is due to the removal of shallow traps near the Schottky gate metal /<font face='roman' || '>AlGaN interface.
    Applied Physics Letters 05/2005; · 3.79 Impact Factor
  • Dongmin Liu, Jaesun Lee, Wu Lu
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    ABSTRACT: In this paper, we report the temperature dependence of microwave noise performances of AlGaN/GaN high electron mobility transistors (HEMTs) up to 573 K and the influences of post-gate-annealing (PGA) technique on temperature dependant characteristics of HEMTs. From room temperature (R.T.) to 573 K, the maximum current of devices with PGA decreased from 998 mA/mm at R.T. to 478 mA/mm while maximum current of devices without PGA dropped from 848 mA/mm at R.T to 641 mA/mm at 393 K. The gate leakage current of devices with PGA is four orders smaller than that of devices without PGA. For RF performance, the cut-off frequency (fT) and the maximum frequency of oscillation (fmax) of devices decreased significantly with increasing temperature. In noise measurements, annealed devices exhibited significant improvements in minimum noise figure (NFmin) and associated gains (Ga). Specifically, NFmin and Ga at 10 GHz are measured at different temperatures. At R.T., the devices after PGA showed an NFmin of 1.16 dB that was 0.7 dB lower than the un-annealed devices. At 573 K, the devices with PGA still exhibit excellent characteristics with an NFmin of 3.94 dB and a Ga of 7.02 dB while devices without PGA can only work up to 393 K. To our knowledge, this is the first report of microwave noise performances of AlGaN/GaN HEMTs for substrate temperatures higher than 473 K. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 03/2005; 2(7):2659 - 2662.
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    ABSTRACT: Not Available
    Semiconductor Device Research Symposium, 2005 International; 02/2005
  • 01/2005;
  • Jaesun Lee, Dongmin Liu, Hyeongnam Kim, Wu Lu
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    ABSTRACT: The effects of postprocessing annealing on direct current, radio frequency small signal, and power performances of AlGaN / GaN high electron mobility transistors with a gate-length of 0.2 μ m were investigated. The postannealing technique can improve the device performance, especially, after 10 min postannealing at 400 ° C , the gate-to-drain breakdown voltage of devices exhibits remarkable improvement from 25 to 187 V . The maximum extrinsic transconductance increases from 223 to 233 mS / mm at a drain bias of 10 V after 10 min annealing at 400 ° C . The maximum drain current at a gate bias of 1 V increases from 823 to 956 mA / mm . After annealing, the values of the unity current gain cut-off frequency and the maximum oscillation frequency increases from 24 and 80 GHz to 55 and 150 GHz , respectively. The output power and gain at 10 GHz were improved from 16.4 dBm and 11.4 dB to 25.9 dBm and 19 dB , respectively.
    Applied Physics Letters 10/2004; · 3.79 Impact Factor
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    ABSTRACT: Self-aligned AlGaN/GaN high electron mobility transistors (HEMTs) are fabricated and the direct current and radio frequency small signal performance of self-aligned devices is characterized in comparison with non-self-aligned devices. An ultra-thin Ti/Al/Ti/Au ohmic metal scheme is used for gate to source and drain self-alignment. To suppress the gate leakage current, the ohmic contact annealing of self-aligned devices is performed in a furnace. The self-aligned devices with 0.25 μm gate-length and 100 μm gate-width exhibit good pinch-off characteristics. The maximum drain current at a gate bias of 1 V is 620 mA/mm for self-aligned HEMTs, and 400 mA/mm for non-self-aligned devices, respectively. A maximum extrinsic transconductance of 146 mS/mm is measured in self-aligned devices, while non-self-aligned HEMTs show only a peak gm of 92 mS/mm. The self-aligned devices exhibit an extrinsic f<sub>T</sub> of 39 GHz and an f<sub>MAX</sub> of 130 GHz, whereas non-self-aligned HEMTs show an f<sub>T</sub> of 15 GHz and an f<sub>MAX</sub> of 35 GHz.
    High Performance Devices, 2004. Proceedings. IEEE Lester Eastman Conference on; 09/2004
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    ABSTRACT: In this paper, we present a combined model which unites both the small and large signal models for resonant interband tunneling diodes (RITD) grown on silicon-on-insulator (SOI) substrates. In this combined model, the dependent current source, I<sub>j</sub>(V), is from the large signal model while R, and C<sub>j</sub>(V) are obtained from the small signal model. The combined model was then implemented using ADS software. A transient simulation was performed to simulate the response of the RITD with a serially connected 50 μH inductor to a ramped voltage from 0 V to 1 V in 0.1 ms. The simulation results show strong oscillations when the diode is biased in its negative differential resistance region (NDR) during DC measurements, which would result in the commonly observed plateau in the NDR region, where the needle probe acts as an inductor.
    Device Research Conference, 2004. 62nd DRC. Conference Digest [Includes 'Late News Papers' volume]; 07/2004
  • Jaesun Lee, Dongmin Liu, Hyeongnam Kim, Wu Lu
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    ABSTRACT: The effects of post-annealing on DC, RF, and power performances of AlGaN/GaN HEMTs with a gate-length of 0.3 μm were investigated. The results show that the post-annealing technique can improve the device breakdown voltage and device uniformity, reduce the trapping centers on AlGaN surface and/or GaN buffer layer after the post-annealing, and adjust the device threshold voltage. Specifically, after 20-min post-annealing at 400 °C, the gate-to-drain breakdown voltage of the device exhibits remarkable improvement from 10 to 187 V. The maximum extrinsic transconductance (gm) increases from 238 to 254 mS/mm at a drain bias of 10 V after 10-min annealing at 400 °C. The threshold voltage shifts from −3.3 to −2.7 V. However, due to the decrease of two-dimensional electron gas concentration, the maximum drain current at a gate bias of 1 V reduces slightly from 918 to 904 mA/mm. The values of the unity current gain cut-off frequency (fT) and the maximum oscillation frequency (fMAX) increase after annealing. The output power and gain at 10 GHz were improved from 16.4 dBm and 11.4 dB to 22.4 dBm and 17.9 dB, respectively.
    Solid-State Electronics 01/2004; · 1.48 Impact Factor
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    ABSTRACT: The influence of annealed ohmic contact metals on the polarisation of the AlGaN barrier layer has been investigated by the Schottky contacts on the AlGaN/GaN HFET structure. The analysed result shows that annealed ohmic contact metals weaken the polarisation of the AlGaN barrier layer. When ohmic contact metals are close to Schottky contact metals, the weakened polarisation decreases the 2DEG sheet carrier concentration in the channel.
    Electronics Letters 10/2003; · 1.04 Impact Factor
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    ABSTRACT: Ir, Ni, and Re Schottky contacts on strained Al0.25Ga0.75N/GaN heterostructures are characterized using capacitance–voltage (C–V) and I–V techniques. Based on the measured C–V characteristics, two dimensional electron gas sheet carrier concentrations at the AlGaN/GaN interface and barrier heights of Ir, Ni, and Re Schottky contacts are calculated. The barrier heights of 1.12, 1.27, and 1.68 eV are obtained for Ir, Ni, and Re Schottky contacts, respectively. The results show that the barrier heights of Schottky contacts on strained AlGaN/GaN heterostructures are strongly dependent on the metal work functions. However, contrary to Schottky contacts on bulk AlGaN or GaN, the barrier height on strained AlGaN/GaN heterostructures is lower for a Schottky contact with a higher metal work function. This is attributed to the stronger wave function coupling between electrons in the Schottky metal and surface donor electrons. The I–V characteristics for Ir, Ni, and Re Schottky contacts confirm the results obtained by C–V characteristics. © 2003 American Institute of Physics.
    Applied Physics Letters 06/2003; 82(24):4364-4366. · 3.79 Impact Factor
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    ABSTRACT: In this study, quasi-enhancement mode AlGaN/GaN HEMT devices with the gate length of 1-μm are fabricated using a conventional method (i.e. without gate-recessing process) and the direct current and radio frequency characteristics of these devices are investigated. The threshold voltages are in the range of −0.3 to −0.5 V. The devices exhibit a maximum drain current of 280 mA/mm at a gate bias of 2 V. The pinch off voltage is about −1.0 V. At the gate bias of 1.5 V and the drain bias of 6 V, the devices exhibit an extrinsic transconductance of 140 mS/mm, a unity current gain cutoff frequency (fT) of 4.3 GHz, and a maximum oscillation frequency (fMAX) of 13.3 GHz, respectively.
    Solid-State Electronics 01/2003; 47(11):2081-2084. · 1.48 Impact Factor

Publication Stats

59 Citations
18.35 Total Impact Points

Institutions

  • 2007
    • Columbus State University
      Columbus, Georgia, United States
  • 2005–2007
    • The Ohio State University
      • Department of Electrical and Computer Engineering
      Columbus, OH, United States