IEEE ELECTRON DEVICE LETTERS, VOL. 30, NO. 9, SEPTEMBER 2009
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AlGaN/GaN HEMT With Integrated Recessed Schottky-Drain Protection Diode Eldad Bahat-Treidel, Richard Lossy, Joachim Würfl, and Günther Tränkle
Abstract—We present an AlGaN/GaN high-electron mobility transistor (HEMT) with an integrated recessed protection diode on the drain side of the transistor channel. Results from our Schottky-drain HEMT demonstrate an excellent reverse blocking with minor tradeoff in the ON-state resistance for the complete device. The excellent quality of the forward diode characteristics indicates high robustness of the recess process. The reverse blocking capability of the diode is better than −110 V. Physical-based device simulations give an insight in the respective electronic mechanisms. This is the first time that a recessed Schottky-drain diode integrated in a HEMT device is presented. Index Terms—AlGaN/GaN high-electron mobility transistor (HEMT), protection diode, recessed Schottky-drain diode.
Fig. 1. Schematic sketch of the HEMT with fully recessed AlGaN in the area of the drain contact and the equivalent circuit.
I. I NTRODUCTION
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AN-BASED high-electron mobility transistors (HEMTs) already show an excellent performance in microwave power amplifiers (PAs), including wireless base stations [1]. For high-efficiency PAs, efforts are presently focusing on the switch-mode type of amplifier architecture, as it allows highest power-added efficiency (PAE). From the different types, the class-S concept, which uses bandpass delta–sigma bit sequences at the input to switch the power transistor between ON and OFF state, becomes more attractive also for microwave frequencies [2]. Class-S switch-mode amplifiers need a reconstruction filter at the output side that may lead to short negative pulses at the drain terminal of the power transistor. The negative pulses depending on circuitry may reach a similar voltage level as in the positive range. For the purpose of highfrequency switching in microwave switch-mode amplifiers, it is, therefore, highly desirable to combine the advantages of the high-speed switching transistor with a fast switching protection diode connected in series with the GaN HEMT. The integration of a fast diode in the transistor protects the switching transistor from transients due to the external circuitry. Due to the minimized tradeoff with respect to the ON-state resistance of the overall device, it maximizes the switching efficiency.
Manuscript received May 29, 2009; revised June 22, 2009. First published August 11, 2009; current version published August 27, 2009. This work was supported by the German Bundesministerium für Bildung und Forschung under Contract 01BU605. The review of this letter was arranged by Editor J. A. del Alamo. The authors are with the Ferdinand-Braun-Institut für Höchstfrequenztechnik, 12489 Berlin, Germany (e-mail:
[email protected];
[email protected];
[email protected];
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2009.2026437
Recently, a discrete protection diode has been combined with an AlGaN/GaN HEMT on the same chip [3]. An AlGaN/GaN HEMT with an integrated recessed Schottky protection diode is demonstrated for the first time. From the simulation, we propose the structure shown in Fig. 1. The Schottky metal drain contact is connected to the HEMT’s 2DEG by recessing the drain contact and completely removing the AlGaN barrier layer so that the Schottky drain contact metal is connected directly to the GaN buffer. The recess reduces the forward voltage onset and can reduce the ON-state power loss in diodes for power applications [4]. The device takes advantage of the good electrical connection between the Schottky metal and the GaN buffer in forward bias that is introduced by the high-mobility electrons spilling into the buffer at the vicinity of the drain electrode during the high-field condition [5]. This results in a high conductivity in the forward direction of the Schottky-drain diode. II. FABRICATION AND C HARACTERIZATION HEMT structures were grown by metal–organic vapor phase epitaxy on semi-insulating SiC substrates having an AlN nucleation layer followed by a 1400-nm unintentionally doped (UID) GaN buffer layer and a 25-nm UID AlGaN barrier layer containing 24% Al on top. Field-effect transistors were fabricated on a wafer. Source ohmic contacts were made by evaporating Ti/Al/Ti/Au with a sputtered WSiN layer on top to provide a smooth pattern delineation after RTP at 830 ◦ C. Mesa isolation was performed using Cl2 /BCl3 reactive ion etching. Drain trenches for the Schottky contacts were defined by electron beam lithography, and the AlGaN layer was recessed by using BCl3 -based reactive ion etching. Two etch durations were investigated which
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IEEE ELECTRON DEVICE LETTERS, VOL. 30, NO. 9, SEPTEMBER 2009
Fig. 2. Output characteristics in the on state (VGS = 0 V) and the threshold state (VGS = −4 V), showing reverse blocking due to the integrated diode. The inset is the same in logarithmic scale.
correspond to 26- and 32-nm etching depth, respectively. The Schottky contacts use Pt/Ti/Au stack metallization aligned to the etched trench. The T-shaped gate defined by electron beam lithography has a footprint of 0.4 μm using a Pt/Ti/Au stack for the Schottky barrier metal. The Pt/Ti/Au stack that serves as a Schottky contact metal provides a barrier height of 0.65 eV. The HEMT structures were passivated with SiNx from PECVD, and, finally, connection and plating layers were fabricated. The tested devices were 2 × 125 μm wide with a gate–drain spacing LGD = 6 μm and LGS = 1 μm. The dc-output characteristics of the ON state (VGS = 0 V) and the threshold state (VGS = −4 V) of forward and reverse bias are shown in Figs. 2 and 3. The reverse blocking capability for both ON and threshold states of the Schottky-drain diode is higher than −110 V. The ON-state operation shows an onset, where IDS = 1 mA/mm, of current flow due to the Schottky-drain diode at 1.0-V drain voltage. Characterization of the HEMT devices reveals an average saturation current of 1.2 A/mm and an average maximum transconductance of 290 mS/mm (Fig. 3). Comparable numbers are obtained for devices using customary ohmic drain contacts. The average device breakdown measured at VGS = −6 V was 40 V limited by a drain current of 1 mA/mm. The ON-state resistance (Ron ) defined as the reciprocal of the I–V curve’s slope at the steepest position and the drain saturation current (IDS max ) of the two recess depths applied to the properties of the reference device are compared. The wafer average measured Ron are 3.97 ± 0.10, 4.48± 0.08, and 3.21 ± 0.16 Ω · mm, and the measured IDS max are 1.15 ± 0.22, 1.16 ± 0.19, and 1.18 ± 0.18 A/mm which correspond to the 26-nm recess depth, the 32-nm recess depth, and the standard ohmic drain contact, respectively. These values reflect the robustness of the contact recess process as will be discussed later. RF characterization from load-pull measurements in class-AB operation at 2 GHz (shown in Table I) reveals a saturated output power density of 9.7 W/mm with 65-V drain bias (2 × 125 μm finger width and 1-μm field plate), 41% PAE, and 24-dB unilateral gain. RF switch-mode operation will not be covered here. The reference device with the same dimensions and measurement conditions leads to 9.9 W/mm, 46% PAE, and 23-dB gain (see Table I).
Fig. 3. (Top) Detail of the wafer average output characteristics showing the onset of drain current at VDS = 1.0 V. (Bottom) Wafer average transfer characteristics and transconductance of the GaN HEMTs with a recessed Schottky drain at VDS = 15 V. The error bars represent the standard deviation of the measurements.
III. S IMULATION I NSIGHT Two-dimensional physical-based device simulations (SILVACO—“ATLAS”) [6] are performed to get an insight to the recessed Schottky-drain-contact HEMTs. The simulation follows the measurement sequence of the measurement for ON -state condition in forward and reverse bias. The simulated logarithmic electron concentration ne in the device for the different conditions is logged. The simulation is consistent with the experimental findings. An onset current flow due to the Schottky-drain diode and the reverse blocking capability for the ON state of the Schottky-drain HEMT is obtained. During forward bias [Figs. 4(a) and 5(a)], most of the electrons are traveling in the 2DEG region; however, some of them are injected from the 2DEG into the GaN buffer due to the presence of high electric fields. Once injected into the buffer, the electrons experience quite a high mobility. This means that, in addition to direct contact of the Schottky drain electrode to the 2DEG, the highly mobile electrons in the GaN volume are able to bypass a potential gap between the 2DEG and the Schottky metal by just being injected into the GaN buffer, thus providing a drain connection with low ON-state resistance. On the other hand, during reverse drain bias [Figs. 4(b) and 5(b)], as in a protection diode mode, strong depletion of electrons in the surroundings of the Schottky drain electrode can be seen, preventing reverse current and, by that, protecting the transistor. IV. D ISCUSSION The intention of the AlGaN/GaN HEMTs with a protection diode is to prevent a current flow in the transistor during
BAHAT-TREIDEL et al.: AlGaN/GaN HEMT WITH INTEGRATED RECESSED SCHOTTKY-DRAIN PROTECTION DIODE
Fig. 4. Simulation using SILVACO “ATLAS,” revealing the electron concentration distribution in the on state during (a) forward-bias-drain condition and (b) reverse-bias-blocking-drain condition. TABLE I RF C HARACTERIZATION F ROM L OAD -P ULL M EASUREMENTS
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Fig. 5. Simulation using SILVACO “ATLAS,” revealing the electric field distribution in the on state during (a) forward-bias-drain condition and (b) reverse-bias-blocking-drain condition.
interface. Some overetching is acceptable with only low penalty in the device performance. This gives the concept of the recessed-Schottky-drain-diode pronounced process robustness. V. C ONCLUSION
reverse-bias conditions in switching devices. Replacing the planar drain ohmic contact by a Schottky metal without recess will show the desired reverse blocking but with the disadvantages of very high ON-state resistance and often poor blocking in reverse bias. The realization of a good Schottky diode is possible when the AlGaN barrier underneath the metal is completely removed and a contact of the Schottky metal to the 2DEG and/or the GaN semiconductor buffer layer is achieved. Simulations and measurements have shown that, in addition to direct contact of the 2DEG from the side of the Schottky drain electrode, electrons are injected to the GaN buffer under the Schottky metal and its contact. Due to high-field-assisted electron injection into the buffer, it is also possible to slightly etch through the 2DEG region without too much penalty in the ON-state resistance. This provides quite a safe process window for device fabrication. At forward-drain-bias conditions, a deep potential drop creates a high-field condition in the vicinity of the drain electrode. This field allows the electrons to spill over into the high resistive GaN buffer, where they have such a high mobility that allows them to flow toward the drain electrode. In reverse drain bias, the Schottky potential barrier depletes the GaN buffer close to the drain, and the electrons cannot get injected into the buffer. Any further increase in the reverse potential increases the depletion depth, and the drain diode blocking is improved. In addition, there is a big advantage in the process robustness of the recessed drain Schottky electrode. Simulation shows that the current flow into the Schottky drain is distributed across some nanometers of depth and extends the thickness of the 2DEG. The complete etching of the AlGaN barrier has therefore not necessarily to be stopped exactly at the AlGaN/GaN
We have presented a demonstration of an AlGaN/GaN HEMT with an integrated protection Schottky diode. The results of the innovative Schottky-drain HEMT have demonstrated an excellent reverse blocking with minor tradeoff in the ON-state resistance for the complete device. The reverse blocking capability of the diode has been better than −110 V, demonstrated with an excellent quality of the forward diode characteristics. The demonstrated recess process has shown high robustness and low influence on the electrical dc characteristics. ACKNOWLEDGMENT The Ferdinand-Braun-Institut für Höchstfrequenztechnik would like to thank the German Bundesministerium für Bildung und Forschung for the support under Contract 01BU605. R EFERENCES [1] H. Shigematsu, Y. Inoue, S. Masuda, M. Yamada, M. Kanamura, T. Ohki, K. Makiyama, N. Okamoto, K. Imanishi, T. Kikkawa, K. Joshin, and N. Hara, “C-band GaN-HEMT power amplifier with over 300-W output power and over 50-% efficiency,” in Proc. Compound Semicond. Integr. Circuit Symp., 2008, pp. 1–4. [2] C. Meliani, J. Flucke, A. Wentzel, J. Würfl, W. Heinrich, and G. Tränkle, “Switch-mode amplifier ICs with over 90% efficiency for class-S PAs using GaAs-HBTs and GaN-HEMTs,” in Proc. MTTS, 2008, pp. 751–754. [3] W. Chen, K.-Y. Wong, and K. J. Chen, “Monolithic integration of lateral field-effect rectifier with normally-off HEMT for GaN-on-Si switchmode power supply converters,” in IEDM Tech. Dig., San Francisco, CA, Dec. 15–17, 2008, pp. 1–4. [4] H. Ishida, D. Shibata, H. Matasuo, M. Yanagihara, Y. Uemoto, T. Ueda, T. Tanaka, and C. Ueda, “GaN-based natural super junction diodes with multi-channel structures,” in IEDM Tech. Dig., San Francisco, CA, Dec. 15–17, 2008, pp. 1–4. [5] E. Bahat-Treidel, O. Hilt, F. Brunner, J. Wurfl, and G. Trankle, “Punchthrough-voltage enhancement of AlGaN/GaN HEMTs using AlGaN double-heterojunction confinement,” IEEE Trans. Electron Devices, vol. 55, no. 12, pp. 3354–3359, Dec. 2008. [6] ATLAS User’s Manual, Device Simulation Software, 2008 ed., SILVACO Int., Santa Clara, CA, 2008.
