Normally Off AlGaN/GaN Low-Density Drain HEMT (LDD-HEMT) With ...

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transistor (LDD-HEMT) that exhibits enhanced breakdown volt- age and reduced .... In this work, we fixed the gate length (LG) at 1 µm and the gate–source ... gate–drain spacing LGD = 3 µm: (a) OFF-state breakdown voltage with and without ...

IEEE ELECTRON DEVICE LETTERS, VOL. 28, NO. 3, MARCH 2007

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Normally Off AlGaN/GaN Low-Density Drain HEMT (LDD-HEMT) With Enhanced Breakdown Voltage and Reduced Current Collapse Di Song, Jie Liu, Zhiqun Cheng, Wilson C. W. Tang, Kei May Lau, Fellow, IEEE, and Kevin J. Chen, Senior Member, IEEE

Abstract—We report a low-density drain high-electron mobility transistor (LDD-HEMT) that exhibits enhanced breakdown voltage and reduced current collapse. The LDD region is created by introducing negatively charged fluorine ions in the region between the gate and drain electrodes, effectively modifying the surface field distribution on the drain side of the HEMT without using field plate electrodes. Without changing the device physical dimensions, the breakdown voltage can be improved by 50% in LDD-HEMT, and the current collapse can be reduced. No degradation of current cutoff frequency (ft ) and slight improvement in power gain cutoff frequency (fmax ) are achieved in the LDD-HEMT, owing to the absence of any additional field plate electrode. Index Terms—AlGaN/GaN, breakdown voltage, current collapse, fluoride-based plasma treatment, HEMT, low-density drain (LDD), normally off surface field modification.

I. I NTRODUCTION

W

IDE BANDGAP AlGaN/GaN HEMTs are promising candidates for next-generation microwave power amplifiers and power electronics components (e.g., switches and converters), owing to their large power handling capabilities [1]–[3]. Tremendous progress has been made in the performance of the conventional AlGaN/GaN depletion-mode HEMTs by an improvement in material growth, epi-structure design, and device processing techniques [4]–[7]. In circuit applications, normally off AlGaN/GaN HEMTs are desirable because they offer simplified circuit configurations and favorable operating conditions for device safety. However, the normally off AlGaN/GaN HEMTs usually exhibit a lower maximum drain current compared to their normally on counterparts, especially when the threshold voltage is increased to ∼ +1 V to assure the complete turn-off of the 2DEG channel at zero gate bias [8]–[11]. To compensate the reduction in maximum current and to maintain similar power handling capability, the breakdown voltage (VBK ) needs to be further improved, preferably not at the cost of increased gate-to-drain distance which inevitably increases the device size. The use of a metallic field plate, connected either to the gate or source electrodes, Manuscript received September 25, 2006; revised January 2, 2007. This work was supported by the Hong Kong Research Grant Council under Grant 611706. The review of this letter was arranged by Editor J. del Alamo. The authors are with the Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong (e-mail: [email protected]). Digital Object Identifier 10.1109/LED.2007.891281

can effectively enhance VBK by modifying the surface field distribution. The gate-terminated field plate [12], however, can introduce additional gate capacitances (CGS and CGD ), leading to reductions in the devices’ gain and cutoff frequencies. The source-terminated field plate [13]–[14], which requires a thickenough dielectric layer between the gate and field plate, proves to be effective in achieving an enhanced VBK and mitigating the gain reduction. In this letter, we present a simple approach of modifying the surface field distribution between the gate and drain without using a field plate electrode. The field modification is achieved by turning the entire or part of the region between the gate and drain into a region with low density of 2DEG, effectively forming a low-density drain (LDD). With the same device dimensions, the OFF-state breakdown voltage VBK improves from 60 V in a device without LDD to over 90 V in a device with LDD. No degradation in ft and slight improvement of power gain and fmax were observed in the LDD-HEMT, owing to the absence of a field plate electrode and increased output resistance (RDS ). In addition, a reduction in current collapse is observed in LDD-HEMTs that are not passivated by a SiN layer, indicating a passivation effect of the fluorine plasma treatment. II. F ABRICATION OF N ORMALLY O FF AlGaN/GaN LDD-HEMT S The AlGaN/GaN HEMT structure used in this work was grown on (0001) sapphire substrates in an Aixtron AIX 2000 HT metal organic chemical vapor deposition system. The HEMT structure consists of a low-temperature GaN nucleation layer, a 2.5-µm-thick unintentionally doped GaN buffer layer, and an AlGaN barrier layer with a nominal 30% Al composition. The barrier layer consists of a 3-nm undoped spacer, a 15-nm carrier supplier layer doped at 2.5 × 1018 cm−3 , and a 2-nm undoped cap layer. Room temperature Hall measurements of the structure yield an electron sheet density of 1.3 × 1013 cm−2 and an electron mobility of 1100 cm2 /V · s. The device fabrication, as shown by the cross section of a finished device in Fig. 1(a), is similar to that of the normally off AlGaN/GaN HEMT reported in [8], with one additional CF4 plasma treatment step added after the gate formation. First, a device mesa was formed using a Cl2 /He plasma dry etching in an inductively coupled plasma reactive ion etching (ICP-RIE) system, followed by the source/drain ohmic contact

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IEEE ELECTRON DEVICE LETTERS, VOL. 28, NO. 3, MARCH 2007

Fig. 2. Breakdown voltage enhancement in LDD-HEMTs for a fixed gate–drain spacing LGD = 3 µm: (a) OFF-state breakdown voltage with and without the LDD region. (b) Breakdown voltage and knee voltage’s dependence on the length of the LDD region for a fixed gate–drain spacing LGD = 3 µm.

III. O PERATING P RINCIPLE OF LDD-HEMT S

Fig. 1. (a) Cross section of a completed LDD-HEMT. (b) Schematic 2DEG distribution along the lateral dimension of an LDD-HEMT.

formation with Ti/Al/Ni/Au annealed at 850 ◦ C for 30 s. The ohmic contact resistance and sheet resistance were typically measured to be 1.0 Ω · mm and 400 Ω/. After gate windows with 1-µm length were opened by a contact photolithography, the sample was treated by a CF4 plasma in an RIE system at an RF plasma power of 150 W for 170 s. This plasma treatment can effectively incorporate fluorine ions into the AlGaN/GaN heterostructure and convert the active device from depletion mode to enhancement mode, yielding a self-aligned normally off 2DEG channel. Ni/Au e-beam evaporation and liftoff were carried out subsequently to form the gate electrodes. Subsequently, the windows of the LDD region are defined [see Fig. 1(a) for definition] by photolithography, and a second CF4 plasma treatment with an RF source power of 150 W is applied for 45 s. The 2DEG density in the LDD region after the fluorine treatment is about 9 × 1012 cm−2 . Finally, the sample was annealed at 400 ◦ C for 10 min. In this work, we fixed the gate length (LG ) at 1 µm and the gate–source spacing (LGS ) at 1 µm. The gate–drain spacing (LGD ) was chosen to be either 1 or 3 µm. The length of the LDD region (LLDD ) is 0.5 µm, and 1 µm for devices with 1-µm LGD , and 0.5, 1, 1.5, 2, and 3 µm for devices with a 3-µm LGD . A schematic sketch of the 2DEG density distribution in different regions of an LDD-HEMT is shown in Fig. 1(b). For comparison, the conventional HEMT devices with LLDD = 0 were also fabricated on the same wafer.

At first, the F− ions incorporated in the AlGaN layer of the LDD region provide negative fixed charges which can modulate the surface electric field and the 2DEG density, enabling the redistribution of the E-field and reduction of the peak field. The function of the LDD region is similar to a metal field plate in terms of improving the breakdown voltage [11], but without introducing any additional capacitances. Second, the fluorine ions incorporated in the AlGaN layer can effectively raise the energy band in the sample surface, effectively suppressing the trapping and detrapping process. In addition, our recent deep level transient spectroscopy experiments revealed that the incorporation of fluorine ions in the AlGaN layer can reduce the shallow trap states associated with the point defects in the AlGaN layer. On the other hand, the fluorine plasma treatment introduces acceptorlike traps with energy levels that are about 1.8 eV below the conduction band. Thus, the current collapse associated with the surface states and shallow traps can be reduced by implementing the LDD with the CF4 plasma treatment. It should be noted that the devices shown in this letter are not passivated by the SiN layer for the purpose of observing the passivation effect of the fluorine plasma treatment. IV. R ESULTS AND D ISCUSSION Fig. 2 shows the breakdown voltage enhancement in the LDD-HEMT. By converting half of the gate-to-drain region into a low-density region, a 50% increase is realized in VBK . As shown in Fig. 2(a), the device with LGD = 1 µm and LLDD = 0.5 µm exhibits a breakdown voltage similar to that achieved in a bulkier device with LGD = 3 µm and LLDD = 0 µm (without the LDD). The dependence of VBK and knee voltage (Vknee , defined as the source–drain voltage when the drain current reaches 90% of the maximum value) on LLDD is shown in Fig. 2(b). For devices with the same LGD = 3 µm and different LLDD (0 or 3 µm), the same Vth = 0.75 V, IMAX = 300 mA/mm, and GM = 150 mS/mm are obtained, as shown in Fig. 3(a). In Fig. 3(b), the LDD-HEMT shows no degradations in current gain and ft (14.1 GHz), slight improvement in power gain (MSG/MAG) and fmax (from 34 to 35.3 GHz) that result from the increased output resistance (RDS ). The only penalty imposed on the LDD-HEMT is the increased ON-resistance that results in an increase of Vknee . This increase is at most 1.6 V in the device with LLDD = LGD = 3 µm, much smaller than the enhancement in VBK (> 30 V).

SONG et al.: NORMALLY OFF AlGaN/GaN LDD-HEMT WITH BREAKDOWN VOLTAGE

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of the gate-to-drain region show commonly observed drain current reduction under a pulsed operation. However, as the fluorine treatment extends to half of the gate-to-drain region (LLDD ≥ 1.5 µm), the pulsed drain current exceeds the dc drain current, a result from both the reduction in the current collapse and the elimination of the self-heating effect under the pulsed operation. The reduction in the current collapse reaches the maximum when the entire gate-to-drain region is treated by the fluorine plasma treatment (LLDD = 3 µm). V. C ONCLUSION Based on the CF4 plasma treatment technique, we have demonstrated a novel approach of modifying the drain-side surface field distribution in normally off AlGaN/GaN HEMTs without using a field plate electrode. Our technique delivers breakdown voltage enhancement and current collapse reduction without any degradation in gains and cutoff frequencies. R EFERENCES

Fig. 3. Comparison of (a) dc transfer and output characteristics and (b) H21 and MSG/MAG between devices with and without the LDD region. All the devices have the same gate length (LG = 1 µm), gate–drain spacing (LGD = 3 µm), and gate–source spacing (LGS = 1 µm).

Fig. 4. DC and pulsed I–V characteristics of the LDD-HEMT with different LLDD . All the devices have the same gate–drain spacing of LGD = 3 µm. The quiescent biases for the pulsed I–V measurement are VG0 = 0.5 V and VDS0 = 10 V. The pulses have a width of 1 µs and a separation of 1 ms.

Fig. 4 plots the dc and pulsed current–voltage (I–V ) characteristics of devices with the same LGD (3 µm) and various LLDD . The devices without fluorine treatment in the entire (LLDD = 0 µm) or large portion (LLDD = 0.5 or 1.0 µm)

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