Effect of Gate Length on DC Performance of AlGaN/GaN HEMTs Grown by MBE

J.W. Johnson ⁽¹⁾, A.G. Baca ⁽²⁾, R.D. Briggs ⁽²⁾, R.J. Shul ⁽²⁾, J.R. Wendt ⁽²⁾, C. Monier ⁽²⁾, F. Ren ⁽¹⁾, S.J. Pearton ⁽³⁾, A.M. Dabiran ⁽⁴⁾, A.M. Wowchack ⁽⁴⁾, C.J. Polley ⁽⁴⁾and P.P. Chow ⁽⁴⁾

⁽¹⁾ Department of Chemical Engineering, University of Florida, Gainesville, FL 32611 USA

⁽²⁾ Sandia National Laboratories, Albuquerque, NM 87185 USA

⁽³⁾ Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611 USA

⁽⁴⁾ SVT Associates, Eden Prairie, MN 55344 USA

ABSTRACT

The DC performance of AlGaN/GaN High Electron Mobility Transistors grown by plasma-assisted molecular beam epitaxy was investigated for gate lengths in the range 0.1 - 1.2 μm. On 0.25 μm gate length devices we obtained 40 V_DS operation with > 50 mA peak I_D. The peak drain current density was 0.44 Aּmm^-1 for 100 μm gate width devices with 1.2 μm gate lengths. The extrinsic transconductance (g_m) decreased with both gate length and gate width and was ≥75 mSּmm^-1 for all gate widths for 0.25 μm devices. E-beam written gates typically produced a slightly lower Schottky barrier height than optically patterned gates.

INTRODUCTION

Remarkable progress has been made in recent years in high performance AlGaN/GaN High Electron Mobility Transistors (HEMTs) grown on a variety of substrates including sapphire, SiC, and Si. ^(1-21) The high current density of the AlGaN/GaN heterostructure (up to 1.6 A/mm has been reported) is a result of the large polarization-induced field and large conduction band offset in the AlGaN/GaN system. This polarization field has both a piezoelectric, strain-induced, as well as a spontaneous polarization component. For the AlGaN HEMTs most widely studied to date, the spontaneous polarization field dominates. This built-in field induces a two-dimensional electron gas (2DEG) that is linearly proportional to the Al-mole fraction x (0 ≤ x ≤ 1) as n_s = x(5 x 10¹³ cm^-2). For the 30% Al-comparison most widely studied, a channel sheet electron density of ~1.5 x 10¹³cm^-2 can be realized that is a factor of 5 - 10 times higher than typical GaAs or InP PHEMTs. The associated mobility at this high current density is in the range of 1000 - 1500 cm² / Vּs, which is well below that achieved in the conventional III-V material. However, the combined μ_cn_s product is competitive. It is the μ_cn_s product that enables this material system to be applicable to low noise amplifiers as well as power amplifiers.

The high voltage capability of AlGaN/GaN HEMTs is the result of a critical electric field of ≥3 MV/cm. This value is 10 times that of Si and 5 times that of GaAs. For typical AlGaN/GaN HEMT structures, this translates to gate-to-drain breakdown voltages of approximately 100 V/μm. While record breakdown voltage over 500 V have been demonstrated for large gate-to-drain spacing, the more important result is for a device layout consistent with good microwave performance (i.e. gate-to-drain spacing of 1-2 μm). In this latter case, a breakdown voltage of ~80 - 100 V is achieved in a microwave transistor that also has the high channel current mentioned previously. ⁽¹⁹⁾

To make full use of the high voltage capability, the transistor must not be thermally limited. GaN also has an advantage over GaAs in this regard, with thermal conductivities up to 2.0 W/cmּK for GaN versus 0.46 W/cmּK for GaAs. Furthermore, GaN HEMTs can be grown on semi-insulating SiC with a thermal conductivity of 3.3 W/cmּK. The high breakdown field and good thermal conductivity allow these devices to be used in high efficiency (theoretical efficiency of 78.5%) class B push/pull amplifiers at full power rating. GaAs microwave devices, on the other hand, can only be implemented in a class B push/pull amplifier by backing off the voltage bias (and hence the power level) to accommodate the higher voltage swing in this configuration as compared to class A, single-ended operation.

Over the past few years, dramatic progress has been made in understanding the AlGaN HEMT device physics and in demonstrating record microwave power performance. The potential of any microwave device technology is first realized in small periphery devices with microwave power density being the relevant figure of merit. The best result to date is 9.8 W/mm with 47% power added efficiency (PAE) at 8 GHz ⁽¹⁹⁾. This power density is close to seven times the best result obtained for GaAs technology and over twice that achieved with SiC MESFETs at this frequency ⁽²⁰⁾. With continued improvements in material quality and device design, a power density of 12 W/mm or higher appears practical.

Recently, it has also been shown that these devices can achieve low microwave added noise figures (NF = 0.6 dB at 10 GHz) while maintaining a large breakdown voltage (>60 V) and hence a large dynamic range. These results imply that AlGaN HEMTs can be used to perform the active transmit and receive functions in more robust, higher dynamic range modules.

One feature of the HFET work to date is the fact that MBE-grown material appears quite competitive with that grown by MOCVD. This has not been the case with photonic devices, in which the generally better optical properties obtained with MOCVD have led to this method being the dominant one for manufacture.

In this paper we report the DC performance of MBE-grown AlGaN/GaN HEMTs as a function of gate length in the range 0.1 - 1.2 μm.

EXPERIMENTAL

The growth details have been given previously ^(22-24), but in brief consist of a low temperature AlN (300Å) buffer on sapphire (0001), 2 μm of undoped GaN grown at 750°C under Ga-rich conditions, 250Å of undoped Al_0.2Ga_0.8N, and a 30Å undoped GaN cap. A growth rate of 0.5 - 1 μmּhr^-1 was used for all depositions.

Ohmic contacts were formed by lift-off of e-beam evaporated Ti/Al/Pt/Au, which was annealed at 850°C for 30 sec. to minimize contact resistance. Schottky gates of e-beam deposited Ni/Au were formed by conventional optical lithography for 1.2 μm gate lengths or e-beam lithography for shorter lengths. Mesa isolation was formed by ICP dry etching. A schematic of the final structure is shown in Figure 1. The DC characteristics were measured at 25°C with an HP4145B parameter analyzer, while the quality of the gate formation was examined by scanning electron microscopy (SEM).

RESULTS AND DISCUSSION

Figure 2 shows an SEM micrograph of a 0.1 μm gate crossing the mesa. Note the use of an anchoring technique at the end of the gate for improved adhesion. A cross-sectional view of the same gate finger (0.1 ´ 100 μm²) is shown in Figure 3. Note the excellent edge definition on both the gate and the source/drain contacts. The surface morphology of the MBE-grown epilayers is also excellent, with no evidence of defects or tilted growth boundaries. This is one of the attractive properties of MBE for GaN electronics applications involving heterostructures with relatively thin active layers. In the case of MOCVD growth, there is more of a tendency to emphasize the three-dimensional growth of GaN on lattice-mismatched substrates, leading to poor uniformity and thickness control.

Figure 4 shows an SEM micrograph of a 0.25 μm gate length device with the mesa step in the background. Once again the edge acuity of the metal is excellent. This micrograph gives a good view of the T-gate and shows that the lithography and lift-off processes produced well-defined gate contacts.

The I_D-V_DS characteristics for devices with five different gate lengths are shown in Figure 5. In each case the gate width was 100 μm, and the devices were biased with +1V gate bias (V_G), with a step of -1V. The drain characteristics are free of kinks in these unpassivated devices, with no sign of the current collapse phenomenon attributed to hot electron injection and trapping in the buffer layer. ⁽¹³⁾ In this mechanism, as a high drain voltage is reached, electrons are injected into the GaN buffer layer where they are trapped in deep states. This trapped charge acts to deplete the two-dimensional electron gas from the buffer side of the channel and thereby reduces the drain current for the subsequent traces.⁽¹³⁾ The shortest gate length devices show a crowding of the I-V traces at high grain source voltage and do not pinch-off. These short-channel effects are common in GaAs- and InP-based HEMTs.

Figure 6 shows the I_D-V_DS characteristics from a 0.25 ´ 100 μm² date device operated at high-bias conditions. Operation at >40 V_DS with >50 mA peak I_D was obtained, i.e. ≥2 W of power in devices not optimized for power performance. It is clear that GaN HEMTs on sapphire substrates will not be as well-suited for power operation as those on SiC substrates. Flip-chip bonding is an attractive option to avoid the poor thermal characteristics of the sapphire.

A summary of the peak I_D values obtained as a function of both gate length and gate width (100, 150, or 200 μm) is shown in Figure 7. A maximum current density of ~0.44 Aּmm^-1 was obtained in 1.2 μm gate length devices for the shortest gate width. The variations in I_D,max at 0.5 μm gate length are likely random and do not represent a trend. There seems to be little change in peak I_D with decreasing gate length for the e-beam written gates, despite the expected I_D increase at shorter lengths.

Typical HEMT transfer characteristics for 0.25 ´ 100 μm² devices are shown in Figure 8 (top). There is little fall-off in g_m over a range of drain-source biases, with a maximum transconductance of ~80 mSּmm^-1. A summary of the transconductance data as a function of gate length is shown at the bottom of Figure 8. Maximum g_m values of ~90 mSּmm^-1 were obtained with 1.2 ´ 100 μm² devices.

We did observe a strong effect of the lithographic patterning technique on the forward I-V characteristics of the HEMTs. Figure 9 (top) shows the I_G-V_G characteristics as a function of gate length for 100 μm gate width devices. At low V_G there is excess leakage in the e-beam patterned HEMTs (i.e. between ~0.8 and 2.5 V), and this is reflected in the Schottky barrier height derived from the relationship

where J_F is the forward current density at voltage V, A^** is Richardson’s constant for n-GaN (26.4 Aּcm^-2ּK^-2), T is the absolute measurement temperature, f_B the barrier height, n the ideality factor, and k is Boltzmann’s constant. The resulting f_B values are shown in Figure 9 (bottom) and show a decrease of ~0.1 eV for the e-beam patterned gates. This is likely due to e-beam-induced damage to the GaN surface, since annealing at ~400°C could restore the barrier height to the value of the optically patterned gates. This result emphasizes the sensitivity of the GaN surface to disruption by energetic particle-enhanced processes (others include dry etching or plasma enhanced chemical vapor deposition). In the early days of GaN device processing it was thought that the material was relatively impervious to these problems. Part of the reason for the dramatic improvement in device performance over the past few years has been the development of processing conditions that minimize surface damage.

SUMMARY AND CONCLUSIONS

RF plasma-assisted MBE growth of AlGaN/GaN HEMTs appears well-suited for producing high-performance devices. The surface morphologies and uniformity of device DC performance are excellent. Reasonable current densities and excellent scaling properties were achieved over a range of gate lengths and gate widths. Further work will focus on the microwave and power characteristics of the MBE-grown HEMTs.

ACKNOWLEDGEMENTS

The authors would like to thank A. Ongstad for assistance with SEM images. The work at UF is partially supported by NSF grants DMR-0101438 and CTS-991173. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000.

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FIGURE CAPTIONS

Figure 1. Cross-sectional view of AlGaN/GaN HEMT structures.

Figure 2. SEM micrograph of 0.1 μm gate length HEMT showing ohmic contacts, mesa step, and gate profile.

Figure 3. SEM front view of 0.1 μm gate length HEMT showing gate and channel definition.

Figure 4. SEM cross-section of 0.25 μm Ni/Au gate contact.

Figure 5. I_D-V_DS characteristics for HEMTs of five different gate lengths. The gate width is a constant 100 μm.

Figure 6. High voltage I_D-V_DS characteristics for 0.25 ´ 100 μm² HEMT.

Figure 7. Summary of peak I_D as a function of obth gate length and gate width for HEMTs.

Figure 8. Transfer characteristics for 0.25 ´ 100 μm² HEMTs (top) and summary of g_m as a function of both gate length and gate width for HEMTs (bottom).

Figure 9. Forward gate current characteristics from e-beam and optically patterned devices for different gate lengths (top) and Schottky barrier height extracted from I-V data (bottom).