Elimination of kink phenomena and drain current hysteresis in InP ...

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IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 50, NO. 2, FEBRUARY 2003

Elimination of Kink Phenomena and Drain Current Hysteresis in InP-Based HEMTs With a Direct Ohmic Structure Ken Sawada, Tomoyuki Arai, Tsuyoshi Takahashi, and Naoki Hara, Member, IEEE

Abstract—We eliminated kink phenomena and Ids hysteresis in a double-doped InP-based HEMT without degrading its frequency performance by fabricating direct ohmic contacts in the InGaAs channel. A direct ohmic structure lets us control current paths in the device and relax the electric field at the recess edge of the drain side. As a result, we can suppress impact ionization and decrease the hole currents that originate from the high electric field region at the recess edge of the drain side. Kink phenomena are eliminated in the direct ohmic structure. We also suggest a hole trap mechanism to explain the appearance of hysteresis in the I–V characteristic of the conventional nonalloyed ohmic structure device. Index Terms—Current control, HEMT, hysteresis, impact ionization, kink, MODFET, ohmic.

may improve I–V characteristics if the kink phenomena are related to impact ionization or to traps. Comparing a direct ohmic structure device and a conventional structure device, we can get the electric field intensities in the channel through a device simulation. Additionally we can evaluate the amount of hole current by hole current measurements and can visualize the area of the recombination of electrons and holes by emission measurements. From these experimental data we discuss the relationship between the ohmic electrode structure and suppressing impact ionization and also describe the mechanism of eliminating kink phenomena by reducing hole current originated from impact ionization in the channel.

I. INTRODUCTION

II. DEVICE FABRICATION AND STRUCTURE

HE rapidly increasing demand for greater transmission capacity has created a need for very high-speed ICs in optical communication systems. To fulfill this need, we are developing InAlAs/InGaAs HEMTs for use in over 40-Gbit/s optical communication ICs [1]. One of the problems of integrating the HEMTs is kink phenomena in the I–V characteristics of the InP-based HEMTs. Kink is associated with reduced gain and excess noise at high frequencies, and is a limiting factor in circuit design. Much research has been done on the relationship between impact ionization and kink phenomena [2], [3]. Due to the presence of the high field, impact ionization is initiated in the high field region under the gate and extends all the way to the drain, resulting in the generation of holes. It easily occurs impact ionization in the case of a narrow band gap channel, as in InGaAs, InAs or InSb. Impact ionization makes it difficult to shorten the gate length and yields a small on-state breakdown voltage. Some authors suggested that the kink phenomena are related to traps [4], while others attributed kink phenomena to the accumulation of holes induced by impact ionization [5]. While the precise mechanism relating impact ionization and kink phenomena is still under study, it is expected that a reduction in impact ionization will result in a suppression of kink phenomena. In this paper we introduce a new viewpoint that is current paths in the device. We have developed InP-based HEMTs that have a direct ohmic structure, which can alter current paths in the device. The ability to changing current paths in the device

We used a double-doped HEMT structure consisting of InGaAs cap layer/InP etching stopper layer/InAlAs carrier supply layer/InGaAs channel layer/InAlAs carrier supply layer/InAlAs buffer layer on InP substrates. We fabricated both a direct ohmic structure device and a conventional nonalloyed ohmic structure device. An alloyed ohmic structure was used for the direct contacts, which were fabricated by etching the n-InGaAs cap layer, evaporating the Ni/AuGe/Au layer and alloying. Etching the cap layer reduced the contact resistance enough to prevent an excessive reaction between the Au and the n-InGaAs cap layer and to obtain optimum alloy conditions. The conventional nonalloyed ohmic structure was fabricated by evaporating Mo/Ti/Pt/Au onto an n-InGaAs cap layer. Fig. 1 shows a schematic diagram of the internal epilayer resistance in the direct ohmic structure device (a) and in the nonalloyed ohmic structure device (b). In this figure, we define the internal epilayer resistances as follows; the resistance of the interface between the alloy ohmic electrode and the semiconductor (Res1), the contact resistance of the nonalloyed ohmic electrode (Res2), the sheet resistance of the cap layer (Rcap), the resistance to the channel (Rch), which represents the resistance through the heterointerface of InAlAs/InGaAs in Fig. 2(a), the sheet resistance of the channel layer (Rcs), the resistance of the potential barrier due to the differences of the band bending in the channel (Rcb), which is indicated in Fig. 2(b), and the sheet resistance of the recess region (Rre). Rcb must be small enough for the total source resistance, otherwise InAlAs/InGaAs heterostructures usually have a large band gap discontinuity, and Rch is larger than Rcap. In the conventional nonalloyed ohmic structure, most of the electrons in the cap layer flow to the recess edge of the source side, then flow into the channel layer, reaching the recess edge of the drain side,

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Manuscript received July 8, 2002; revised October 23, 2002. The review of this paper was arranged by Editor C.-P. Lee. The authors are with Fujitsu Laboratories, Ltd., Kanagawa 243-0197, Japan (e-mail: [email protected]). Digital Object Identifier 10.1109/TED.2002.808555

0018-9383/03$17.00 © 2003 IEEE

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Fig. 3. Schematic diagram of a simulated electric field in a device with two high electric field regions: region 1: the drain edge of the gate electrode, region 2: the recess edge of the drain side. TABLE I SIMULATED ELECTRIC FIELD INTENSITIES OF THE REGIONS INDICATED IN Fig. 2 (Vg = 0 V, Vds = 2 V) Fig. 1. Schematic diagram of the internal epilayer resistance: (a) in a direct ohmic structure and (b) in a nonalloyed ohmic structure.

mm. The resistance of the interface between the alloy ohmic mm. Comelectrode and the semiconductor (Res1) was 0.23 paring these resistances leads us to consider that in a direct ohmic structure, most of the electrons flow into the channel directly from the source electrode and flow out to the drain electrode. The curRcs Rcb Rre in Fig. 1(a). Typrent path is Res1 ical characteristics of a double-doped InP-based HEMT with a direct ohmic structure and a nonalloyed ohmic structure that have m m gates are maximum transconductance of 910 mS/mmand880mS/mm,andfTof180GHzand185GHz,respectively. The reason to which the device characteristic was almost equal to two types of structures is that in the epi and the device structure we used, the values of the contact resistivity (in other words the values of source resistance) were almost same as cm for a direct ohmic structure and cm for a nonalloyed ohmic structure. III. RESULTS AND DISCUSSION

Fig. 2. Schematic diagram of conduction band profiles corresponding to the A A section and B B section in Fig. 1 explaining the internal epilayer resistances of (a) Rch and (b) Rcb.

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and then flow out to the cap layer. (The current path is Res2 Rcap Rch Rcb Rre in Fig. 1(b).) In this case, the current concentrates at the recess edge of the drain side and triggers impact ionization. In contrast, in the direct ohmic structure, ohmic electrodes directly contact the channel. We calculated the internal epilayer resistance by using the TLM method. We measured the resistances of 5 m, 10 m, 15 m gaps with recess and without recess patterns. The internal epilayer resistance of Rch was 0.40

To clarify the effect of changing the current path on the electric field, we simulated the electric field in a device with a direct ohmic structure and in one with a nonalloyed ohmic structure using Silvaco-ATLAS. The main features of this simulation are as follows. The simulated devices had a double-doped epilayer structure that was doped to the heterointerface so that the current paths might vary with ohmic structure, as we described. Selberherr’s model [6] was used as the impact ionization model. The Arora model [7] and the field-dependent model were used as the mobility models for a low electric field and high electric field, respectively. Fig. 3 shows the schematic diagram of a simulated electric field in a device with two high electric field regions: the drain edge of the gate electrode and the recess edge of the drain side. Table I shows the electric field intensities of these regions in the device with the direct ohmic structure and in the device with the nonalloyed ohmic structure

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Fig. 4. I–V characteristics of (a) a direct ohmic structure and (b) a nonalloyed ohmic structure. (Lg = 0:13 m, Wg = 40 m, Vg max = 0 V, Vg step = 0:1 V).

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at Vg V and Vds V. In particular, there is a large difference in the electric field intensity of the recess edge of the drain side between the two structures. The value for the direct V/cm, and for the nonalloyed structure structure is V/cm. These results indicate that in the direct it was ohmic structure, the electric field is reduced by the lower current path concentration across the heterointerface of the recess edge of the drain side. Next, the I–V characteristics of the fabricated HEMTs were evaluated. Fig. 4 shows the I–V characteristics of the direct ohmic structure device (a) and the nonalloyed ohmic structure device (b). The gate length is 0.13 m and the gate width is 40 m. The maximum gate bias voltage is 0 V and the voltage steps are 0.1 V each. Even with the double doped structure, kink phenomena V, especially at and drain current hysteresis around at Vds V, can be observed in the nonalloyed ohmic structure. Vg However, in the direct structure no kink phenomena is evident, and drain current hysteresis occurs only when the gate bias is set to nearly the threshold voltage. The drain conductance in both structures was about 46 mS/mm. Since the appearance of kink phenomena and drain current hysteresis differs greatly with the type of ohmic structure used, it is assumed that the mechanism for these factors depends on the current paths and the electric field intensity at the recess edge of the drain side. Gate hole currents induced by impact ionization were measured by subtracting the Schottky current from the total measured gate current [8]. Fig. 5 shows the total gate current contours and Fig. 6 shows the gate hole currents due to impact ionization versus gate to source voltage with drain voltage as a parameter. Fig. 5(a) and Fig. 6(a) show the current contours of the direct ohmic structure device, and Fig. 5(b) and Fig. 6(b) show that of the nonalloyed ohmic structure device. There are double peaks in the gate current [9]. Although in [9] the authors mentioned the second peak, observed at positive

Fig. 5. Total gate currents versus gate to source voltage with drain voltage as a parameter (a) a direct ohmic structure and (b) a nonalloyed ohmic structure (Lg = 0:13 m, Wg = 80 m).

gate bias, was attributed to the linearly constant effective length of the high field region, we consider the two peaks of the hole currents indicate that there are two high electric field regions in the device. Since the electric field at the drain edge of the gate V/cm to V/cm electrode increased from as the gate bias was added to minus voltages (from 0 V to 0.4 V) in our simulation, we consider that the peak around V (peak1) relates to the drain edge of the gate elecVg V, there are trode. When the device is operating around Vg many carriers in the channel and the impact ionization occurs frequently in the current concentrated region. Thus, the peak V (peak2) relates to the recess edge of the around Vg drain side. Peak1 is dominant in the direct structure, and peak2 is dominant in the nonalloyed structure. The maximum value of V is 14.4 A for the the hole current of peak2 at Vds direct structure and 26.5 A for the nonalloyed structure. This conclusively demonstrates that current paths change according to the ohmic structure, that the electric field can be reduced and impact ionization suppressed at the recess edge of the drain side, and that hole currents induced by impact ionization are reduced in the direct ohmic structure. Since holes generated at the recess edge of the drain side have to drift to a gate and source region affected by electric fields and traps between the gate and the drain, otherwise the holes generated at the drain edge of the gate electrode only flow into the gate or drift to a source region without

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Fig. 8. Measured integrated emission intensities along the gate finger, comparing the direct ohmic structure with the nonalloyed ohmic structure (Vds = 1:2 V, Vg = 0 V, Lg = 0:13 m, Wg = 40 m, Lgsr = 0:02 m, Lgdr = 1:0 m).

Fig. 6. Gate hole currents induced by impact ionization of (a) a direct ohmic structure and (b) a nonalloyed ohmic structure (Lg = 0:13 m, Wg = 80 m).

Fig. 7. Spatial distribution of emission intensities for a direct ohmic structure device (Vds = 1:2 V, Vg = 0 V, Lg = 0:13 m, Wg = 40 m, Lgsr = 0:02 m, Lgdr = 1:0 m).

being affected by the state between the gate and the drain, it is also considered that the I–V characteristics are more strongly affected by the hole current of peak2 compared with that of peak1. To discuss the relationship between ohmic structures and the electric field, we measured the emission intensities from an operating device. Fig. 7 shows spatial distribution of the emission intensities for a direct ohmic structure device. For this measurement, we used a cooled CCD camera equipped with a microV, scope that had a 0.3 m spatial resolution and biased Vg V for operating the device. Since the CCD and Vds camera’s sensitivity was 400–1100 nm we couldn’t measured the spectrum like that in [10]. But mainly signals emitted in the visible region could be obtained. The luminescence mechanisms between gate and drain are not only interband transitions in the recombination process in which holes generated by impact ionization drift to the source side, where they accumulate [11], but are intraband transitions of channel electrons that have gained a large amount of energy in the high electric field region [12]. Fig. 8 compares the measured integrated emission intensity along the gate finger of the direct ohmic structure and that of nonalloyed ohmic structure. The direct ohmic structure and nonalloyed ohmic structure had almost the same drain current with V and Vds V. The the bias conditions, Vg

emission intensity is weaker in the direct ohmic structure and appears to be uniform between the gate and drain. The emission intensity of the device with the nonalloyed ohmic structure was strongly distributed in the gate width direction, but is small in the direct ohmic structure. Comparison of the emission intensities between gate and drain shows that the electric field intensity is uniformly reduced in the device with the direct ohmic structure. This is in agreement with the tendency evident in the simulation result shown in Fig. 3. Thus we can conclude that the number of holes generated by the impact ionization in the recess edge of the drain side of the direct ohmic structure is smaller than in the nonalloyed ohmic structure. Next we evaluated the hysteresis of the drain current in the nonalloyed structure [Fig. 4(b)]. We consider that such a drain current feature originates from traps because it becomes remarkably visible when it is measured with a curve tracer and several Hz pulse I–V characteristics are measured. Fig. 9 indicates the result of pulse I–V measurements in which we monitored Ids on the condition to which the duration of Vds was changed. The period and height of Vds pulses were 250 ms (4 Hz) and 2 V, respectively, and the duration of a Vds pulse was changed from 20 ms to 240 ms. The Ids increased monotonically as the duration of a Vds pulse increased. The measurements indicated that the frequency of trapping/detrapping processes was on the order of several Hz or above. And we assumed these traps were related at the interface of InP/InAlAs or the surface of InP, but it’s not clear now present. Moreover the drain current hysteresis in the curve tracer does not appear if the drain voltage is applied within 2 V. To investigate the influence of current paths that flow near the recess surface on hysteresis, we compared the I–V characteristics of two kinds of direct ohmic structure devices having different recess lengths between the gate and drain (Lgdr). The hysteresis increased with Lgdr, from 0.04 m to 0.50 m. This shows that the hysteresis may be strongly affected on both the current paths near the recess edge and traps in the gate-drain region. Therefore, we consider that the drain current hysteresis occurs through capture the holes that have generated on the recess edge of the drain side by impact ionization in traps and subsequent release. Depending on the frequency at which the curve tracer is operating (100 Hz), the capture and release processes occur by turns and drain current hysteresis appears. When we use nonalloyed ohmic structure devices for broadband optical

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Fig. 9. Dependence of the Ids in a nonalloyed structure on a Vds pulse duration (The pulse period and height of Vds is 250 ms and 2 V, respectively. See insertion figure. Vg = 0 V, Lg = 0:13 m, Wg = 40 m).

communication ICs, this hysteresis may causes a drain conductance frequency dispersion that prevents stable operation. In the direct ohmic structure, holes are not generated at the recess edge and the current paths are set apart from the hole traps, which eliminates the hysteresis. We also consider that the small hysteresis observed when negative bias is applied to Vg in the I–V characteristics of Fig. 4(a) and (b) relates to the hall generation at the drain edge of the gate electrode. The detailed mechanisms of this hysteresis are currently being examined. IV. CONCLUSION In summary, we fabricated InP-HEMTs with a direct ohmic structure that consists of Ni/AuGe/Au electrodes directly contacting the channel. We controlled the current paths in the device and decreased the electric field concentration at the recess edge of the drain side by reducing the current path concentration across the heterointerface. Kink phenomena and drain current hysteresis were eliminated in the I–V characteristics of the transistor. We believe the reason for this is that the number of holes generated by impact ionization on the recess edge of the drain side can be controlled, and the electric field between the gate and the drain can be reduced. We also believe that a hole trap mechanism could explain the elimination of hysteresis in the I–V characteristic. By using this direct ohmic structure for InP-HEMTs, we’ll be able to improve the characteristic of the transistor by shortening the gate lengths while maintaining the excellent drain conductance of these devices, and so achieve greater flexibility of circuit design. Improved reliability, which is a key issue for commercial applications, is also expected. ACKNOWLEDGMENT The authors would like to thank K. Imanishi, K. Makiyama and N. Okamoto for their fruitful discussions and M. Takikawa for his encouragement. REFERENCES [1] T. Takahashi, M. Nihei, K. Makiyama, M. Nishi, T. Suzuki, and N. Hara, “Stable and uniform InAlAs/InGaAs HEMT IC’s for 40-Gbit/s optical communication systems,” in Proc. IPRM, 2001, p. 614. [2] A. Ernst, M. Somerville, and J. del Alamo, “Dynamics of the kink effect in InAlAs/InGaAs HEMT’s,” IEEE Electron Device Lett., vol. 18, pp. 613–615, Dec. 1997. [3] M. Somerville, A. Ernst, and J. del Alamo, “A physical model for the kink effect in InAlAs/InGaAs HEMT’s,” IEEE Trans. Electron Devices, vol. 47, pp. 922–930, May 2000.

[4] B. Georgescu, M. A. Py, A. Souifi, G. Post, and G. Guillot, “New aspects and mechanism of kink effect in InAlAs/InGaAs/InP inverted HFET’s,” IEEE Electron Device Lett., vol. 19, pp. 154–156, May 1998. [5] T. Suemitsu, T. Enoki, N. Sano, M. Tomizawa, and Y. Ishii, “An analysis of the kink phenomena in InAlAs/InGaAs HEMT’s using two-dimensional device simulation,” IEEE Trans. Electron Devices, vol. 45, pp. 2390–2399, Dec. 1998. [6] S. Selberherr, Analysis and Simulation of Semiconductor Devices. New York: Springer-Verlag, 1984. [7] N. D. Arora, J. R. Hauser, and D. J. Roulston, “Electron and hole mobilities in silicon as a function of concentration and temperature,” IEEE Trans Electron Devices, vol. ED-29, pp. 292–295, 1982. [8] R. Webster, S. Wu, and A. F. M. Anwar, “Impact ionization in InAlAs/InGaAs/InAlAs HEMT’s,” IEEE Electron Device Lett., vol. 21, pp. 193–195, May 2000. [9] U. Auer, R. Reuter, P. Ellrodt, W. Prost, and F. J. Tegude, “Characterization and analysis of a new gate leakage mechanism at high drain bias in InAlAs/InGaAs heterostructures field-effect transistors,” in Conf. Proc., 8th Int. Conf. on InP and Related Materials, April 1996, p. 650. [10] N. Shigekawa, T. Enoki, T. Furuta, and H. Ito, “Electroluminescence measurements of InAlAs/InGaAs HEMT’s lattice-matched to InP substrates,” in Conf. Proc., 8th Int. Conf. on InP and Related Materials, April 1996, p. 681. [11] N. Shigekawa, T. Furuta, T. Suemitsu, and Y. Umeda, “Optical characterization of impact ionization in flip-chip-bonded InP-based high electron mobility transistors,” Jpn. J. Appl. Phys., vol. 38, p. 5823, 1999. [12] N. Shigekawa, K. Shiojima, and T. Suemitsu, “Electroliminescence characterization of AlGaN/GaN high-electron-mobility transistors,” Appl. Phys. Lett., vol. 79, p. 1196, 2001. Ken Sawada was born in Osaka, Japan, on November 11, 1973. He received the B.S. and M.S. degrees in electronic science and engineering from Kyoto University, Kyoto, Japan, in 1996 and 1998, respectively. In 1998, he joined Fujitsu Laboratories, Ltd., Kanagawa, Japan. Since then, he has been studied and developed InP-based HEMT devices and their fabrication techniques. Mr. Sawada is a member of the Japan Society of Applied Physics. Tomoyuki Arai was born in Kobe, Japan, in 1974. He received the B.S. and M.S. degree in materials science and engineering from Kyoto University, Kyoto, Japan, in 1998 and 2000, respectively. In 2000, he joined Fujitsu Laboratories, Ltd., Kanagawa, Japan, where he has been working on development of InP-based HEMTs devices.

Tsuyoshi Takahashi was born in Tochigi, Japan, in 1963. He received the B. E. and M. E. degrees in science and engineering from University of Tsukuba, Ibaraki, Japan, in 1985 and 1987, respectively. In 1987, he joined Fujitsu Laboratories, Kanagawa, Japan. There he has been engaged in research on fabrication technology for InP-based HEMTs and InGaP-emitter HBTs. Mr. Takahashi is a member of the Japan Society of Applied Physics and the Institute of Electronics, Information, and Communication Engineers of Japan. Naoki Hara (M’99) was born in Tokyo, Japan, on February 9, 1963. He received the B. E., M. E., and Ph. D. degrees from the University of Tokyo, Tokyo, Japan in 1985, 1987, 1990, respectively. In 1990, he joined Fujitsu Laboratories, Ltd., Kanagawa, Japan, where he has been engaged in the research and development of HEMTs and other heterostructure devices. Currently, he is also with the Nanoelectronics Collaborative Research Center, Institute of Industrial Science, University of Tokyo. Dr. Hara is a member of the Institute of Electronics, Information, and Communication Engineers of Japan and the Japan Society of Applied Physics.