Effect of Source, Drain and Channel Spacing from Gate of HEMT

Effect of Source, Drain and Channel Spacing from Gate of HEMT Shashank Kumar Dubey and Aminul Islam Abstract This paper presents an AlGaN/GaN HEMT on 6H–SiC substrate. The impact of horizontal optimization of gate terminal with source-to-gate spacing and drain-to-gate spacing on DC and RF characteristic of the high-electron-mobility transistor (HEMT) and impact of vertical optimization of gate terminal with recessing have been represented in this paper. The 1.25×/1.32×/1.204× improvements in the drain current/transconductance/gate-source capacitance have been reported as the source-to-gate spacing varied from 1.833 to 0.633 µm. The 0.78× decrement in the ON-resistance of the device has been recorded as the drain-to-gate spacing varied from 3.327 to 1.327 µm. The recessing of the gate resulted in a 2.45× increment in the transconductance at the cost of severe decrement in the drain current as the recessing depth varied from 0 to 22 nm. Keywords HEMT · Source-to-gate spacing · Drain-to-gate spacing · Recessed gate 1 Introduction With the rise in demand for RF and microwave technology in the last couple of decades, the conventional semiconductors could not meet the demand due to various limitations in their high frequency and high-power applications. This has paved up the way for wide bandgap materials like GaN, SiC, etc. Among these, GaN-based heterostructure devices have shown great potential for the present and upcoming high power and microwave frequency application. This is because of its remarkable physical and electrical properties like high breakdown electric field, high saturation velocity and electron mobility. The breakdown field of GaN devices is about ten times S. K. Dubey · A. Islam (B) Department of Electronics and Communication Engineering, Birla Institute of Technology, Mesra, Ranchi, Jharkhand, India e-mail: aminulislam@bitmesra.ac.in S. K. Dubey e-mail: dubey.shashank1991@gmail.com © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2020 A. Sikander et al. (eds.), Energy Systems, Drives and Automations, Lecture Notes in Electrical Engineering 664, https://doi.org/10.1007/978-981-15-5089-8_8 81 82 S. K. Dubey and A. Islam higher than that of Si devices, observed as high as 3.3 MV/cm. Further, improvement in the breakdown voltage and high-temperature operation capability is achieved with the introduction of wider bandgap semiconductor material SiC. High electron mobility transistor (HEMT) is a field-effect transistor where a channel is formed by a junction formed by two different wide bandgap materials. In case of GaN HEMTs, GaN is the thick epitaxial layer over which resides the thin barrier layer, and a highly mobile two-Dimensional Electron Gas (2-DEG) is present at the heterointerface [1]. Due to their highly linear performance at high frequency, GaN-based HEMTs are widely utilized for high-power applications, RF applications like cellular telecommunications, imaging, RADAR and radio astronomy. Low noise amplifiers, oscillators and mixers are designed using GaN HEMTs attributing to their low noise and better high-frequency performance [2]. Due to technical advancements, HEMT devices have been used extensively for modelling of powerful devices because of its high-speed switching operations [1, 2]. These devices with high cut-off frequency are also widely used for power electronics applications such as power conditioning and microwave transceiver for communications [3]. The term high-electron mobility in HEMT signifies the higher mobility the electrons exhibit in the HEMT devices in comparison to that of other transistors, e.g. MOSFETs. The electrons from the heavily doped wide bandgap material diffuse into the undoped narrow-bandgap material and form a channel. Since, the channel is isolated from the area where the electron concentration is in bulk thus, HEMTs exhibit higher mobility. The AlGaN/GaN-based HEMT shows better performance than others on account of its existing material characteristics such as high-electron density, high breakdown voltage, large electric breakdown field (1.5 × 107 V/m, as compared to 2.5 × 105 V/m of GaAs), high-electron mobility (1000 cm2 V−1 s−1 ), high-electron saturation velocity (1.5 × 107 cm/s) and large bandgap energy (3.42 eV, as compared to 1.4 eV of GaAs) [4–7]. The AlGaN/GaN HEMT has high spontaneous and piezoelectric polarization, which gives a two-dimensional electron gas (2-DEG) with electron density of about 1013 cm−2 for Al content in the range of 30–40% in barrier layer, without the need of the doping. Moreover, the GaN-based device gives 50× higher power capacity than the GaAs-based device [8]. HEMT devices find wide applications in the fields of imaging, radio astronomy, wireless communications, military (missile, RADAR seeker) applications, mobile communication and short-wavelength lasers and LEDs, radio detection and ranging (RADAR) [3, 4, 7]. HEMT-based devices are formed by combining hetero-materials (i.e. junction between two materials with different bandgaps and nearly equal lattice constants). When AlGaN and GaN materials are brought close to each other, a spontaneous polarization [9] occurs that leads to trapping of electrons in the narrowbandgap material referred to as 2-DEG thereby forming a conducting channel [4]. Thus, AlGaN/GaN HEMT devices exhibit 2-DEG without doping. The key reason behind the formation of potential well (i.e. 2-DEG) at the interface is the mismatch of the lattice constants of AlGaN and GaN which eventually results in the piezoelectric polarization [10]. The potential well forms channel in which electrons flow. The HEMT is a three-terminal device namely source, drain and gate terminals. The electrons flow from the source to the drain. The source and drain terminals form Effect of Source, Drain and Channel Spacing … 83 the ohmic contact whereas, the gate terminal forms the Schottky contact. The gate terminal is a terminal which controls the movement of carriers by varying the channel width. The proper distances of source and drain with respect to gate are of great importance as they affect the performance of the device [11]. The source-to-gate spacing influences the drain current by varying the source resistance and impacts the transconductance while the threshold voltage remains almost the same. It also influences the gate-source capacitance and cut-off frequency of the HEMT device. The drain-togate spacing affects the breakdown voltage [12] which as a result impacts the ONresistance. The recessing of gate is also an important technique to optimize the DC performance of a HEMT device. The contribution of this work is as follows • This paper studies impact of gate-to-source spacing to optimize the DC parameters such as drain-to-source current (I D ) and transconductance (gm ). • It also examines effect of gate-to-drain spacing to optimize the ON-resistance of the device. • It investigates the impact of gate recessing on the DC parameters such as drainto-source current (I D ) and transconductance (gm ). The entire work has been ordered as follows: Sect. 2 presents the structure of the proposed device. The simulation results are discussed in Sect. 3. The paper is finally concluded in Sect. 4. 2 Device Structure The simulated device was modelled on a 6H-SiC substrate shown in Fig. 1. A 23-nm heavily doped and wide bandgap barrier layer of AlGaN was designed above the 2µm channel layer of undoped and narrow-bandgap GaN. Since, there exists a lattice mismatch between GaN and 6H–SiC, a thin nucleation layer of AlN was designed. The fraction of Aluminium in the AlGaN barrier layer was 0.22. Moreover, a 3-nm GaN capping layer was designed on top of the AlGaN barrier layer to reduce the gate leakage current. 3 Simulation Results and Discussion 3.1 Effect of Source-to-Gate Spacing Figure 2 illustrates the effect of source-to-gate spacing L SG on the drain current of the device. The drain current has been observed to increase as the L SG decreased. The increase in drain current with the decrease in L SG occurs since [13] 84 Fig. 1 The simulated proposed AlGaN/GaN HEMT on 6H-SiC Fig. 2 Drain current versus source-to-gate distance S. K. Dubey and A. Islam Effect of Source, Drain and Channel Spacing … ID = 85 VDS 2RC + RS + RD (1) where V DS is drain-to-source voltage, RS is the source resistance, I D is drain current, RC is channel resistance and RD is drain resistance. Thus, if RS can be made smaller, the I D can be made larger. The idea to decrease RS is in accordance with the following relation given by RS = L SG qn 0 µ0 (2) where L SG is source-to-gate spacing, n0 is electron concentration, µ0 is electron mobility. Thus, by decreasing the L SG , we have improved the drain current of the HEMT device. The threshold voltage (V t ) is found to vary from −1.6 to −1.5 V as L SG is decreased from 1.833 to 0.633 µm. Thus, analyzing a typical transfer characteristic curve, it is evident that higher currents obtained with downscaled L SG where the V t effectively remains the same lead to a high transconductance gm . This effect can be seen in Fig. 3. The increase in the gate-source capacitance C GS with the reduction in L SG occurs since as source and gate terminals are kept closer, the capacitance increases, and this is shown in Fig. 4. The cut-off frequency f T remains almost the same as C GS and gm both increase with the reduction of L SG as fT = Fig. 3 Transconductance versus source-to-gate distance gm 2π (CGS+ CGD ) (3) 86 S. K. Dubey and A. Islam Fig. 4 Gate source capacitance versus source-to-gate distance Therefore, the increase in gm and C GS compensate for any change in the f T . This is shown in Fig. 5. As can be observed from Table 1, the I D increases as L SG decreases. Fig. 5 Cut-off frequency versus source-to-gate distance Effect of Source, Drain and Channel Spacing … Table 1 Effect of L SG on DC response 87 L SG (µm) I D (A/µm) gm max (S/µm) C gs max (fF/µm) 1.833 0.00036312 0.00023 1.678 1.633 0.00037376 0.000239 1.6769 1.533 0.00037874 0.000244 1.711 1.133 0.00041195 0.000266 1.7818 0.933 0.00042379 0.00028 1.9618 3.2 Effect of Drain-to-Gate Spacing Our investigation on variation of drain-to-gate spacing (L DG ) reveals that the drain current is mostly not affected. As the gm and C GD remain unaffected due to the variation in L DG the f T remains unchanged. The impact of L DG variation on the RF performance of the device is reported in Fig. 6. As can be visualized from the figure, the cut-off frequency remains almost unchanged even though the L DG is downscaled from 3.327 to 1.327 µm. The effect of L DG variation on the DC parameter such as ON-resistance of the device is reported in Fig. 7. As can be seen from Fig. 7, the ON-resistance increases as the L DG increases. The increased L DG increases the breakdown voltage [14]. The breakdown voltage V BR impacts on ON-resistance RON of the device [15] as RON = 2 4VBR . εS µn E c3 (4) where V BR is the breakdown voltage, E c is the critical electric field, µn is the mobility of electrons and εS is the permittivity of the semiconductor. Fig. 6 Cut-off frequency versus Drain-to-gate distance (L GD ) 88 S. K. Dubey and A. Islam Fig. 7 ON-resistance versus Drain-to-gate distance (L GD ) 3.3 Impact of Gate Recessing The impact of position of the terminals in a direction parallel to channel has been discussed earlier in this paper. The position of a gate terminal in a direction perpendicular to the channel is also important as it changes the characteristics of the device. In this work, the gate to channel spacing is varied from 0 to 22 nm to observe its effect on transconductance, drain current and electron concentration in the channel. The results of this investigation are plotted in Figs. 8 and 9. Fig. 8 Transconductance versus different recess depth Effect of Source, Drain and Channel Spacing … 89 Fig. 9 Electron concentration in channel versus recessing depth Figure 8 shows the transconductance versus recess depth curve. The transconductance increases as the recess depth increases. This is because the gate terminal has better control over the two-dimensional electron gas (2-DEG) as the gate to channel distance reduces. The same is modelled by [15] gm = ε0 εr veff W h (5) where gm is transconductance, veff is the effective velocity, W is the device width and h is the gate channel separation. The recessing of gate is done in order to achieve small h. The layer under the gate foot is cut and gate terminal is extended into it. This leads to a decrease in h those results in an increase of gm . However, higher transconductance is obtained at the cost of lower drain current. This reduction is justified by the decreased electron concentration in the channel with deeper recessing of gate as shown in Fig. 9. Since the role of a gate terminal is to deplete the channel having the carriers, therefore, deeper recessing gives the gate a better control over the channel and increases the depletion. 4 Conclusion This paper presents the impact of source-to-gate spacing and drain-to-gate spacing on DC and RF performance of the HEMT. The impact of recessed gate has also been reported. Thus, this work reports in brief about the source, drain and gate terminal designing of the HEMT. 90 S. K. Dubey and A. Islam Acknowledgements This material is based on work supported by Defence Research and Development Organization (DRDO) under Sanction letter no. ERIP/ER/DG-MED & CoS/990216301/M/01/1675 dated 04 July 2017. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the DRDO. References 1. Chakroun A et al (2017) AlGaN/GaN MOS-HEMT device fabricated using a high quality PECVD passivation process. 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