JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ Vol. 13 No 3, 03014(4pp) (2021)  13  3, 03014(4cc) (2021) The results were presented at the International Conference on Innovative Research in Renewable Energy Technologies (IRRET-2021) 2077-6772/2021/13(3)03014(4) 03014-1  2021 Sumy State University Utility of a Reverse Double-drift Structure for Fabricating GaN IMPATT Diode Operating in the Terahertz Regime Sahanowaj Khan 1,* , Rishav Dutta 2,† , Aritra Acharyya 3 , Arindam Biswas 2 , Hiroshi Inokawa 4,‡ , and Rudra Sankar Dhar 1 1 Department of Electronics and Communication Engineering, National Institute of Technology, Chaltlang, Aizawl, Mizoram 796012, India 2 School of Mines and Metallurgy, Kazi Nazrul University, Asansol, Burdwan, West Bengal 713340, India 3 Department of Electronics and Communication Engineering, Cooch Behar Government Engineering College, Cooch Behar, West Bengal 736170, India 4 Research Institute of Electronics, Shizuoka University, Hamamatsu 4328011, Japan (Received 21 March 2021; revised manuscript received 16 June 2021; published online 25 June 2021) Utility of the reverse double-drift region (DDR) structure has been studied for fabricating the gallium nitride impact avalanche transit time (IMPATT) diode operating at 1.0 terahertz (THz). Static and large- signal simulations have been carried out in order to verify the THz capabilities of conventional (normal) and reverse DDR structures. It is revealed that IMPATT operation is only possible in a reverse GaN DDR structure due to the lower value of series resistance of it as compared to the normal GaN DDR structure. Normal DDR GaN IMPATT cannot be operational at THz regime. Earlier, the authors had calculated the series resistance of conventional GaN DDR IMPATT diode designed to operate at 1.0 THz, however. They did not take into account the current crowing and spreading resistance at the ohmic metal contacts. That is why, the results were misleading. Those results lead to the conclusion that conventional THz GaN DDR IMPATT may produce sufficient effective negative resistance since the series resistance of it remains with- in the range of 1.5-2.0 Ω. In this paper, authors have proposed a reverse DDR IMPATT structure exclusive- ly for GaN material and THz frequency bands. By using this reverse DDR structure, p + -GaN ~ Ni/Au con- tact can obtain a sufficient contact area, so that the anode-contact resistance can be minimized. A non- sinusoidal voltage-excited large-signal model developed by the authors has been used to study the static (DC) and large-signal properties of conventional and reverse DDR structures at 1.0 THz. The present study on the evaluation of THz source seems to open a new horizon for THz researchers and scientists. Keywords: Double-drift region, GaN, IMPATT, Reverse DDR, Terahertz. DOI: 10.21272/jnep.13(3).03014 PACS numbers: 42.55.Px, 45.70.Ht * khannowda84@gmail.com † dutta.rishav17@gmail.com ‡ inokawa.hiroshi@shizuoka.ac.jp 1. INTRODUCTION Several researchers have already explored the te- rahertz (THz) proficiencies of gallium nitride (GaN). Especially, the potentialities of impact avalanche transit time (IMPATT) diodes based on GaN as THz sources are already well known [1-4]. The IMPATT diode can produce very small magnitude of negative resistance (|Rd| < 10 Ω) at THz regime. Therefore, the positive parasitic series resistance of the THz diode must be very small (Rs < |Rd|) in order obtain signifi- cant output power at THz frequencies. The primary components of Rs are un-swept depletion layers, n + - and p + -contact layers and contact resistances due to anode and cathode ohmic metal contacts [5]. Very low resistivity of n + -GaN and Ti/Al/Ti/Au ohmic contact can be achieved (~ 10 – 8 -10 – 6 Ωcm 2 ) [6]. But sufficiently low resistivity of p + -GaN and Ni/Au ohmic contact is very difficult to achieve; the minimum achievable resis- tivity remains of the order of 10 – 3 -10 – 2 Ωcm 2 [7]. Therefore, in order to keep the overall series resistance sufficiently low, the area of p + -GaN ~ Ni/Au contact must be as high as possible. In conventional n + -n-p-p + DDR structure having p + -layer at the top may not pro- vide sufficient contact area to the p + -GaN ~ Ni/Au con- tact, especially at the THz frequencies. For this study, the material parameters of GaN are taken for the sim- ulation from recently published literature [8-15]. 2. REVERSE DOUBLE-DRIFT STRUCTURE A conventional GaN based DDR IMPATT structure is shown in Fig. 1a [8]. The initial substance is a p-type GaN substrate. An n + -buffer layer of around 10 m thickness can be grown on the p-GaN substrate; the donor concentration of n + - buffer layer is 2.0×10 24 m – 3 . Next, the n + , n, p, and p + - layers are successively grown on the n + -buffer layer in order to form a conventional DDR structure; thickness and doping concentration of each layer are shown in Fig. 1a. Anode is formed by depositing Ni/Au on p + - layer, and similarly the cathode is formed by depositing Ti/Al/Ti/Au on n + -buffer layer (a ring-shaped cathode structure is elaborately described in an earlier paper [8]). In the reverse DDR IMPATT structure, the order of layers of conventional DDR is reversed; it is shown in Fig. 1b. Here, the initial substance is n-type GaN substrate over which a p + -buffer layer can be deposited.