A SPICE Large Signal Model for Resonant Tunneling Diode and Its Applications Mohammad Javad Sharifi and Yasser Mohammad! Banadaki Electrical and computer faculty of Shahid Beheshti university, Velanjak, Tehran, IRAN m i
[email protected].
[email protected].
[email protected] Abstract. In This paper, a new large signal SPICE model for RTD is presented which includes large signal capacitances for the first time. Then the usefulness of the model is considered in design, simulation and characterization of a large signal oscillator, a RF amplifier and a frequency multiplier. Keywords: Resonant TunneUng Diode, SPICE, Large Signal Circuit Model, Non-linear capacitance. PACS: Computer modeling and simulation, 07.05.Tp, Resonant tunneling devices, 85.30.Mn, Electronic circuits, 07.50.Ek, 84.30.-r, Microwave circuits, 84.40.Dc
INTRODUCTION Resonant Tunneling diode (RTD) is a quantum device with special features such as high current density, high speed and negative differential resistance that makes it suitable for designing many ultra high-speed analog and digital circuits with minimum complexity. In this regard a SPICE model is needed and gready helps designers by making possible SPICE simulations of proposed circuits. Up to now, many circuit models have introduced for RTD but all of them were large signal DC models [1, 2] or small signal RF models [3], and no AC large signal model introduced yet. This happened because SPICE hasn't any nonlinear capacitance elements such as CTABLE or QTABLE etc. So a new method is needed to insert the nonUnear capacitance of RTD in SPICE. This method is presented in this paper by using GTABLE and ETABLE capabiUties and some additional elements. Parameters that are used in this model are valued with the results of a device level simulator such as NEMO [4] or with direct measurements. After this introduction, in section 2 the detailed description of the proposed SPICE model is explained. Then time and frequency simulation for some large signal circuits using our introduced model is presented in section 3.
LARGE SIGNAL MODEL OF RTD There are different ways to incorporate a new device such as RTD in SPICE that some of them are more usual, like as GPOLY, GVALUE and GTABLE [2, 5]. Among them, GTABLE is the easiest approach [5]. It uses a table consisting of pairs of numbers for voltages and currents of the element. It must note that GTABLE is a four terminals part and the correct way to use it as a model for RTD which has only two terminals is very important. To this end we connect the input and output terminals of GTABLE together in a way that is shown in fig.l [5] but it seems that some previous works that have used GTABLE in their model didn't take care of this point. In figure (3) of [2], the I-V characteristics of two series connected identical RTD is shown. We see two maxima and two minima in the figure but this is not correct, because two serially connected identical RTDs must have a I-V characteristics similar to a single RTD with a scaling in the voltage axes. As we said the failure is perhaps due to a mistake in GTABLE connections in the modeling. IN+OUT, IN- OIJ
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FIGURE 1. Correct connection of GTABLE for RTD modeUng.
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The AC large signal RTD model must contain the large signal capacitance. This capacitance may be obtained from small signal capacitance c(v) that may achieved by simulation results or by direct measurements [3]. In order to add the AC large signal behavior of RTD to the DC model of fig.l, some additional elements are needed (see fig 2). To obtain this model, as the first step, we calculate Q(v), from the small signal capacitance c(v), according to equation (1).
(?(v) = I c(v') dv' Then, this Q(v) is placed in another SPICE element, ETABLE, as shown in figure 2 such that its output voltage will be a signal that is proportional to Q(t) at any time. Next a RC differentiator circuit is added which gives a signal proportional to the current of the capacitance from the output voltage of ETABLE. After that a two-port linear element, G, is used to compensate the RC circuit loss and to transform the voltage to current. In this way the output current of G is the large signal current of the RTD capacitance. So it is added to the two-port GTABLE output current which was used for conductance. Finally, a small resistance (Rs) is added to the circuit to improve simulation stabiUty and convergence. At the end, a schematic symbol is selected for the whole circuit and added to the library of PSPICE as a new bilateral device (see figure 2-B).
(A)
(B)
FIGURE 2. The SPICE large signal AC model of RTD. In this paper we have used a specific RTD [3] for simulation of some different circuits at the next sections. Fig.3 shows the C-V and Q-V characteristics of this RTD. CiiV&OrsV
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FIGURE 3. C-V and Q-V characteristics of RTD are used in the simulations. The geometry of this InAs/AlSb RTD is 1.5 nm emitter barrier width, 7.9 nm well width and 0.9 nm collector barrier width. The C-V characteristics were obtained from the simulation results of NEMO [3].
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APPLICATION TO SOME CIRCUITS SIMULATIONS The introduced large signal SPICE model for RTD is used for simulation of three large signal circuits in time and frequency domains. The circuits including a large signal oscillator at about 400GHz, A RF wideband amplifier with cut-off frequency about 50GHZ and a frequency multiplier that the ratio of the third harmonic to the first harmonic is about 3.33. Fig.4 shows the schematic diagram of the oscillator and its simulation result in time and frequency domains. Fig.5 show RTD's DC curve and a sample of input and output voltages in time domain when used as RF amplifier and Fig.6 show output signal in frequency domain and frequency response of RF ampUfier respectively. The schematic circuit diagram of wideband frequency multipUer and the simulation results of the circuit in time domain and its Fourier transform are shown in Fig 7. Detail of these circuits and their simulations are given in the full paper. wotma*ftM- m r t ^ miJiii
(a) FIGURE 4. (a) Circuit sdiematic diagram of large signal THz oscillator, (b) Output voltage of THz oscillator in time and frequency domains.
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