1290 IEICE TRANS. ELECTRON., VOL.E93–C, NO.8 AUGUST 2010 PAPER Special Section on Heterostructure Microelectronics with TWHM 2009 A Third Order Harmonic Oscillator Based on Coupled Resonant Tunneling Diode Pair Oscillators Koichi MAEZAWA †a) , Senior Member, Takashi OHE † , Student Member, Koji KASAHARA † , and Masayuki MORI † , Nonmembers SUMMARY A third order harmonic oscillator has been proposed based on the resonant tunneling diode pair oscillators. This oscillator has signifi- cant advantages, good stability of the oscillation frequency against the load impedance change together with capability to output higher frequencies. Proper circuit operation has been demonstrated using circuit simulations. It has been also shown that the output frequency is stable against the load impedance change. key words: resonant tunneling diode, oscillator, coupled oscillators, THz sources 1. Introduction A resonant tunneling diode (RTD) is an ultrahigh frequency device showing a negative differential resistance (NDR). This NDR works as a basis for oscillators, and they are promising for future THz signal sources. The RTD oscilla- tors were intensively studied from 1980s to early 1990s [1]– [4]. Ultrahigh frequency oscillations up to 712 GHz were demonstrated in 1991 [5]. However, research activities on this field declined after those days due to the lack of real applications. Recently, increasing attention to THz wave technology revived the interests in RTD oscillators [6]. A record high-frequency oscillation of 831 GHz has been re- ported most recently [7], and also the THz signal generation based on harmonics has been also reported [8]. The RTD oscillators, however, still have some difficult problems for practical applications. The first is a bias insta- bility due to the NDR itself, which exists from dc to THz fre- quency range [9]. This often causes a low frequency spuri- ous oscillation. The second is a frequency instability caused by the variation of the load impedance. RTDs have only two terminals, so that the isolation of the core oscillation cir- cuit from the output load is difficult. Though an isolator or circulator can be used for this purpose, conventional isola- tors/circulators are difficult to apply in the frequency range we are interested in (higher than 100 GHz). Optical isolators are also difficult to apply this frequency range. To overcome the first problem, we proposed and demonstrated the RTD pair oscillators consisting of series connected RTDs [10]–[12]. In this paper, we propose a har- monic oscillator based on the coupled RTD pair oscillators Manuscript received November 17, 2009. Manuscript revised February 22, 2010. † The authors are with the Graduate School of Science and En- gineering, University of Toyama, Toyama-shi, 930-8555 Japan. a) E-mail: maezawa@ieee.org DOI: 10.1587/transele.E93.C.1290 to solve the second problem. 2. RTD Pair Oscillators Figure 1 shows the basic circuit configuration of the RTD- pair oscillator [10]. This oscillator consists of two RTDs connected serially and a resonator connected to the node be- tween the two RTDs. The serially-connected RTDs are bi- ased by voltages with the same absolute value with the op- posite sign. The most significant advantage of this is that it separates the oscillation node from the bias nodes. This per- mits us to connect large capacitors (C S ) to the bias nodes, which stabilize these nodes, while they have no effect on the high-frequency oscillation. This is in contrast with the conventional RTD oscillators. We first fabricated the circuits based on the RTDs on InP substrates and demonstrated basic operations [11]. Next, we fabricated the circuits on the AlN ceramic sub- strates employing novel heterogeneous integration technol- ogy [12]. This integration enabled us to mount large chip capacitors close to the core circuit, which further stabilized the circuits. 3. Coupled RTD Pair Oscillators As described above, there are two problems when applying RTD oscillators to real applications. The first problem, the bias instability, can be solved by the RTD pair configura- tion. To solve second problem, sensitivity of the oscillation Fig. 1 A series-connected RTD oscillator. Copyright c  2010 The Institute of Electronics, Information and Communication Engineers