ABSTRACT
Resonant tunneling diodes (RTDs) have a potential for compact and coherent terahertz (THz) sources operating at room temperature. In this chapter, a basic analysis of oscillation characteristics and recent progress of RTDs for THz sources are described. For high-frequency oscillation, reductions of capacitance, and electron delay time are important. Room-temperature fundamental oscillations up to 1.3-1.4 THz have been achieved until now. By a structure optimization, a fundamental oscillation close to 2 THz is feasible. Although the output power is usually 1-10 mW, an output power of more than 1 mW is possible by impedance matching between the antenna and the RTD. At present, 420 mW has been obtained at 550 GHz in a single oscillator by the offset-fed slot antenna. In a preliminary experiment on two-element arrays, 610 mW has been obtained at 620 GHz
with the mutual injection locking and coherent power combining between the oscillator elements. Spectral characteristics, including the linewidth and frequency change with bias voltage, are shown. A preliminary experiment on wireless data transmission by direct bias modulation is also demonstrated. 7.1 IntroductionFor various applications expected in the terahertz (THz) frequency range [1], compact and coherent solid-state sources are important key components. Because the THz range is located between light waves and millimeter waves, both optical and electronic devices are being investigated for THz sources. Figure 7.1 shows output power versus oscillation frequency for several semiconductor single oscillators at present. From the optical device side, p-type Ge lasers [2] and THz quantum cascade lasers (QCLs) [3-5] are studied. Oscillators with two-terminal devices, such as impact ionization avalanche transit-time (IMPATT) diodes, tunneling transit-time (TUNNETT) diodes, Gunn diodes, and resonant tunneling diodes (RTDs), are being investigated from the electron device side [6-17]. Heterostructure bipolar transistors (HBTs), high-electron-mobility transistors (HEMTs), and Si complementary metal oxide semiconductor (CMOS) transistors are rapidly approaching the THz range [18-26]. Devices based on Bloch oscillation, the plasma effect, and velocity modulation are also being studied [27-30]. RTDs have the highest oscillation frequency among the above electron devices. Room-temperature operation, which has not yet been achieved in a single optical device at present, is also an attractive feature. Research of RTDs began with the theoretical prediction by Tsu and Esaki in 1973 [31], and their behavior of negative differential resistance was experimentally demonstrated at liquid nitrogen temperature in 1974 [32] and at room temperature in 1985 [33]. Oscillation in the microwave range was demonstrated at a low temperature in 1984 [34]. The oscillation frequency was then updated many times to several hundred GHz [35], and a room-temperature fundamental oscillation at 712 GHz was reported in 1991 [9]. A 64-element arrayed oscillator containing RTDs and
its oscillation at 650 GHz were reported in 1997 [10]. A harmonic oscillation above 1 THz was also demonstrated [12]. The frequency of the room-temperature fundamental oscillation increased to 831 GHz in 2009 [13] and then exceeded 1 THz in 2010 [14]. The increase in oscillation frequency is still continuing, and oscillations of 1.3-1.4 THz have been obtained until now [15-17]. The output power is also increasing by structure improvement [36, 37].
