The conversion efficiency for planar Al0:7GaAs/ GaAs heterostructure barrier varactor triplers is shown to be reduced from a theoretical efficiency of 10% to 3% due to selfheating. The reduction is in accordance with measurements on planar Al0:7GaAs/GaAs heterostructure barrier varactor (HBV) triplers to 261 GHz at room temperature and with low temperature tripler measurements to 255 GHz. The delivered maximum output power at 261 GHz is 2.0 mW. Future HBV designs should carefully consider and reduce the device thermal resistance and parasitic series resistance. Optimization of the RF circuit for a 10-m diameter device yielded a delivered output power of 3.6 mW (2.5% conversion efficiency) at 234 GHz.

In this paper, we propose a simple set of accurate frequency-domain design equations for calculation of optimum embedding impedances, optimum input power, bandwidth, and conversion efficiency of heterostructure-barrier-varactor (HBV) frequency triplers. A set of modeling equations for harmonic balance simulations of HBV multipliers are also given. A 141-GHz quasi-optical HBV tripler was designed using the method and experimental results show good agreement with the predicted results.

By the Schrödinger and Poisson equations, we have theoretically investigated AlGaAs/GaAs and InAlAs/InGaAs single barrier varactors. The energy band structure, carrier distribution, and conduction current are fully exploited for varactor design. We have explained the experimental current–voltage and capacitance–voltage measurements very well. A simple analytical model for energy band structure is derived based on the Schrödinger and Poisson equation calculation. It is found that a barrier structure of 3 nm Al0.3Ga0.7As/3 nm AlAs/3 nm Al0.3Ga0.7As for an Al0.3Ga0.7As/GaAs varactor and a barrier structure of 8 nm In0.52Al0.48As/3 nm AlAs/8 nm In0.52Al0.48As for In0.52Al0.48As/In0.47GaAs are optimal for minimal conduction currents.

We in¨estigate efficiency limitations of frequency multipliers with the use of a simple model for symmetric¨aractors. Our ( ) calculations show that the con¨ersion efficiency is impro¨ed for a C V shape with large nonlinearity at zero¨olt bias. For quintuplers, the optimal embedding impedance at the third harmonic is an inductance in.w x to-back barrier-N-layer-N diode bbBNN 3 . These varac-Ž . tors will only produce odd harmonics 2 n q 1 и f when p pumped at the frequency f . The absence of even harmonics p simplifies the realization of higher-order multiplier circuits. For the tripler case, it is possible to realize a multiplier Ž . circuit considering only the pump frequency f and the p Ž . output frequency 3 и f . For the quintupler case only three p frequencies have to be considered: the pump frequency, the Ž . output frequency 5 и f , and the idler at the third harmonic p Ž . 3иf . Currents at frequencies higher than the output frep quency have usually only a minor influence on multiplier performances. Multipliers using symmetric varactors may therefore be useful as future millimeter-and submillimeterwave sources.Limiting factors for varactor performances are the breakdown voltage, resistive losses, and capacitance magnitude and shape. A commonly used figure of merit for predicting multiplier efficiencies is the cutoff frequency, defined in Section III. However, the cutoff frequency does not consider the Ž . shape of the C V characteristic. In this article a simple Ž . analytical model is adopted to describe the C V characteristic and a frequency-and bias-independent series resistance is Ž . assumed. We investigate how the shape of the C V characteristic influences the maximum multiplier efficiency, and how to choose the idler load for maximum efficiency. Moreover, we exemplify how our results can be important for the design of varactor diodes. The varactors mentioned in the introduction have in common that the elastance nonlinearity originates from a bias-de-Ž . pendent depletion-layer thickness. For Eq. 1 to describe one of the mentioned varactors, the differential elastance must increase monotonously for increasing charge. From this constraint it follows that for fixed S and S we can min max Ž . identify two extreme cases, which we denote flat S V and Ž . sharp S V . Each case has their own set of coefficients ␤ and ␥ , as shown in Table 1. II. THE POLYNOMIAL MODELGoing beyond the values for ␥ given in Table 1 tance can be tuned out with a series-connected inductance in resonance, the maximum varactor efficiency is only dependent on the nonlinear part of the elastance, and is therefore independent of S . min III. TRIPLER ANALYSESWe made the calculation with the use of an in-house harmonic balance program. Losses were introduced with a resistance R in series with the nonlinear elastance. The currents at harmonics higher than the output frequency were assumed to be zero, which is the optimal condition. To describe the efficiency and output power versus R , S , V , and S ,it is convenient to nor...

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