Lars Dillner

Effects of self-heating on planar heterostructure barrier varactor diodes

The conversion efficiency for planar Al/sub 0.7/GaAs-GaAs heterostructure barrier varactor triplers is shown to be reduced from a theoretical efficiency of 10% to 3% due to self-heating. The reduction is in accordance with measurements on planar Al/sub 0.7/GaAs-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 /spl mu/m diameter device yielded a delivered output power of 3.6 mW (2.5% conversion efficiency) at 234 GHz.

Heterostructure-barrier-varactor design

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.

A distributed heterostructure barrier varactor frequency tripler

We present a broadband nonlinear transmission line (NLTL) frequency multiplier at F-band. The multiplier consists of a finline section periodically loaded with 15 heterostructure barrier varactor (HBV) diodes. Tapered slot antennas are used to couple the fundamental signal from a WR-22 rectangular waveguide to the distributed multiplier as well as radiate the output power into free space. The frequency tripler exhibits 10-dBm peak radiated power at 130.5 GHz with more than 10% 3-dB bandwidth and 7% conversion efficiency. The tripler can be used as an inexpensive broad-band solid-state source for millimeter-wave applications.

Frequency multiplier measurements on heterostructure barrier varactors on a copper substrate

We have fabricated heterostructure barrier varactors (HBV) on a copper substrate, which offers reduced spreading resistance, and improved thermal conductivity compared to an InP substrate. The devices are fabricated without degrading the electrical characteristics. The three-barrier HBV material grown by MOVPE has a leakage current of only 0.1 /spl mu/A//spl mu/m/sup 2/ at 19 V. The maximum capacitance is 0.54 fF//spl mu/m/sup 2/. In a frequency tripler experiment a maximum output power of 7.1 mW was generated at 221 GHz with a flange-to-flange efficiency of 7.9%.

Analysis of Symmetric Varactor Frequency Multipliers

We investigate efficiency limitations of frequency multipliers with the use of a simple model for symmetric varactors. Our calculations show that the conversion efficiency is improved for a C(V) shape with large nonlinearity at zero volt bias. For quintuplers, the optimal embedding impedance at the third harmonic is an inductance in resonance with the varactor diode capacitance.

AlGaAs/GaAs and InAlAs/InGaAs heterostructure barrier varactors

By the Schrodinger 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 Schrodinger and Poisson equation calculation. It is found that a barrier structure of 3 nm Al0.3Ga0.7As/3 nm AlAs/3nm 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.

Capacitance analysis for AlGaAs/GaAs and InAlAs/InGaAs heterostructure barrier varactor diodes

By self-consistently solving Schro¨dinger and Poissons equations, we have investigated the capacitances of AlGaAs/GaAs and InAlAs/InGaAs heterostructure barrier varactors. When compared with semiclassical particle model, quantum mechanics show that the maximal capacitance of the varactor is saturated when the spacer and barrier are thin. When the spacer and barrier are very thick, both the quantum mechanics and semiconductor particle approach result in the same conclusion, namely, the maximal capacitance is inversely proportional to the sum of the spacer and barrier thicknesses. We have also shown that the maximal capacitance increases for high carrier effective mass.

Heterostructure barrier varactors at submillimetre waves

We present an overview of recent results concerning quantum barrier varactors, also called heterostructure barrier varactors (HBV). This device is normally constructed as a mesa device, with one or several thin (ca. 200 A) barriers made from a larger bandgap material. The diodes are excellent as multipliers and have several advantages over the common Schottky barrier varactor. The hbv only generates odd harmonics of the input power, can be built to handle large powers and permits a large device area at very high frequencies. We present theoretical design and performance characteristics for triplers and quintuplers and give an overview of hbv multiplier experiments.

A 141-GHz integrated quasi-optical slot antenna tripler

We present a quasi-optical frequency tripler with heterostructure barrier varactor (HBV) diodes soldered across two slot antennas and located at the focal plane of a dielectric lens. The slot antennas are fed from a WR-22 waveguide connected to a Gunn oscillator. A quasi-optical high-pass filter is used to tune the slot impedance and increase the conversion efficiency. The symmetric capacitance-voltage and asymmetric current-voltage characteristics of the HBV diodes only allows odd harmonics of the applied signal to be generated, and thus simplifies the frequency tripler design.

n-Si/SiO2/Si heterostructure barrier varactor diode design

Symmetric C–V and antisymmetric I–V characteristics are essential for a heterostructure barrier varactor (HBV) to generate odd harmonics in a frequency multiplier. Practically high multiplier efficiency is obtained when the shape of the C–V characteristic is sharp near zero bias and the conduction current is low. Here we present the design of an n-type Si/SiO2/Si-based HBV and its state-of-the-art device performance. Self-consistent solutions of the Schrodinger and Poisson equations show a drastic decrease of the conduction current due to the large electron effective mass and the SiO2 barrier height. The shape of the C–V curve can be easily tuned by modifying the thickness of the SiO2 layer. By the techniques compatible with the conventional Si technology, a Si/SiO2/Si varactor junction (having a SiO2 layer of 20 nm) has been processed and the device characteristics are very promising.