Gunn and mixer diodes will ‘power’ future terahertz security-imaging systems. The Gunn diode's low phase noise, moderate output power, reliable long-term operation, and relatively low cost make it ideal for millimeter-wave imaging systems. Our newly developed gallium arsenide (GaAs) Gunn diodes incorporate a step-graded aluminum-GaAs (AlGaAs) hot-electron launcher (see Figure 1). They are commercially mass-produced by e2v Technologies Ltd. (UK) for use in millimeter-wave systems. Tight specifications are placed on parameters such as radio-frequency (RF) output power, applied voltage-controlled frequency-tuning range, and the oscillation frequency's temperature dependence. These depend on hot-electron-injector composition and carrier concentrations in the injector's doping spike and transit region.1 We developed a physically based predictive device model to complement experimental data and perform sensitivity analysis on the effects of variations in epitaxial composition (especially carrier concentration in the doping spike and transit region).
Figure 1. Scanning-electron-microscopy image of a fabricated Gunn diode (right) and the 3D cylindrical model (left). WD: Working distance.
To develop our model, we used SILVACO, a physics-based software platform that provides a virtual wafer-fabrication (VWF) environment for 2D or 3D device simulations using the Atlas simulation software.2 It allows simulations of the electrical, optical, and thermal behavior of semiconductor devices under given bias conditions to obtain their DC, RF, and time-domain responses. Since accuracy of the simulated response depends on one's choice of material parameters and the selection of physical models (and their associated parameters),3 we extensively researched the material and model parameters. Where the software did not allow sufficient flexibility to accurately account for mutual parameter dependence, we wrote appropriate functions in C and incorporated them through the built-in C interpreter.
We grew Gunn-diode wafers epitaxially using a Riber V100 molecular-beam-epitaxy reactor (see Figure 2). The hot-electron injector, situated between the n+ cathode contact layer and the n− transit region, consists of two main components, a graded AlxGa(1−x)As launcher and a doping spike. In the absence of a spike, a depletion region forms behind the launcher (see Figure 3). This prevents nucleation of high-field domains, thus decreasing device efficiency.
Figure 2. Epitaxial structure and conduction-band diagram for the GaAs Gunn diode with step-graded AlGaAs launcher.
Figure 3. Simulation results showing the effects of the doping spike (1×1018/cm3) on electron concentration in the transit region (solid/dashed lines: spike present/absent).
A 77GHz (second harmonic) model3 provided ~45GHz fundamental frequency, which is in line with the calculated oscillation frequency for a free-running device. We used the current asymmetry (the ratio of reverse to forward bias-threshold current) to evaluate injector performance, particularly doping-spike carrier concentration (n). It yielded 1×1018/cm3 as optimal value,2 which we used for higher-frequency model development. The transit-region length (L) was reduced, while the region's nL product was kept constant.
We show the simulated forward- and reverse-bias current-voltage (IV) characteristics of ~100GHz fundamental mode in Figure 4, which match the measured data extremely well. We measured a fundamental power of ~21mW at 98GHz.
Figure 4. Simulation results showing the measured and simulated current-voltage (IV) characteristics at 300K.
In summary, we successfully extended a 2D physical model for an advanced step-graded AlGaAs hot-electron-injector Gunn diode to higher frequencies (>100GHz). The good match between measured and simulated IV curves validates our choice of physical models and material parameters. Future work will include development of a fully functional multiplier as a high-power terahertz source for frequencies up to 600GHz.
The UK's Science and Technology Facilities Council and the Defence Science and Technology Laboratory are gratefully acknowledged for supporting this program through the industrial program support scheme (PIPSS). Faisal Amir is funded on a scholarship from the National University of Sciences and Technology (Pakistan) through this PIPSS project. Thanks are also due to Mike Carr of e2v Technologies (Lincoln, UK) for supplying the experimental data.
Mohamed Missous, Faisal Amir, Colin Mitchell
School of Electrical and Electronic Engineering
University of Manchester
Mo Missous leads the departmental semiconductor efforts. He has a proven track record in innovation coupled with technology transfer. He maintains close industrial involvement with leading players in the micro- and millimeter-wave fields worldwide.
Faisal Amir is undertaking a PhD degree using physically based, predictive modeling of advanced, graded-gap Gunn diodes for use at millimeter waves and terahertz frequencies.
Colin Mitchell is a senior research associate. He is developing millimeter-wave Schottky multipliers using both GaAs and advanced indium gallium arsenide/indium aluminum arsenide.