Planar Varactor and Mixer Diodes Fabricated Using InP-Based Materials

This paper reports on a planar integrated technology which, unlike previous approaches, utilizes InP-based materials and airbridge technology to reduce parasitics by avoiding the use of a bridge-supporting dielectric. Verticalheterojunction varactors (VHV) and mixers were grown by the in-house Metalorganic-Chemical Vapor Deposition (MOCVD) system on S.1. InP substrates. A novel process is presented which allows fabrication of planar diodes with very short airbridges, a low risk of anode/ohmic undercutting, and without need for replanarization. Methods have been investigated here for replacing the InP substrates with glass and quartz. Systematic studies on VHV's revealed the effects of varying the barrier thickness (d 1 ), epilayer thickness (d2) and barrier indium concentration (x). Control of Cmin , Crnax/Cinia, reverse saturation current density (J), forward and reverse leakage, and Schottky barrier height (ybe) was demonstrated via choice of device layer structure and alloy composition. The technology allowed demonstration of varactors with leakage currents which could be substantially reduced by design and material choice. Microwave characterization permitted the study of cutoff frequencies (f c) as functions of anode diameter for several values of d 1 , d,, and x. One gm VHV's showed fc 's in the THz range. First results on the 1 gm InGaAs/InP mixers showed much lower forward turn-on voltages than for similarly-sized GaAs diodes. This suggests the possibility of diode mixing using smaller local oscillator power levels than GaAs diodes. IL Introduction Traditionally, whisker anode contacting technology has been the de-facto standard for submillimeter mixers and varactor multipliers. While whisker contacting produces low parasitics, it is suitable only for discrete devices, has limited reliability, and consequently is difficult to space qualify. To address these problems, GaAs planar diodes have been proposed and demonstrated M. In many GaAs-based planar diode technologies [1], [2], anode openings are etched through a film of Si02 which also supports the evaporated airbridges. Usually, isolation is achieved by laterally undercutting the n+ GaAs under the airbridge. In short-airbridge designs, the isolation etch runs the risk of undercutting the anode. In [2], a solution is proposed using a trench isolation technique in which the trench is anisotropically etched before the airbridge is fabricated. However the trench in [2] also requires a planarizing step. Planar varactor and mixer diodes have traditionally relied on GaAs and AlGaAs/GaAs material systems to achieve high performance at submillimeter and THz frequencies. In recent years, Work supported by NASA contract NAGW-1334 Fifth International Symposium on Space Terahertz Technology Page 515 improvements in growth technology have made it feasible to utilize the performance advantages offered by the InP-based material systems. The high mobility and conductivity of InP-based materials (InGaAs) enables the reduction of access resistance (R s). The low barrier heights (91,e) of mixers using InGaAs and InP Schottky junctions are especially useful in reducing LO power requirements. The InAlAs/InGaAs heterojunction of InP-based varactors achieves an electron barrier of 0.5eV vs. 0.26-0.3eV for AlGaAs/GaAs, resulting in potentially reduced leakage relative to GaAs/AlGaAs varactors. A first demonstration of the InAlAs/InGaA.s varactor principle was recently reported by the authors [3]. To gain full advantage of the InP-based system for planar diodes, the authors have developed a novel isolation technique that avoids the use of Si02 and the BHF Si0 2 etch, thus avoiding etching damage to the InAlAs barrier layers used in InP-based varactors. By avoiding use of the Si02 under the bridge this novel planar diode process also reduces parasitic capacitance by approximately ifE which is significant compared to the 3-6fF of 11.tm diodes. Furthermore, this process avoids the need of undercutting the airbridge while not requiring a replanarization step. Section III. of this paper covers layer structures of the demonstrated InP-based varactor and mixer diodes. A novel planar diode fabrication technique is discussed in section IV. Several advantages, such as reduction of parasitic capacitance and ease in dicing accrue from the replacement of the InP substrate. Therefore section V illustrates the replacement of the diodes' InP substrate with glass. Finally, sections VI and VII present measured dc and microwave performance of the planar varactors and mixers. III. Varactor and Mixer Layer Structures Figure 1. illustrates the layer structures for the varactor and mixer diodes. All structures covered here were grown in-house via Metalorganic-Chemical Vapor Deposition (MOCVD) on S.I. InP substrates. The 1.1,tm thick n+ InGaAs layer serves to provide a low sheet resistance and good ohmic contacts. The heterojunction varactors here use an undoped InGaAs active layer (thickness d 2 = 400A and 800A) to support a variable-thickness depletion region. In all cases, the InGaAs is lattice-matched to the InP substrate. The In„Al l As barrier layer thickness (d 1 ) is 100A. or 200A, Decreasing x is expected to increase the electron bather height, and decrease varactor leakage. To study the impact of x, some varactors have been fabricated with x = 0.4 (strained) and x = 0.52 (latticematched). Optimization of the InAlAs significantly affects performance and required a special study as reported in [4] and [5]. The mixer structure uses an 800A i-InP active layer. The InP/InGaAs heterojunction region is Varactor Mixer Schottky metal )11011DITUMEITII Active Layer