Monolithic RTD Array Oscillators at 100 GHz and 200 GHz with On-Wafer Bias Stabilization

We report 100 GHz and 200 GHz monolithic slot antenna coupled RTD array oscillators. The IC process for the array oscillators incorporates 0.1μm InGaAs/AlAs Schottky-collector RTDs (SRTDs) vertically integrated above AlInGaAs Schottky-diode stabilizers which suppress parasitic oscillations in the DC bias circuit. A quasi-optical transmitter/receiver set up using the RTD oscillator as transmitter and a Schottky-diode mixer as receiver is employed for testing these oscillators. Designs above 200 GHz have shown signals on a bolometer and frequency measurements of these are currently in progress. 712 GHz wave guide RTD oscillators have been demonstrated [1]. Power levels however are low due to constraints imposed on device area for suppressing parasitic bias circuit oscillations and also due to the difficulty in integrating several discrete waveguide oscillators for power combining. Higher power levels can be obtained with monolithic RTD arrays in which parasitic oscillations in the bias circuit are suppressed by on-wafer bias stabilizers [2]. The monolithic RTD oscillator [Fig.1] consists of a low impedance Schottky-diode bias stabilizer located λosc/4 from the SRTD. The shunt combination of SRTD and the Schottky-diode has no net negative differential resistance in its DC characterisitics [Fig.4]. At the design frequency of oscillation fosc, the low impedance Schottky-diode bias stabilizer is decoupled from the SRTD and the SRTD admittance is now shunted only by the slot antenna admittance. The layer structure [Fig.2] consists of a graded bandgap AlInGaAs Schottky-diode layers grown beneath the SRTD layers. The SRTDs have 0.1μm Schottky-collectors, 350 Å space-charge layers, peak current density of 5 × 10 A/cm, and have an estimated fmax of ≈ 2.2 THz [3]. Array fabrication demands an aggresive 8 mask IC process for monolithic integration of the SRTDs, the Schottky-diode bias stabilizers, SiN MIM capacitors, resistors and airbridges [Fig.3]. SRTD oscillators were tested with a quasi-optical transmitter/receiver set up [Fig.5]. The receiver is a broad band bowtie-antenna-coupled 0.1μm×0.5μm×150 Å Schottky-diode operated as a harmonic mixer with a 48 − 60 GHz LO for downcoverting the RF oscillations to a 2 − 12 GHz span of the spectrum analyzer. Signals were detected at 100 GHz and 200 GHz [Fig.6]. The Si lens on which the oscillator array is placed forms the array’s external resonant cavity, determining the oscillation frequency and the oscillator Q. The oscillation frequency was determined by shifting the LO frequency and observing the magnitude and sign of the frequency shift of the IF spectrum. The 100 GHz, 2×1 array oscillated at 108.92 GHz with −34 dBm IF output power while the 200 GHz, 2×1 array oscillated at 196.4 GHz with −44 dBm IF output power. The 100 GHz, 4×4 array showed oscillations at 94.46 GHz (−28dBm) and at 107.48 GHz (−41dBm) which correspond to two cavity resonant frequencies (95% of oscillator output power in dominant mode). The measured IF power level which includes coupling losses in the quasi optical set up, conversion loss of the mixer and the preamplifier gain is roughly −20 to −30 dB below the oscillator output power. The resulting 100μW to 1mW estimated oscillator output power must be verified using a calibrated bolometer. Extensive testing of higher frequency oscillators using a bolometer and a Fabry-Perot interferometer are currently in progress. This work is supported by ONR under contract N00014-93-0378, JPL President’s Fund, AFOSR and QUEST. The authors wish to acknowledge discussions with H.S. Tsai and R.A.York from University of California, Santa Barbara.