725 GHz sampling circuits integrated with nonlinear transmission lines

W e have measured 0.68 p s electrical step functions, the fastest reported to date, generated by nonlinear transmission lines (NLTLs) integrated with sampling circuits which have a 3dB bandwidth of at least 725GHz. From the measured waveforms, the average velocity of the depletion edge of the varactor diodes on the NLTL is calculated to be 2.1. lo7 cm / sec. Because the other time constants in the circuit are much smaller than the measured 0.68 p s , this velocity saturation is believed to be the limiting phenomenon of the circuit performance. An NLTL is an electrical step function generator consisting of a high-impedance transmission line periodically loaded with varactor diodes [ I ] . The NLTL per-diode propagation delay q,Tllr,ov = [ L,,,,y (C,,,,, + Cdrode ( V))]1'2 is a function of the diodes' capacitance, and both decrease with increasing reverse bias voltage. The falling edge of the waveform becomes steeper during propagation since the delay for the waveform peak is greater than for the trough. A shock wave is formed whose transition time is limited by the diodes' cutoff frequency f , = (27~i?~C~/) ' and the Bragg frequency f,, = ( 7 ~ . Tlelrrv)-' that arises from the NLTL's periodicity. To make f,, high the diode separation must be decreased, and this spacing becomes limited by the diodes' physical size. As the line dimensions are reduced, the diodes' pad parasitics contribute a large fraction of C,,,,, and lower f,, . To address both these problems, coplanar waveguide (CPW) transmission lines were fabricated with the center conductor raised off' the substrate. Others have used a similar technique to reduce loss at the line input [2], but the impact is much greater at the high frequency output end. By elevating the center conductor 3 p m above the substrate, the wave velocity is doubled and, hence, for the same physical separation between diodes, f ' , is doubled. The process flow consists of forming ohmic contacts, ion implanting to provide isolation, depositing metal for the Schottky contacts and CPW ground planes, and then applying a layer of polyimide. The polyimide is subsequently etched in an 0 2 R.I.E. system until = 0 . 2 p m of the Schottky metal is exposed. The posts for the air bridge lines are formed on top of the polyimide and provide the contacts to the tops of the diodes. After electroplating the air lines, the polyimide is removed, leaving the contacts between the CPW and the diodes in air, substantially reducing the parasitic capacitance. By contacting the diodes this way, a small Schottky contact can be placed in the middle of a larger active region. The regions outside the active areas are H + implanted to render them semi-insulating, and the lateral straggle of the ions damages the active regions near the mask edge. By contacting the diodes from the top, l p m x l p m sampling diodes were fabricated that suffered no performance degradation from the Hf lateral straggle. The NLTLs are designed to have f,, = 1500GHz and the Ipin diodes on GaAs with an active layer of N , = 1,1017 cm-' have f, = 4THz [3]. NLTLs providing separate strobe and test signals are integrated with sampling circuits to provide for on-wafer measurements. The bandwidth of the sampling circuits is determined by the aperture time of the strobe pulse and the RC time constant of the sampling diodes, both of which are < 0 . 2 p s , The sampled waveform has a 3.7 V step with a 0.68 p s 10% 90% falltime. In previous work, where the time constants of the sampling circuits and the NLTLs were comparable, a simple sum-of-squares deconvolution has been used to determine the sampler bandwidth. Using that method here gives a conservative estimate for the sampler bandwidth of 725 GHz . In the NLTL's varactor diodes, the depletion edge moves 145nm in 0.68 p s , giving an average velocity of 2.1.107 cm / sec. Because all other time constants in the circuit are much lower than this, the velocity saturation, which has been analyzed by others 141, appears to be the limiting phenomenon. Similar circuits will be fabricated on material with N , = 3. cm-' and a speed enhancement is expected because of the & reduction i n the distance the depletion edge will have to move.