J. Son

Picosecond pulse propagation on coplanar striplines fabricated on lossy semiconductor substrates: modeling and experiments

A simple model for the propagation of high-frequency signals on coplanar striplines with lossy semiconductor substrates is proposed and demonstrated. This model incorporates the effect of a conductive substrate through the loss tangent in a distributed-circuit analysis extended to high frequencies. Very strong attenuation and dispersion due to the substrate are observed even when the GaAs conductance is only 1 mho/cm, corresponding to a doping density of around 10/sup 15/ cm/sup -3/. The accuracy of this model is tested with a direct comparison to experimental data of picosecond pulse propagation on a doped GaAs coplanar stripline (CPS) measured in the time domain using the electro-optic (EO) sampling technique. Good agreement is found in terms of the attenuation and phase velocity of the distorted pulses at four propagation distances up to 300 mu m. The pulse propagation on a multiple modulation-doped layer is also studied experimentally as a prototype of high-frequency signal propagation on the gate of a modulation-doped field-effect transistor (MODFET). The attenuation shows linear frequency dependence up to 1.0 THz, contrary to the cubic or quadratic dependence of coplanar transmission lines on low-loss substrates. >

Transient velocity overshoot dynamics in GaAs for electric fields ≤ 200 kV/cm

We have experimentally studied the transient velocity overshoot dynamics of photoexcited carriers in GaAs for electric fields as great as 200 kV/cm. Time domain waveforms proportional to the velocity and the acceleration of carriers have been acquired, respectively, from guided and free‐space radiating signals which contain terahertz frequency components. The measurements demonstrated that the degree of overshoot was maximized for an electric field on the GaAs between 40 and 50 kV/cm when 1.44‐eV photons in an 80‐fs laser pulse excited the sample. For carriers excited with higher initial energy (1.55 eV), the degree of overshoot decreased and the maximum degree of overshoot occurred at a higher electric field.

Time-domain network analysis of MM-wave circuits based on a photoconductive probe sampling technique

A photoconductive probe sampling technique with 2-ps temporal resolution and microvolt sensitivity has been developed. The photoconductive probe sampling technique combines the ultrafast optical technology of the 120-fs Ti-sapphire short pulse laser and the microfabrication technology of the silicon-on-sapphire photoconductive sampling probe, which consists of a high-impedance interdigitated photoconductive switch. This technique can be applied to the measurement of the S-parameters of millimeter-wave circuit components with a 120-GHz measurement bandwidth. The probe technology was applied to the characterization of millimeter-wave band-block filters used in InP-based heterostructure MMICs (monolithic microwave integrated circuits) for 90-GHz to 180-GHz frequency doubling. Millimeter-wave delay lines have also been characterized, and the properties of transmission lines on thin semiconductor substrates have been studied.<<ETX>>

Comments on "Picosecond pulse propagation an coplanar striplines fabricated on lossy semiconductor substrates: modeling and experiments"[with reply]

The objective of this correspondence is to point out that the propagation of picosecond pulses with superluminal speed has been reported in the original paper [see ibid, vol. 41, p. 1574-80, 1993]. A reply is given. >

Picosecond Pulse Propagation On Coplanar Striplines With Lossy Semiconductor Substrates

The widely used microstrip, as well as the higberbandwidtb coplanar transmission lines, may both be fabricated on semiconductor substrates that could become lossy due to unintentional doping during the growth of thin-film layer. The effect of these substrate losses, particularly on the ultra-short-duration signal propagation which will be required in future high-speed circuits, has not been thoroughly considered. ”his is the case even though the distortion of ultrawide-bandwidth signals would severely degrade the performance of microwave and digital circuits. We demonstrate the attenuation and dispersion characteristics of coplanar striplines (CPS) fabricated on poorly insulating semiconductor substrates both experimentally and computationally by observing the evolution of picosecond electrical pulses guided by these structures. In our computations, a time-domain waveform propagating on a transmission line has been represented as a signal V(f,z). Considering V(f,O) as the waveform at a spatial origin, V(f,z), was thus given by