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Photoconductive Switches

Tiny black chip leans against the edge of a dime.
Credit: Feldman/NIST
Er-doped GaAs photoconductive switch that generates ultra-fast electrical pulses used for high-speed waveform next to a dime.

The Technology

The technology for transporting high-speed, high-bandwidth communications signals is improving at a rapid pace — and so is the challenge of ensuring that the waveform measurement instruments processing those signals at up to 145 GHz can do so accurately. That testing is performed using transfer standards in the form of calibrated photodiodes that send a well-characterized stream of pulses to the measurement instruments for use as a performance benchmark.

However, commercially available photodiodes are fundamentally limited in bandwidth by two factors: the switching speed/bandwidth of the semiconductor technologies that generate the pulses; and the frequency limit of single-mode operation of coaxial interconnects used to carry the calibration signal from the generator to the waveform measurement instrument.

NIST researchers are developing solutions to both problems in a package that could be used as next-generation transfer standards. For one, they are creating high-speed photoconductive switches (PCS) that can operate at much higher frequencies than traditional photodiodes. A PCS utilizes the fact that the ultrafast incident light very briefly increases electrical conductivity, enabling ultra-fast switching. The NIST team has demonstrated on-wafer photoconductive switches that operate at 150 GHz and beyond, with 2 ps pulse widths.

For the other, NIST scientists are at work on a chip-scale, on-wafer pulse generator suited to calibrating next-generation connectorless electronics. Circuits operating in an on-wafer environment have been demonstrated at frequencies into the terahertz regime.

Future goals include embedding the PCS devices as pulse generators for quantum science and quantum network applications.

Advantages Over Existing Methods

As electronics evolve away from band-limited connectorized devices, on-wafer test methodologies will require an on-chip reference source to characterize those circuits. This could be achieved via an optoelectronic chip-scale reference pulse generator, where an on-wafer calibration is made and transferred to the device under test through a wafer probe or other coaxial interconnect.  

Photodetectors operating in an on-wafer environment and integrated with other standards for scattering parameters and power could be used to characterize broadband on-wafer circuits. Photodetectors operating as reference signal sources in the range of 150 GHz are needed to characterize new and near-future instruments.

NIST’s photoconductive switches and circuits operating in an on-wafer environment are not limited in bandwidth or operating frequency by coaxial connectors.


Photodetectors are widely used to characterize the complex frequency response of waveform measurement instruments such as vector signal analyzers, sampling oscilloscopes and light-wave component analyzers. NIST’s high-speed photoconductive switch technology can be used at frequencies up to 150 GHz and beyond. It is expected to be very attractive to high-speed electronics and optoelectronics manufacturers.

Key Papers

J.A. Jargon, C.J. Long, A. Feldman and J. Martens. Developing Models for a 0.8 mm Coaxial VNA Calibration Kit within the NIST Microwave Uncertainty Framework. 2020 94th ARFTG Microwave Measurement Symposium (ARFTG). Jan. 2020. DOI: 10.1109/ARFTG47584.2020.9071653

J.A. Drisko, A.D. Feldman, F. Quinlan, J.C. Booth, N.D. Orloff and C.J. Long. Impedance tuning with photoconductors to 40 GHz. IET Optoelectronics. Jan. 24, 2019. DOI: 10.1049/iet-opt.2018.5102

A. Mingardi, W-D. Zhang, E.R. Brown, A.D. Feldman, T.E. Harvey and R.P. Mirin. High power generation of THz from 1550-nm photoconductive emitters. Optics Express. May 23, 2018. DOI: 10.1364/OE.26.014472

P.D. Hale, D.F. Williams and A. Dienstfrey. Waveform metrology: signal measurements in a modulated world. Metrologia. Aug. 3, 2018. DOI: 10.1088/1681-7575/aad1cd 

E.R. Brown, A. Mingardi, W.D. Zhang, A.D. Feldman, T.E. Harvey and R.P. Mirin. Abrupt dependence of ultrafast extrinsic photoconductivity on Er fraction in GaAs:Er. Applied Physics Letters. July 18, 2017. DOI: 10.1063/1.4991876

E.R.R. Brown, A. Feldman, T. Harvey, R.P. Mirin and W.-D. Zhang. Model for Ultrafast Extrinsic Photoconductivity in Er-Doped GaAs. 2016 41st International Conference on Infrared, Millimeter, and Terahertz waves (IRMMW-THz). Sept. 2016. DOI: 10.1109/IRMMW-THz.2016.7758557 

D.F. Williams. A prescription for THz transistor characterization. 2014 International Workshop on Integrated Nonlinear Microwave and Millimetre-wave Circuits (INMMiC). April 2014. DOI: 10.1109/INMMIC.2014.6815113  

T. Borsa, D.F. Williams, P.D. Hale and B. Van Zeghbroeck. Novel nano-structured metal-semiconductor-metal photodetector with high peak voltage. Japanese Journal of Applied Physics. June 22, 2009. DOI: 10.1143/JJAP.48.06FD03 

D. Williams, P. Hale and K.A. Remley. The sampling oscilloscope as a microwave instrument. IEEE Microwave Magazine. July 16, 2007. DOI: 10.1109/MMW.2007.383954 

D.F. Williams, P.D. Hale and T.S. Clement. Calibrated 200 GHz waveform measurement. IEEE Transactions on Microwave Theory and Techniques. April 18, 2005. DOI: 10.1109/TMTT.2005.845760 

D.C. Driscoll, M.P. Hanson, A.C. Gossard and E.R. Brown. Ultrafast photoresponse at 1.55 μm in InGaAs with embedded semimetallic ErAs nanoparticles. Applied Physics Letters. Jan. 27, 2005. DOI: 10.1063/1.1852092


Created June 10, 2020, Updated July 20, 2020