Photoconductive Switches

Dr. Rüdiger Paschotta

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Photoconductive Switches

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Definition: electric switches controlled by light via photo-induced conductivity

Categories: article belongs to category photonic devices photonic devices, article belongs to category optoelectronics optoelectronics

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DOI: 10.61835/r7j Cite the article: BibTex plain text HTML Link to this page! LinkedIn

A photoconductive switch is an electrical switch which is based on the photoconductivity of a material, i.e. an increase in its electrical conductance as a consequence of irradiation with light. In nearly all cases, one uses a semiconductor material, where the absorbed light (with a photon energy above the band gap energy) generates free carriers, which then contribute to the conductivity. Frequently used materials are chromium-doped gallium arsenide (Cr-GaAs), low-temperature grown gallium arsenide (LT-GaAs), indium phosphide (InP), amorphous silicon, and silicon on sapphire (SoS). In order to reduce the recovery time of the switch (determined by the lifetime of photoexcited carriers), one typically uses low-temperature growth (often followed by rapid thermal annealing), some doping (e.g. chromium in GaAs), or ion bombardment for producing crystal defects. Apart from the recovery time, important criteria are the bandgap energy, dark resistivity, and electrical breakdown resistance.

There are different designs of photoconductive switches:

bulk devices several millimeters or even centimeters long with electrical contacts on the end faces, used for switching very high voltages (sometimes above 100 kV)
devices with a small gap in a microstrip; the gap can be straight or interdigitated and has a width between a few microns and tens of microns; for low-power applications with very high speed
sliding contact devices for the highest speed, where a point between the two parallel strips of a coplanar stripline is illuminated

All such devices are of the metal–semiconductor–metal (MSM) type.

Photoconductive switches are used for various purposes:

for photoconductive sampling, particularly for testing of high-speed integrated electronic circuits (even before dicing the wafer because electrical contacts are required only for DC and low-frequency signals)
for the generation of terahertz pulses
for the generation of microwaves and millimeter waves via direct DC to RF conversion, in both continuous-wave and pulsed mode (e.g. with a frozen waveform generator)
as high-speed photodetectors in optical fiber communications
in very fast analog-to-digital converters

More to Learn

Photoconductive sampling

Electro-optic sampling

Bibliography

[1]	F. W. Smith et al., “Picosecond GaAs-based photoconductive optoelectronic detectors”, Appl. Phys. Lett. 54 (10), 890 (1989); https://doi.org/10.1063/1.100800
[2]	D. Krokel et al., “Subpicosecond electrical pulse generation using photoconductive switches with long carrier lifetimes”, Appl. Phys. Lett. 54, 1046 (1989); https://doi.org/10.1063/1.100792
[3]	C. H. Lee, “Picosecond optics and microwave technology”, IEEE Trans. Microwave Theory Technol. 38 (5), 596 (1990); https://doi.org/10.1109/22.54928

(Suggest additional literature!)

Questions and Comments from Users

2024-07-12

We develop LT-GaAs photoconductive switches in our laboratories for pump and probe measurements. These devices are rather long (about 2 mm × 100 µm). Until now, we have always fully illuminated the “strip”, but now we would like to try measurements where we will illuminate only the central area confined to a few tens of microns.

How does the size of the illuminated spot affect the photocurrent? I see two different ways of thinking:

The energy band view leads me to say that it should not be decisive; once the electrons reach the conduction band, they move under the influence of the electric field.
However, an “electrical engineer” perspective suggests that the drop in resistivity should be practically negligible because I am simply creating a small low-resistivity area “immersed” between two regions where, since light has not reached them, the resistivity remains very high.

What is now true?

The author's answer:

This depends on how far carriers can drift in the normally non-conductive gap. Their lifetime in LT-GaAs is normally very small, possibly less than a picosecond. On the other hand, the drift velocity with an applied electric field can be quite high – on the order of 10⁵ m/s. Multiplying these numbers leads to a drift distance on the order of 0.1 μm only. From that, I conclude that the carriers will not be able to drift through the full gap width, and that you will presumably not see significant photocurrents when focusing the radiation to such a small spot.

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