Photoconductive Semiconductor Switches - Materials, Physics, and Applications

At a Glance

Metadata	Details
Publication Date	2025-01-10
Journal	Applied Sciences
Authors	Vincent Meyers, Lars F. Voss, Jack Flicker, Luciano Garcia Rodriguez, Harold P. Hjalmarson
Institutions	University of New Mexico, Lawrence Livermore National Laboratory
Citations	3
Analysis	Full AI Review Included

Photoconductive Semiconductor Switches (PCSS): Technical Analysis

Executive Summary

Value Proposition: PCSS devices offer unique advantages for high-power, high-frequency switching, including ps-scale jitter, remote optical triggering (immunity to EMI), and high voltage/current handling, making them significantly more compact than conventional switches (e.g., IGBTs or spark gaps).
High-Gain Operation (Lock-on): Direct bandgap materials (GaAs, GaN) exhibit a non-linear, high-gain mode (“lock-on”) where conduction persists after the optical trigger ends, drastically reducing optical energy requirements. This mode is not observed in indirect bandgap materials like SiC or Diamond.
GaN Lock-on Physics: The proposed mechanism for GaN involves deep acceptor defects (Mn or Fe) creating a high-field region near the anode. Optical triggering initiates avalanche injection, leading to sustained current filamentation independent of the external optical source.
Material Tradeoffs: GaAs offers high mobility but poor durability (<10⁴ shots). SiC and UWBG materials (Diamond, beta-Ga₂O₃) offer superior thermal properties and breakdown fields but typically operate only in the lower-gain linear mode.
SSCB Demonstration: The technology was successfully integrated into a 6 kV Medium-Voltage DC Solid-State Circuit Breaker (SSCB), where a GaN PCSS diverted fault current away from cascaded SiC JFETs, confirming its utility in demanding electrical utility applications.
Engineering Challenge: Commercial viability requires fundamental efforts to understand and engineer the lock-on mechanism across different materials to improve shot lifetime (currently limited, especially in GaN/GaAs) and reduce electrical damage.

Technical Specifications

Material Properties Comparison (4H-SiC, GaN, Diamond, beta-Ga₂O₃)

Parameter	Value	Unit	Context
GaN Bandgap	3.39	eV	Direct bandgap.
4H-SiC Bandgap	3.26	eV	Indirect bandgap.
Diamond Bandgap	5.47	eV	Ultra-wide bandgap (UWBG).
beta-Ga₂O₃ Bandgap	4.5	eV	UWBG, indirect bandgap.
GaN Breakdown E-Field	3.3	MV/cm	High critical field.
Diamond Breakdown E-Field	10	MV/cm	Highest critical field listed.
GaN Electron Mobility	900	cm² V^-1s^-1	High saturation drift velocity (2.5 x 10⁷ cm/s).
Diamond Electron Mobility	4500	cm² V^-1s^-1	Highest mobility listed.
4H-SiC Thermal Conductivity	4.5	W/cmK	Excellent thermal management capability.
GaN Thermal Conductivity	1.3	W/cmK	Lower than SiC or Diamond.

GaN PCSS Performance (Mn-Doped, Lateral Geometry, 600 µm Gap)

Parameter	Value	Unit	Context
Bias Field	40	kV/cm	Used for lock-on testing.
Minimum Trigger Energy (Mn)	22.5	µJ	Required to initiate high-gain mode (800 nm).
Mean Delay (Mn)	27.8	ns	Time between laser pulse and switch closing.
Jitter (Mn)	2.1	ns	Low jitter achieved at high field.
Reliability (Mn)	100	%	Reliability at 40 kV/cm bias.
Lock-on Threshold Voltage	~40	V	Observed for specific n-i-n structure and capacitor combination.
Lock-on Trigger Wavelength	800	nm	Extrinsic triggering via deep traps.

Medium-Voltage DC Circuit Breaker (SSCB)

Parameter	Value	Unit	Context
System Voltage	6	kV	Prototype demonstration voltage.
Load Current (Test)	5	A	Line current during turn-off transient test.
HV Switch Topology	Cascaded	N/A	Series connection of 8 x 1.2 kV SiC JFETs.
Normally-Off Leg	GaN PCSS + C	N/A	PCSS diverts current to shunt capacitor (C) during fault.
PCSS Trigger Energy (SSCB)	1.35	mJ/pulse	Laser energy used for successful 6 kV turn-off.

Key Methodologies

Material Selection and Doping:
- High-quality, semi-insulating (SI) substrates were used: GaAs, SiC:V, GaN:Mn, GaN:Fe, and Fe-doped beta-Ga₂O₃.
- GaN substrates (~300 µm thick) were grown via ammonothermal or Hydride Vapor-Phase Epitaxy (HVPE) to achieve high resistivity (10⁹-10¹⁴ Ω·cm) using Mn or Fe deep acceptors.
Device Fabrication and Geometry:
- Lateral PCSS: Constructed as unipolar two-contact devices with Ti/Al/Ni/Au contacts separated by 0.6 to 3 mm gaps.
- Vertical PCSS: Fabricated with identical coaxial metallization on the front (anode) and back (cathode). Low-fill factor hole grids were used on the anode for nonlinear mode testing to seed current filaments.
Optical Triggering System:
- A 5 ns pulsed laser (Nd:YAG or similar) was used for triggering.
- Wavelengths were varied (650 nm to 1050 nm) to study extrinsic absorption pathways via deep traps (e.g., Mn or Fe levels in GaN).
Lock-on Characterization:
- Nonlinear switching (lock-on) was confirmed when the applied field and pulse energy exceeded material-specific thresholds (e.g., >25 kV/cm and >22.5 µJ for Mn-doped GaN).
- Wavelength-resolved cameras monitored luminescence to distinguish between reflected laser light, surface flashover, and internal current filamentation (a signature of high-gain lock-on).
Circuit Integration (SSCB):
- A 6 kV DC SSCB prototype was assembled with two parallel legs: a normally-on leg (cascaded 1.2 kV SiC JFETs) and a normally-off leg (GaN PCSS in series with a shunt capacitor).
- Precise timing coordination between the JFET gate driver and the PCSS optical driver was critical to ensure the PCSS activated at the minimum required voltage threshold (V_th) to achieve lock-on and divert current safely.

Commercial Applications

The unique characteristics of PCSS devices, particularly those operating in high-gain mode, position them for use in several high-power and high-frequency domains:

MVDC Power Systems:
- Solid-State Circuit Breakers (SSCBs) for rapid fault protection in medium-voltage DC grids (5 kV to 65 kV).
- High-efficiency power conversion and switching in DC distribution systems.
Pulsed Power and Directed Energy:
- Generating ultra-wideband (UWB) electromagnetic pulses.
- High-repetition-rate pulse amplifiers and kHz amplifiers, leveraging the ps-scale jitter.
Renewable Energy Integration:
- Advanced switching components for energy storage systems and efficient conversion of renewable energy sources (e.g., solar, wind) into grid power.
High-Temperature/Radiation Environments:
- Utilizing UWBG materials (SiC, GaN, Diamond) for reliable switching in extreme conditions where conventional silicon devices fail.
Electric Vehicle (EV) Infrastructure:
- High-speed, high-power switches required for advanced EV charging stations and power management.

View Original Abstract

Photoconductive semiconductor switching (PCSS) devices have unique characteristics to address the growing need for electrically isolated, optically gated, picosecond-scale jitter devices capable of operating at high voltage, current, and frequency. The state of the art in material selection, doping, triggering, and system integration in PCSSs is presented. The material properties and doping considerations of GaN, GaAs, SiC, diamond, and β-Ga2O3 in the fabrication of PCSS devices are discussed. A review of the current understanding of the physics of the high-gain mode known as lock-on is presented.

Tech Support

Original Source

DOI: https://doi.org/10.3390/app15020645

References

2018 - Current state of photoconductive semiconductor switch engineering [Crossref]
2015 - High Power Lateral Silicon Carbide Photoconductive Semiconductor Switches and Investigation of Degradation Mechanisms [Crossref]
1997 - Photoconductive semiconductor switches [Crossref]
2018 - Lock-on physics in semi-insulating GaAs—Combination of trap-to-band impact ionization, moving electric fields and photon recycling [Crossref]
2019 - Numerical studies into the parameter space conducive to “lock-on” in a GaN photoconductive switch for high power applications [Crossref]
1996 - Impact ionization model for full band Monte Carlo simulation in GaAs [Crossref]