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, Sandia National Laboratories
Citations	3
Analysis	Full AI Review Included

Technical Documentation & Analysis: Photoconductive Semiconductor Switches

Executive Summary

This review highlights the critical role of Wide Bandgap (WBG) and Ultra-Wide Bandgap (UWBG) materials in advancing Photoconductive Semiconductor Switches (PCSS) for high-power, high-frequency applications.

UWBG Material Superiority: Diamond (5.5 eV bandgap, 10-20 MV/cm breakdown field, 22-24 W/cm·K thermal conductivity) is identified as the ideal material for next-generation PCSS due to its intrinsic properties, despite current technological immaturity.
High-Gain Switching Confirmed: Nonlinear (lock-on) switching mode, previously observed in GaAs, is confirmed in Mn-doped GaN PCSS devices, demonstrating high reliability (100%) and low jitter (2.1 ns) at high fields (40 kV/cm).
Extrinsic Triggering Requirement: To overcome the high bandgap of UWBG materials (Diamond, SiC, GaN), extrinsic doping (e.g., N in Diamond, V in SiC, Mn/Fe in GaN) is necessary to create deep donor/acceptor levels, enabling triggering using more accessible visible/IR laser wavelengths.
Application Validation: The GaN PCSS was successfully integrated into a 6 kV Medium-Voltage DC Solid-State Circuit Breaker (SSCB) prototype, demonstrating the practical utility of high-gain PCSS devices in commercial electrical utility applications beyond traditional pulsed power.
Material Engineering Focus: Future commercial viability hinges on precise control of material quality, defect concentrations, and doping profiles to optimize carrier generation and recombination lifetimes.

Technical Specifications

Parameter	Value	Unit	Context
Diamond Bandgap	5.47	eV	UWBG Material
Diamond Breakdown Electric Field	10	MV/cm	Critical strength field
Diamond Thermal Conductivity	2.2	W/cm·K	Superior heat dissipation
Diamond Electron Mobility	1060	cm² V^-1s^-1	High carrier transport
GaN Bandgap	3.39	eV	Direct bandgap material
GaN PCSS Bias Field (Mn-doped)	40	kV/cm	High-gain (lock-on) operation
GaN PCSS Minimum Trigger Energy (Mn)	22.5	µJ	Required for high-gain mode (800 nm)
GaN PCSS Mean Delay (Mn)	27.8	ns	Time from optical trigger to switch closure
GaN PCSS Jitter (Mn)	2.1	ns	Low timing variability
SiC PCSS Maximum Blocking Voltage	370	kV/cm	Lateral 4H-SiC:V device [4]
SiC PCSS Minimum On-State Resistance	<1	Ω	Vertical 6H-SiC:V device [36]
SiC PCSS Maximum Off-State Resistance	12 x 10¹²	Ω	Radial 4H:SiC:V device [34]
GaN n-i-n Structure Thickness	2	µm	Simulated structure thickness
GaN n-region Donor Density	10¹⁹	cm^-3	Heavily doped contacts
GaN p⁺-region Acceptor Density (Mn)	5 x 10¹⁹	cm^-3	High-gain mechanism reliance

Key Methodologies

The research focused on material characterization, device fabrication, and high-voltage testing, particularly for GaN PCSS devices operating in the high-gain (lock-on) mode.

Material Growth and Doping: High-quality, semi-insulating (SI) GaN substrates (~300 µm thick) were grown using ammonothermal or Hydride Vapor-Phase Epitaxy (HVPE). Deep acceptor dopants (Mn or Fe) were incorporated at high concentrations (near 10¹⁹ cm^-3) to achieve high dark resistivity (10⁹-10¹⁴ Ω·cm) and enable extrinsic sub-bandgap absorption.
Device Fabrication: Both lateral (Ti/Al/Ni/Au contacts, 0.6-3 mm gap) and vertical (coaxial metallization) geometries were utilized. Vertical structures incorporated low-fill factor hole grids designed to facilitate the optical seeding of current filaments necessary for nonlinear mode operation.
Optical Triggering and Bias: Switches were tested under high bias fields (up to 40 kV/cm). Triggering was achieved using a 5 ns pulsed laser, with wavelengths varied between 650 nm and 1050 nm to investigate the energy requirements for initiating nonlinear conduction.
Lock-on Confirmation: Nonlinear switching was confirmed when the applied field and laser energy surpassed specific thresholds (e.g., 25 kV/cm and 22.5 µJ for Mn-doped GaN). The lock-on state was verified by monitoring current filamentation and near-bandgap emission using wavelength-resolved cameras.
Circuit Integration: A GaN PCSS device was integrated as the normally-off leg in a 6 kV Medium-Voltage DC Solid-State Circuit Breaker (SSCB) prototype, working in tandem with a normally-on leg composed of cascaded SiC JFETs to manage fault current diversion.
Computational Modeling: Device physics were analyzed using the Ensemble Monte Carlo (EMC) method and the Radiation Effects in Oxides and Semiconductors (REOS) solver to model transient behavior, confirming that defect-enhanced field regions near the anode enable avalanche injection in GaN.

6CCVD Solutions & Capabilities

6CCVD provides the advanced MPCVD diamond materials and customization services necessary to replicate, optimize, and scale the UWBG PCSS research presented in this paper, particularly focusing on the high-performance potential of diamond.

Applicable Materials

To achieve the high breakdown fields and superior thermal management required for next-generation PCSS, 6CCVD recommends the following materials:

Optical Grade Single Crystal Diamond (SCD): Essential for intrinsic PCSS studies requiring the highest purity, highest breakdown field (10-20 MV/cm), and lowest surface roughness (Ra < 1 nm) to mitigate surface flashover.
Custom Doped SCD/PCD: Required for extrinsic PCSS operation. We offer controlled incorporation of deep-level dopants (e.g., Nitrogen or Boron) necessary to achieve sub-bandgap triggering using visible/IR lasers, mirroring the defect engineering approach used with GaN:Mn and SiC:V.
High-Quality Polycrystalline Diamond (PCD): Ideal for scaling up high-power devices. PCD offers excellent thermal properties and can be manufactured in large formats up to 125 mm diameter, addressing the need for commercial-scale, high-current PCSS.

Customization Potential

The complexity of PCSS fabrication, involving specific geometries and multi-layer contacts, aligns perfectly with 6CCVD’s custom engineering capabilities:

PCSS Fabrication Requirement	6CCVD Custom Capability
Large-Area Substrates	Plates/wafers up to 125 mm (PCD) and custom SCD dimensions, enabling scale-up for MVDC utility applications.
Precise Thickness Control	SCD and PCD thicknesses from 0.1 µm to 500 µm for active layers, and substrates up to 10 mm for robust vertical structures.
Custom Electrode Design	In-house metalization capabilities including Au, Pt, Pd, Ti, W, and Cu. We can deposit the multi-layer contacts (e.g., Ti/Al/Ni/Au) required for lateral and vertical PCSS geometries.
Surface Quality	Precision Polishing: SCD surfaces polished to Ra < 1 nm and inch-size PCD polished to Ra < 5 nm, crucial for minimizing scattering losses and suppressing high-field surface flashover.
Doping Profile Engineering	Ability to control doping concentrations and profiles (e.g., heavy doping for contacts, semi-insulating for the intrinsic region) critical for achieving the n-i-n or m-i-m structures described in the research.

Engineering Support

The successful implementation of high-gain switching relies heavily on understanding the interaction between deep defects, electric fields, and carrier dynamics. 6CCVD’s in-house PhD team specializes in MPCVD diamond growth and defect engineering. We can assist researchers and engineers with:

Material Selection: Optimizing the choice between SCD and PCD based on required breakdown voltage, thermal load, and active area.
Defect Control: Tailoring doping recipes to achieve specific deep-level concentrations (e.g., N or B) necessary to replicate the extrinsic triggering mechanisms demonstrated in GaN:Mn and SiC:V projects.
Design Consultation: Providing technical guidance on material integration for high-power applications, such as the Medium-Voltage DC Solid-State Circuit Breaker (SSCB), ensuring optimal performance and longevity.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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]