High Current Density Diamond Photoconductive Semiconductor Switches With a Buried, Metallic Conductive Channel

At a Glance

Metadata	Details
Publication Date	2024-04-10
Journal	IEEE Electron Device Letters
Authors	Zhuoran Han, J. Lee, Stephen Messing, Thomas Reboli, Andrey E. Mironov
Institutions	University of Illinois Urbana-Champaign
Citations	8
Analysis	Full AI Review Included

Executive Summary

Novel Device Structure: Reports on laterally configured diamond Photoconductive Semiconductor Switches (PCSS) featuring a buried, metallic p+ conductive channel to enhance ON-state current density.
High Current Density: Achieved a peak current density of 43.5 A/cm under a 60 V DC bias, significantly higher than typical linear-mode diamond PCSS (which are often < 0.2 A/cm).
Linear Mode Operation: The device operates in linear mode (uniform current conduction), avoiding carrier multiplication, impact ionization, and damaging filamentation common in high-gain GaN/GaAs switches, promising better reliability.
Ultra-Fast Switching: Exhibits fast rise and fall times of approximately 2 ns (10-90%), limited primarily by the 4 ns FWHM of the triggering laser pulse.
High Switching Contrast: Achieved extremely high ON/OFF ratios exceeding 10¹¹, leveraging the high OFF-state resistance of the low-impurity semi-insulating diamond layer.
Above-Bandgap Triggering: Utilizes above-bandgap optical excitation (wavelengths < 226 nm) to maximize responsivity, reaching over 130 mA/W.
Current Conduction Mechanism: TCAD simulation and experimental data confirm that 92% to 93% of the current conduction flows through the highly conductive buried p+ channel during the ON-state.

Technical Specifications

Parameter	Value	Unit	Context
Substrate Material	Type IIa HPHT Diamond	N/A	500 µm thick
Diamond Bandgap (E_G)	5.47	eV	Ultrawide bandgap (UWBG)
Critical E-Field (E_critical)	10 - 20	MV/cm	Intrinsic diamond property
Thermal Conductivity (κ)	22 - 24	W·cm^-1·K^-1	Intrinsic diamond property
p+ Channel Thickness	500	nm	Heavily boron-doped layer
p+ Channel Doping	5 x 10²⁰	cm^-3	Buried conductive channel
Semi-Insulating Layer Thickness	1.5	µm	Unintentionally doped (UID) layer
UID Layer Doping (Boron)	5 x 10¹⁵	cm^-3	Active absorption layer
Optimal Excitation Wavelength	212	nm	Above bandgap excitation
Laser Pulse FWHM	4	ns	Optical Parametric Oscillator (OPO) source
Maximum Peak Current Density	43.5	A/cm	PCSS A (8 µm spacing) at 60 V DC bias
Responsivity (PCSS A)	130.3	mA/W	At 60 V DC bias, 212 nm excitation
ON/OFF Ratio (PCSS B)	3.3 x 10¹¹	N/A	Highest measured switching contrast
Rise/Fall Time (10-90%)	~2	ns	Limited by laser pulse rise/fall time
Contact Metal Stack	Ti/Pt/Au	N/A	Deposited by e-beam evaporation
Conduction through Buried Channel	92 - 93	%	Estimated via equivalent circuit model

Key Methodologies

Substrate Preparation: Started with a 500 µm thick, 4 x 4 mm² Type IIa high-pressure, high-temperature (HPHT) diamond substrate.
MPCVD Growth (p+ Layer): Grew a 500 nm thick layer of heavily boron-doped p+ diamond using Microwave Plasma Enhanced Chemical Vapor Deposition (MPCVD), achieving an atomic doping concentration of 5 x 10²⁰ cm^-3. This layer serves as the buried, metallic conductive channel.
MPCVD Growth (UID Layer): Followed the p+ layer with a 1.5 µm thick layer of unintentionally doped (UID) diamond, having a low atomic boron concentration of 5 x 10¹⁵ cm^-3, serving as the active absorption layer.
Metallization: Rectangular metal contacts (Ti/Pt/Au stack) were deposited via e-beam evaporation.
Annealing: The contacts were thermally annealed at 450 °C under an Argon (Ar) atmosphere for 1 hour to ensure Schottky contact behavior in the dark, transitioning to Ohmic contact under illumination.
Optical Triggering: Photoconductive measurements were performed using a tunable Optical Parametric Oscillator (OPO) laser (210 nm to 230 nm range) with a 4 ns FWHM pulse and 10 Hz repetition rate.
Electrical Measurement: The PCSS was connected in series with a DC power supply and a 50 Ω load resistor. The transient response was monitored using a Tektronix DPO 7254C oscilloscope (50 Ω input impedance).

Commercial Applications

Pulsed Power Systems: The combination of ultra-fast switching (~2 ns) and high current density makes these devices ideal for generating high-power, short-duration pulses used in radar, directed energy, and particle accelerators.
High-Voltage Power Grid Protection: Diamond’s high critical E-field (10-20 MV/cm) and superior thermal properties enable the development of compact, high-voltage switches necessary for rapid protection against grid faults and outages.
High-Frequency Switching: Diamond’s high thermal conductivity allows for operation at high power densities and high temperatures without thermal runaway, enabling next-generation high-frequency power converters and inverters.
Reliable Power Electronics: By operating in linear mode and eliminating filamentation, these PCSS devices promise significantly enhanced operational reliability and longer life spans compared to conventional high-gain semiconductor switches (e.g., GaN, GaAs).
Extreme Environment Electronics: Leveraging the inherent radiation hardness and thermal stability of diamond for applications in aerospace, nuclear facilities, or deep-well drilling.

View Original Abstract

Laterally configured diamond photoconductive semiconductor switches (PCSS) with a buried, metallic p+ current channel are reported. Above bandgap ( λ ≤ 226 nm) optical triggering enables responsivity of over 130 mA/W. The use of low-impurity semi-insulating diamond as an active absorption layer enables fast rise and fall times (~2 ns) and on/off ratios greater than 10<sup>11</sup>. The PCSS excited with a laser energy of 20 nJ per pulse passes a high current density (44 A/cm) under a DC bias of 60 V, thanks to the buried metallic p+ current channel. The reported devices promise high current carrying capacity without the need for filamenting while leveraging the excellent optical, electronic, and thermal properties of diamond.

Tech Support

Original Source

DOI: https://doi.org/10.1109/led.2024.3387325