Abstract
In this article, we focus on the physical modeling of the nonlinear operation of intrinsic photoconductive semiconductor switches (PCSS) based on 4H-SiC using coupled electrical and optical simulations to provide performance bounds of the switch as a function of material and geometry parameters, as well as applied bias. We also conduct a full design-space exploration to identify the optimal operating and design conditions to maximize the compound metric $f_{\mathrm {op}} P_{\mathrm {out}}$ , where $f_{\mathrm {op}}$ is the maximum operating frequency, and $P_{\mathrm {out}}$ is the maximum output power the switch can provide. We quantify that a 10- $\mu \text{m}$ long and 5- $\mu \text{m}$ thick 4H-SiC PCSS can deliver output power density greater than 2W/mm at 150 GHz when triggered by a 0.325- $\mu \text{m}$ laser with intensity of 3 kW/cm2. The output power density can be significantly enhanced by increasing the optical generation rate as well as by using thicker SiC to improve its absorption characteristics. A brief discussion of signal distortion and electrostatic screening effects at high optical bias is included. Finally, we present an analytic model of charge cloud propagation and the frequency of operation based on the physics, material parameters, and geometry of the PCSS. The model accurately captures $f_{\mathrm {op}}$ of 4H-SiC PCSS over a broad range of laser spot size, device length, and electrical bias applied at the contacts.
Highlights
Silicon Carbide (SiC) is a wide bandgap semiconductor material with several polytypes, such as 3C, 2H, 4H, and 6H, depending on the stacking order of the Si-C bilayers along the c-axis of its lattice [1], [2]. 3C-SiC is the only form of SiC with a cubic crystal structure, while the overall crystal structure of 4H-SiC and 6H-SiC is hexagonal
The electrical bias is fixed at 2000 V for all device lengths to ensure that the charge cloud propagation occurs predominately due to drift even for the longest device (100 μm separation between the contacts) that is analyzed in this work
Due to the high voltage applied across the device, drift dominates over diffusion, and due to very low recombination in the channel, the peak value of the charge cloud is nearly constant with time
Summary
Silicon Carbide (SiC) is a wide bandgap semiconductor material with several polytypes, such as 3C, 2H, 4H, and 6H, depending on the stacking order of the Si-C bilayers along the c-axis of its lattice [1], [2]. 3C-SiC is the only form of SiC with a cubic crystal structure, while the overall crystal structure of 4H-SiC and 6H-SiC is hexagonal. Owing to its wide bandgap, high breakdown field, and high electron saturation velocity, 4H-SiC based PCSS can operate at high repetition rate while sustaining high output power [15]. We present the physics of operation and performance limits of intrinsic 4H-SiC PCSS using a coupled electrical-optical numerical simulation framework, an extension of our previously published work [22]. The switch is optically triggered using above-bandgap laser which generates excess electrons and holes that drift in opposite directions based on the electric field distribution across the device. Generation rate in the device exceeds 1027 cm−3s−1, there is significant signal distortion due to electrostatic screening effects This leads to a slower turn-off transient and a wider pulse width at the contact. The spot size of the laser is denoted as S for simplicity in Fig. 1, while for practical purposes and in simulation, the laser spatial profile
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