The Crucial Role of Epitaxial Layers in Semiconductor Devices

1. The Cause of Its Appearance

In the realm of semiconductor device manufacturing, the quest for materials that can cater to the evolving demands has continually posed challenges. By the end of 1959, the development of thin-layer monocrystalline material growth techniques, known as epitaxy, emerged as a pivotal solution. But how exactly has epitaxial technology contributed to material advancement, particularly for silicon? Initially, the fabrication of high-frequency, high-power silicon transistors encountered significant hurdles. From the perspective of transistor principles, achieving high-frequency and high-power necessitated a high breakdown voltage in the collector region and minimal series resistance, translating to a reduced saturation voltage drop.

These requirements presented a paradox: the need for high resistivity materials in the collector region to increase breakdown voltage, versus the need for low resistivity materials to decrease series resistance. Reducing the thickness of the collector region material to lessen series resistance risked making the silicon wafer too fragile for processing. Conversely, lowering the material’s resistivity contradicted the first requirement. The advent of epitaxial technology successfully navigated this dilemma.

2. The Solution

The solution involved growing a high-resistivity epitaxial layer on a low-resistivity substrate. Device fabrication on the epitaxial layer ensured a high breakdown voltage thanks to its high resistivity, while the low-resistivity substrate reduced the base resistance, thereby diminishing the saturation voltage drop. This approach reconciled the inherent contradictions. Furthermore, epitaxial technologies, including vapor-phase, liquid-phase epitaxy for materials like GaAs, and other III-V, II-VI group molecular compound semiconductors, have significantly advanced. These technologies have become indispensable for the manufacture of most microwave devices, optoelectronic devices, power devices, and more. Notably, the success of molecular beam and metal-organic vapor-phase epitaxy in applications like thin films, superlattices, quantum wells, strained superlattices, and atomic layer epitaxy has laid a solid foundation for the new research domain of “bandgap engineering.”

3. Seven Key Capabilities of Epitaxial Technology

(1) Ability to grow high (low) resistivity epitaxial layers on low (high) resistivity substrates.

(2) Capability to grow N § type epitaxial layers on P (N) type substrates, directly forming PN junctions without the compensation issues associated with diffusion methods.

(3) Integration with mask technology to selectively grow epitaxial layers in designated areas, paving the way for the production of integrated circuits and devices with unique structures.

(4) Flexibility to alter the type and concentration of dopants during the growth process, with the possibility of abrupt or gradual changes in concentration.

(5) Potential to grow heterojunctions, multilayers, and variable composition ultra-thin layers.

(6) Ability to grow epitaxial layers below the melting point of the material, with controllable growth rates, enabling atomic-level thickness accuracy.

(7) Feasibility of growing single-crystal layers of materials that are challenging to pull, such as GaN, and ternary or quaternary compounds.

In essence, epitaxial layers offer a more controllable and perfect crystal structure compared to substrate materials, significantly benefiting material application and development.**

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