Phototransistors are light-sensitive transistors that act as a switch when illuminated, converting light into electrical signals. Infrared phototransistors are specifically engineered to detect weak infrared light, and are attracting an increasing amount of attention within the shortwave infrared photodetector world due to their low cost, compatibility, and effectiveness in a wide variety of applications. Current methods used to produce phototransistors are advancing; however, complicated and expensive manufacturing processes provide challenges for scaling up production.

A Novel Phototransistor Design

A recent study has developed a transformative new design for shortwave infrared detection that uses carbon nanotubes (CNTs). The fabrication process involves coating an initial layer of hafnium oxide with a CNT solution. A second layer of hafnium oxide and a layer of zinc oxide are added to form a self-aligned gate structure. The self-aligned heterojunction gate ensures that the gate is positioned over and fully encapsulates the CNT channel, without the need for the precise alignment that was essential in previous fabrication methods. 

Importance of CNTs

CNTs form an important aspect of this novel phototransistor design. Phototransistors using heterojunction-gating rely on signal amplification mechanisms to increase responsivity to weaker light signals. The use of CNTs is advantageous in this due to their exceptionally high carrier mobility – the speed at which electrons move through the material when subjected to an electric field – which is faster than that of most conventional semiconductors. In addition, CNTs are compatible with the silicon-based fabrication workflows commonly used in semiconductor manufacturing. 

The use of CNTs in this novel phototransistor design contributed to its:

- High responsivity – enabling it to generate a large electrical signal from a small amount of light.

- High detectivity – it can detect very weak infrared signals, enabling its use in ultra-low-light applications.

- Fast response time – enabling rapid, low-light detection and improving gain-speed trade-offs.

Future Implications

This design has the potential to transform infrared phototransistors, enabling their easy integration into various applications and unlocking further technological advances. These phototransistors have the potential to advance night vision applications, where their high sensitivity enhances the ability to see in ultra-low-light environments. The ability of these phototransistors to detect low light intensities may also contribute to space exploration, enabling the observation of distant objects that emit infrared light. Biological imaging is a further application for these phototransistors, enabling non-invasive medical imaging for diagnostics. 

This novel production method improves sensitivity and response time while using relatively simple fabrication methods, making it both scalable and cost-effective while maintaining high performance.