Radiation Tolerant MOSFET: Structure, Working Principle & Key Advantages

Introduction

As electronic systems are increasingly deployed in space, nuclear, and defense environments, the demand for components that can withstand harsh radiation exposure continues to grow. One such critical device is the radiation tolerant MOSFET. It is specially engineered transistor designed to resist performance degradation caused by ionizing radiation and energetic particles. We will explore internal structure, working principles and key benefits of these radiation hardened MOSFETs.

Radiation Tolerant MOSFET Structure

Radiation-tolerant MOSFETs are specialized transistor designs engineered to operate reliably in high radiation environments such as space, nuclear reactors and high energy physics experiments. Structurally, these devices are based on conventional MOSFETs but incorporate several enhancements to mitigate radiation induced degradation as follows.

• Encapsulated Gate Configuration (ELT): This design features a gate electrode that fully encircles the active region, effectively eliminating edge related leakage paths that can be induced by ionizing radiation. Such a configuration enhances the device’s resilience to radiation induced degradation.

• Enhanced Isolation Techniques: Employing thick field oxides in conjunction with shallow trench isolation (STI) structures serves to suppress leakage currents and provide effective isolation between devices. This combination is particularly effective in mitigating radiation induced parasitic conduction paths.

• Peripheral Protective Structures: Incorporating guard rings and channel stop implants around sensitive regions of the MOSFET helps in controlling parasitic surface currents that may be triggered by radiation exposure. These structures act as barriers, preventing unwanted charge accumulation and conduction.

• Silicon-on-Insulator (SOI) Technology: Utilizing SOI substrates reduces the volume of silicon available for charge collection, thereby minimizing the device’s susceptibility to single event effects (SEEs). This structural approach enhances the overall radiation hardness of the MOSFET.

• Radiation Resistant Gate Oxides: Developing gate oxides with specialized processing techniques reduces the likelihood of charge trapping and threshold voltage shifts caused by total ionizing dose (TID) effects. This ensures stable operation of the MOSFET in radiation rich environments.

Radiation Tolerant MOSFET Working

Radiation tolerant MOSFETs are essential building blocks in the design of robust electronic systems for high radiation environments. Let us understand its working principle. The basic operation is similar to standard MOSFET. It controls flow of current between drain and source terminals via electric field established by gate voltage. P-channel MOSFETs need negative voltage (Vgs) to activate where as N-channel need positive Vgs to activate.

By implementing modifications in layout and processing, radiation hardened (Rad-hard) power MOSFET devices effectively reduce charge trapping, protect sensitive regions and manage electric field distributions. Such enhancements contribute to maintaining stable threshold voltages and suppressing unintended leakage currents. This ensures consistent and reliable operation even under prolonged exposure to high radiation environments like space or nuclear facilities.

Figure : Radiation Tolerant MOSFET, Courtesy : Infineon Technologies

Example

• International Rectifier (IR) HiRel and its parent company Infineon Technologies manufacture rad-tolerant MOSFETs. N-channel MOSFETs are avaolable in 60V and 150V options with surface mount design packages such as TO-263 and TO-247. Product BUP15CN027E-01 N-channel MOSFET supports 150V (Vds), 98A (Id), 27 milli-ohm (Rds) and -40deg.C to +125deg.C (temperature). The figure depicts BUP06CP038F-01 mosfet device which is of P-type and requires Vds of about -60V to activate.

Advantages of Radiation Tolerant MOSFET

Following are some of the benefits of radiation tolerant MOSFET.

1. Offers high reliability in space and nuclear applications. It maintains performance in environments with high total ionizing dose (TID) and single event effects (SEE).
2. It has minimal threshold voltage shifts.
3. Advanced isolation and layout techniques prevent radiation induced parasitic conduction paths. Hence device has low leakage current.
4. Withstands long duration missions or installations with minimal degradation.
5. Variants are available with optimizations for digital logic, analog circuits and power switching.

Conclusion

With specialized layout techniques, oxide processing and shielding mechanisms, Radiation Tolerant MOSFET devices deliver long term stability, low leakage and reliable switching under extreme conditions.