Speaker
Description
Bipolar Junction Transistors (BJTs) are fundamental semiconductor components utilized extensively for switching, amplification, and signal control in modern electronics. However, their electrical performance in high-radiation environments, such as space and nuclear facilities, is often compromised by degradation.
A 2N2907A pnp silicon BJT was subjected to electron irradiation under a strontium-90 source with an overall flux of $2.3\times10^{7}$ electrons/cm$^2$ for roughly 18 days. The base–collector junction of the BJT was electrically characterized using Current–Voltage (I–V), Capacitance–Voltage (C–V) and a transistor tester, which revealed degradation due to defect formation, including increased leakage current, and reduced barrier height. Specifically, the $\beta$ current gain dropped from 204 to 10 after 18 days of irradiation. Results showed a loss in the ability of the silicon transistor to amplify signals effectively compared to before radiation exposure. Capacitance data indicated a complex evolution of carrier concentration, where an initial slight increase during short-term exposure was followed by a sharp decrease after prolonged irradiation due to the formation of deep-level defects.
Deep-Level Transient Spectroscopy (DLTS) was applied to the transistor before and after irradiation to detect electrically active defects within the bandgap of the semiconductor. A specialized three-layer sample holder was designed in order to securely mount the transistor in place and overcome certain challenges including the thermal lag and electrical isolation when modifying the DLTS setup. The sample holder consisted of an aluminium base, an alumina insulating layer, and an aluminium top cavity to secure the device.
This study represents one of the first applications of Laplace DLTS applied to a BJT, providing significantly enhanced resolution of overlapping defect peaks compared to conventional DLTS techniques. The conventional and Laplace DLTS techniques provide a method to determine the corresponding activation energies and apparent capture cross-sections from the measured data, allowing for the identification of defect complexes. Multiple native and irradiation-induced defects were identified, most notably the divacancy $V_{2}(+/0)$ defect. The activation energy of the divacancy was detected via conventional and Laplace DLTS, with conventional DLTS yielding an activation energy of $E_a=0.194$ eV and an apparent capture cross-section of $\sigma_a= 5 \times 10^{-16}$ cm$^2$ for the $V_{2}(+/0)$ defect. Other observed signatures were tentatively assigned to boron-, oxygen-, or vacancy-related complexes.
While some defects could not be assigned due to signal overlap or uncertainty in their identification based on previous literature, the application of Laplace DLTS proved essential for distinguishing complex defect structures. These findings demonstrate the feasibility of applying advanced spectroscopic techniques to commercial bipolar semiconductor devices and provide a critical framework for predicting device reliability and guiding the design of radiation-tolerant electronics in harsh environments.
| Apply for student award at which level: | Honours |
|---|---|
| Consent on use of personal information: Abstract Submission | Yes, I ACCEPT |