1. Voltage and Power Handling Capability: The AU variant offers a higher Collector-Emitter breakdown voltage (100V vs. 80V) and maximum power dissipation (2.6W vs. 1.3W). Within the same SOT-223 package, the AU can withstand higher voltage stress (e.g., spikes from inductive load switching) and supports higher continuous current or elevated ambient temperatures. Its thermal design margin is more generous, resulting in higher reliability. 2. Saturation Voltage Test Conditions: Both devices are rated for a nominal Vce(sat) of 500mV, but the AU is tested under more stringent conditions (Ic=1A, Ib=100mA vs. Ic=500mA, Ib=50mA). This implies that at the same 1A operating current, the AQ may exhibit higher actual saturation voltage and conduction loss. Moreover, its DC current gain (hFE) likely degrades more significantly near 1A, affecting drive efficiency. 3. Quality Certification: The AU variant is explicitly marked as AEC-Q101 compliant, whereas the AQ datasheet does not indicate this qualification. The AU has undergone more rigorous reliability verification (e.g., temperature cycling, HAST) and is suitable for harsh environments such as automotive electronics. If the AQ is to be used in such applications, additional quality documentation from the manufacturer must be obtained. Summary: The core distinction is that the AQ is a “down‑rated” version in terms of voltage, power, and reliability class. Feasibility of substitution depends entirely on the actual voltage, current, thermal conditions, and industry standards of the specific application. If the original design does not fully utilize the AU’s performance margin and the application falls within consumer or industrial domains, substitution may be considered—provided thermal performance and long‑term reliability are thoroughly re‑assessed.