Question

The logic family which is least affected by temperature variations, is

• A. DTL
• B. TTL
• C. CMOS
• D. ECL

Hint:

To remember temperature characteristics of logic families: - Bipolar junctions (BJTs) are fundamentally sensitive to thermal runaway and voltage drifts. - MOSFETs exhibit a negative temperature coefficient on current at higher voltages, giving them robust protection against temperature variations.

Answer 1

The Correct Option is C

Understanding the Concept: Logic families are categorized by their underlying semiconductor components (diodes, bipolar transistors, field-effect transistors). The stability of their characteristics (like threshold voltage, power dissipation, propagation delay) with temperature varies considerably:
• Bipolar Families (DTL, TTL, ECL): Rely on BJTs where carrier parameters like $V_{BE}$ drop by approximately $-2\text{ mV/}^\circ\text{C}$ and current gain $\beta$ varies drastically, leading to high thermal sensitivity.
• MOS Families (CMOS): Rely on MOSFETs where thermal variations cause two primary competing effects—carrier mobility reduction and threshold voltage drop—which partially self-compensate, ensuring stable logic margins over a wide operational range.

Step 1: Analyzing Bipolar Logic Families (DTL, TTL, ECL).
In Diode-Transistor Logic (DTL) and Transistor-Transistor Logic (TTL), saturation parameters, input thresholds, and leakage currents depend directly on the base-emitter junction voltages ($V_{BE}$) of diodes and transistors. Because $V_{BE}$ drops strongly with temperature increments, the noise margins and switching thresholds shift drastically. Emitter-Coupled Logic (ECL) operates in the active region to achieve maximum speed; however, its operating bias current points are intrinsically linked to thermal behavior, requiring complex external compensation networks to balance voltage levels.

Step 2: Analyzing Complementary Metal-Oxide-Semiconductor (CMOS).
CMOS logic uses pairs of complementary p-channel and n-channel enhancement MOSFETs. The two fundamental characteristics that shift with an increase in ambient temperature are: 1. The carrier mobility ($\mu$) decreases due to increased lattice scattering. 2. The gate threshold voltage ($V_{th}$) decreases. The reduction in carrier mobility natively reduces the drainage current, which counteracts runaway currents and provides inherent thermal stability. Thus, the noise margin thresholds change less symmetrically, leaving CMOS highly stable and the least affected overall by thermal fluctuations.