20 transistors, which is the minimum number for a full adder in CMOS technology.

A full adder is a digital circuit that performs addition of three binary bits: two significant bits and a carry-in bit. In CMOS technology, the implementation of a full adder requires a specific arrangement of NMOS and PMOS transistors.

1. Understanding Full Adder Structure: A full adder has three inputs (A, B, and C_in) and two outputs (Sum and C_out). The logic functions for a full adder can be expressed as:

   • Sum = A ⊕ B ⊕ C_in
   • C_out = (A AND B) OR (C_in AND (A ⊕ B))

2. Transistor Count: In CMOS design:

   • Each basic gate (AND, OR, XOR) can be implemented using a certain number of transistors.
   • Typically, an XOR gate requires 6 transistors, an AND gate requires 6, and an OR gate also requires 6 transistors.

3. Implementation for Full Adder:

   • To construct the full adder, you will need:
     • 2 XOR gates for Sum.
     • 1 AND gate for generating partial carry.
     • 1 AND gate to combine the carry outputs.
     • 1 OR gate to generate the final carry output.

4. Calculating Total Transistors:

   • 2 XOR gates: $2 \times 6 = 12$ transistors
   • 2 AND gates: $2 \times 6 = 12$ transistors
   • 1 OR gate: $1 \times 6 = 6$ transistors
   • Total Transistor Count = 12 + 12 + 6 = 30 Transistors

Final Answer

The minimum number of transistors needed for a full adder in CMOS technology is 30 transistors.