How are logic gates implemented using transistors?

Logic gates, the building blocks of digital circuits, can be implemented using transistors, which serve as the fundamental switching elements. There are two main types of transistors used in digital circuitry: Bipolar Junction Transistors (BJTs) and Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs). Here’s how logic gates can be implemented using each type:

BJT Implementation:

NOT Gate: A NOT gate can be implemented using a single BJT transistor configured as a common-emitter amplifier. When the input is high, the transistor saturates and the output is pulled low. When the input is low, the transistor is cutoff and the output is pulled high.
NAND and NOR Gates: NAND and NOR gates can be implemented using multiple transistors arranged in a parallel or series configuration. For example, a NAND gate can be constructed by connecting multiple transistors in parallel on the input side and a single transistor in series on the output side.
AND and OR Gates: AND and OR gates can be implemented using combinations of NOT, NAND, and NOR gates. For example, an AND gate can be constructed by cascading a NAND gate followed by a NOT gate, while an OR gate can be constructed by cascading a NOR gate followed by a NOT gate.
MOSFET Implementation:

NOT Gate: A NOT gate can be implemented using a single NMOS or PMOS transistor configured as a pull-down or pull-up resistor, respectively. When the input is high, the NMOS transistor conducts and pulls the output low. When the input is low, the NMOS transistor is cutoff and the output is pulled high. The operation is reversed for a PMOS transistor.
NAND and NOR Gates: Similar to BJT implementation, NAND and NOR gates can be constructed using combinations of NMOS and PMOS transistors arranged in parallel and series configurations.
AND and OR Gates: AND and OR gates can be implemented using combinations of NOT, NAND, and NOR gates, similar to BJT implementation.