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BJT bipolar, MOSFET unipolar; CMOS NOT

Short answer

BJT is bipolar because its current involves both electrons and holes; MOSFET is unipolar because conduction uses only one carrier type (electrons or holes). A CMOS NOT gate connects a PMOS and NMOS in series, gates tied as input, drains as output; it inverts with almost zero static power draw.

Why BJT is bipolar and MOSFET is a unipolar device? Draw the circuit diagram of NOT gate using CMOS.

Why BJT is bipolar

In a BJT, current is carried by both electrons and holes. In an NPN transistor, the emitter injects majority electrons into the p-type base, where they become minority carriers, and some of them recombine with holes in the base. Both majority and minority carriers take part in conduction, so the device is bipolar.

Why MOSFET is unipolar

In a MOSFET, the current flows through the induced (or built-in) channel using only one type of carrier: electrons in an n-channel device or holes in a p-channel device. There is no minority carrier injection and no gate current, since the gate is insulated by SiO₂. It is therefore a unipolar device.

CMOS NOT gate

                VDD
                 |
               S |
          +--G-o|   PMOS
          |    D |
 A o------+      +------o Y = A'
          |    D |
          +--G--|   NMOS
               S |
                 |
                GND

The PMOS source is at VDD and the NMOS source is at ground. The two gates are joined to form the input A, and the two drains are joined to form the output Y.

  • A = 0: PMOS is ON, NMOS is OFF, so Y = VDD (logic 1).
  • A = 1: PMOS is OFF, NMOS is ON, so Y = 0 (logic 0).

So Y = A' (inversion), with almost zero static power, since one transistor is always OFF.

✓ Answer checked by Claude Opus 5.5 (high effort)

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