CMOS are voltage control current sources while BJTs are current-controlled current sources. Also, MOS is highly integrable wherein billions of MOS transistors can be integrated into a chip of the size of our fingertips which is difficult to achieve with BJTs.
CMOS and BJTs#
Advantages of CMOS transistors over BJTs
- A near-ideal switching device
- Bi-directional capability (drain and source are interchangeable)
- High delay sensitivity to load (fan-out limitations)
- High input impedance (low drive current)
- High yield with large integrated complex functions
- Higher noise margins
- Higher packing density, i.e., the lower manufacturing cost per device
- Low output drive current (the issue in driving large capacitive loads)
- Low transconductance, where transconductance gm ∝ Vin
- Lower static power dissipation
- Scalable threshold voltage
Advantages of BJTs over CMOS transistors
- Better analog capability
- Essentially unidirectional
- Generally better noise performance and better high-frequency characteristics
- High power dissipation n Lower input impedance (high drive current)
- High unity gain bandwidth at low currents
- Higher current drive per unit area, higher gain
- Higher switching speed
- Improved I/O speed (particularly significant with the growing importance of package limitations
- Low delay sensitivity to load n High gm (gm ∝ Vin)
- Low packing density
- Low voltage swing logic