CMOS vs. Bipolar in Analog Chip Design

Which is better for analog design, CMOS or bipolar? The debate is as old as the devices themselves. Below are some of the main points:

Bipolar transistors require an input (base) current, and CMOS devices don’t.
Bipolar transistors have lower offset voltages.
Bipolar transistors have lower noise.
CMOS devices have smaller dimensions.
Bipolar transistors are better for low-voltage design.

Let's examine each of these.

Bipolar Transistors Require a Base Current, and CMOS Devices Don’t

This is strictly true only at DC. At higher frequencies, the input capacitance present in a CMOS transistor results in a current. Also, some analog designs manage to bring this current down to a very low level.

Bipolar Transistors Have Lower Offset Voltages

This is generally true, but offset voltage depends on size. You can achieve equally low offset voltage by making a CMOS transistor larger than a bipolar one or using trimming.

Bipolar Transistors Have Lower Noise

This is also generally true, especially at low frequencies (1/f noise). There is one exception: the auto-zero, or chopper-stabilized, input.

CMOS Devices Have Smaller Dimensions

This is generally not true. To get the required matching, gain, and noise performance in an analog design, CMOS transistors need to be much larger than the minimum dimensions allowed by the CMOS process. At reasonably high (3 V and above) supply voltages, CMOS and bipolar devices end up about equal in size.

Bipolar Transistors Are Better For Low-Voltage Design

This is true. Transconductance in a CMOS device increases as the square of the gate voltage above the threshold. If the gate voltage can only go 0.5 V above the threshold, for example, it will take a painfully large gate width to get a substantial drain current.

In a bipolar transistor, a tenfold increase in collector current is obtained at room temperature with only a 60 mV increase in base voltage. It’s ironic that CMOS is marching toward lower and lower voltages, where it’s at a serious disadvantage.

Other Semiconductor Process Options

So far, we’ve considered two simple, basic processes: CMOS and bipolar. These processes can require as few as eight masks. There are many variations, all based on these two. They include:

Mixed-mode CMOS, which includes devices for somewhat higher operating voltages and additional poly (and metal) layers.
BiCMOS processes, which add full-fledged bipolar transistors to CMOS.
Bipolar processes with vertical, high-speed PNP transistors.
CMOS processes with some high-voltage (500 V) devices.

All of these variations have one factor in common: they increase the number of masks and processing steps required and are thus more expensive. However, they tend to make the design of high-performance analog circuits easier, especially when both CMOS and bipolar transistors are available.