Multiple fingers vs single finger layout (MOSFET transistor)

Question

Please, provide a summary of advantages and disadvantages of a transistor layout with multiple fingers (MF) vs single finger?

When laying out a MOSFET with a particular width and length, in an EDA tool, one has two options with regards to the shape of the gate:

1) Single stripe (classical case) (one finger);
2) Several stripes (several fingers).

Hypotheses (based on various Internet forums):

1) MF provide more flexibility in layout planning for transistor with high W/L or L/W. In other words, allows making a layout more square-like.

2) MF allow better matching of transistors, when needed. For example, if using common-centroid techniques.

3) MF layout reduces gate resistance (for AC). If so, can you explain why?

4) MF reduce current density in the gate if there are technology limitations on this.

Can someone with knowledge do a comparison of the two approaches?

Figure 1: One finger.

Figure 2: Two fingers. Two transistors in parallel (widths are summed).

Figure 3: Two fingers. Two transistors in series (lenghts are summed).

Answer 1

Long transistors are split into smaller ones for various reasons. One reason could be to fit them better into the overall layout of a block or simply to have a better aspect ratio. Another reason is to reduce the gate resistance of the device. The problem is that the gate-channel capacitance forms a lowpass with the gate resistance and switching speed is reduced. In addition the drain area is reduced because two fingers share a common drain and therefore parasitics are reduces as well.

Update: Splitting a transistor with some W/L into a transistor with multiple fingers is done such, that the width (W) stays the same and the length of finger is L/k, where k is the total number of fingers. The effective length and width stay the same, but the gates are now in parallel and therefore the resistance is reduced. In addition, gates are often contacted at both ends to reduce the effective resistance.

Splitting transistors can either be done by using multiple transistors with a single gate or with transistors that have multiple gate fingers.

Transistors with multiple fingers have the disadvantage that the current direction is different for two neighbouring fingers. E.g. if for the first finger the source is to the left then the source for the next finger will be to the right. The properties of transistor can change depending on the current direction. Therefore extra care has to be taken when trying to achieve good matching.

Using multiple fingers to obtained scaled current sources for example in a current mirror is also considered inferior to having multiple single gate transistors because of slightly different properties.

An analog designer therefore has to decide which option is the best for a given problem.