Matching Concepts for MOSFET
MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors), several factors introduce variations that can affect their electrical behavior. These variations are critical to consider when designing circuits that rely on matched MOSFETs for consistent performance. Here are some key points to understand.
Channel Length and Width Variations
- In MOSFETs, the channel width (w) and channel length (l) are subject to stochastic edge shifts.
- Deterministic fringe effects also influence channel lengths.
- Sub-diffusion of source and drain zones under the poly gate shortens the channel length.
- In processes with LOCOS field oxide, the field-oxide opening edge shift due to the bird’s beak effect reduces the effective electrical channel width.
Matching MOSFET
- To achieve matching in MOSFETs, they should be split into “basic transistors” with the same channel lengths and widths.
- Even without these effects, MOSFETs should have a uniform channel length for sensible matching due to circuit constraints.
- MOSFET matching typically involves maintaining uniform channel length and setting w/l ratios through folding to ensure consistent channel widths among single transistors.
Rule of Thumb
- Matching MOS-FETs should always have a uniform channel length.
- The w/l ratios are adjusted through folding so that the channel widths of individual transistors are as uniform as possible.
- This approach helps mitigate the impact of fringe effects on matching, leading to improved performance.
Example
- Illustrated in a simple current mirror circuit (Fig.1), where transistor M1 is used as a reference and transistor M2 mirrors the current in a 1:2 ratio.
- The channel width of M2 is twice that of M1.
- In the first layout variant where transistors have different widths, fringe effects worsen the channel width ratio, resulting in poor matching.
- In the second layout variant, splitting the larger transistor through folding offsets fringe effects, resulting in better matching.
Matching MOS-FETs with uniform channel lengths and appropriate w/l ratios is essential to achieve consistent and predictable performance in analog circuits while accounting for variations and fringe effects.