Chapter 4 · Visualization 5 of 6 · Sections 4.4.4 and 4.6 (Fig. 4.19)

Regions of Operation and Forward/Reverse Currents

For a given gate-body voltage, the levels of inversion at the source and drain ends decide the region. The current is a forward part set by the source minus a reverse part set by the drain.

Region map in the (VSB, VDB) plane: lightest = depletion, then weak, moderate, strong (dot: your bias)

Forward and reverse components versus VDB, at your VGB and VSB

What to look for

IDS = IF(VGB, VSB) − IR(VGB, VDB) (4.4.23)

  • The map is symmetric about the diagonal: swap source and drain and only the sign of IDS changes.
  • Strong inversion fills the region where at least one end is below VQ; raising VGB pushes all boundaries outward.
  • IR falls to zero as VDB passes VP: forward saturation, IDS = IF.

Try this

  1. Start at VGB = 2 V, VSB = 0.3 V. Raise VDB from 0 to 3 V. Through which regions does the dot pass? Is the transistor ever not in strong inversion?
  2. Now raise VSB to 1.3 V with VDB = 3 V. What region is the transistor in?
  3. Set VDB < VSB. Which component now dominates, and what is the sign of IDS?
  4. Lower VGB to 1 V. How do the region boundaries move?