Chapter 4 · Visualization 1 of 6 · Sections 4.2 and 4.3

Transistor I–V Characteristics

The complete all-region model gives the drain current for any terminal voltages, in every region of inversion, from one expression. Sweep the bias and watch the output and transfer characteristics.

Output characteristics IDS(VDS): grey VGS = 0.8 … 2.4 V, blue your VGS (dot: your bias, dashed: V′DS)

Transfer characteristic log IDS(VGS) at your VDS, with the regions of inversion at the source

What to look for

IDS = IDS1 + IDS2, from ψs0 = ψS(VGB, VSB) and ψsL = ψS(VGB, VDB)

  • Every curve flattens: saturation exists in all regions of inversion. In strong inversion it starts near V′DS; in weak inversion after a few φt.
  • On the log plot weak inversion is a straight line (diffusion), strong inversion bends over (drift). The components cross in moderate inversion.
  • gm/IDS is largest in weak inversion (up to 1/(nφt) ≈ 30 V⁻¹) and falls in strong inversion.

Try this

  1. Set VGS = 1.5 V and sweep VDS. Where does the dot reach the dashed V′DS line, and does the current really stop growing there?
  2. Set VDS = 0.05 V. How does the transfer curve change compared with VDS = 2 V in weak inversion? In strong inversion?
  3. Raise VSB from 0 to 1.5 V. How far does VT move, and how do the output curves shrink?
  4. Find the VGS where drift and diffusion carry equal current. Which region is it in?