This interactive simulation was written to gain an understanding of transconductance
(or Gm).
When is it fixed?, When does it vary?
Parametric (or Etest) testing sweeps the
Vgs (gate to source) voltage for a given Vd (drain) voltage, to derive the (curve sweep) plot on the right.
The transistor does not "Turn-on" (or conduct current through the drain = Id) until Vgs reaches the "Threshold Voltage" (or Vt).
Note that Vd changes only effect Id in the "Linear" region (hence "Saturation" means Vd changes do not effect the Id current flow through the transistor).
Vd (the drain voltage, or voltage across the whole transistor) must be less than Vgs (the gate to source voltage) - Vt (the theshold or turn on voltage) to be in this (ohmic or) linear region.
If this does not make sense to you, please see Semiconductor Basics under Ken's Technology Notes (for an explaination of how transistors are used in analog amplifier and digital circuits).
How a BJT Works | This one has the best (electron flow in a) field animation work I've seen in a device demo. I hope to replicate it on a future project. | Flash |
How a JFET Works | Similar interactive application, no Gm | Flash |
MOSFET Operation-1 | Basic Vgs, Vd interactivity. | Java Applet |
MOSFET Operation-2 | Basic Vgs, Vd interactivity, nice curve tracing (better than mine!) | Java Applet |
Semiconductor Simulation Library | Extensive (and good!) library demonstrating most of the basics | Java Applet |
Funktionsweise des MOS-Transistors | German website containing a series demos | Java Applet |