This Java applet illustrates a MOS capacitor and also can be used to understand basic MOS transistor function. Three scrollbars for the lattice temperature, doping and oxide thickness are provided for interaction. A voltage supply scrollbar is also provided to bias the MOS capacitor. Finally two buttons for altering the gate doping (i.e. work-function/Fermi level in control gate) and making MOS capacitor (field effect) are also provided.

When uploaded with the browser the applet initially displays separate Control Gate - Dielectric (oxide) and P-Silicon materials (i.e. no MOS field effect). By clicking on the button "Isolated/MOS" the MOS capacitor is created. The Button N-Type/P-Type is used to switch between the N-Type and P-Type doping in the control Gate to show the effect of the work function on the MOS capacitor or transistor threshold voltage.

When making the contact among the Control Gate - Dielectric and Silicon materials a MOS capacitor is created. The gate is initially N-Type with heavy doping to reduce depletion effect in the gate. Assuming that the gate oxide is a perfect dielectric, therefore no leakage exists in the dielectic, the Fermi level is not altered by the contact of the materials forming the capacitor. Conduction, intrinsic and valence energy levels are altered (bended) however due to the difference in work-functions or Fermi levels between that of the silicon and the control gate. By applying a voltage between the capacitor nodes one can alter this bending. The voltage necessary to make the energy levels flat (no bending accross the silicon) is called flatband voltage. The flatband condition is the limit between accumulation and depletion. Large flatband voltage generally suggests easy to convert (inversion) the silicon carrier population from P-Type to N-Type in the surface of the P-substrate.

The carriers forming this inversion layer are the minority carriers and excited valence charge to the conduction band, and attracted to the surface of the semiconductor. However, these carrier densities are in general small with limited lifetime making the inversion hard to sustain. Therefore, a source of minority carriers is needed to assert this inversion; hence the source diffusion in MOS devices. MOS capacitors without source of minority carriers are measured at frequencies much lower than 1/τ where τ is the lifetime of minority carriers in the semiconductor.

The threshold voltage is the voltage necessary for the onset of the inversion layer. By convention this voltage needed to set the Fermi level at the semiconductor surface above the intrinsic level in P-Type silicon (below in N-Silicon) and the surface voltage is twice the difference between Fermi and Intrinsic level in bulk (i.e. q*Vsurf = 2*(Eip - Efp) ). The surface voltage is shown by the vertical arrow at the interface oxide-silicon. By scrolling slowly the supply voltage these conditions can be satisfied and the threshold voltage is displayed.

It is worth noting the effect of the gate doping or type on the flatband voltage and therefore the threshold voltage. For a given doping concentration in the silicon, the threshold voltage increases by over a volt when the N-Type gate is switched to P-Type. The same observation in the P-type substrate with N-Poly gate. Notice that flatband is affected by the oxide charges. Ideal flatband value is obtained when the oxide charge is null.