Hints, Tips and Solutions

Volume 12, Number 5, May 2002

Q. Can ATLAS simulate a Schottky contact ?

A. ATLAS allows the user to define a contact with a number of different boundary conditions; ohmic, Schottky, current controlled, floating or reflecting.

The Schottky contact boundary condition realizes that at the metal semiconductor interface a barrier exists due to the presence of interface states. A special surface potential is applied at this contact that is calculated according to

where AFFINITY is the electron affinity of silicon and WORKFUN is the workfunction of the metal contact. For example, if the Schottky contact was aluminum with a workfunction difference to the silicon of 4.2 eV and the barrier height was 0.7 eV, then the user would define the Schottky contact with the statement

CONTACT NAME=GATE WORKFUN=4.9

The WORKFUN parameter is the aluminum workfunction plus the barrier height. This is all that is needed by the simulation to simulate a Schottky contact.

As an example of this type of simulation a Schottky diode and a regular p+/n+ diode have been simulated within ATLAS. The Schottky diode has been simulated with the first technique described above and consists of an aluminum contact on 1e14 cm-3 n- type silicon where a barrier height of 0.7 eV has been assumed at the contact. The mesh was carefully created to ensure that under the Schottky contact the mesh density was sufficient to resolve the depletion region. The chosen set of models were

MODELS FERMI SRH CVT AUGER

Fermi-Dirac statistics were chosen to ensure proper simulation of high carrier concentration statistics, SRH to model thermal recombination and generation, CVT was the chosen mobility model and AUGER to model carrier-carrier recombination.

Figure 1 shows the two structures that were simulated and Figure 2 shows the I-V characteristics of the Schottky and regular diodes on a log scale. The simulation shows several important effects

• the Schottky diode turns on at a lower voltage than the p+/n+ diode
  
  
• the Schottky diode exhibits forward current saturation at much lower current levels than the diode
  
  
• the Schottky diode has much higher leakage current than the p+/n+ diode

These are the well known features of a Schottky diode which are made use of in many modern technologies.

Figure 1. The structures simulated were a Schottky diode and a pn diode.
In the Schottky case the anode contact was the Schottky contact.

Figure 2. Simulated IV characteristics of the Schottky and pn diodes.