Tunnelling Through a Polysilicon-Oxide-Silicon Diode

diodeex12.in : Tunnelling Through a Polysilicon-Oxide-Silicon Diode

Requires: S-Pisces
Minimum Versions: Atlas 5.28.1.R

This example shows how to self-consistently include oxide barrier tunneling in the I-V characteristics of a MIS diode. For forward bias there are three different components to the tunnel current, and in reverse bias the tunnel current is electronic in nature and limited by the thermal generation rate of carriers in the silicon.

The example is split into several sections, each starting with
go atlas
The first section sets up a structure with a polysilicon gate, a 2.5 nm thick oxide and a lightly p-doped Silicon substrate. The substrate is contacted with an ohmic aluminum contact.

The next section reads in the structure and sets material parameters for the oxide and silicon. The effective masses in the oxide strongly effect the value of the tunnel current. The contact workfunction is set to be that of n-type polysilicon, and the effective masses in the contacts are also set. On the MODELS statement some mobility and recombination models are set for the silicon. The parameter QTNLSC.EL enables self-consistent electron tunneling, whereby electrons tunneling through the oxide are injected into the electron continuity equation in the silicon. The parameter QTNL.DERIVS puts non-local derivatives into the system matrix to help achieve a converged solution, and is essential in this example. The anode bias is then ramped to 2.4 V and the resulting currents saved in a logfile. The current components are also saved using the j.elec, j.hole and j.tun parameters.

The next section is essentially the same as the last except that hole tunnelling is included as well, by specifying QTNLSC.HO . In the following section band-to-band tunneling is initially enabled by using the QTNLSC.BBT parameter. This will be significant when the valence band in the polysilicon is at a greater electron energy than the silicon conduction band. The forward biased I-V curves show that at low biases the electron injection dominates, then hole tunneling from the silicon valence band to the polysilicon valence band becomes dominant. At even higher biases the band-to-band tunneling switches on and dominates.

The example continues with negative bias being applied to the anode. This causes the electrons to be removed from the device by tunnelling to the gate, and because the electrons are minority carriers they are only supplied through SRH generation processes. Even at large negative biases the current is relatively small because of the rate limiting effect of the SRH generation of minority carriers. To illustrate this the final section repeats the reverse bias simulation but with an SRH lifetime that is two orders of magnitude longer and the tunnel current is much smaller because of the lower thermal generation rate. The hole and band-to-band tunnel rates are negligible in this case. The contact Fermi-level is near to the polysilicon conduction band and consequently hole injection is negligible. The I-V characteristics of the diode between Anode voltages of -2.4 V and 2.4 V are shown in the final plot. This example thus demonstates the rectifying properties of the tunneling diode, and the physical processes which lead to this behavior.

To load and run this example, select the Load button in DeckBuild > Examples. This will copy the input file and any support files to your current working directory. Select the Run button in DeckBuild to execute the example.