Small Signal AC Simulations — Lesson 1

Simulation Settings

In this unit, we will review the solver settings that need to be specified when running small-signal AC simulations. The “Small Signal AC” tab in the property editor window of the CHARGE solver contains the small-signal AC simulation controls which determine the characteristics of the AC signal used to perturb the system. These characteristics include:

Perturbation amplitude: The amplitude of the AC signal should be very small. The default value is 1 mV, which is suitable for most cases.
Frequency spacing: The frequency of the AC signal can be chosen from three different modes.

• Single: The signal will only have a single frequency.
• Linear: A range of frequencies can be chosen for the signal where the spacing between frequency points will be linear with equal intervals between points.
• Log: A range of frequencies can be chosen for the signal where the spacing between frequency points will vary in a logarithmic fashion to accommodate for a large range of frequencies in the simulation.

Example Device

As was mentioned earlier in the course, the CHARGE solver can simulate the frequency-dependent characteristics of semiconductor devices using a small signal AC analysis. In a small signal analysis, a small AC voltage is applied on top of the DC (steady-state) bias and the small-signal currents and voltages at the contacts are reported.

In this example, we will focus on the electrical simulation of a germanium-based pin photodiode. The geometry of the photodiode is simplified to be a single germanium slab sandwiched between two metal contacts as shown in the diagram below. The top and bottom surfaces of the germanium slab are doped with p- and n-type dopants respectively. The rest of the slab is left undoped, creating a pin device. We will perform a small-signal analysis of the pin diode to extract its frequency-dependent capacitance in reverse bias.

Small Signal AC Simulation

In this video lesson, we will demonstrate a simple small-signal AC simulation to calculate the changes in the reverse-bias capacitance of a PIN diode as a function of frequency. The simulation project and script file used in this unit can be downloaded here.