PNP Gummel Plot and Ic/Vce Characteristic

bjtex08.in : PNP Gummel Plot and Ic/Vce Characteristic

Requires: SSuprem 4/S-Pisces
Minimum Versions: Athena 5.22.3.R, Atlas 5.28.1.R

This example demonstrates the formation and characterization of a PNP bipolar transistor. It consists of:

  • Structure formation in Athena
  • Gummel plot simulation in Atlas
  • Ic/Vce simulation in Atlas

The first part of this input file uses Athena to construct the geometry and doping of the PNP transistor. The device is a polysilicon emitter PNP with an emitter length of 0.4um. Since the device is symmetrical through the center of the emitter, only half the device needs to be simulated. The resulting currents from Atlas can always be doubled to account for this reflection. The intrinsic base is formed by a 100keV phosphorus implant at the start of the process. Extrinsic base is formed by spacers on the sidewall of the emitter polysilicon. The emitter area is formed by p+ doped polysilicon directly deposited onto the silicon.

At the end of the process, metal contacts are patterned to give an emitter contact on top of the polysilicon and a base contact to the right. These metal regions are defined as electrodes in Athena to prepare the structure for Atlas.

The final step in the process simulation is to extract some process parameters. The base width is extracted by measuring the base/collector and emitter/base junctions individually and subtracting the two results. Using the 1D Poisson solver in DeckBuild, the sheet resistances of the base and polysilicon emitter region are obtained.

The first of the two Atlas runs in this file is to extract the Gummel plot characteristics. From this plot the gain and fT can be obtained. The material parameters for polysilicon and silicon regions are set in separate lines in the Atlas input file.

The material parameters for polysilicon reflect the low mobility and short carrier lifetimes seen in this material. They also contribute to the reduction in base current and consequent increase in gain seen in polysilicon emitter bipolar transistors. The effect of the native oxide layer that exists between the polysilicon and silicon is emulated by these material parameters. The effect of the native oxide is to force recombination of carriers at the polysilicon/silicon interface rather than the carriers diffusing further in to emitter region. The short lifetime and low mobility in the polysilicon accomplish the same effect and have been widely used in polysilicon emitter bipolar device simulation.

The Gummel plot is simulated at Vce=-2.0V by ramping Vbe from -0.4 to -1.0V. In addition to the DC solution, a small AC signal is also applied. This enables fT vs. Ic to be extracted. The AC signal is specified using the parameter AC on the solve statement. The frequency of this signal is 1MHz. At the end of the solve sequence several device parameters are extracted: peak gain, peak collector current and peak fT.

The second Atlas run extracts a family of Ic/Vce curves for different base currents. For each curve it is necessary to force a constant current through the base electrode. This is done using the current boundary condition set by contact name=base current .

Since the base currents used are quite high, it is easier to first ramp the base voltage up close to the initial current level required. In this case Vbe is ramped to -0.7V. Then the current boundary conditions are applied and solutions for base currents of 1uA/um to 5uA/um were obtained. At each step in Ib a solution file is stored for later use.

During the ramp up and storing of files with different Ib the collector voltage is left at ground. Next a sequence of statements is used to LOAD the stored file for a particular Ib value, open a log file for the results, and solve a ramp of collector voltage to -5V. This sequence is repeated for each Ib considered. Separate results files for each Ib value are recommended for use in TonyPlot and for extracting parameters from the results. It is possible to store all IV data output for this simulation into one file. A full bipolar extraction for SPICE models can be done by interfacing these log files with UTMOST.

Finally some extraction on the Ic/Vce data with Ib=5uA/um is done. The peak current in mA, the peak slope in the linear region and the slope of the saturation region can all be obtained using extract.

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.