InGaAs/InP HBT DC and High Frequency Characteristics

hbtex06.in : InGaAs/InP HBT DC and High Frequency Characteristics

Requires: DevEdit/Blaze
Minimum Versions: Atlas 5.28.1.R

In this example, an HBT structure based on the InGaAs-InP material system is constructed using DEVEDIT. The structure is then passed to Atlas for electrical testing. The input file consists of the following 2 main portions:

  • Construction of the device in DEVEDIT
  • Simulation of the Gummel plot in Atlas
  • Simulation of the AC parameters
  • S-parameter extraction

The first stage of the input constructs the HBT geometry, material regions, doping profiles, and electrodes in DEVEDIT. The n-p-n HBT device is based on a lattice matched InGaAs-InP material system. It consists of a highly doped InGaAs cap region followed by another cap region made of InP. The next region, also made of InP, constitutes the actual emitter. As usual in HBTs the emitter (InP) has wider energy band gap than the base and collector (InGaAs). The base is followed by the n- subcollector and n+ collector regions. The substrate is made of undoped InP.

The structure was created in DEVEDIT by drawing the device regions in interactive mode and specifying 2D doping distribution. In this example each region was uniformly doped. The Ga composition fraction of 0.47 was also specified in DEVEDIT for each of the InGaAs regions. Finally the mesh was generated automatically by specifying basic mesh constraints and refining it along x- and/or y-directions in the important areas of the device.

The Atlas simulation begins by reading in the structure from DEVEDIT. DeckBuild provides an automatic interface between DEVEDIT and Atlas so that the structure produced by DEVEDIT is transferred to Atlas without having to indicate the mesh statement.

The first active statements in the Atlas portion of the input file are material parameters and models definition. In this example the energy band gap, densities of states, and other fundamental material parameters for InGaAs are calculated based on the composition specified in DEVEDIT. For InP the respective default values are used. The band alignment is defined using the align parameter on the material statement. Material and model parameters can be specified in Atlas on material-by-material or region-by-region basis. The latter possibility is used here to define carrier lifetimes, low field mobilities, and saturation velocities taking into consideration the doping levels in the respective regions.

The same set of physical models is applied here to all the regions/materials: Shockley-Read- Hall recombination, electric field dependent mobilities with GaAs-like velocity-field characteristic, and Fermi-Dirac statistics. To reflect the different properties of materials with regard to the critical field, ecritn is specified in separate model statements for InP and InGaAs.

The simulation is first performed to obtain the Gummel plot by biasing simultaneously the base and collector with respect to the emitter up to 1.2 V. At the same time, a small signal AC perturbation is applied at a frequency of 1 MHz to calculate the AC parameters (conductances and capacitances) as functions of Vbe. This also allows the user to obtain the cutoff frequency using the low frequency approximation in TonyPlot or using the extraction feature of DeckBuild. Currents, voltages, and AC parameters are saved in a log file, and internal structure information is saved for a bias condition (Vbe=Vce=0.95 V) where the cutoff frequency is close to its maximum. The calculation of s-parameters is specified on the log statement with the s.param command.

At the end of the simulation, extract statements are used to determine the maximum cutoff frequency, the base bias, the input (base) capacitance, and the transconductance - all at the bias of Vbe=Vce=1.0 V where the maximum cutoff frequency was observed.

The structure plot, the Gummel plot, the AC current gain versus frequency, and S-parameters are then displayed using TonyPlot.

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.