InP/InGaAs/InP Double HBT with varying doping profile : InP/InGaAs/InP Double HBT with varying doping profile

Requires: Blaze
Minimum Versions: Atlas 5.28.1.R

In this example a DHBT structure based on the InP / InGaAs material system is constructed using Atlas. The DHBT is then electrically tested, and properties that characterise the device's DC and high frequency performance are calculated and presented. The DC performance is shown through a gummel plot, and the DC current gain is calculated using the functions property in TonyPlot. The AC performance is evaluated via the cutoff frequency ft, as well as the maximum oscillation frequency fmax. The cutoff frequency is defined as the frequency at which the magnitude of AC current gain ( i.e. h21), decreases to unity. It is important to note that the transistor is connected in common-emitter configuration for this calculation. The maximum oscillation frequency fmax is the frequency at which the unilateral power gain of the transistor tends to unity. The unilateral power gain effectively represents the maximum power gain achievable by the transistor. These properties can be improved though appropriate use of heterostructures, stoichiometry and doping to lower collective resistance, epitaxial strain as well as several other properties, all of which are incorporated within HBTs and indeed DHBTs offering superior performance to BJTs.

The superior performance encountered with HBTs and DHBTs is mainly due to the presence of a quantum well in the emitter / base junction arising through the use of two materials of different band gap. In this example InP / In(0.53)Ga(0.47)As have been used. The quantum well stops back injection of holes from the base into the emitter which will reduce the base current and consequently increase the current gain. The quantum well will also permit the base doping to be increased which will lower the resistance encountered within the base and reduce the transit time for electrons thus increasing frequency performance. By adding a second heterostructure within the collector / base region (i.e. In(0.53)Ga(0.47)As / InP) epitaxial relationships are improved throughout the structural growth with an overall improvement in device performance.

Another technique to improve a device's performance is to use a graded doping profile. This will aid in controlling electric fields concomitant with depletion regions and will improve breakdown characteristics. This example demonstrates this ability by importing doping profiles previously specified in an ASCII text editor. These files are called upon during the construction stage and the doping profiles will be replicated accordingly. In this example the files used are hbtex08_n.dat and hbtex08_p.dat for n and p type doping respectively. The user can simply create an ASCII text file having two columns. The left column specifies the depth location and the right column specifies the concentration at that location.

An example would be:

0.0 1e16
1.0 1e20
1.001 0.0
2.0 0.0
( Must put a carriage return here )

As this is an ASCII file, a carriage return must be placed at the end of the file. To use this file would require the commands:

doping (specify type) ascii infile=(filename)

It is also noted that interpolation is used between specified locations. Consequently the doping profile must be set to zero through certain locations if so desired. Otherwise an interpolated value will be used until the end of the device is encountered. This is demonstated in the above example.

The accuracy of the simulations is also improved by the use of Atlas-Interpreter mobility models for the materials. The models are based on "Empirical low-field mobility model for III-V compounds applicable in device simulation codes", M. Sotoodeh, A. H. Khalid, and A. A. Rezazadeh, J. Appl. Phys., 87, 2890 (2000).

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.