Hints, Tips and Solutions

 

Volume 12, Number 7, July 2002

 

Q. How can I measure low current FT for Bipolar devices?

A. The input impedence of most network analyzers are around 1 MOhm, which is relatively small. Due to this low impedence the leakage current on the Base port (Typically Port1) becames high and makes it impossible to find the correct collector current bias when "Linear IC" or "Log IC" measurement modes are selected in FT_CE routine.

For example, if the base voltage biased at 0.5V the leakage current to the ground will be 0.5uA. For an NPN device with DC gain of 100, this will translate into a collector current loss of 50uA. This means that the device can not be biased using current sources (Linear IC and Log IC modes in FT_CE routine). During the "Linear IC" or "Log IC" modes of operation, UTMOST automatically iterates the base current and measures the collector current until collector current is within the bias error percentage defined in the setup screen. (Figure 1.) For the low collector currents the majority of the base current will leak to the ground and the bias iteration will fail to reach to the targeted collector current value.

 

Figure 1. FT_CE routine measurement setup screen.

 

In order to solve this issue, the "VBE,VCE constant" or "VBE, VCB constant" measurement sweep modes were developed for the FT_CE routine. If one of these modes are selected, the FT_CE routine will search for the bias currents within the previously measured DC "bias" files. These "bias" files contain the gummel (IC and IB data) plot data. (Figure 1.) The gummel plot data is automatically stored in a file named "bias.xxx" when data is measured, simulated or retreived from a logfile using the "Gummel"routine (routine# 14). The bias file should be generated using the "Gummel" routine for each VCE bias that will be used for the FT_CE measurements. The suffix "xxx" of the bias file is computed as VCE * 100. For example, "bias200" corresponds to gummel plot data measured for VCE=2V. Therefore if the user measures, simulates or retrieves data using the "Gummel" routine at VCE=2V and VCE=5V, the files named "bias200" and "bias500" will appear in the UTMOST user directory.

Figure 2. Gummel plot data.

 

When the "VBE,VCE constant" or "VBE, VCB constant" options are selected the measurement variables "VBE_start", "VBE_stop" and IC_points will be used to determine base voltage VBE. However the collector current will not be measured and the base current will not be iterated. The value of the base and collector currents will be obtained from the "bias.xxx" file. The linear regression method will be used to determine the exact base and collector currents for the given VBE. This approach extends the low collector current measurement limits to 5uA to 10uA. If the "bias.xxx" is not present during the FT_CE measurement then an error message will be displayed on UTMOST screen.

 

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