A New Method For Extracting Reverse Transit Time (TR) in Bipolar Transistors

 

Introduction

The reverse transit time (TR) is used to model the excess charge stored in a bipolar transistor when its collector-base junction is forward biased.TR is an important parameter for modeling the delay of a bipolar transistor operating in the saturation region. The new extraction technique is based on the use of ac current gain (h21) measurements taken using a network analyzer. The extraction method is simple and easy to implement for on-wafer measurements.

 

Existing Measurement and Extraction Methods

  1. The pulse measurement technique [1] is a simple method of calculating TR from the measured saturation delay time constant. However, this method requires that the forward transit time (TF) be known. Therefore, any errors made in the TF measurement can lead to an inaccurate TR extraction.
  2. In cases where the reverse dc gain (BR) is much greater than 1, TR can be extracted by using the same method that is used to extract forward transit time (TF). In today's bipolar transistors the BR is less than or slightly greater than 1 and thus this method can not be directly applied to a TR extraction.

New Extraction Method and Measurement Technique

In the proposed method [2] the extraction of TR is based on the h21(ac gain) measurements taken in the reverse active mode of operation. During the measurement the collector-base junction should be forward biased (VBC > 0V) and VBE should be equal to 0V (for an NPN transistor).

The FT_CC routine in the UTMOST Bipolar module can be used for the data acquisition and the TR extraction [3]. In this example VBC is set to 0.78V and VBE is set to 0V. The "Bias Sweep Button" in the AC measurement screen should be toggled until it reads "VBE,VCE const". The "VEC_start" and "VBC_start" variables should have the same value (i.e. 0.78V) to provide the VBE=0V bias condition (Figure 1).

 

Figure 1. The FT_CC routine AC Measurement Screen.

 

 

After the DC biasing is provided and the h21 measurement is completed the following equations are used to extract TR (Please refer to [2] to obtain the complete equations):

 

 

BR can be extracted from h21(0) using (1). Then the magnitude of h21 at w=wp can be calculated using (2) and (3). Now wp can be obtained from the h21 frequency response plot (Figure 2). Finally TR can be calculated using (4).

 

Figure 2. h21 frequency response plot.

 

When the h21 measurement using the FT_CC routine is completed, press the "Fit" button from the "Options" menu. This will extract TR and the value will be copied to the parameters screen and the graphics screen (Figure 2). The extracted TR value will vary slightly with changing DC bias conditions. In the Gummel-Poon model TR does not vary with changing bias, hence it is recommended to extract TR near the operating bias point.

 

Conclusions

A new method of extracting TR, and its implementation in the UTMOST Bipolar FT_CC routine is presented. This technique is based on h21 measurements taken when the bipolar device is in the reverse-active mode of operation.

 

References

[1] "Modeling the Bipolar Transistor", 
Ian Getreu, Published by Tektronix Inc. 1979.

[2] "A New Method for Determining the Reverse 
Transit Time in Bipolar Transistors.",
Yeung-Seuk Kim, David Burnett, Craig S. Lage, 
IEEE Transactions on Electron 
Devices, Vol. 39, No. 10, October 1992. 

[3] UTMOST Extraction Manual, Volume 2, Bipolar 
Device Modeling Routines, Page 249-253, 
January 1995.