Second Order Intermodulation Linearity of a 200 GHz SiGe HBT Technology

Intermodulation linearity is an important consideration for RF technologies. Third order intermodulation linearity is typically considered, as third order output is close to the fundamentals. Second order intermodulation outputs are far away from the fundamentals, and thus can be filtered [1]. However, the same second order nonlinearity also produces dc offset, which presents a significant problem for direct-conversion receivers [2]. While differential circuits can decrease second order nonlinearity, cancellation is not complete, and is worsened by transistor mismatch.  

We present here characterization of the second order intermodulation linearity on 200 GHz peak f_T SiGe HBTs. The second order input intercept point, IIP2, is characterized on I_C-V_CE plane for different breakdown voltages. The measurement system is set up and calibrated according to [3].

Fig. 1 (a) and (b) show the measured IIP2 and f_T versus I_C for V_CE = 1.0, 1.3, 1.6, and 1.9 V on a standard breakdown voltage (SBV) HBT. For comparison, the third order input intercept point (IIP3) is shown in Fig. 1 (c). IIP2 increases with I_C before f_T roll-off, and then decreases. A higher V_CE effectively delays IIP2, f_T and IIP3 roll-off overall. The impact is more obvious for IIP2 and IIP3 than for f_T. Observe that the IIP3 peak is reached at a much smaller I_C, well before peak f_T is reached. IIP2 peaks either reside inside the flat f_T region or closely lag behind f_T roll-off. At high I_C, higher V_CE shifts IIP2 to higher level, implying that a higher V_CE can be used to effectively improve IIP2 at higher currents. 

In Fig. 2 (a) and (b) , the current dependence of IIP2 and f_T are compared for three HBTs of different breakdown voltage. IIP2 first increases with I_C, peaks, and then decreases sharply before rising again. The IIP2 peak occurs approximately at the same current as the f_T peak for all breakdown voltages. Before 1 mA, IIP2 is the same for all, indicating that the collector-base junction difference does not affect IIP2 so long as high injection does not occur. A higher V_CE could be used with higher breakdown devices to extend the current range of high f_T and IIP2, which is also confirmed by measurement.

References

[1] P. Wambacq, W. Sansen, Distortion Analysis of Analog Integrated Circuits, Kluwer, 1998

[2] B. Razavi, RF Microelectronics, Prentice-Hall, 2012

[3] G. Niu, et al., BCTM, pp. 1-4, 2006.