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6.5 Influence of Backside Doping on the CG(VGS) Characteristics

The previous investigations demonstrated the capability of predictive device simulations to improve the DC and RF characteristics. In the following section simulation is used to explain some specific device characteristics and its impact on voltage controlled oscillators (VCO).

The physical origin of CGS is a change in electron concentration in certain semiconductor layers due to a change in the gate voltage. Thus different contributions can be attributed to the different semiconductor layers. These are sketched schematically in  Figure 6.50. In HEMTs with doping only above or in the channel, contributions of channel and upper barrier doping sum up to a well known monotonous increase of CGS with VGS. With an additional doping at the backside of the channel, a third contribution is added. Depending on the doping concentration and the energy level relative to the channel this can result in a curve with a local maximum shown by the bold line in  Figure 6.50.

Figure 6.50 Contributions to CGS due to backside doping (dots), channel (short dashes) and upper barrier doping (long dashes).

In  Figure 6.51, both simulated and extracted CGS(VGS) curves are shown which compare very well. If VGS is increased from pinchoff CGS increases until it reaches a maximum. Both, simulated and extracted CGS show a negative gradient over more than 400 mV of VGS which is the largest part of the usable VGS swing of the device.

Figure 6.51 Simulated and extracted CGS of the investigated millimeter wave HEMT at VDS=3.0 V.

Simulations of the same device were performed where only the backside doping NDb was changed.  Figure 6.52 proves that NDb is the reason for the local maximum of CGS(VGS). It demonstrates that the location and magnitude of the maximum depends on the concentration of NDb. No local maximum can be observed in the case of NDb = 0. With increasing NDb the pinch off voltage decreases and the local maximum in CGS(VGS) appears. The higher the contribution of NDb to the total doping, the more pronounced is the local maximum in the CGS curve. This behavior has an impact on circuits whose properties strongly depend on the CGS(VGS) characteristics such as some types of VCOs.

Figure 6.52 Simulated CGS of the same HEMT but with different backside doping at VDS=3.0 V.

If a voltage controlled oscillator (VCO) is tuned directly by VGS variations, the change in CGS is one of the most important parameters for its frequency characteristics.  Figure 6.53 shows a photograph of such a monolithic VCO with buffer amplifier [72]. In this type of VCO the output frequency usually decreases with increasing VGS if HEMTs with doping only in or above the channel are employed [83]. This can be different in the same type of VCO if backside doped HEMTs are used [72]. The change in the CGS(VGS) curve due to the backside doping changes the tuning behavior. In particular, the frequency response can be reversed for a certain interval of VGS.

Figure 6.53 Layout of the measured VCO with buffer amplifier.

Figure 6.54 shows the oscillation frequency fosc and the corresponding CGS of the HEMT employed in the VCO, both as a function of VGS. In this case fosc increases over the whole range in which the VCO is oscillating. This clearly coincides with the range in which CGS is decreasing with rising VGS. In the case of the measured VCOs no oscillation could be observed for VGS below 0.2 V and above 0.65 V.

Figure 6.54 Measured fosc of the VCO versus the tuning voltage VGS and the extracted CGS of the HEMT used in the VCO.

next up previous contents
Next: 7 Conclusion Up: 6 Applications Previous: 6.4 Comparison between Low Noise, Power, and Millimeter Wave HEMTs

Helmut Brech