6.5 Influence of Backside Doping on the C_G(V_GS) 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 C_GS 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 C_GS with V_GS. 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.
 

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

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

If a voltage controlled oscillator (VCO) is tuned directly by V_GS variations, the change in C_GS 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 V_GS 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 C_GS(V_GS) curve due to the backside doping changes the tuning behavior. In particular, the frequency response can be reversed for a certain interval of V_GS.
 

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