High-frequency Characteristics

To characterize the high-frequency performance, the S-parameters were measured on-wafer from 0.1 to 30 GHz using an HP8510 network analyzer and then de-embedded by subtracting the OPEN dummy. Fig. 2.5 shows the high-frequency characteristics of circle structure. The maximum stable gain/maximum available gain (MSG/MAG) and short-circuit current gain (h21) were calculated from S parameters. The cutoff frequency (fT) and maximum oscillation frequency (fmax) were determined as the frequency where the current gain was 0 dB

and the frequency where MAG was 0 dB, respectively. The transistors were measured at drain voltage VDS=20 V with different gate voltages. From Fig. 2.6, fT had maximum values at VGS=2.5 V, where the transconductance showed a peak. With increasing the gate voltage, both fT and fmax decreased owing to the mobility degradation and quasi-saturation effects.

The cutoff frequency and maximum oscillation frequency for the LDMOS with different layout structures are compared in Fig. 2.6. The transistors were biased at VGS=2.5 V and VDS=20 V to obtain the maximum value of fT. It is observed that the values of fT in these layout structures are circle > octagon > square > fishbone and the values of fmax are square >

circle > octagon > fishbone.

By analyzing a MOSFET small-signal equivalent circuit, we can determine the effect of device parameters on high-frequency characteristics more clearly. We adopted a simple model (shown in Fig. 2.7) and extracted the equivalent circuit parameters of the LDMOS by the method described in ref. 22. After de-embedding the extrinsic parasitic resistances and the substrate-related parameters, the intrinsic components could be directly extracted from intrinsic Y-parameters (Yi) by the following equations [23]:

,12

Using extracted parameters from the existing device and altering one parameter at the time, the effect of model parameters on the cutoff frequency and maximum oscillation frequency can be visualized. The influences of model parameters on fT and fmax are shown in Fig. 2.8. The x-axis showed the parameter value departure from the initial value in percent.

The y-axis showed the change in frequency in percent. Parameters not shown in the figure had approximately the same value for the ring and fishbone structures or had a minor influence on fT and fmax. As shown in Fig. 2.8(a), the intrinsic transconductance (gm), gate-source capacitance (Cgs) and gate-drain capacitance (Cgd) have large effect on fT. The cutoff frequency can be expressed in a simple way of fT = gm/ 2π (Cgs+ Cgd) which is related to the gm and input intrinsic capacitances (Cin= Cgs+ Cgd). The extracted model parameters that affect the fT more significantly are listed in Table 2-2. For ring structures, we find that the slight increase of fT in circle device is mainly due to the slight increase of gm. When gm increases from 34.58 mA/V to 35.92 mA/V and 36.7 mA/V for octagon and circle respectively contrasting with square structure, fT is improved by about 3.7% and 5.9% (see Table 2-3). The gate capacitances are nearly unchanged. For fishbone device, however, the drain resistance (Rd) is large and its effect on fT cannot be ignored. Rd represents the drain contact resistance and part of the drift region. As compared to square device, Rd has been increased from 2.6 Ω to 9.05 Ω for fishbone device, and thus fT becomes worse about -3.9%. In addition, the lower Cgd and higher Cgs in the fishbone structure cause about 3.7% and -7.2% changes in fT due to different layout design. Otherwise the intrinsic gm of fishbone is the biggest among four structures and makes fT increase about 6.5%. As illustrated in Fig. 2.8(b), in addition to the intrinsic parameters, the Rd, gate resistance (Rg), drain-source capacitance (Cds) and drain-substrate junction capacitance (Cjdb) have apparent effects on fmax [24]. The extracted model parameters that affect fmax more significantly are listed in Table 2-4. The square structure has the highest fmax due to lower parasitic drain junction capacitance (Cjdb) and gate to source capacitance (Cgs). The Cjdb can be separated into two parts; one is between P-body

and DNW, another is between DNW and substrate (see Fig. 2.9), and then the Cjdb is relative to area of DNW. Because the square structure has small area, the DNW area and thus the Cjdb

are small (see Table 2-6). Contrary to square, the fishbone structure has the lowest fmax due to higher Rd and Cgs in addition to lower fT. Comparing to the square structure, the octagon and circle structures have lower fmax due to larger Cjdb and drain to source capacitance (Cds).A part element of Cds represents the overlap of metal conducting wires. Although fmax should be improved with the gm increasing, the other parameters will also affect the fmax. For ring structures, the Cds increases from 97.62 fF to 119.9 fF and 119.8 fF for octagon and circle respectively contrasting with square structure, both fmax become worse about -2.9% (see Table 2-5). In addition, the Cjdb increase from 52.13 fF to 129.9 fF and 132.3 fF, the fmax also become worse about -7.6% and -7.8%. For fishbone structure, we are easy to find the Rd has a great impact on fmax and becomes important. As compared to square device, Rd has been increased from 2.6 Ω to 9.05 Ω for fishbone device, and thus fmax becomes worse about -23.8%. In addition, the lower Cgd and higher Cgs in the fishbone structure cause about 8.6%

and -6.9% changes in fmax due to different layout design. Otherwise the extrinsic parameters (Rg and Cds) also make fmax increase about 5.7% and 6.8% respectively. Consequently, the fmax

for circle structure is lower than the one for square structure by about -3.5% (fmax was 14.74 GHz for the square structure and 14.22 GHz for the circle structure). The fmax for octagon structure is lower than the one for square structure by about -10.7% (fmax was 14.74 GHz for the square structure and 13.16 GHz for the circle structure). The fmax for fishbone structure is lower than the one for square structure by about -11.2% (fmax was 14.74 GHz for the square structure and 13.09 GHz for the fishbone structure).