**3.2 Small signal characteristics**

The DC output simulation parameters have been used as the input for simulation of small signal analysis. The significant high-frequency or small signal parameters obtained from this analysis are optimum frequency (fp), peak negative conductance (−Go), negative resistance (ZR), RF power output (PRF) and output power density (PD). These parameters are obtained from the high-frequency simulation of GaAs and Si DDR IMPATTs at several biased current density and represented in **Table 4**.


**Table 4.**

*Small signal characteristics of Si and GaAs DDR IMPATT diodes at design frequency of 94, 140 and 220 GHz.*

**Figure 2.** *Variation of negative conductance with frequency for DDR IMPATT operating at 94 GHz.*

The diode negative conductance and negative resistance as a function of frequency for the different DDR IMPATT diodes of GaAs and Si are mentioned at different operating frequency. GaAs IMPPAT shows less negative conductance as compared to Si IMPATT. The diode negative conductance (−Go) as a function of frequency for GaAs and Si DDR IMPATT is plotted in **Figures 2**–**4**. From the figures it is observed that, as the diodes are optimized with the current density, the peak of the negative conductance lies at the design operating frequencies 94 GHz, 140 GHz and 220 GHz and also it is noticed that the peak negative conductance of Si is remarkable higher. Subsequently in negative resistance case the behavior is directly reverse. The GaAs based IMPATT DDR diode gives less value of negative resistance (−ZR) than that of Si DDR diodes.

The power density of GaAs bas IMPATT shows high value as compared to Si based IMPATT. The high value of power density indicates GaAs IMPATT diode is capable of high output power. Again it is noticed that the power density increases with increase in the operating frequency. This high power density of GaAs generates more noise in the device which is discussed in the subsequent section.

Over all, the variation of negative conductance with the different operating frequencies in **Figure 5** and it is also observed that the negative conductance of all the IMPATT diode based on Si and GaAs increases with the increase in operating frequency keeping area of the diode constant.

### **3.3 Noise properties**

Since noise is an important aspect of IMPATT study, and hence, the noise characteristic of GaAs IMPATT diode has been analyzed and compared the results with Si based IMPATT diodes. In **Table 5**, the mean square noise voltage per band width of the three different diodes based on GaAs and Si at different

*IMPATT Diodes Based on GaAs for Millimeter Wave Applications with Reference to Si DOI: http://dx.doi.org/10.5772/intechopen.95874*

**Figure 3.** *Variation of negative conductance with frequency for DDR IMPATT operating at 140 GHz.*

**Figure 4.** *Variation of negative conductance with frequency for DDR IMPATT operating at 220 GHz.*

operating frequencies of 94 GHz, 140 GHz and 220 GHZ are represented. From **Table 5** it is seen that the peak values of mean square noise voltage per band width (<v2 >/df)max are found at the frequencies (fp) of 75 GHz, 100 GHz and 160 GHz for Si for the operating frequency of 94 GHz, 140 GHz and 220 GHz,

#### **Figure 5.**

*Variation of negative conductance with operating frequency for DDR IMPATT diodes design to operate at 94, 140 and 220 GHz.*


#### **Table 5.**

*Noise properties of GaAs and Si DDR IMPATT diodes.*

for GaAs IMPATT the frequencies of peak values of <v2 >/dfmax are 60 GHz, 110 GHz and 155 GHz. The corresponding values of mean square voltage per band width (<v2 >/df)max at the operating frequency of 94,140 and 220 GHz for all the diodes are given in **Table 5**. GaAs DDR IMPATT shows minimum mean square noise voltage per band width. The variation of mean square noise voltages per bandwidth (MSNVPBW) of the IMPATT diodes based on the semiconductors under consideration as a function of frequency are plotted in **Figures 6**–**8** for operating frequency of 94 GHz, 140 GHz and 220 GHz respectively.

Noise measure (NM), which is an indicator of noise to power ratio, is an important aspects for the study of noise behavior. The values of computed noise measure at the designed frequency are presented in **Table 5**. We have plotted noise measure as a function of frequency in **Figures 9**–**11** for both the IMPATT diodes based on GaAs and Si at different operating frequency of 94 GHz, 140 GHz and 220 GHz respectively. The comparative study of noise measure of Si and GaAs is

*IMPATT Diodes Based on GaAs for Millimeter Wave Applications with Reference to Si DOI: http://dx.doi.org/10.5772/intechopen.95874*

**Figure 6.**

*Variation of mean square voltage per bandwidth with frequency for DDR IMPATT diodes operating at 94 GHz.*

**Figure 7.**

*Variation of mean square voltage per bandwidth with frequency for DDR IMPATT diodes operating at 140 GHz.*

given in **Table 5**. GaAs IMPATT produces less noise as compare to Si IMPATT. The main reason for the low noise behavior of GaAs for a given electric field is the same value of the electron and hole ionization rates. Whereas, in Si the ionization rates are quite different. It may be mentioned here, the high power generation mechanism is such that if we wish to get more output power then we are supposed to get more noise.

**Figure 8.** *Variation of mean square voltage per bandwidth with frequency for IMPATT DDR operating at 220 GHz.*

**Figure 9.** *Variation of noise measure with frequency for DDR IMPATT diodes operating at 94 GHz.*

*IMPATT Diodes Based on GaAs for Millimeter Wave Applications with Reference to Si DOI: http://dx.doi.org/10.5772/intechopen.95874*

**Figure 10.** *Variation of noise measure with frequency for DDR IMPATT diodes operating at 140 GHz.*

**Figure 11.** *Variation of noise measure with frequency for DDR IMPATT diodes operating at 220 GHz.*
