Chapter-8 Comparative Study of Transfer Electron Device and Avalanche Transit Time Device

Transcription

1 Chapter-8 Comparative Study of Transfer Electron Device and Avalanche Transit Time Device

2 Comparative Study of Transfer Electron Device and Avalanche Transit Time Device 8.1 Introduction It is seen from our study that Impact Avalanche Transit Time {IMPATT) and Gunn diodes which is a Transfer Electron Device are considered to be two powerful solid state sources operating in the sub-mm-wave and THz frequencies. They are widely used in various communication systems as signal generators in recent years. The feasibility of using GaN for Transfer Electron Device and Avalanche Transit Time Devices have been explored in this work in the previous chapters. Now, these investigations need a detailed comparison between both Transfer Electron Device and Avalanche Transit Time Devices i.e IMPATTs and Gunn devices respectively at the same operating conditions to operate at sub-mm and THz frequency. Keeping it in mind, the author proposes to take a detailed and comparative study on the DC and High-Frequency characteristics of Transfer Electron Device and Avalanche Transit Time Devices in this chapter. This work will definitely be helpful to evaluate the potential of each device based on GaN material before significant resources are dedicated to material growth, device fabrication and characterization. The method of formulation for wide band gap semiconductors based Transfer Electron Device and Avalanche Transit Time Devices to study the static and dynamic characteristics are as described in the previous chapter of the thesis. Here the results are summarized and presented with a detail comparison [A3], It may be mentioned here that SiC can not be used for Gum diode as it do to have transfer electron characteristics and hence can not have Negative Differential Mobility Region [120], This justifies our comparison of GoAf-based IMPATTs and Gunn diode. 188

3 8.2 Discussion and Comparison of Transfer Electron Device and Avalanche Transit Time Device To compare the results, the diode structures (both Gunn and IMPATT) are optimized first for the same operating conditions and are presented in table 8.1 and table 8.2 for Gunn and IMPATT diodes. The diodes were simulated using these structures and the DC and High frequency characteristics were determined using the same method as described in the previous chapters. Some of the obtained results are presented in tables 8.3 and 8.4 for Gunn and IMPATT diodes respectively. Some of the simulated results for Gunn and IMPATT at different temperatures are also presented in figures8.1 and 8.2 respectively before the analysis of comparison. Table 8.1: Optimized design parameters for GaN-based Gunn diode Structure Length of the Anode,La (pm) Length of the Cathode,Lc (pm) Length of the Active Region, Lactive (ptn) Background doping 3 concentration (/cm ) Active region doping (n) > "cc > c Ol 'v. cc s "O O SZ -i» a o O c c * Z <D >; O C C C O l g 'a. o Q Strl '3 10' Table 8.2: Optimised structural parameters for GaN-based IMPATT diode Structure Width (pm) Doping{10ncm3) n-side p-side n-side p-side Znb GaN n+npp+ DDR Wz GaN nnpp+ DDR

4 It is seen from the table 8.3 that the maximum electric field at the junction (Emax) is 3.21xl06V/cm and diode breakdown voltage is found to be 95.77V for Znb structure IMPATT devices which will be compared with the Znb structure Gunn diode. From the table it is also seen that efficiency (r ) is 11.4% for IMPATT where it is only 3.21% for Gunn diode (table 8.4). Using our simulation scheme, the power generated from such devices is also determined. A power of 2.78x107W/cm2 is generated from Znb Ga/V-based IMPATTs. For same kind of structure of Gunn diode, it is seen that the maximum efficiency is 3.21 and power generated is around 1.40xl07W/cm2. Table 8.3 Device properties for different DDR IMPATT Structure Wz n+npp+ Znb n+npp+ EPeak(MV/cm) Vb (Volt) p(%) Power Density (10/W/cm"!) Table 8.4 Device properties of Gunn Diode at different bias voltage Bias Voltage 70V 80V 90V 100V Tl(%) Power Density (107W/cm2) It is well known that the diode properties degrade with increase in temperature. Hence we have analyzed and computed the diode parameters at different temperatures for both Gunn and IMPATT and is presented in Figures8.1 and 8.2 respectively. It is seen from figure8.2 that the diode efficiency decreases from 12.5% to 10.7% when the temperature increases from 300K to 600K indicating power loss as heat. However, the breakdown voltage increases from 158V to 205V when the temperature increases 190

5 from 300K to 600K in IMPATT case (also shown earlier). Similar tendency is seen in case of Gunn diode also {figure 5.1). However, as mentioned in table 8.4, the power generated is found to be more than 2 to 10 times in IMPATTs compare to Gunn and efficiency of IMPATT is at least 5 to 20 times more than Gunn diode at the same operating conditions U.U Frequency,GHz Fig.8.2 IMPATT device characteristics at different temperatures. The bracketed values show the efficiency and breakdown voltage 191

6 To analyze the characteristics of Gunn and IMP ATT, the electric field curves at different positions (for IMPATTs from p-end and for Gunn from cathode end) are drawn and are shown in figure8.3. The electric field drawn for the Gunn diode is captured at lops simulation time (to have the operating frequency of 94GHz). Figure8.3 shows that the maximum electric field obtained from IMPATT is around 4.5kV/cm. However, the electric filed at this time is more in Gunn diode at the anode end. In Gunn diode, the electric field remain almost constant throughout the active region and thus the integrated bias voltage is less than that of IMPATT diode leading to high power generation from IMPATT than Gunn diode. Electric Fjetd,E(x) (kv/cm) Absolute Distance, x(m) Fig.8.3 Gunn electric field formations from cathode end to anode end and for IMPATT from p-end at different time interval The integration of electric field determines the breakdown voltage and determines the efficiency of IMPATT diode whereas the DC component in current density curves obtained determines the efficiency of Gunn diode. Thus efficiency is determined for both the cases and a comparative figure is drawn and shown in 192

7 figure8.4 at different temperatures keeping all other operating conditions same. As mentioned, in previous chapters, the efficiency in both the cases remains almost constant at different temperatures showing the stability of GaN-based diode. However, the same figure shows that efficiency is at least 5 to 10 times more in GaN-based IMP ATT diode compare to Gunn diode. This shows that IMPATT device gives better properties than Gunn diode. Fig.8.4 Efficiency comparison of Gunn and IMPATT at different temperatures. Before concluding the chapter, the power density is also determined for both the diodes at different frequency. The peak power density at optimum frequency of operation at different temperatures is drawn in figure8.5. It is seen from the figure that the power is found to be 1.48xl07W/cm2 for GaN-based IMPATTs clearly indicating the power obtained from GaN-based IMPATTs will be almost ten times more than that of GaN-based Gunn diode. It is seen from the figure that with increase in temperature, the frequency of operation remains the same in all these cases unlike that of GaAs-based devices. Though the power generated decreases with increase in 193

8 temperature, the dynamic properties remain the same for temperature up to 600K. This indicates that GarV-based Gunn diodes and IMPATT diodes can be operated at higher temperature. However, at high temperatures also the power obtained from GaN-based Gunn diode is less than that of GaiV-based IMPATTs indicating clearly that GaiV-based IMPATTs have better advantages than GaiV-based Gunn diode. Power Density, PfkW/cm*) OO IMPATT diode Same operating conditions at different temperatures Gunn diode / p Temperature, T(K) Fig.8.5 Power Density at different temperatures for Gunn and IMPATT diodes. 8.3 Conclusion Simulation studies based on the newly emerging wide bandgap material, GaN- based Gunn and IMPATT diodes are presented. All possible type of structural variations of the diodes is considered to explore possibility of improving the performance. A 2 to 10 times higher power output for GaiV-based IMPATTS compared to the GaN- based Gunn diode is noteworthy. It can be concluded that Ga/V-based IMAPTT diode is expected to generate high power at high frequency compare to Gunn diode. 194