|Year : 2011 | Volume
| Issue : 4 | Page : 351-356
A New mm-Wave GaAs~Ga0.52 In0.48 P Heterojunction IMPATT Diode
SR Pattanaik1, JK Mishra2, GN Dash3
1 Department of Electronics and Telecommunication Engineering, KIST, Bhubaneswar, Orissa, India
2 Department of Physics, Govt. Women's College, Sambalpur, Orissa, India
3 Department of Physics, Sambalpur University, Jyoti Vihar, Sambalpur, Orissa, India
|Date of Web Publication||20-Oct-2011|
S R Pattanaik
Department of Electronics and Telecommunication Engineering, KIST, Bhubaneswar, Orissa
| Abstract|| |
The potentials of a new lattice-matched material combination, GaAs~Ga 0.52 In 0.48 P, is explored using a computer simulation method for application as an IMPact ionization Avalanche Transit Time diode at mm-wave frequencies. It is observed that by suitably adjusting the ternary layer width in the drift region of the diode not only is the power output enhanced from 0.38W to 0.50W but also the noise measure reduces from 26 dB to 21 dB.
Keywords: GaInP, Heterojunction, IMPact ionization Avalanche transit time, Mm-wave
|How to cite this article:|
Pattanaik S R, Mishra J K, Dash G N. A New mm-Wave GaAs~Ga0.52 In0.48 P Heterojunction IMPATT Diode. IETE J Res 2011;57:351-6
| 1. Introduction|| |
In the recent years, communication systems use IMPact ionization Avalanche Transit Time (IMPATT) diodes as one of the most powerful solid-state sources for microwave power at frequencies ranging from 10 GHz to 300 GHz. Research activities are focused on the search for the best-suited base semiconductor material(s) that would give high efficiency, high microwave power and low noise. With the advancement of technology, heterojunction IMPATT diodes have been conceptualized by researchers for localizing the avalanche zone using lattice-matched semiconductors, one with a higher ionization rate for the high-field avalanche region and the other with a lower ionization rate for the low-field drift region. During the last decade, several theoretical and experimental reports on the potentiality of heterojunction IMPATT diodes, both double drift region (DDR) ,,, and double avalanche region (DAR) , , have found place in the published literature. The results of the performance of a special kind of DDR heterojunction, the IMPATT diode based on Si~SiGe (lattice mismatched) investigated by our group,  has been found to be remarkably encouraging. As a step forward in the search of a new heterojunction, we have explored the potentiality of a new combination of materials, GaAs and Ga0.52 In0.48 P because no report (to the authors' knowledge) on the performance of this heterostructure for IMPATT application is seen in the published literature.
A systematic study of the microwave and noise behavior of GaAs~Ga0.52 In0.48 P heterojunction IMPATT diode is reported in this paper. For this study, we have considered GaAs having higher ionization rates for the avalanche region, while the ternary material Ga0.52 In0.48 P with lower ionization rates for the drift region. The results of our simulation indicate that with a proper choice of the ternary layer width for the drift region not only the microwave properties but also the noise behavior of the diode can be improved.
| 2. Design Considerations|| |
The operational frequency for the lattice-matched GaAs~Ga0.52 In0.48 P heterojunction IMPATT diode has been chosen around the atmospheric window frequency of 140 GHz. The total width of the diode has been designed accordingly. A one-dimensional schematic diagram of the proposed device structure is shown in [Figure 1]. It is a symmetrical DDR structure with doping distribution of the form n + npp + . In the present study, we have considered six different structures (S0, S1, S2, S3, S4 and S5) by varying the ternary layer width symmetrically in the drift regions. The structure S0 is a GaAs homojunction IMPATT diode. S1, S2, S3, S4 and S5 are GaAs~Ga0.52 In0.48 P heterojunction IMPATT diode structures with a total ternary layer width of 100 nm, 200 nm, 300 nm, 400 nm and 500 nm, respectively. The total depletion region width for all the diode structures is considered to be 800 nm, with the n- and p-regions consisting of 400 nm each. The n + and p + regions, which are to be used for ohmic electrical contacts to external circuit, are the highly doped regions that cause the electric field to drop to zero suddenly and thus they contribute negligible value to the active width of the diode. For all the heterojunction diode structures, the doping concentrations for each n - and p-region are taken to be 1.1×10 23 m -3 , while the doping concentration for each n + - and p + -region is taken as 1.0×10 26 m -3 . The diode area and current density for the present study are considered to be 10 -10 m 2 and 7.5×10 8 Am -2 , respectively. IMPATT diode performance is basically limited by the diode junction operating temperature. However, our simulation will be limited to the availability of material parameter data for Ga0.52 In0.48 P, which are available for 300 K. Therefore, we have simulated the diode properties at 300 K. The vital material parameters like ionization rate, drift velocity, mobility etc. for both GaAs and Ga0.52 In0.48 P used for the present simulation study have been taken from the experimental reports  and are presented in Appendix-I.
|Figure 1: 1-D schematic diagram of the proposed heterojunction DDR IMPATT diode (active region=0 to W, Avalanche region=xA1 to xA2, junction=x0, drift regions=0 to xA1 and xA2 to W).|
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| 3. Simulation Method|| |
The simulation method used for the present analysis of the GaAs~Ga0.52 In0.48 P heterojunction IMPATT diode has three stages, namely DC analysis, small signal analysis and noise analysis. In heterojunction IMPATT, tunneling plays a vital role. Therefore we have incorporated this phenomenon into our simulation programs  . The Poisson's equation, carrier continuity equations as well as the space charge equation are solved simultaneously at each space point of the active layer of the diode subject to the necessary boundary conditions to obtain the electric field profile and carrier current profiles. The results thus obtained from DC analysis are used as input for the small signal as well as noise analysis.
In order to analyze the microwave power performance of the proposed heterojunction IMPATT diode, a double-iterative computer simulation program developed by our group  is used to solve two second-order differential equations on the diode resistance and diode reactance with necessary boundary conditions. From this, the device parameters such as negative conductance (-G), susceptance (B) and negative resistance (-ZR) of the diode are determined taking into account the contribution from each space point using the simulation program. A realistic power output from the device can be estimated from a full-cycle large signal simulation of the diode. We have however kept such simulation outside the scope of this paper. Nonetheless, a rough estimate of the power output can easily show direction for future improvement. Thus, based on our small signal simulation, we have adopted the following method to compute an output power from the device. This however should only be taken as an indicator of power output from the device. When the susceptance of the device is resonated, the maximum RF power output (PRF) from the device can be obtained following the relation 
where VRF is the amplitude of the RF swing and is considered to be VB /2 for 50% modulation of the DC bias voltage, VB is assumed to be the breakdown voltage, − Go is the device-negative conductance at the operating frequency and A is the area of the diode.
The noise behaviors of the heterojunction IMPATT diode are analyzed following a generalized noise simulation scheme developed by our group  . The simulation program is used to solve two second-order differential equations for the real and imaginary part of the noise electric field e(x, x /) due to a noise source γ(x/) located at space point x / . The noise source is first considered to be at the beginning of the depletion region. From this, the noise source is then shifted to the next space step and the procedure is repeated until the noise source spans the entire depletion region. The terminal voltage produced by a noise source γ(x/) located at x / is given by
Now, the transfer impedance can be computed from the above relation as
where dIc (x /) is the current generated due to the noise source γ(x/) located at x / in the space interval dx / of the depletion layer. The mean square noise voltage per bandwidth and the noise measure (NM) are calculated using the relations
The validity of our simulation scheme has been discussed elsewhere  and has also been supported by experimental trends.
| 4. Results and Discussions|| |
The computer simulation scheme employed for the present study computes efficiently the DC characteristics, expected microwave power and noise behavior. The results obtained from the simulation of all the IMPATT diode structures with different ternary layer width are summarized and presented in [Table 1]. It is observed from [Table 1] that as we go on increasing the ternary layer width in the drift region, the breakdown voltage VB as well as the efficiency η of the proposed heterojunction diode structure increases while the maximum electric field (Emax) remains nearly same for all the structures. It is interesting to obtain the highest efficiency of 18.4% for the diode structure S5 with 500 nm of ternary layer width in the drift region and 300 nm of binary layer width in the avalanche region. This value of efficiency is very high as compared with the efficiency of 12.6% for a GaAs homojunction IMPATT diode S0 designed to operate at the same frequency of 140 GHz. The reason for this observation may be explained in the following way. GaInP has a wide bandgap as compared with GaAs. Thus, the drift region voltage increases as also the breakdown voltage. This is also understood from [Figure 2], where we have plotted the electric field profile of the heterojunction IMPATT diode with different ternary layer width. It can be observed from [Figure 2] that at the heterojunction a jump in the value of electric field occurs due to the differences in dielectric constants for GaAs and Ga0.52 In0.48 P. The region consisting of Ga0.52 In0.48 P is seen to have high values of electric field as compared with the expected electric field for the GaAs material for the same region. Further, the voltage drop across the diode is shared by the drift region and the avalanche region. As estimated by Scharfetter and Gummel  , the high voltage drop across the avalanche region reduces the efficiency whereas the increase in net breakdown voltage is supposed to increase the same. As we increase the ternary layer width, the breakdown voltage increases, with consequent decrease in the relative value of the avalanche voltage drop. Thus, the efficiency of the diode increases with an increase in the ternary layer width. However, we cannot go on increasing the ternary layer width indefinitely to get higher efficiency. This is because for higher value of the ternary layer width, it is observed that the ternary layer enters into the avalanche region. This will increase the avalanche voltage drop and the diode efficiency will degrade. This is thus indicative of the existence of a critical value of ternary layer width for which the diode is supposed to exhibit the highest efficiency. In this case, this critical ternary layer width is observed to be 500 nm.
|Figure 2: Electric field profiles of the GaAs~Ga0.52In0.48P heterojunction IMPATT diodes with different ternary layer widths.|
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|Table 1: Microwave and noise properties of the GaAs homojunction and GaAs/GaInP heterojunction IMPATT diode structures at the design frequency of 140 GHz|
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The microwave properties like device-negative resistance and negative conductance are computed from the small signal high-frequency analysis. The values of device-negative conductance (Go), at the frequency of interest, 140 GHz, for all the IMPATT diode structures are presented in [Table 1]. It can be seen from the table that the structure S4, with ternary layer width of 400 nm, exhibits the highest value of device-negative conductance recording a value of 7.71×10 7 Sm -2 . The breakdown voltage for this structure is also found to be high as compared with the GaAs homojunction IMPATT diode structure. We have also computed the expected power output (PRF) from each of the diode structures. The results have been presented in [Table 1]. It can be observed from [Table 1] that the power output from the device has also shown improvement with the incorporation of heterojunction in the classical IMPATT structure. The expected power output from the proposed device structure increases with an increase in the ternary layer width in the drift region, records optimum value of 0.50 W for the ternary layer width of 400 nm and then decreases with further increase in the ternary layer width. The low value of device-negative conductance for the structure S5 is the reason for low power output, although the structure exhibits high efficiency. It thus indicates that with a localized avalanche region, high-breakdown voltage and high device-negative conductance, the diode is capable of exhibiting optimum microwave power performance at 140 GHz for a ternary layer width of 400 nm in the drift region. In order to have a comparative account of device performance, the variations of device-negative conductance with frequency for the GaAs homojunction IMPATT diode structure and GaAs~Ga0.52 In0.48 P heterojunction IMPATT diode structure S4 are plotted in [Figure 3]. It is well seen from the figure that the heterojunction diode has high values of negative conductance over the homojunction diode around the frequency of 140 GHz. Thus, the heterojunction IMPATT diode structure S4 would be a good candidate for microwave power generation at 140 GHz. This fact is also supported with the negative resistivity plots of the diodes. The variation of negative resistivity with distance at 140 GHz is plotted in [Figure 4] for the GaAs homojunction IMPATT diode and GaAs~Ga0.52 In0.48 P heterojunction IMPATT diode with ternary layer width of 400 nm. The two negative-resistivity peaks for the heterojunction diode are observed to be higher than those for the homojunction diode. This in turn would boost the power performance of the heterojunction diode.
|Figure 3: Plots of negative conductance versus frequency for the GaAs homojunction and GaAs~Ga0.52In0.48P heterojunction IMPATT diode with ternary layer width of 400 nm.|
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|Figure 4: Negative-resistivity profiles for the GaAs homojunction and GaAs~Ga0.52In0.48P heterojunction IMPATT diode with ternary layer width of 400 nm.|
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The material parameter data of semiconductors, although taken from experimental reports, suffer from inaccuracy owing to various experimental conditions. These will thus have a substantive impact on the simulated results. We have therefore taken up a sensitivity study of the device properties on material parameters. The ionization rates are the most important parameters for the IMPATT device study. Therefore we varied the ionization rates by 5% and computed the device properties of the optimized structure S4. We observed that ionization rate variation for Ga0.52 In0.48 P has almost no effect on the device to 7% variation properties. However, 5% variation in ionization rates of GaAs gives rise in device-negative conductance. We have got similar results with variation of drift velocity also. Thus, the device properties are highly sensitive to material parameters present in the avalanche region. The material parameters of GaAs are more accurately known than those of Ga0.52 In0.48 P. But, we observed that the material parameter variation of Ga0.52 In0.48 P has little influence on the device properties of the diode. Thus, our results to a great extent are immune from inaccuracy arising out of uncertainty in material parameter data of Ga0.52 In0.48 P.
We have emphasized here on the noise behavior of the proposed heterojunction diode structure as compared with the homojunction diode structure. The noise properties like mean square noise voltage per bandwidth and NM are computed for all the diode structures under consideration. [Figure 5] shows the variations of mean square noise voltage per bandwidth with operating frequency for the heterojunction IMPATT diode with different values of ternary layer width. It can be seen from the figure that the GaAs homojunction IMPATT diode structure exhibits the highest peak of mean square noise voltage per bandwidth. As we go on increasing the ternary layer width, it is also seen from the figure that the peak noise value reduces and the peak vanishes for the ternary layer width of 400 nm. The noise curve gets flattened. It is further interesting to note that a similar trend is also reflected when we look for the value of mean square noise voltage per bandwidth at the frequency of interest (140 GHz). These values are presented in [Table 1]. Although we found that the noise values go down with further increase in ternary layer width, we have not considered those structures because they are found to be low-power and low-efficiency structures. It is worth to mention here that IMPATT diodes are generally meant for high-power applications. The reason for low noise with increase in ternary layer width can be well explained. It is very well understood that the values of electron and hole ionization rates of the ternary material are too small to carry out ionizing events in the drift region. With increase in ternary layer width in the drift region, the number of ionizing events becomes almost nil and thus the noise in the device reduces remarkably. Another contributing factor to the low noise of the device is the reduction in avalanche region width.
|Figure 5: Variation of mean square noise voltage per bandwidth with frequency for the GaAs homojunction and GaAs~Ga0.52In0.48P heterojunction IMPATT diodes.|
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One of the important parameters to assess the noise to power performance of the diode is the NM. The NMs for all the structures are determined following the relation as described in the previous section. It is interesting to note from [Table 1] that the NM decreases with increase in ternary layer width. The lowest NM of 21 dB at 140 GHz is recorded for the heterojunction diode with ternary layer width of 400 nm. This value of NM is also less as compared with the NM of 26 dB for the GaAs homojunction IMPATT diode.
| 5. Conclusion|| |
A new heterostructure DDR IMPATT diode based on the lattice-matched GaAs~Ga0.52 In0.48 P combination with Ga0.52 In0.48 P for the drift region is suggested and designed. A detailed study of the DC, microwave and the noise behavior of the diode for different values of the Ga0.52 In0.48 P layer widths are computed and compared with those of a pure homostructure GaAs DDR diode for operation at 140 GHz. Our results indicate that with a suitable choice of Ga0.52 In0.48 P layer width, the device efficiency, breakdown voltage and the microwave power become remarkably high compared with those of its GaAs homostructure counterpart. Further, the heterostructure IMPATTs in general are found to be less noisy as compared with the pure homostructure diode. The lowest NM of 21 dB has been observed for GaAs~Ga0.52 In0.48 P heterostructure IMPATT with an optimum ternary layer width of 400 nm as compared with 26 dB for the pure GaAs homostructure diode. This indicates that the incorporation of heterojunction in the classical structure of GaAs IMPATT diode proves to be beneficial with significant improvement in the small signal as well as noise behavior of the diode for its applications at the atmospheric window frequency of 140 GHz.
The ionization rates are determined using the formulae of the type
for holes, where E is the electric field and other parameters for GaAs and Ga0.52 In0.48 P are listed in [Table 2]. The electron drift velocity is determined using the formula
whereas the hole drift velocity uses the relation
where, μn.p are the motilities and vsn,sp are the saturated drift velocities for electrons and holes, respectively. These and other parameters along with the permittivity ε are presented in [Table 2].
| References|| |
|1.||R U Khan, P Chakrabarthy, and B B Pal, "MITATT mode in DDR heterostructure IMPATT", Appl. Phys. A, vol. 42, pp. 303, 1987. |
|2.||G N Dash, and S P Pati, "Studies on the prospects of GaInAs and GaInAsP for double drift region heterostructure IMPATTs", Appl. Phy. A, vol. 58, pp. 211, 1994. |
|3.||J K Mishra, A K Panda, and G N Dash, "An extremely low noise heterojunction IMPATT", IEEE Trans. Electron. Devices, vol. ED-44, pp. 2143, 1997. |
|4.||J F Luy, H Jorke, H Kibbel, A Kasel, and E Kasper, "Si/SiGe hetero-structure MITATT diode", Electron. Lett, vol. 24, pp. 1386, 1988. |
|5.||J K Mishra, G N Dash, S R Pattnaik, and I P Mishra, "Computer simulation study on the noise and millimeter wave properties of InP/GaInAs heterojunction double avalanche region IMPATT diode", Solid states electronics, vol. 48, pp. 401, 2004. |
|6.||S R Pattnaik, I P Mishra, G N Dash, and J K Mishra, "Study of Si/SiGe heterostructure DDR IMPATTs for operation at 94GHz", IETE Journal of Research, vol. 50, pp. 163, 2004. |
|7.||J K Mishra, A K Panda, and G N Dash, "Design optimization of a single sided Si/SiGe heterostructure mixed tunneling avalanche transit time double drift region", Semicond. Sci. Technology, vol. 12, pp. 1635, 1997. |
|8.||M Levinshtein, S Rumyantsev, and M Shuv, "Hand book series on semi-conductor parameters", Singapore: World Scientific; 1999. |
|9.||G N Dash, and S P Pati, "A generalized simulation method for MITATT mode operation and studies on the influence of tunnel current on IMPATT properties", Semicond. Sci. Tecnology, vol. 7, pp. 222, 1992. |
|10.||S R Pattnaik, G N Dash, and J K Mishra, "Prospects of 6H-SiC for operation as IMPATT diode at 140GHz", Semicond. Sci. Technology, vol. 20, pp. 299, 2005. |
|11.||G N Dash, J K Mishra, and A K Panda, "Noise in mixed tunneling avalanche transit time (MITATT) diodes", Solid State Electronics, vol. 39, pp. 1473, 1996. |
|12.||D L Scharfetter, and H K Gummel, "Large signal analysis of a Silicon Read diode oscillator", IEEE Trans. Electron. Devices, vol. ED16, pp. 64, 1969. |
| Authors|| |
S. R. Pattanaik was born in 1977 at Sambalpur. He received his M.Sc and Ph.D degrees from Sambalpur University in 1999 and 2010 respectively. He has been doing extensive research work on various aspects of IMPATT diode. He is now working as Associate Professor at Apex Institute of Technology and Management, Bhubaneswar. His research interest includes microwave power devices and antenna. He has published 34 research papers in various journals of international repute and proceedings of national and international seminar/workshop.
J. K. Mishra was born in the year 1963 and has completed his Post graduation in the year 1985 from sambalpur University, Odisha. He obtained his M. Phill and Ph.D degree from Sambalpur University in the year 1988 and 1998 respectively. He joined as a Lecturer in the year 1988 in a Govt. College of Odisha. He is now working as a Reader in Govt. Women's College, Sambalpur. He has published 52 research papers in various journals of international repute and proceedings of national and international seminar/workshop. His area of interest is the study of the Semiconductor devices like IMPATT, MITATT and APD.
G. N. Dash was born in 1955. He received M.Sc., M.Phil., and Ph.D. degrees in 1977, 1983 and 1992 respectively. He joined as a Lecturer in Physics under the Govt. of Odisha in 1978. Subsequently he joined the Physics faculty of Sambalpur University in 1984 as a Lecturer. He was promoted to Reader in 1993 and currently he is a Professor since 2001. He has published more than 150 papers in journals of repute and proceedings of international and national conferences. Six scholars have completed Ph.D. under his supervision and six more are working. His research interest includes mm-wave and terahertz semiconductor devices and signal processing for bioinformatics. Prof Dash is a senior member of IEEE (USA), Member of IET (UK) and Life Fellow of IETE (New Delhi).
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2]