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Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

Effects of Impurity Charge Bump on Dynamic Properties of InP Based Pulsed IMPATT Diode at Window Frequency Moumita Mukherjee Centre of Millimeter Wave Semiconductor Devices and Systems, University of Calcutta, 1, Girish Vidyaratna Lane, Kolkata 700009, India Email: mm_drdo@yahoo.com, mukherjee_mita@hotmail.com Abstract— Current work is presented on the computer based design, study and analysis of InP DDR (Double Ddrift Region) IMPATTs for operation around 35 GHz. Two InP DDR IMPATT diode structures having i) flat doping profile, ii) SLHL (single low-high-low bump on the n-side only) have been designed. The DC and small-signal properties of designed p+-p-n-n+ DDR InP IMPATT diodes have been investigated for operation at Ka-band. The simulation study indicates that a large amount of pulsed output power can be obtained from both of the designed diodes at Ka-band with appreciable DC to MM-wave power conversion efficiency. It is also found from the computer study that SLHL IMPATT diode is superior to flat profile counterpart as far as negative conductance, efficiency and peak output power is concerned.

IMPATTs, if properly designed, may generate higher output power as compared to Si and GaAs IMPATTs. Owing to its excellent material properties, InP thus emerged as an attractive semiconductor material for fabrication of high power and high efficiency avalanche diodes [1]. There has been very limited experimental research on the development of InP IMPATTs, mostly because of complexity involved in the growth of InP epilayers. In 1978, Berenz et al. first reported fabrication of X-band InP SDR diodes and obtained CW output power of 1.6 W with 11.1% power conversion efficiency at 9.78 GHz [2]. Recently Ho Ki Kwon et al. [3] reported the fabrication of a InP IMPATT diode using the low-pressure MOCVD technique. The test results yielded a maximum efficiency of 12.6 % and pulsed power output of 2.2 W at 34 GHz. So a complete theoretical study of InP IMPATT devices accounting for all InP features is of interest at this point to evaluate their potential in support of ongoing and future experimental attempts. The authors have, therefore designed InP DDR IMPATT , to study the DC and high frequency (Ka-band) characteristics. The authors have modified the doping profile in order to get more pulsed output power with much better power conversion efficiency. The objective has been achieved by introducing single impurity (charge) bump selectively on either side of the n and p regions of the depletion layer of the DDR diode. The schematic diagram of SLHL DDR diode is shown in Figure 1. The authors in this paper present the results of computer studies on the DC and small-signal properties of DDR (p+-p-n-n+ ) type InP IMPATT devices suitable for operation at Ka-band . Two DDR structures having :i) flat doping profile , ii) SLHL (single low-high- low bump on either side of n and p region) doping profile have been studied.

Index Terms— IMPATT diodes, InP, flat and SLHL DDR, high power, Ka-band, MM-Wave properties, window frequency

I. INTRODUCTION Impact-Ionization-Avalanche-Transit-Time (IMPATT) diodes have been used for microwave power generation at frequencies between 10 GHz and 300 GHz. They are the most powerful solid-state sources at millimeter wave frequencies and are widely used in mmwave civilian and space communication systems as well as in high power radars, missile-seekers etc. IMPATT diodes are usually made from silicon and GaAs. Under pulsed conditions, the peak output power of an IMPATT diode at a given frequency is limited by its underlying material-properties. Compared to conventional III-V GaAs, the material parameters of III-V compound semiconductor Indium Phosphide (InP) are highly suitable for developing high power IMPATT devices. It is found that although InP has a lower electron mobility compared to GaAs (0.54 vs. 0.85 m2 V-1 s-1), the electron exhibits a higher saturation velocity (vS: 2.2x105 vs. 1.0 x 105 m s-1). Moreover, compared to GaAs, InP has higher critical electric breakdown field (EC: 4x107 Vm-1 vs. 5x107 Vm-1) and lower tunneling rates. These are the reasons for selecting InP as a base semiconductor material for developing high power IMPATT diode, since RF power output from a IMPATT into a load having reactance Xc at frequency f is given by – PM = (EC2 vS2)/ (4πXCf2) , (1) where, EC is the critical field for avalanche breakdown and vS is the saturation drift velocity, PM is the maximum power output. It is evident from equation (1) that InP

II. COMPUTER METHODOLOGY The IMPATT diode is basically a p-n junction diode that operates when it is reverse-biased to avalanche breakdown condition. A one-dimensional model of the pn junction has been considered in the present analysis. The following assumptions have been made in the simulation of DC and small signal behavior of InP DDR

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Proc. of Int. Conf. on Control, Communication and Power Engineering 2010 TABLE 1.

method. From the DC field and current profiles, the spatially dependent ionization rates that appears in the

DESIGN DATA OF INP BASED DDR AT 300K DDR diode (flat doping profile)

Flat layer doping concentration (on either side of p and n region) (m-3) Depletion layer width (on either side of p and n-region) (m) Position of the bump [on either side of p and n region] (distance from metallurgical Junction) (m)

0.230x1023

DDR (SLHL)(single low-high-low bump on either side of n and p region) 0.210x1023

0.180x10-5

0.180x10-5

High (bump) layer Doping concentration (m-3) Current densities (Am-2)

---

0.110x1024

1.20x108

1.20x108

---

0.050x10

TABLE 2. OUTPUT RESULTS FOR INP DDR DIODE STRUCTURES AT CURRENT 8 2 DENSITY (J) = 1.2 X 10 A/M DDR diode (flat doping profile) Field Maximum (Em) (Vm-1) Total Breakdown Voltage (VB) (V) Voltage across Avalanche Layer(Va) (V) Avalanche frequency (GHz) Peak frequency (GHz) Conductance(G) at 35 GHz (Sm-2) Susceptance(B) at 35 GHz (Sm-2) Conductance (G) at peak frequency(Sm-2) Susceptance (B) at peak frequency(Sm-2) Quality Factor

-5

IMPATT diodes: (i) the electron and hole velocities have been taken to be saturated and independent of the electric field throughout the space-charge layer, (ii) the effect of carrier diffusion has been considered. The effects of mobile space charge have been incorporated in the simulation scheme. The simulation scheme also incorporates the material parameters from recent reports. The experimental values of material parameters, viz., realistic field dependence of ionization rates [4], saturated drift velocities [5], and mobility of charge carriers [5] in InP have been incorporated in the analysis.

114.49

112.83

54.7

48.7

33.8

34.8

35.7 -3.47 x 106

35.7 -4.40 x 106

1.70 x 106

0.24 x 106

-3.61 x 106

-4.9 x 106

2.59 x 106

1.97 x 106

-0.72

- 0.4

appear s in the Gummel-Blue equations are evaluated, and fed as input data for the small signal analysis. The edges of the depletion layer of the diode, which are fixed by the DC analysis, are taken as the starting and end points for the small signal analysis. On splitting the diode impedance Z (x,ω) obtained from Gummel–Blue method, into its real part R (x,ω) and imaginary part X (x,ω), two differential equations are framed [6]. A double-iterative simulation scheme incorporating modified Runge-Kutta method is used to solve these two equations simultaneously. The small signal integrated parameters like negative conductance (-G), susceptance (B), impedance (Z) and frequency band width of the diodes are obtained satisfying the boundary conditions derived elsewhere [6]. The avalanche frequency (fa) is the frequency at which the susceptance (B) of the devices changes its nature from inductive to capacitive. Again, it is the minimum frequency at which the conductance (G) of the device becomes negative. At the avalanche frequency, oscillation starts to build up in the circuit. Again, at a given bias current density, the peak frequency (fp) is the frequency at which the negative conductance of the diode is a maximum. The peak output power (pulse mode) is calculated from the following formula: P(pulse mode) = [(VB ) x (Current density ) x (Area of the diode)x (η(in %))]/ (duty cycle (in %)) (3).

A. DC Analysis The DC method, described in details elsewhere [6], considers a generalized (n+ n p p+) structure. Here, n+ and p+ are highly doped substrates and n and p are epilayers. Summarily, in the DC method, the computation starts from the field maximum near the metallurgical junction. The distribution of DC electric field and carrier currents in the depletion layer are obtained by a doubleiterative simulation method, which involves iteration over the magnitude of field maximum (Em), and its location in the depletion layer. The method is used for a simultaneous solution of Poisson and carrier continuity equations at each point in the depletion layer. The DC to RF conversion efficiency (η) [7] is calculated from the approximate formula, η ( %) = (VD x 100 ) /(π x VB) (2) where, VD = voltage drop across the drift region . Also, VD = VB-VA, where, VA = voltage drop across the avalanche region, and, VB = breakdown voltage. The results of the DC analysis are used in the small signal analysis, described briefly in the next sub-section. B. Small Signal Analysis The small signal analysis of the IMPATT diode provides insight into the high frequency performance of the diode. The range of frequencies exhibiting negative conductance of the diode can easily be computed by Gummel-Blue

III. RESULTS AND DISCUSSIONS The design data of DDR diodes (both flat and SLHL types) are reported in Table 1. DC and high frequency output results of the two DDR structures are summarized in Table 2. Figure 1 shows the E(x) profiles of the optimized flat and SLHL type InP DDR IMPATTs. 318

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0.575x108

DDR (SLHL)(single low-high-low bump on either side of n and p region) 0.571 x 108


Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

The authors have found with the help of equation (3) that if the designed DDR devices are pulse driven, with 0.5% duty cycle, then appreciable amount of high pulsed power (914 Watt for flat diode structure and 975 Watt for SLHL structure) at Ka band can be generated. The RF power output from these designed diodes are considerably larger than the latest designed GaAs and Si DDR IMPATTs as the fabricated Si IMPATT diodes (p + p n n +) are providing microwave power of 10W-20W in the 33-37 GHz [8] frequency range and Ka-band oscillation testing on the fabricated GaAs DDR IMPATT diodes yielded average performance of 3.5W with 16% efficiency for pulse lengths > 1 μs and 10.5 W with 13 % efficiency for pulse length < 1 μs [9]. IV. CONCLUSION Figure 1. Electric field profile of (a) flat profile and (b) SLHL

A full-numerical small-signal simulation study has been performed on InP DDR IMPATT diodes ( for both flat and Lo-Hi-Lo doping profile). For Ka-band InP IMPATTs, our study predicted peak pulse power capability of at least 914 W for DDR structures. These power levels are much higher than the best reported experimental results for Si and GaAs DDR diodes at the same frequencies.

IMPATT diodes To generate more pulsed output power, the device is driven at a higher bias current density. The effect of large space charge at such a high current density can be reduced if charge spikes are introduced in the depletion layer. This fact is reflected in the E(x) profiles of the diodes, as shown in Figure 1. Due to the introduction of charge spike, voltage across avalanche layer of SLHL diode reduces to 48.7 V (Table 2), whereas the same for its flat profile counterpart is much higher (54.7V). This in turn, increases the efficiency of the SLHL diode to 18.0%, where as maximum efficiency of flat type DDR IMPATT is 16.63%. Figure 2 represents the conductance (G) against susceptance (B) plot (admittance plots) of flat profile and SLHL DDR diode structures. From Table 2, it is clear that due to the introduction of charge spike in flat profile DDR diode, the conductance values of the resultant SLHL DDR IMPATT is increased by 35.7 % than its flat counterpart. Quality factor of SLHL IMPATT is also found to be much better than its flat profile counterpart, as shown in Table 2. The simulation study predicts that both the diodes may oscillate at 35.7 GHz, which is very near to the atmospheric window frequency of 35 GHz.

ACKNOWLEDGMENT The author is grateful to University Grants Commission (UGC) and Defence Research and Development organization (DRDO), India, for providing her research fellowship to carry out this work. REFERENCES [1] J. P. Banerjee and R. Mukherjee, Electronics Letters, Vol. 30, pp. 1716-1717, 1994. [2] J J Berenz, F B Fank and T L Hiert, “Ion Implanted p-n junction Indium Phosphide IMPATT diodes”, Electron Lett (GB) , vol. 14, p. 683, 1978. [3] H. K. Kwon, J. Park and R. D. Dupuis, Paper J9,41st “ Growth of high performance InP IMPATT diodes by Metallorganic Chemical Vapour Depodition”, Electronic Materials Conference, Santa Barbara CA (30 June - 2 July 1999) 1999. [4] L. W. Cook, G. E. Bulman, and G. E. Stillman, “ Electron and hole impact ionization coefficients in InP determined by photo-multiplication measurements”, Appl. Phys. Lett., vol. 40, 7 pp. 589-591, 1982. [5] S. Kang and Ch. W. Myles, Phys. Stat. Sol.(a), vol. 181, p219, 2000. [6] S.K. Roy, M. Sridharan, R. Ghosh and B. B. Pal, “ Computer methods for DC field and current profiles in IMPATT diodes starting from the field extremum in the depletion layers”, Proc. Conf. on Num. Anal. of Semiconductor Devices (NASECODE I),(Dublin) (Dublin: Boole)(Ed. J.H. Miller), p-266, 1979. [7] D. L. Scharfetter and H. K. Gummel, “Large signal Analysis of a Silicon Read Diode Oscillator”, IEEE Trans. Electron Devices, vol.16, p.64, 1969. [8] D.K.Gaskill, O.J. Glembocki, R.T.Holm, and A.Giordana, Jr.of Electronic Materials, vol. 28, No. 12, pp-1424, 1999. [9] N.S.Boltovets, V.V.Basements, V.N.Ivanov, V.A.Krivutsa, Semiconductor physics ,Quantum electronics and Optoelectronics ,vol. 3, pp-359-370 , 2000.

Figure 2. Admittance plots of (a) flat and (2) SLHL diodes

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