International Journal of Soft Computing and Engineering (IJSCE) ISSN: 2231-2307, Volume-4, Issue-1, March 2014 Dynamic Characterization and Noise Analysis of 4H-SiC Impatt Diode at Ka Band Joydeep Sengupta, Girish Chandra Ghivela, Monojit Mitra Abstract--The microwave as well as the small signal noise properties on a one dimensional n+npp+ DDR structure 4H-SiC IMPATT Diode have been studied using advanced computer simulation program developed by us and compared at different 82 frequency of Ka band by taking the area of the diode as 10 m . Also the theory for the diode current noise associated with the electron hole pair generation and recombination in the space charge region of the diode is presented. This paper can help to know about the small signal behavior as well as noise behavior of IMPATT diode along with power density at the Ka band and will be helpful for designing the 4H-SiC based IMPATT diode depending upon the microwave applications. Index Terms--Impact ionization, efficiency, mean square noise voltage, quality factor, noise spectral density, power density, noise measure. I. INTRODUCTION Figure1:The Active Layers of a Reverse Biased p-n Junction In recent years, IMPATT(Impact Avalanche and Transit Time) diodes have proved as a major source of microwave 1 DC ANALYSIS and millimetre wave power generator. Thus, designers are concentrating on the best semiconductor materials for the To obtain the DC parameters such as break down voltage, optimized design of the diode. For our design, we have carrier current profiles, electric field profiles etc., dc analysis choose 4H-SiC material for the IMPATT because of high is done here by solving Poisson equation, the space charge electron mobility and have recently been established as equation and the carrier continuity equation simultaneously technologically important materials for both electronic and over the double drift structure of the diode[5]. The values of optoelectronic devices[1]-[2]. Also the advantages of SiC- the material parameters of 4H-SiC are taken as a standard based IMPATT devices over traditional Si,GaAs,InP based one, which are enlisted in table1 and the design parameters IMPATT at Ka-band were presented in [3]. However, the are shown in table 2 . The Computer simulation method for noise generated by the statistical nature of impact ionization DC analysis is initiated from the position X 0 of the field in the active space charge region limits the potential of maximum E in the depletion layer which is situated near IMPATT diode as a microwave generator for various m applications[4]. So, the understating of the noise generation the metallurgical junction. In the location of the field mechanism and its behaviour at small signal conditions is maximum i.e. at xX 0 ,where dE dx 0 ,the value of very important in establishing optimized design and operating conditions for IMPATT diodes. the maximum electric field and its location X 0 in the depletion layer are initially chosen suitably for the taken II.SIMULATION METHOD doping profile and dc current density( J 0 )(table1(B)). Then The three stage computer simulation method using by using this the initial value of the mobile space MATLAB consists of DC analysis, small signal analysis and charge( P(X ) ) is determined. After that the Poisson noise analysis are carried out on a one-dimensional n+npp+ 0 DDR model of 4H-SiC based IMPATT diode throughout the equation and the carrier continuity equation are solved Ka band. The device structure is shown in figure1.The simultaneously through the numerical approach[6] by taking equations involved in the analysis are nonlinear in nature, space steps of very small width(0.055nm). Iteration and thus their solutions are more complex. So the IMPATT over and are carried out till boundary conditions are diode is considered as consisting of several small space being satisfied at both edges (at and W) of the points for the analysis. The active layer width of the diode is divided into number of steps each of size 0.055 nm. depletion layer. The numerical approach is started from the point (field maxima) and moving towards the right Manuscript Received March, 2014. side of the junction(p-region) till the field boundary Joydeep Sengupta, Department of Electronics Engineering, conditions, in carrier current and electric field are satisfied at Visvesvaraya National Institute of Technology, Nagpur-440010, India. Girish Chandra Ghivela Department of Electronics Engineering, xW . Now the Visvesvaraya National Institute of Technology, Nagpur-440010, India. computation again started at DR. Monojit Mitra, Department of Electronics and Telecommunication field maximum point and it Engineering, Bengal Engineering and Science University , Shibpur, proceed towards the left side Howrah-711103, India. Published By: Retrieval Number: A2119034114/2014©BEIESP Blue Eyes Intelligence Engineering 145 & Sciences Publication Dynamic Characterization and Noise Analysis of 4H-SiC Impatt Diode at Ka Band of the junction (n-region) till boundary condition is satisfied. 2 Small Signal Analysis Thus, the DC electric field and carrier current distribution The high frequency performance of the IMPATT diode can profiles for the 4H-SiC IMPATT diode operating at the be obtained by doing the small signal analysis of the diode. taken current density ( 4 1082Am ) are obtained from the The DC parameters such as electric field profile and current final solutions of the and . The method described profile, which are obtained from the DC analysis, are fed as input data for the small signal analysis. In small signal above gives the avalanche breakdown characteristics of the condition the ac field developed across the diode is assumed IMPATT diode. The avalanche layer width X A is obtained to be much smaller than the breakdown field. As a result the ionization rates with electric field can be assumed to be by taking the condition P(x) =0.95, i.e. 95% growth of the linear. Therefore, small signal solutions can be obtained by electron and hole carrier current. The operating frequency of linear zing the complex diode equations. The drift velocities IMPATT diode essentially depends on the transit time of the of the charge carriers are assumed to be saturated and the charge. Computer simulation using MATLAB is carried out diffusion current is not considered in the small signal at operating frequencies of 26.5 GHz to 40 GHz and the analysis, but considered in the noise analysis. The small width of the epilayers are accordingly chosen using the signal parameters like negative conductance( ), transit time formula [Sze and G susceptance ( B ), the diode impedance Zx(,) , quality Ryder,(1971)[7]]Wn,, p0.35 V ns ps f ; where Wnp, is factor ( ) and range of frequency over which the diode the penetration of depletion layer towards n and p- QP sides,V ; are the saturation velocity for electron, hole exhibit the negative conductance can easily be computed ns, ps numerically through the Gummel-Blue approach, where the respectively and f is the operating frequency. The model takes into account, the contribution from each space depletion layer width of the diode is obtained as points and effectively determines the parameters of the diode after satisfying the boundary conditions. The real part WWWand drift region width is obtained as nP Rx(,) and imaginary part Xx(,) are obtained by WX A . The voltage drop across the diode zones, i.e. splitting the diode impedance using Gummel-Blue breakdown voltage of the diode VB and avalanche voltage method and thus two different equations are framed [8]-[11]. Then by using modified Runga-Kutta method the solutions drop V are determined by integrating the electric field over A of these two equations are found through a double iterative the corresponding zone layer. Then, the voltage drop across simulation scheme. the drift zone is calculated by as VVV. The DC to DBA The diode negative resistance ( ZR ) and reactance ( Z X ) RF conversion efficiency of the diode can be obtained from are computed through numerical integration of the Rx() the expression, 2mV and Xx() profiles over the active space charge layer. (%) D (1) Thus VB where m is the modulation index and the breakdown W W X voltage is calculated by integrating the spatial field profile Z Rdx and Z Xdx 0 R X over total depletion layer width as then the results obtained 0 Em 0 from the DC analysis are used in the small signal analysis. The diode impedance Z is obtained by, W W VB E() x dx (2) Z()(,) Z x dx ZRX jZ (3) 0 0 The diode admittance is obtained as Table1(A,B): Material parameters used for the analysis (at 300K) (A) 11 Y G jB or, An Bn Ap Bp Vns Vps Z ZRX jZ ( 1010m 1 ) ( 1091Vm ) ( 1081m ) ( 1091Vm ) ( 1051ms ) ( 1051ms ) ZR G 22 4.57 5.24 5.13 1.57 2.508 2.5 ZZRX Z and B X (4) ZZ22 (B) RX J 0 ND NA n p and are both normalized in accordance with the 82 23 3 23 3 21 21 ( 10Am ) ( 10m ) ( 10m ) ()m Vs ()m Vs diode area (1082m ) taken in our analysis. The small signal quality 4 2.8 2.9 0.12 0.12 factor (QP ) is calculated by Published By: Retrieval Number: A2119034114/2014©BEIESP Blue Eyes Intelligence Engineering 146 & Sciences Publication International Journal of Soft Computing and Engineering (IJSCE) ISSN: 2231-2307, Volume-4, Issue-1, March 2014 B p 2 j Q (5) D , H (p n ) DE m , =( n p ) , P G x P The expected RF power delivery from the diode can be n p n p p p n n calculated by using the relation rrnp, , = 2 2 2 2 PVGARF( RF ) P / 2 (6) Initially, the noise generation source is assumed to be The power density P is taken as PA,.