S. P. Pati

A generalized simulation method for MITATT-mode operation and studies on the influence of tunnel current on IMPATT properties

A generalized method of DC and high-frequency analysis for microwave transit time diodes in mixed tunnelling and avalanche mode, which can be applied to any type of diode structure is reported. Taking a purely field-dependent tunnel generation rate for electrons, the same is computed for holes from a simulated energy band diagram within the depletion layer of the diode. The method has been applied to a variety of Si, GaAs and InP diode structures. The results show a substantial degradation of IMPATT properties due to phase distortion caused by the tunnelling current.

High frequency numerical analysis of double avalanche region IMPATT diode

A realistic and accurate computer method for high frequency numerical analysis under small signal conditions of a double avalanche region (DAR) (n+p nu np+) IMPATT diode is described. The method is used to determine microwave properties of several silicon DAR diodes under various operating conditions. The results indicate the existence of discrete and widely separated negative conductance bands in the DAR diode which can provide selective tuning in addition to wide frequency coverage for the device. The magnitude of DAR negative conductance, which is found to be smaller by an order of magnitude than that for the corresponding double drift diode, can be enhanced by the introduction of suitable asymmetry in the doping concentration of n and p of the DAR diode structure. The common drift layer for both types of charge carriers in the case of a DAR diode would result in space charge wave cancellation which may make it possible to push the input current to a high value and reduce the noise generation. Thus, a DAR diode may provide an appreciable amount of microwave power with low noise level over several frequency bands covering a wide frequency range.

Computer-aided studies on the wide-band microwave characteristics of a silicon double avalanche region (DAR) diode

The results of accurate and realistic high-frequency numerical analysis of a silicon DAR (double avalanche region) diode indicate some unique and useful microwave characteristics. The DAR diode under any structural condition exhibits multiband microwave negative resistance characteristics between 8 and 350 GHz which would make it possible to realize wide-band microwave oscillations (8 to 350 GHz) from any single DAR diode with a multituning facility. The negative resistance space distribution profiles of the diode at high frequencies of operation shows near sinusoidal variation, which suggests that a DAR diode can have several optimum diode widths for generation of a particular frequency. The results have been explained on the basis of computation of the total avalanche delay produced in both avalanche regions and transmit time delay produced in the drift region.

A Computer Study on the High Frequency Properties of Silicon mm-Wave Single Drift Impatt Diodes

A detailed investigation on dc and high frequency properties of silicon single drift Impatt devices is presented for device operation at the frequencies of 35, 94, 140 and 220 GHz which are the atmospheric window frequencies for mm—wave signals. A comparative account of device properties of n+ pp+ and p+ nn+ SDRs is also presented. Single drift Impatts at frequencies beyond 100 GHz are characterised by very high electric fields at the metallurgical Junction, wide avalanche zone and low breakdown and drift voltages. The magnitude of field maximum for Si n+ pp+ SDR increases from a value of 38.5 V/μm to 93.7 V/μm when the frequency of operation increases from 10 GHz to 140 GHz. The ratio VD/VA for n+ pp+ diode which is nearly unity for X-band operation drops down to only 0.16 for operation at frequencies around 150 GHz. The computed values of normalized avalanche zone width of Si n+ pp+ diodes are found to be only 30%, 60% and 70% respectively for operation at 10 GHz, 94 GHz and 140 GHz as against the corre...

OPTIMIZATION OF ION IMPLANTED LOW-HIGH-LOW IMPURITY PROFILE FOR SILICON N+PP+ SDR DIODE

A reaslistic silicon low-high-low SDR doping profile has been generated through computer similation while introducting impurity bump (high doping level) into flat low doping level by implantation of appropriate ion beam. The IMPATT characteristics of these doping profiles have been studied for different energy values and for different doses. The ion Implantation parameters for V-band silicon low- high-low SDR are optimized.

Effect of the diffusion impurity profile on the microwave properties of silicon p+nn+ IMPATT diodes

The duration (t) and temperature (T) of impurity diffusion in the process of junction formation determine the diffusion doping profile on the p+ and n sides of the p+nn+ junction. The diffusion profile can be computed from the error function involving the parameter s=2 square root (D(T)t) which takes different shapes for different values of s. Microwave properties of Ku, Ka, V and W band p+nn+ diodes in IMPATT mode for different values of s have been computed through DC and high-frequency analyses of the diodes following an accurate simulation program. The results indicate that the diode properties are optimized for a particular value of s for one particular frequency band. The diode properties deteriorate for both small and large values of s on either side of the optimum value. The results can be utilized for determination of appropriate diffusion parameters in the process of p+nn+ diode fabrication for realization of optimum RF performance at different frequency bands.

Si/SiC-based DD hetero-structure IMPATTs as MM-wave power-source: a generalized large-signal analysis

A full-scale, self-consistent, non-linear, large-signal model of double-drift hetero-structure IMPATT diode with general doping profile is derived. This newly developed model, for the first time, has been used to analyze the large-signal characteristics of hexagonal SiC-based double-drift IMPATT diode. Considering the fabrication feasibility, the authors have studied the large-signal characteristics of Si/SiC-based hetero-structure devices. Under small-voltage modulation (~ 2%, i.e. small-signal conditions) results are in good agreement with calculations done using a linearised small-signal model. The large-signal values of the diodes negative conductance (5 × 106 S/m2), susceptance (10.4 × 107 S/m2), average breakdown voltage (207.6 V), and power generating efficiency (15%, RF power: 25.0 W at 94 GHz) are obtained as a function of oscillation amplitude (50% of DC breakdown voltage) for a fixed average current density. The large-signal calculations exhibit power and efficiency saturation for large-signal (> 50%) voltage modulation and thereafter decrease gradually with further increasing voltage-modulation. This generalized large-signal formulation is applicable for all types of IMPATT structures with distributed and narrow avalanche zones. The simulator is made more realistic by incorporating the space-charge effects, realistic field and temperature dependent material parameters in Si and SiC. The electric field snap-shots and the large-signal impedance and admittance of the diode with current excitation are expressed in closed loop form. This study will act as a guide for researchers to fabricate a high-power Si/SiC-based IMPATT for possible application in high-power MM-wave communication systems.

Possible realization of near optimum efficiency from n-Si-Ge/p-Ge-Si DDR Hetero Structure IMPATT Diode

A p-n junction under reverse bias avalanche breakdown condition is capable of producing high frequency rf power in Impatt mode. With the advancement of Device Technology, the present state of art reports realization of alloy Si-Ge junction, Si-Ge hetero junction. Introduction of a n-Ge and p-Ge impurity bumps near the junction face on respective side of Si p-n junction leaves an asymmetrical hetero structure junction which has become the scope of study of this paper for operation at 15 and 96 GHz. Three tier sophisticated computer algorithm has been framed and used for Impatt analysis of resulting n-Si-Ge/p-Ge-Si Hetero Structure reveals realization of device efficiency as high as 29.6% (Theoretical Optimum Efficiency of Impatt Diode=31%) and also high value of negative conductance. Presence of Ge layer near junction and an order high carrier ionization rate in Ge compared to Si localizes the avalanche zone, which pushes the efficiency and RF power generation. Similar results are also noticed for 96 GHz operations. The performance from this structure is observed to be superior by considerable extent as compared to Si and Ge homo structure. However the complementary hetero structure having the form n-Ge-Si/p-Si-Ge is observed to exhibit performance almost on par to Si and Ge homo structures. The results are highly encouraging which may make Si-Ge Hetero Structure Diode as a microwave generator.

MM-wave characteristics of SiC-based IMPATT oscillators

Wide band gap semiconductor SiC with their superior electrical properties are likely candidates to replace conventional low band gap materials like Si and GaAs in the near future for RF power applications. The authors have therefore studied this prospects through advanced computer simulation experiment on hexagonal (both 4H and 6H) SiC based double drift region IMPATT diodes. The study indicates that around 300GHz, 4H-SiC based Impatt devices is capable of generating high microwave power with efficiency as compared to 6H-SiC based IMPATT diodes for the same frequency of operation.

Phase Distortion Assisted MITATT Design for Compensation of Performance Deterioration due to Tunnel Current

A method for design of high frequency IMPATT, operating in MITATT mode is presented which can compensate for the deterioration in device performance due to loss of carrier build up phase delay caused by tunneling current. The results for 94 GHz Si DDR indicate that the device performance of MITATT diodes would be improved considerably with the modulation of diode design parameters based on phase distortion. The suggested method would provide a realistic approach for design consideration of any form of high frequency IMPATTs operating in mixed tunneling and avalanche mode.