“Application of transit time effect in IMPATT diodes”

Ramjee Prasad

RH6802B54

10804900

B.Tech(LEET)

ECE-304

Introduction:

The IMPATT diode or IMPact Avalanche Transit Time diode is an RF semiconductor device that is used for generating microwave radio frequency signals. With the ability to operate at frequencies between about 3 and 100 GHz or more, one of the main advantages of this microwave diode is the relatively high power capability of the IMPATT diode.

The structure of the diode is shown below:

IMPATT diodes are used in a variety of applications from low power radar systems to alarms and many other microwave radio applications. In fact IMPATT diodes are ideal where small cost effective microwave radio sources are needed. The main drawback of generators using IMPATT diodes is the high level of phase noise they generate. This results from the statistical nature of the avalanche process that is key to their operation. Nevertheless these microwave diodes make excellent signal sources for many RF microwave applications.

IMPATT diode fabrication:

IMPACT avalanche transit time (IMPATT) diodes are well known for their performance at frequencies extending into the millimetre-wave range. Many radar systems have a need for high power microwave sources in their transmitters that can only be addressed by the use of IMPATT diodes .

A fabricated IMPATT diode generally is mounted in a micro wave package. The diode is mounted with its high – field region close to a copper heat sink so that the heat generated at the diode junction can be readily conducted away by the copper heatsink. Similar microwave packages are used to house other microwave devices. Their small size and

light weight are advantageous in these systems when compared with alternatives. So far, monolithic millimetre-wave IMPATT oscillators have been fabricated in GaAs, InP, Si, and SiGe processes. There is a variety of structures that are used for the IMPATT diode. All are variations of a basic PN junction and usually there is an intrinsic layer, i.e. a layer without any doping that is placed between the P type and N type regions. Typically the N type layer is around one or two microns thick and the intrinsic layer between 3 and 20 microns. In the very high frequency versions of the diodes the intrinsic layer will be very much thinner and dimensions of only 0.5 microns are not unknown.

A variety of semiconductor materials are used for the fabrication of IMPATT diodes. Silicon and gallium arsenide are the most commonly used semiconductors, although germanium, indium phosphide and other mixed group semiconductors can be employed.

Most IMPATT diodes are produced in discrete form and operated in external circuits, limiting their widespread use in compact, lightweight, and low-cost systems that require higher degrees of integration. Recently, monolithic integration of lateral IMPATT diodes with a microstrip patch antenna at 77 GHz has been achieved in standard complementary metal–oxide–semiconductor (CMOS) technology . This CMOS transmitter is particularly appealing from a cost reduction and system integration standpoint. In this work, the antenna is designed as a radiator and a resonator at the same time to minimize the required chip area. In addition, a study of the effect of different feed type and feed locations to match the IMPATT impedance is presented. Such a topology reduces parasitic losses, since no microstrip line is necessary to connect the oscillator to the antenna. At the same time, matching the

IMPATT to the antenna is facilitated by integrating three diodes and three stubs along one of the radiating edges. In this study, the antenna impedance seen by the IMPATT diode was estimated by the electromagnetic (EM) field solver Sonnet, while the impedance of the IMPATT diode was characterized by on-wafer measurements in a standard CMOS process .

The fabricated IMPATT diodes are generally mounted in microwave packages to ensure that their performance is not impaired by an inferior package. The package itself is key to the performance of the IMPATT, especially as these devices may operate at frequencies of many tens of GHz. For thermal reasons, the diode is mounted so that its high field region around the junction is close to a copper heat sink area in the package. This enables the heat generated within the device to be removed effectively so that it can run at its rated power without the junction temperature rising too high. Often the package is coaxial in format so that the correct transmission line properties are presented to the RF signal which may be at many tens of GHz. As a result the package is often quite intricate and accordingly very expensive, especially when very high frequencies are used.IMPATT diode operation:

In terms of its operation the IMPATT diode can be considered to consist of two areas, namely the avalanche region or injection region, and secondly the drift region.

These two areas provide different functions. The avalanche or injection region creates the carriers which may be either holes of electrons, and the drift region is where the carriers move across the diode taking a certain amount of time dependent upon its thickness.

If a free electron with sufficient energy strikes on silicon atom, it can break covalent bond of silicon and liberate an electron from the covalent bond. If the electron gained energy by electric field and liberated other electrons from other covalent bonds then this process can cascade (avalanche) very quickly into chain reaction producing a number of electrons and large current flow in diode. This phenomenon is called impact avalanche. At breakdown, the n – region is punched through and forms the avalanche region of the diode. The high resistivity i – region is the drift zone through which the avalanche generated electrons move toward the anode.

The IMPATT diode is operated under reverse bias conditions. These are set so that avalanche breakdown occurs. This occurs in the region very close to the P+ (i.e. heavily doped P region). The electric field at the p-n junction is very high because the voltage appears across a very narrow gap creating a high potential gradient. Under these circumstances any carriers are accelerated very quickly. As a result they collide with the crystal lattice and free other carriers.

These newly freed carriers are similarly accelerated and collide with the crystal lattice freeing more carriers. This process gives rise to what is termed avalanche breakdown as the number of carriers multiplies very quickly. For this type of breakdown only occurs when a certain voltage is applied to the junction. Below this the potential does not accelerate the carriers sufficiently.

Once the carriers have been generated the device relies on negative resistance to generate and sustain an oscillation. The effect does not occur in the device at DC, but instead, here it is an AC effect that is brought about by phase differences that are seen at the frequency of operation. When an AC signal is applied the current peaks

are found to be 180 degrees out of phase with the voltage. This results from two delays which occur in the device: injection delay, and a transit time delay as the current carriers migrate or drift across the device.

The voltage applied to the IMPATT diode has a mean value that means the diode is on the verge of avalanche breakdown. The voltage varies as a sine wave, but the generation of carriers does not occur in unison with the voltage variations. It might be expected that it would occur at the peak voltage. This arises because the generation of carriers is not only a function of the electric field but also the number of carriers already in existence.

As the electric field increases so does the number of carriers. Then even after the

field has reached its peak the number of carriers still continues to grow as a result of the number of carriers already in existence. This continues until the field falls to below a critical value when the number of carriers starts to fall. As a result of this effect there is a phase lag so that the current is about 90 degrees behind the voltage. This is known as the injection phase delay.

When the electrons move across the N+ region an external current is seen, and this occurs in peaks, resulting in a repetitive waveform.

Practical operation

The main application for IMPATT diodes is in microwave generators. An alternating signal is generated simply by applying a DC supply when a suitable tuned circuit is applied. The output is reliable and relatively high when compared to other forms of microwave diode. In view of its high levels of phase noise it is used in transmitters more frequently than as a local oscillator in receivers where the phase noise performance is generally more important. It is also used in applications where phase noise performance is unlikely to be of importance.

To run an IMPATT diode, a relatively high voltage, often as high as 70 volts or higher may be required. This often limits their application as voltages of this order are not always easy to use in some pieces of equipment. Nevertheless IMPATT diodes are particularly attractive option for microwave diodes for many areas.

Effect of Ionizing Radiation on the Silicon IMPATT Diode Characteristics:

We investigated the effect of 60Co γ-irradiation (doses from 102 to 2×106 Gy), both without and with heat annealing, on silicon IMPATT diode parameters. It is shown that such treatments improve the

diode characteristics (particularly decrease the reverse current and increase both the output and the diffusion length of the minority charge carriers) due to radiation-enhanced processes.

Introduction:At present a number of technological procedures are known that enable to purposefully change the parameters of device structures based on different semiconductor materials. Various methods of gattering and structural - impurity ordering have been developed that can improve the device structure parameters. During manufacturing devices often accumulate various structural defects generated in them during chip formation, dissipative welding and/or thermo-compression bonding. This results in both degradation of parameters in the finished product and yield reduction. For such devices the restoration of their properties poses a problem. Conventional heat treatment would not do in this case because the annealing temperature is limited (as the contacts must not be fused). To apply other treatments, one has to know with certainty the nature of the defect as well as the treatment peculiarities, themselves neither of which is often taken into account in practice. We did manage to improve the parameters of silicon impact-avalanche and transit-time (IMPATT) diodes by -irradiation of _nished production (packaged diodes) or generator units (IMPATT diodes in a cavity), both with and without heat annealing. Such treatments do not generate structural defects, nor do they result in material compensation and widening of the space charge region.Experimental Procedure:

The diodes studied were fabricated using boron diffusion from vapor phase into the n − n+ Si substrate (the charge carrier concentration in the n layer was (3 + 5) × 1016 cm−3). The reverse mesa diameter was 5×10-3 cm, and the avalanche-breakdown

voltage VB was 19-20 V. The starting microwave output P* out did not exceed 35 mW at operating current Iop = 100 mA. The reverse branches of I −V curves were taken and the diffusion length of minority charge carriers Lp and output Pout were measured both before and after the corresponding treatments. To treat the diodes studied we used 60Co –radiation (doses from 102 to 2×106 Gy, dose rate 3 Gy/s), both with and without heat annealing, at temperature T = 200-250 0C for 40-60 min.Experimental Results and Discussion:

Shown in Figure are the dose dependencies of the reverse current IR (at the reversebias VR = 0.9 V), diffusion length Lp and the relative change of the mean microwaveoutput, Pout/P *out. One can see that both IR and Lp dose dependencies correlate with change of Pout/P*out change due to 60Co γ-irradiation. According to

Dependencies of the silicon IMPATT diode parameters on the absorbed dose of 60Co γ-irradiation: 1- Reverse current IR; 2- Diffusion length of the minority charge carriers Lp; 3- Output Pout.

Pout = (ΔTw/ϵVB RT S1/2)2µ eff qB; - -------- (1)

where ΔT is the difference between the temperatures of the SCR and the ambient, w is the SCR width, ϵ is permitivity of the diode material, RT is the diode heat resistance, S is the p − n junction area, µeff

is the effective mobility of charge carriers and qB is the diode resistivity.

From Expression (1) it is evident that if w; VB and qB remain constant, then the Poutincrease due to 60Co -irradiation may result from changes in RT; S and/or µeff . Indeed,the direct heat resistance measurements for the IMPATT diodes studied have shown that RT was decreasing during γ-irradiation. This results from the increase of the effective cross section area of the active region where heat dissipation occurred. In addition, the effective mobility µeff may grow as a result of the radiation-enhanced gettering due to a decrease in the number of scattering centers. The last statement is circumstantially evidenced by the results of direct electron-probe measurements of the diffusion length Lp both before and after -irradiation of test structures fabricated from the same wafers that were used to fabricate the IMPATT diodes studied (see Figure, curve 2).

We have studied also the effect of the low-temperature (T = 200-280 0C) annealing on the output Pout and the reverse current IR

for the IMPATT diodes reradiated by the 60Co -quanta. The corresponding results are given below:

Effect of 60Co γ-irradiation (without and with heat annealing at 200 0C for 1 hour)on the Pout=P* out and IR=ISR ratios for silicon IMPATT diodes.

Effect of the temperature of heat annealing (for 1 hour) on the Pout=PSout and IR=ISR ratios for 60Co -irradiated (dose of 5 _ 105 Gy) silicon IMPATT diodes.

Effect of the duration of heat annealing (at 200 0C) on the Pout=Psout and IR=ISR ratios

for 60Co γ-irradiated silicon IMPATT diodes (dose of 7 × 105 Gy) .

One can see that the low-temperature annealing after 60Co γ -irradiation increases Pout as well. It should be noted that the above heat treatment of the non-irradiated IMPATT diodes either resulted in a decrease of the output Pout, or, at best, did not change it.

In impact avalanche transit time (IMPATT) diodes fabricated in 0.18- m standard complementary metal–oxide–semiconductor technology to enable operation at 77 GHz. The lateral IMPATT diodes are integrated with a microstrip patch antenna, modified to provide impedance matching and widen the tuning range. The antenna dimensions and the impedance matching are designed using the high-frequency electromagnetic field solver Sonnet. The output spectrum has no visible spurious components. The transmitted power is 62 dBm at 76 GHz. The measured frequency is within 1.3% of the simulated value. It is hoped that this device will find application in automotive and communication systems.

Monolithic Integrated Millimeter-Wave IMPATT Transmitter in Standard CMOS Technology:

INTRODUCTION:IMPACT avalanche transit time (IMPATT) diodes are well known for their performance at frequencies extending into the millimeter-wave range. Many radar systems have a need for high power microwave sources in their transmitters that can only be addressed by the use of IMPATT diodes . Their small size and light weight are advantageous in these systems when compared with alternatives. So far, monolithic millimeter-wave IMPATT oscillators have been fabricated in GaAs, InP , Si , and SiGe processes. Most IMPATT diodes are produced in discrete form and operated in external circuits, limiting their widespread use in compact, lightweight, and low-cost systems that require higher degrees of integration. Recently, monolithic integration of lateral IMPATT diodes with a microstrip patch antenna at 77 GHz has been achieved in standard complementary metal–oxide–semiconductor (CMOS) technology .

This CMOS transmitter is particularly appealing from a cost reduction and system integration standpoint. In this work, the antenna is designed as a radiator and a resonator at the same time to minimize the required chip area.

In addition, a study of the effect of different feed type and feed locations to match the IMPATT impedance is presented. Such a topology reduces parasitic losses, since no microstrip line is necessary to connect the oscillator to the antenna. At the same time, matching the IMPATT to the antenna is facilitated by integrating three diodes and three stubs along one of the radiating edges.

In this study, the antenna impedance seen by the IMPATT diode was estimated by the electromagnetic (EM) field solver Sonnet, while the impedance of the IMPATT diode was characterized by on-wafer measurements in a standard CMOS process.

OSCILLATOR DESIGN:A necessary condition for steady-state oscillation is a zero sum for the circuit and device impedances at the steady-state operating point. The microwave negative resistance of an IMPATT diode arises out of a phase difference between the RF voltage and RF current. This phase difference is produced by the lagging RF current generated in the space charge layer with respect to the applied RF voltage.

The time delay inherent in the build up of the avalanche current is augmented by the transit delay experienced by the charge

carriers in crossing the drift region at saturated drift velocity. In order for the diode to operate as a stable oscillator, the negative conductance of the diode must decrease with increasing RF voltage. The RF voltage across the diode will grow until the admittance of the diode is balanced by the admittance of the microwave circuit.Lateral IMPATT Diode:

The cross section of the lateral IMPATT diode as shown in fig.. The diode has a single drift region. The p , n, and n regions of the IMPATT diode are implemented using standard source/drain, n-well, and ohmic contact diffusion regions, respectively. The impedance of the diode is measured up to 110 GHz by means of a vector network analyzer (VNA) connected to a CASCADE wafer prober. To minimize the influence of the measurement setup on the diodes, a constant VNA output power of 20 dBm is used. The Smith chart in Fig. 3 provides the reflection coefficient prior to deembedding. At certain frequencies, it becomes greater than 1, as needed to enable oscillation. With open and short structures, the impedance of the parallel and series parasitics are estimated to allow deembedded measurements of diode impedance.

CONCLUSION:

A monolithic integrated IMPATT transmitter built in standard CMOS technology and operating in the millimetre-wave range has been

investigated experimentally. Using a patch antenna driven by a lateral IMPATT diode, the 4-mm transmitter delivers a radiated power of 62 dBm at 76 GHz. By using this particular configuration, area requirements and parasitic losses of the integrated transmitter are reduced. Because of the cost efficiency and the robustness of standard CMOS manufacturing, this type of monolithic integrated transmitter may be well suited for use in millimetre-wave systems for various applications ranging from communications to automobile anticollision radar systems.

COMPLEX NONLINEAR MODEL FOR THE PULSED-MODE IMPATT DIODE:

INTRODUCTION

A complex nonlinear model for high-power pulsed IMPATT diode oscillator analysis is presented. This model is suitable for the analysis of different operational modes of the oscillator. It takes into account the main electrical and thermal phenomena in the semiconductor structure and the functional dependence of the governing equations' coefficients on the electrical field and temperature. The temperature distribution in the semiconductor structure is obtained using the special thermal model of the IMPATT diode, which is based on the numerical solution of the non-linear thermal conductivity equation. The complex model presented in this work can be applied for the practical design of pulsed-mode millimetre IMPATT diodes. It can also be utilized for diode thermal regime estimation, for the proper selection of feed-pulse shape and amplitude, and for the development of complex doping-profile high-power pulsed millimetric IMPATT diodes with improved characteristics.

IMPATT (IMPact Avalanche ionization and Transit Time) diodes are principal

active elements for use in millimetric pulsed-mode generators. Semiconductor structures suitable for fabrication of continuos-mode IMPATT diodes have been well known for a long time.They have been utilized successfully in many applications in microwave engineering. The possibilities of using the same structures for pulsed-mode microwave generators are very interesting because the pulsed-mode IMPATT-diode generators can successfully operate at high current densities without deterioration of reliability.

The cross section of the pulsed-mode IMPATT diode may be larger than that of continuous-mode diodes. Therefore, a pulsed-mode oscillator can provide a larger output power. Considering that the increase of the output power of millimetric generators is one of the main problems of microwave electronics it is important to optimize the diode's active layer to obtain the generator's maximum output power.

One of the main problems in the operation of high-power IMPATT-diode pulsed-mode generator is the large variation of the diode's admittance during the pulse. This variation is significant during each current pulse due to the temperature change in the diode's semiconductor structure.

Therefore, diffusion coefficients, ionization rates and charge mobility experience large variations during the pulse. These changes strongly affect the amplitude and phase of the first harmonic of the diode's avalanche current. Therefore, the admittance value also changes. This results in the instability of the generator's output power and frequency within each generated microwave pulse. Pulsed-mode IMPATT diodes that are utilized in microwave electronics are, most frequently, single drift and double-drift structures similar to continuous-mode ones . The typical diode structure is shown in the Fig. 1 by curve 1; where N is the

concentration of donors and acceptors and l is the length of the diode active layer. In this type of diode, the electrical field is strongly distorted when the avalanche current density is sufficiently high. This large space charge density is one of the main reasons for the sharp electrical field gradient along the charge drift path. Because of this field gradient, the space charge avalanche ruins itself and consequently the optimum phase relations degrade between the microwave potential and the current. This factor is especially important when the IMPATT diode is fed at the maximum current density, which is exactly the case for pulsed-mode operation.

The idea to utilize a complex doping profile semiconductor structure for a microwave diode was originally proposed in the first analysis of IMPATT diodes by Read [6]. This proposed ideal structure has never been realized till now. However, modern

semiconductor technology provides new possibilities for the fabrication of sub micron semiconductor structures with complex doping profiles. This stimulates the search for IMPATT-diode special structure optimization for pulsed-mode operation.

The proposed new type of IMPATT diode doping profile is shown in Fig. 1 by the

curve 2. This type of semiconductor structure can be named a quasi-Read-type structure. This type of doping profile provides a concentration of the electrical field within the p-n junction. This measure helps to decrease the destruction of the avalanche space charge and therefore permits an improvement of the phase stability between the diode current and voltage. Historically, many analytical and numerical models have been developed for the various operational modes of IMPATT diodes. However, they are not adequate for very high current density values and different temperature distributions inside the structure, which is exactly the case for the pulsed-mode IMPATT diode oscillator. For this reason, we have developed a new complex numerical model of the IMPATT diode that is composed of the advanced thermal model and the modified local-field model. The thermal model provides the exact theoretical temperature distribution along the diode active region. The local-field electrical model calculates the functional dependence of equation coefficients from the electric field and the temperature, and using all these data finally derives the IMPATT diode dynamic characteristics.

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