PROJECT ON IMPATT & TRAPATT DIODE

INTRODUCTION

HISTORY - Ever since the development of modern semiconductor device theory scientists have speculated on whether it is possible to make a two-terminal negative resistance device.

The TUNNEL DIODE  was the first such device to be realised in practice. Its operation depends on the properties of a forward biased p-n junction in which both the p and n regions are heavily doped.

Transferred electron devices are another type of these devices. The transferred electron device or GUNN OSCILLATORS operate simply by the application of a dc voltage to a bulk semiconductor. There are no p-n junction in this device. Its frequency is a function of the load and of natural frequency of the circuit.

The third type of these device are AVALANCHE TRANSIT-TIME DEVICES. The avalanche diode oscillator uses carrier impact ionization and drift in the high field region of a semiconductor junction to produce a negative resistance at microwave frequencies.

An avalanche transistor is a bipolar junction transistor designed for operation in the region of its collector-current/collector-to-emitter voltage characteristics beyond the collector to emitter breakdown voltage, called avalanche breakdown region . This region is characterized by avalanche breakdown, a phenomenon similar to Townsend discharge for gases, and negative differential resistance. Operation in the avalanche breakdown region is called avalanche mode operation: it gives avalanche transistors the ability to switch very high currents with less than a nanosecond rise and fall times (transition times).

The device was originally proposed in a theoretical paper by READ in which he analysed the negative-resistance properties of an idealised n+ -p-i-p+ diode.

BASIC PRINCIPLE AND MODES

Avalanche transit-time diode oscillators rely on the effect of voltage breakdown across a reverse-biased p-n junction to produce a supply of holes and electrons.

Two distinct modes of avalanche oscillator have been observed. One is the IMPATT mode, which stands impact ionization avalanche transit time operation. In this mode the typical dc-to-RF conversion efficiency is 5 to 10%, and frequencies are as high as 100 GHz with silicon diodes.

The other mode is the TRAPATT mode, which represents trapped plasma avalanche triggered transit operation. Its typical conversion efficiency is from 20-60%.

Another type of active microwave device is the BARITT diode, which stands for barrier injected transit-time diode. It has long drift regions similar to those of IMPATT diodes. The carrier traversing the drift regions of BARITT diodes are generated by minority carrier injection from forward biased junctions rather than being extracted from the plasma of an avalanche region. Some structures of BARITT diodes are p-n-p , p-n-v-p , p-n-metal etc.

IMPATT diode

IMPact Avalanche Transit Time diode is a high power radio frequency (RF) generator operating from 3 to 100 gHz. IMPATT diodes are fabricated from silicon, gallium arsenide, or silicon carbide.

An IMPATT diode is reverse biased above the breakdown voltage. The high doping levels produce a thin depletion region. The resulting high electric field rapidly accelerates carriers which free other carriers in collisions with the crystal lattice. Holes are swept into the P+ region. Electrons drift toward the N regions. The cascading effect creates an avalanche current which increases even as voltage across the junction decreases. The pulses of current lag the voltage peak across the junction. A “negative resistance” effect in conjunction with a resonant circuit produces oscillations at high power levels (high for semiconductors).

IMPATT diode: Oscillator circuit and heavily doped P and N layers.

The resonant circuit in the schematic diagram of Figure is the lumped circuit equivalent of a waveguide section, where the IMPATT diode is mounted. DC reverse bias is applied through a choke which keeps RF from being lost in the bias supply. This may be a section of waveguide known as a bias Tee. Low power RADAR transmitters may use an IMPATT diode as a power source. They are too noisy for use in the receiver. [YMCW]

Gunn diode

A gunn diode is solely composed of N-type semiconductor. As such, it is not a true diode. Figure shows a lightly doped N- layer surrounded by heavily doped N+ layers. A voltage applied across the N-type gallium arsenide gunn diode creates a strong electric field across the lightly doped N- layer.

. Gunn diode: Oscillator circuit and cross section of only N-type semiconductor diode

As voltage is increased, conduction increases due to electrons in a low energy conduction band. As voltage is increased beyond the threshold of approximately 1 V, electrons move from the lower conduction band to the higher energy conduction band where they no longer contribute to conduction. In other words, as voltage increases, current decreases, a negative resistance condition. The oscillation frequency is determined by the transit time of the conduction electrons, which is inversely related to the thickness of the N- layer.

The frequency may be controlled to some extent by embedding the gunn diode into a resonant circuit. The lumped circuit equivalent shown in Figure is actually a coaxial transmission line or waveguide. Gallium arsenide gunn diodes are available for operation from 10 to 200 gHz at 5 to 65 mw power. Gunn diodes may also serve as amplifiers.

COMPARISON BETWEEN GUNN DIODE & IMPATT DIODE

Gunn diodes and IMPATT diodes use high field effects in semiconductor materials to drive a negative resistance mode of operation. Gunn diodes use the Gunn effect to produce microwave oscillations when a constant voltage is applied. Gunn diodes are a type of transferred electron device (TED). They generate relatively low-power microwave radio signals at frequencies from a few GHz up to 200 GHz. As a discrete component, a Gunn diode can be used as an oscillator or amplifier in applications that require low-power radio frequency (RF) signals, such as proximity sensors and wireless local area networks (LAN). IMPATT diodes are semiconductor devices that generate relatively high-power microwave signals at frequencies between about 3 GHz and 100 GHz or more. IMPATT is an abbreviation for impact avalanche transit time. IMPATT diodes are used in low-power radar systems and alarms. The main drawback of using an IMPATT diode is the high level of phase noise that the device generates.

Gunn diodes and IMPATT diodes are similar, but not interchangeable. Gunn diodes that are made from gallium arsenide can operate at frequencies up to 200 GHz. A Gunn diode made from gallium nitride can reach 3 THz. Specifications for Gunn diodes include frequency range, minimum power, typical operating voltage, operating current, and packaging. For higher output power, a Gunn diode can be pulsed or stacked. Specifications for IMPATT diodes include frequency range (GHz), bandwidth range (GHz), power output (W), breakdown voltage, and gain (dB). To help maintain frequency and power stability over wide temperature ranges, the diode manufacturer can supply heatsink stands and temperature controllers. An IMPATT diode can be stable (linear) or injection-locked. A pulsed IMPATT diode operates with short bias current pulses and low duty cycles, and can produce higher output power than a continuous wave (CW) IMPATT diode.

IMPATT DIODE

An IMPATT diode (IMPact ionization Avalanche Transit-Time) is a form of high power diode used in high-frequency electronics and microwave devices. They are typically made with silicon carbide owing to their high breakdown fields.

They operate at frequencies between about 3 and 100 GHz or more. A main advantage is their high power capability. These diodes are used in a variety of applications from low power radar systems to alarms. A major drawback of using IMPATT diodes is the high level of phase noise they generate. This results from the statistical nature of the avalanche process. Nevertheless these diodes make excellent microwave generators for many applications.

Device structure

The IMPATT diode family includes many different junctions and metal semiconductor devices. The first IMPATT oscillation was obtained from a simple silicon p-n junction diode biased into a reverse avalanche break down and mounted in a microwave cavity. Because of the strong dependence of the ionization coefficient on the electric field, most of the electron–hole pairs are generated in the high field region. The generated electron immediately moves into the N region, while the generated holes drift across the P region. The time required for the hole to reach the contact constitutes the transit time delay.

The original proposal for a microwave device of the IMPATT type was made by Read and involved a structure. The Read diode consists of two regions (i) The Avalanche region (a region with relatively high doping and high field) in which avalanche multiplication occurs and (ii) the drift region (a region with essentially intrinsic doping and constant field) in which the generated holes drift towards the contact. A similar device can be built with the configuration in which electrons generated from the avalanche multiplication drift through the intrinsic region.

An IMPATT diode generally is mounted in a microwave package. The diode is mounted with its high–field region close to a copper heatsink so that the heat generated at the diode junction can be readily dissipated. Similar microwave packages are used to house other microwave devices.

Principle of operation

Impact ionization

If a free electron with sufficient energy strikes a silicon atom, it can break the covalent bond of silicon and liberate an electron from the covalent bond. If the electron liberated gains energy by being in an electric field and liberates other electrons from other covalent bonds then this process can cascade very quickly into a chain reaction producing a large number of electrons and a large current flow. 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 region is the drift zone through which the avalanche generated electrons move toward the anode.

Consider a dc bias VB, just short of that required to cause breakdown, applied to the diode. Let an AC voltage of sufficiently large magnitude be superimposed on the dc bias, such that during the positive cycle of the AC voltage, the diode is driven deep into the avalanche breakdown. At t=0, the AC voltage is zero, and only a small pre-breakdown current flows through the diode. As t increases, the voltage goes above the breakdown voltage and secondary electron-hole pairs are produced by impact ionization. As long as the field in the avalanche region is maintain above the breakdown field, the electron-hole concentration grows exponentially with t. Similarly this concentration decays exponentially with time when the field is reduced below breakdown voltage during the negative swing of the AC voltage. The holes generated in the avalanche region disappear in the p+ region and are collected by the cathode. The electrons are injected into the i – zone where they drift toward the n+ region. Then, the field in the avalanche region reaches its maximum value and the population of the electron-hole pairs starts building up. At this time, the ionization coefficients have their maximum values. The generated electron concentration does not follow the electric field instantaneously because it also depends on the number of electron-hole pairs already present in the avalanche region. Hence, the electron concentration at this point will have a small value. Even after the field has passed its maximum value, the electron-hole concentration continues to grow because the secondary carrier generation rate still remains above its average value. For this reason, the electron concentration in the avalanche region attains its maximum value at, when the field has dropped to its average value. Thus, it is clear that the avalanche region introduces a 90o phase shift between the AC signal and the electron concentration in this region.

With a further increase in t, the AC voltage becomes negative, and the field in the avalanche region drops below its critical value. The electrons in the avalanche region are then injected into the drift zone which induces a current in the external circuit which has a phase opposite to that of the AC voltage. The AC field, therefore, absorbs energy from the drifting electrons as they are decelerated by the decreasing field. It is clear that an ideal phase shift between the diode current and the AC signal is achieved if the thickness of the drift zone is such that the bunch of electron is collected at the n+ - anode at the moment the AC voltage goes to zero. This condition is achieved by making the length of the drift region equal to the wavelength of the signal. This situation produces an additional phase shift of 90o between the AC voltage and the diode current.

TRAPATT diode

A p-njunction diode, similar to the IMPATT diode, but characterized by the formation from a trapped space-charge plasma within the junction region; used in the generation and amplification of microwave power.Derived from trapped plasma avalanche transit time diode.

I. INTRODUCTION

The trapped plasma avalanche transit time (TRAPATT)diode was developed as a pulsed high power microwaveoscillator.1.2 Oscillators built using TRAPATT diodes must operate at high power levels to generate the trapped plasma.

The high currents involved tend to filament through the device causing "hot spots" and device failure. Since the device operates in the breakdown region a good portion of the oscillationperiod, fabrication while maintaining controlled,

hard breakdown is desirable. If the breakdown voltage and device capacitances are not constant from device to device the external circuit design can be difficult. Currently the TRAPATT diode is used very little because of these problems with reliability, fabrication, circuit design, and because

of the avalanche mechanism involved, phase noise. Although, the traditional use of the TRAPATI diode is in oscillator design, the work presented here uses the diode in the time domain to generate high speed and high voltage

pulses. Operation of the TRAPAIT diode is at low pulse repetition frequencies (PRF), on the order of 1 kHz, with risetimes less than 300 ps and amplitudes greater than 1 kYoThe pulses contain wide spectral content needed in many

areas of instrumentation such as high speed photography, gating of microchannel plate image intensifiers,3-s and ultrawide and radar.6 Other researchers have reported similar behaviour in devices'-lO which have been called avalanche diodes. This work wiIl describe, for the first time, selection of a commercial diode for TRAPAlT operation and the design of a driver circuit. These limitations, until now, have been the main reason low jitter pulse generators with kV level amplitudes and picosecond risetimes have not been widely available for use in physics research instruments.

II. OPERATION IN THE TIME DOMAIN

Figure 1 shows the basic scheme used to generate these picosecond-kilovolt signals. A pulse generator with an amplitudelarger than the breakdown voltage of the diode is applied to the diode in the reverse direction. When the pulse is applied to the circuit the diode will first break down, i.e., the diode wiIllook like a zener diode, and then if the amplitude of the driving signal is large enough, so that a large current flows in the circuit, the diode will go into second BREAKDOWN

Second breakdown can be thought of as a change in diode voltage from the primary breakdown voltage to some much lower value. Since KVL must be maintained in the circuit this is usually accompanied by an increase in the current flowing through the device. Destruction of the device is usually associated with second breakdown. However, if the amount of energy passed through the diode is limited, destructionis avoided. This usually means narrow pulses, <10 ns, must be used. The transition from primary breakdown to

second breakdown can occur in tens of picoseconds. During this time the trapped plasma is being formed within the diode. IT a p+ N-N+ diode is used as a TR.APATI diode then the formation of the electron-hole plasma begins at the

p+ N- junction and travels across the N- region to the N-N+ junction leaving the N- region filled with the trapped plasma, Fig. 2. The apparent velocity of the plasma should be much larger than the saturation velocity of electrons and

holes in the semiconductor for proper operation. The plasma is formed by exciting electrons below the valence band into states above the conduction band. The result is generation of a gaseous conductor (plasma) in picoseconds. Externally this appears as a switch closing in a time dependent on the

plasma formation.

III. REQUIREMENTS FOR GENERATION OF THE

TRAPPED PLASMA

Following the above discussion it might be concluded that the input pulse risetime is not critical to the operation of the circuit. This, however, is not the case. Second breakdown (plasma formation) is triggered by a current level. That is, when the device is in primary breakdown the current flowing

in the diode must reach a critical level, I crit' for second breakdown to occur. Also the velocity of the plasma, v z' is dependent on the current level present when the diode begins to go into second breakdown. This was derived by! Assuming the diode was driven with a constant current source as

Vz=  I/Q Nd A

where I is the current through the diode, q is electron charge,

N D is doping concentration of the N- region, and A is the

cross sectional area. The larger the current flowing in the

diode, the faster the plasma will sweep across the device and

thus the switch will close in less time. Since v z should be

greater than the saturation velocity of carriers in the semiconductor,

v s' the critical current density is given by setting

the saturation velocity equal to the plasma velocity, or

lcri'= vsqNoA. (2)

This equation is useful in understanding how the various diode parameters affect I cril' However, from a circuit design point of view an alternate description of the maximum input transition time of the drive signal is desirable. The current flowing in the diode prior to breakdown is a displacement current due to the diode junction capacitance.

HENCE THE STUDY ON IMPATT AND TRAPATT DIODE IS DONE.