======================COMPONENTS_DIODES====================== Schottky Rectifiers Schottky very low forward voltage drop and switching speeds that approach zero time switching diode requirements of less than 100 picoseconds. require small Schottky devices with low capacitance. reverse recovery time of Schottky diodes are extre fast (but soft) recovery characteristics. What little reverse recovery time is primarily dictated by their capacitance rather than minority carrier recombination as in conventional pn junction rectifiers. provides very little reverse current overshoot when switching forward mode to reverse blocking state. combination of very "fast-soft switching properties" can also eliminate the need for snubber circuits . maximum rated junction temperatures typically in range of 125ƒC to 175ƒC, compared to 200ƒC Schottky rectifiers seldom exceed 100 volts in their working peak reverse voltage (VRWM), Schottky rectifier properties described above primarily determined by metal energy barrier height of material deposited on silicon by manufacturer. RF/ Microwave Diodes At lower frequencies behave like ordinary diodes. Varactor Diodes In P-N junction as the junction is formed holes and electrons at junction diffuses to either sides leaving behind immobile charged ions. (capacitance is inversely proportional to distance between plates ),capacitance decreases. This capacitance is called transition capacitance. useful in tuning circuits, Automatic Frequency Control ( AFC ) circuits, etc. Abrupt Abrupt Voltage 10 20 ( Volts ) capacitance variation is roughly inversely proportional to the reverse voltage. The variation is represented by the following equation. C = K.A.( V + v )-n ........................(1) where: C = capacitance of the diode at voltage, V. K = Constant. A = Area of cross-section of the diode. v = Built in potential of the diode. n = Slope exponent. In the case of Hyperabrupt junction diodes, the capacitance variation is represented by the following equation. C = C0 .( 1 + V/v )- g ........................(2) where: C = capacitance of the diode at voltage, V. C0 = capacitance of the diode at voltage, V= 0. v = Built in potential of the diode. g = Slope exponent. PIN Diodes ( Positive Intrinsic Negative ) PIN diode is a semiconductor device that operates as a variable resist or at RF and Microwave frequencies. also be used as a switch and Limiter. variable resistor usable as Attenuator. P -type and N -type separated by intrinsic region. thickness of intrinsic region has a major role forward biased,holes and electrons are injected from P and N regions to Intrinsic region. The se charges do not immediately recombine, instead a finite quantity of charge always remain stored effectively lowering resistivity of I - region. stored charge depends on carrier life time ( t ), and forward current IF. Charge Stored Q = IF . t ......................... (1) resistance of intrinsic region under forward bias is given by Resistance RS = W2 / ( mN + mP ). Q ...............( 2 ) W = I - region width mN = electron mobility mP = hole mobility velocity of electrons under an applied electric field is directly proportional to electric field. If E is the electric field intensity and v is the electron velocity, then v µ E or, v = m E where m = Mobility of electrons ] Combining both equations we get : RS = W2 / ( mN + mP ). IF . t as forward current increases = resistance decreases. PIN diode resistance is controlled by forward current, diode can be used as switch at RF and Microwave freq If forward current is sufficient to reduce resistance to a very low value, it behaves like a short circuit. If reverse biased the diode is an open circuit and the signal is not allowed to pass through. 2. Harmonic generation When step voltage applied to diode, it generates series of current pulses, applied to a tuned circuit to get desired frequency. Depending on resonant frequency of tuned circuit, any harmonic component of input frequency can generated. 3. Comb generation Comb generator gives very sharpand narrow pulses for Spectrum analysers to produce locking signals. Step recovery signals can be used to generate comb signals by applying a step voltage to the diode. In this case tuned circuit is not required, as required signal should be rich with harmonics . ( 4 ). Mixer diodes Mixer is an important part of a communication receiver. If the RF signal coming from transmitter is fed to a demodulator and amplifiers, the received signal will not be an exact replica of the transmitted signal. As the circuit components are not linear operation at RF frequencies, this results in introduction of unwanted noise, unstable operation of amplifiers due to the feedback caused by capacitors (i.e, at RF frequencies,capacitive reactance is very less, causing in a short circuiting effect. ). Gunn Diodes works under principle of transferred electron effect invented by the scientist Gunn. compound semiconductors like GaAs, InP, etc. are compound semiconductors, a complex energy band structure. If voltage is applied across GaAs slice, electrons flow towards positive end. But applied field makes electrons able to get transferred to the upper energy band, as the electrons require very less energy to jump to this level ( because of the complex energy band structure ). Thus instead of moving faster under the applied field, these electrons slows down. because, as they acquire more energy, become less mobile. This is called transferred electron effect . So in effect, current is reduced even though there is an increase in voltage. results in negative resistance property. In a semiconductor slice, the doping of impurity will not be uniform throughout the slice. So there is a chance that number of electrons in an area ( possibly nearer to the negative end ), is less compared to the other areas of the slices. So this region becomes less conductive than the other regions. So the potential across this region will be greater than the average potential across the slice. As the applied potential is increased, this region will be the first to have a voltage large enough to introduce transfer of electrons to higher energy band. these electrons form a bunch in which the electrons in the front level moves faster while the ones behind buch up. So in effect, the bunch of electrons travel towards the positive end of the slice. As the bunch of electrons reach the positive end, it generates a current pulse there. This pulse is rich in harmonics and if it is applied to a tank circuit ( LC tuned circuit ), it results in oscillations at the resonant frequency of tank circuit. GUNN diodes can be used as oscillators. Drift Velocity Current Eth = Threshold Field Ev = Valley Field Stable Unstable region region Electric field Voltage Eth Ev figures given above shows the variation of drift velocity with electric field, and v ariation of current with voltage. If the applied field is less than the Threshold value ( Eth ), the specimen is stable. In this region, the Gunn diode can be used to amplify the input signal. If the diode is to be used as an oscillator, then the applied field should be more than the threshold value. At the initial formation of the domain, as explained earlier, the field behind domain decreases and the field in front increases. As the layer approaches the anode, the field behind it begins to increase again. When the high field domain disappears at the anode, a new dipole field starts at the cathode and start moving towards the anode. So a series of pulses are obtained at the anode. oscillation frequency is given by frequency ( f ) = vd / Leff Where: vd = Velocity of the domain,( approximately equal to the drift velocity of electrons ) Leff = Effective length that domain travels ( 7 ). IMPATT diodes ( IMPact Avalanche Transit Time diodes ) sufficient reverse voltage electrons acquire energy to make free. free electrons move faster the applied electric field and the process is continued. This goes on resulting in avalanche multiplication of electrons. After application of the field, there will be a time delay for generation of the multiplication. Similarly electron velocity will not be sufficient to cop up with variation of voltage (transit time effect ). two factors results in delay in generation of current. If it sufficiently high to make a 180 0 phase difference between the voltage and the current, results in negative resistance effect IMPATT diode works under this principle. includes impact ionisation of atoms, avalanche multiplication of electrons, and the transit time effect. The structure of the diode is shown below. Avalanche region Drift region Anode Cathode P+ n+ n+ R F and Microwave Diodes - An Introduction Anoop N. K Page: 8 Operation: A d.c biasing voltage is applied just enough to start avalanche multiplication. When an AC voltage is applied over the DC, during the negative half cycle, the diode gets more reverse biased, and avalanche multiplication starts. For the entire half cycle, it develops. When the AC voltage decreases to zero, ( voltage across the diode equals DC reverse voltage ) avalanche current becomes maximum ( at the junction ). Because of the positive voltage at the cathode, electrons are attracted towards cathode. During this time applied AC voltage decreases to zero. While constructing the diode, the length and area of the semiconductor is selected such that, electrons reaches the cathode when the applied voltage is reaching the negative peak. This is indicated in the figure. + Vm - Vm t Current Current pulse reached the cathode when v = - Vm Current pulse at the t junction is Maximum at v = 0 Applications The negative resistance property can be made use of in the application as an oscillator, similar to a Gunn diode. stable region of operation can amplify RF signals. Compared to Gunn ,IMPATT diodes give more output power. But the noise immunity of IMPATT diodes is very less than Gunn diodes. it requires higher supply voltage for operation. IMPATT diodes are more efficient and powerful than Gunn diodes. ( 8 ).Parametric Amplifier Parametric amplifier diodes are similar to varactor diodes. Here, parametric variation ( reactance variation with the voltage ) is made use of to amplify RF signals. rectangular wave pumping signal is applied to the diode to vary the reactance. This signal frequency is selected such that it is twice the frequency of the signal to be amplified. Consider an LC circuit oscillating at resonant frequency. If the capacitance plates are pulled apart at the instant when the voltage reaches the positive peak, the capacitance decreases. The charge stored remains the same and voltage across the capacitor increases ( to satisfy the relation Charge = Voltage x Capacitance ). Similar to this case, apply a reverse biasing voltage across a varactor diode( which is used as a part of an LC circuit ), when the reverse bias is maximum at positive peak, voltage across the diode gets increased. input pumping signal amplified output RF and Microwave Diodes - Tunnel Diode Before discussing of tunnel diode, let us see the energy band structure of semiconductors. Electrons in semiconductor bar may different energies. energy band diagram,electrons lie in different positions. probability finding an electron in particular energy band can be calculated using Fermi - Dirac probability function . Probability of finding an electron in energy band E , f (E ) = 1/ ( 1+ e (E - E F )/ kT ) where: E F = Fermi energy, in eV k = Boltzmann s constant= 8.62 X 10 -5 eV/ 0K T = Temperature , in 0K In the above equation, if E = EF is substituted, we get f( E ) = 1/ 2 = 50 %. So Fermi level can be defined as the energy level at which there is 50 % probability of finding an electron. semiconductor having impurities, energy band diagram will be as shown in figure. . P N conduction band EF valence band space charge region In the case of tunnel diode, the doping is very high that the tunneling occurs. If reverse bias is applied, the minority carriers of P - region ( electrons ) gains energy and move to the upper energy level. So the percentage of finding electrons in the higher energy level increases. In turn the Fermi level rises. So the electrons finds the conduction band of N-region to be in the same level as them and they tunnel through the depletion region. Thus the reverse current increases unlike other P-N junctions. In case of forward biasing, holes manages to tunnel across the junction ( electrons tunnel to P region ). As the higher energy band electrons tunnel to the other side, the Fermi level goes down. So current reaches maximum value and then starts decreasing. results in formation of the negative resistance region in characteristics curve. As tunneling of electrons are over, diode starts behaving similar to an ordinary junction diode and current increases. negative resistance property of tunnel diodes can be use of in oscillators, amplifiers , etc. as explained in case of Gunn and IMPATT diodes. ***********************