Microwave tunnel diode Some anomalous phenomena were observed in diode which do not follows the classical diode equation. This anomalous phenomena was explained by quantum tunnelling theory. The tunnelling time of carrier through the potential energy barrier is not governed by the classic transit time concept (i.e., the transit time equal to barrier width divided by the carrier velocity) rather by the quantum transition probability per unit time. Application-microwave amplification, oscillation. Low cost, high weight, high speed, low power operation, low noise Principle of operation- Doping of p and n junction in very high i.e. 10 19 to 10 20 atoms/cm 3 Depletion layer is very thin, of the order of Angstrom. Classically only those charge particles can pass through the depletion layer whose energy is equal to greater than the potential barrier. Quantum mechanically, if the barrier is of the order of Angstrom, then charge carrier not having enough kinetic energy can pass over the same barrier. In addition to barrier thickness, there must also be filled energy state on the side from which particle will tunnel and allowed empty state on the other side into which particle penetrate through the same energy level. The voltage at which we obtain minimum tunnelling current (valley current) is known as valley voltage. Under open circuit condition or at zero biased equilibrium, the Fermi level of p-type and n- type are at same energy level. At this equilibrium, there is no filled energy state at one side of the junction i.e. at same energy state as empty allowed state on the other side. Therefore, there is no flow of charge across the junction and current is zero. In ordinary diode the Fermi level exist in the forbidden band. Tunnel diode is heavily doped, the Fermi level exist in the valance band in p type and conduction band in n type. As the forward bias is applied and voltage is just above the zero, the potential barrier is decreased by the amount of applied forward bias voltage. Due to this a difference in Fermi level in both sides is created. Now, there are filled states in the conduction band of n type at the same energy level as allowed empty states in the valence band of p type, the electron tunnel through the barrier from the n type to the p type, giving rise to forward tunnelling current from the p type to the n type. As the forward bias is increased to V p, a maximum numbers of electrons can tunnel through the barrier from the filled state in the n type to empty state in p type giving rise to the peak current I p. If the bias voltage is further increased, the tunnelling current decreases and finally becomes zero as there are no allowed empty state in the p type at the same energy level as filled state in the n type therefore, no electrons can tunnel through the barrier. When the forward bias voltage V is increased above the valley voltage V v the ordinary injected carrier I at the p-n junction start to flow. The maximum value of peak current to valley current ratio (i p/ i v ) can be 50 to 100; however the typical value is 15 for GaAs. Page 1 of 5
MESFET (Metal semiconductor field effect transistor) MESFET uses metal gate: gate source junction is schottky contact, also called as schottky barrier FET Semiconductor may be Si or GaAs: preference is GaAs, due to higher drift velocity and electron mobility MESFET are also known as GaAs FET GaAs allows to use MEFET at higher temperature and high power compare to Si based MOSFET N-type GaAs material is used due to high mobility of electron than hole if we use p type GaAs. Therefore n-type GaAs MESFET will be low noise and high speed device. MESFET is used as low noise and power amplifier. For high frequency operation, gate length should be very small. For 1 micrometer gate length GaAs FET can operate upto 32 GHz. GaAs MESFET is a very common device in MIC (microwave integrated circuit) and MMIC (Monolithic MIC) Technology as low noise amplifier. Noise figure of 2dB and below at 10 GHz. Principle of operation: SUBSTRATE: Semi insulting GaAs (100 micrometer thickness) Doped with chromium (cr) Resistivity 10 7 to10 8 ohm/cm N-type GaAs: doped with sulphur or tin 0.15 to 0.30 micrometer Schottky gate: Aluminium (Al) Maximum frequency of oscillation: vs=saturation drift velocity=2*10 7 cm/s If l=1µm Page 2 of 5
TRAPATT (Trapped Plasma Avalanch Triggerd Transit): n + p p + (or p + n n + ) structure. In TRAPATT, during part of a cycle of oscillation, plasma is created in diode depletion layer and is trapped by the low electric field. At point A, the E-field is uniform throughout the sample and its magnitude is large but less than the value required for avalanche breakdown. At the instant of time A, the diode current is turned on. Since the only charge carrier present are those caused by the thermal generation, the diode initially charges up like a linear capacitor driving the magnitude of the E-field above the value required for breakdown voltage. When a sufficient number of carriers are generated, the particle current exceeds the external current and E-field is depressed throughout the depletion region, causing the voltage to decrease. This portion of cycle shown from point B to C During this time interval the electric field is sufficiently large for the avalanche to continue and dense plasma of electron and hole is created. As some of the electrons and holes drift out of the ends of the depletion layer the field is further depressed and traps the remaining plasma. The voltage decreases to point D. A long time is required to remove the plasma because the total plasma charge is large compared to the charge per unit time in external circuit. At point E plasma is removed, but residual charges of electron remain in one side of depletion layer and a residual charge of hole in the other side. As the residual charge is removed, the voltage increases from point B to F. At point F, all the charges that were generated internally have been removed. This charge must be greater than or equal to that supplied by the external current otherwise the voltage will exceed that at point A. This places a limit on the allowable magnitude of the diode current to prevent a second avalanche within the same period. From point F to Point G, the diode charges up again like a fixed capacitor. A point G the diode current goes to zero for half a period, and voltage remain constant V A until the current comes back and cycle repeats. Page 3 of 5
Avalanche Transit Time Devices It is a semiconductor junction with highly doped p and n regions. Applied E-field is high enough to create avalanche of carrier through Impact ionization. Generated carrier passes through a drift space and collected by the anode. If the transit time through the drift space is such that it causes a delay due to which the current is out of phase with the applied voltage then a negative resistance appears across the terminal of the device. Three type of devices: 1) IMPATT: Impact Ionization Avalanche Transit Time Efficiency:- 3% CW :- 60% pulsed power frq n :- 500 MHz to 100 GHz Power output:- CW power 1-3 W at X-band 10-20 W pulsed power at 94 GHz 2) TRAPATT: Trapped Plasma Avalanche Triggered Transmit Efficiency:-20-60% frq n :- 1-3 GHz power output:- several hundred watt 3) BARITT: Barrier Injected Transit Time Low noise figure (<< 15 db) Low power and smaller bandwidth Read Diode: W.T. Read proposed a reversed biased p + nin + diode structure. o p + and n + are highly doped region o p is moderately doped o i (intrinsic) slightly n-type Principal of operation: p + nin + diode is reverse biased. Diode can be divided in the zone (a) Avalanche zone (b) Drift zone. Diode is negatively biased with voltage V DC and to form an oscillator a resonant cavity is added to the circuit. The dimension of cavity decides the resonant frequency. Therefore, the total voltage applied across the device is D.C. bias voltage superimposed by an AC voltage V AC (due to RF signal in the cavity) In positive half cycle (V DC + V AC ) the diode will be in breakdown condition whereas in negative half cycle (V DC - V AC ) it will not be in breakdown condition. In positive half cycle the avalanche ionization process will take place in p + n junction due to very high electric field (order of KV/cm). The avalanche phenomenon is exponential in nature therefore the avalanche current I AV reached its maximum when the RF voltage at ωt=π. Hence, it may be observed that avalanche current I AV lags in phase with respect to RF voltage by 90. During the negative half cycle the avalanche process decays. The generated avalanche current now injected into drift region of the diode. In this way, the avalanche region acts as an injector of carrier producing a pulse of current at each RF cycle, which lags the applied RF voltage By 90. Page 4 of 5
The holes are immediately swept into the n + region and do not contribute any further phase lag to the current. Electrons drifting across the intrinsic region (also known as drift region) with saturated drift velocity V d will cause an additional phase lag of the current (I ext ) in the external circuit depending on the length L of the drift space. If L is chosen in such a way that it provides an additional phase lag of 90, then the total phase lag of current in the external circuit will be 180 with respect to RF voltage. Thus IMPATT diode exhibits a negative resistance. The effective transit Time of the carrier must be equal to half the time period of the oscillator voltage t d = T/2 = 1/2f T=time period L/v d = 1/2f f = v d /2L where v d is drift velocity and L is length of intrinsic region Determine the width of the intrinsic region (i-region) of an Si Read diode for an operation at 10 GHz. f = v d /2L L = v d /2f v d =10 7 cm/s Therefore, L = 5 μm V DC V AC p + n i n + (a) Figure (a) Reversed biased IMPATT (Read) diode with AC voltage across it (b) avalanche and drift zone of Read diode (c) RF voltage giving AC signal across the diode (d) Avalanche current and induced current in the external circuit Page 5 of 5