Class Notes by. K.Elampari, Associate Professor of Physics, S.T.Hindu college, Nagercoil 1 CHAPTER V- Micro Wave Devices Microwaves are a form of electromagnetic radiation with ranging from 1m to 1mm (or) between 300 MHz (0.3 GHz) and 300 GHz. This includes both UHF and EHF (millimeter waves). Applications Microwaves are extensively used for point-to-point telecommunications. Microwaves are especially suitable for this use since they are more easily focused into narrower beams than radio waves. Microwaves are used in long distance communications like TV and telephone communications. They are extensively used in very long distances communications such as spacecraft communication and between ground stations and communications satellites. Their higher frequencies allow broad bandwidth , high data transmission rates, and smaller antenna sizes (since antenna size is inversely proportional to frequency). Microwaves are also employed in microwave ovens and in radar technology. Solid State Microwave Devices As the operating frequency increases, the stray reactance, device capacitance and transit time as well as cable losses are increasing. For example at microwave frequencies, a thin film resistor looks like a complex RLC network with distributed LC values and a different R value. Ordinary electronic components behave differently at microwave frequencies and hence special devices were developed to use in microwave region. Class Notes by. K.Elampari, Associate Professor of Physics, S.T.Hindu college, Nagercoil 2 MW Solid state devices are classified into different categories as MW Bipolar Transistors - MW Heterojunction Transistor (HBT) MW FET such as High Mobility Transistors (HEMT) Microwave Special Diodes MW Diodes The MW special diodes are classified into different groups based on their operating principle Quantum Mechanical Tunnel effect - Tunnel Diode Transfer Electron Device(TED) – Characterised by bulk effect of semiconductor -Gunn Diode Avalanche and Transit time effects – IMPATT and TRAPAT Parametric Excitation effect – Parametric Amplifiers Class Notes by. K.Elampari, Associate Professor of Physics, S.T.Hindu college, Nagercoil 3 Tunnel Diode (Or ESAKI diode) Invented by Leo Esaki in 1958. A tunnel diode is associated with quantum mechanical tunnelling phenomena. Quantum tunnelling is the quantum-mechanical effect of transitioning through a classically-forbidden energy state. (ie) an electron or charge carrier pass through a potential barrier without having sufficient energy to do so. A tunnel diode is a PN junction that exhibits negative resistance between two values of forward voltage. Both P and N sides are degenerately doped (very heavily doped) with impurities several 1000 times that of a typical PN junction diode (1019 to 1020 impurity atoms /cm3). Because of heavy doping the depletion region is very thin and the tunnel distance ‘d’ is very small (5 to 10 nm). Also the heavy doping causes, the Fermilevel within the valence band on P side and within the Conduction band on N side. The amount of degeneracy qVp and qVn is typically 50mev to 200 mev. When there is no voltage (V=0) is applied to the tunnel diode, the diode is in the thermal equilibrium. Under this unbiased condition, the probability of electrons going from states in the conduction band on N-side to states in the valance band on the P-side and in opposite directions are the same. Therefore the net tunneling of the thin barrier is then zero. When a forward bias is VF applied, the energy levels on N side are raised relative to those on P-side. Now, there exists a band of occupied energy states on the N-side (electrons in the conduction band of N-side) and corresponding unoccupied states on the P-side (empty states in the valence band of P-side). The electrons can tunnel from the N-side to the P- side. Class Notes by. K.Elampari, Associate Professor of Physics, S.T.Hindu college, Nagercoil 4 FIG: Tunneling at various Biasing levels TTunneling stops Class Notes by. K.Elampari, Associate Professor of Physics, S.T.Hindu college, Nagercoil 5 This tunneling current will reach a maximum value IP (peak current) at a forward voltage bias voltage VF = VP (Peak Voltage) which is approximately equal to (Vp +Vn) / 3). When the forward bias is further increased (VP < VF < VV valley voltage), the energy levels on N side are further raised and there are fewer available unoccupied states on the P-side and tunneling decreased. Further increase in forward voltage (V = VV) makes the band uncrossed, and at this point tunneling current no longer flow or the current is minimal (IV valley current). With still further increase in forward voltage (VF > VV) the normal thermal current will flow. From the operation of tunnel diode, in the forward bias condition the tunnel current increases from zero to a maximum value IP as the forward voltage increases. With the further increase in voltage the current decreases to IV. The decreasing portion after the peak current is negative differential resistance region. The tunnel diode at various bias conditions are shown in Figures 1 through 5 to the corresponding points in the characteristic curve. The values of IP and IV determine the magnitude of the negative resistance and the ratio IP/IV is known the figure of merit of the tunnel diode. Class Notes by. K.Elampari, Associate Professor of Physics, S.T.Hindu college, Nagercoil 6 Tunnel diode Characteristics Symbol and Equivalent circuit of Tunnel diode Ls- Lead inductance (nH) , Rj - Negative Resistance (100 ohm) , Rs – Bulk Resistance (few ohm) , Cj – Junction capacitance (pf) The empirical form of Tunnel diode current is given by The first term of the above equation is the Tunnel current and the second term is the normal thermal current. The negative differential Class Notes by. K.Elampari, Associate Professor of Physics, S.T.Hindu college, Nagercoil 7 resistance can be obtained from the first term of the equation and is given by The values of VP and VV depend upon the diode materials. The following figure shows typical V-I characteristics of some important materials at room temperature. The current ratios of IP/IV are 8:1 for Ge and 12.1 for GaSb and GaAs. Because of its smaller effective mass (0.042M0), the smaller bandgap (0.72eV), the GaSb tunnel diode has the largest negative resistance of the three devices. Applications 1. The negative resistance gives negative power (= -I2R) and is the source of power. (ie) instead of absorbing the signal it boost the signal. This property found applications in Microwave amplifiers and oscillators. 2. Since the tunneling phenomenon is very fast, tunnel diodes are used in high speed switching circuits (The time of switching of a tunnel diode is only few nano seconds) Class Notes by. K.Elampari, Associate Professor of Physics, S.T.Hindu college, Nagercoil 8 Advantages Insensitive to temperature - hence tunnel diodes are used in place of normal diodes in military applications . Very low junction Capacitance Extreme speed in switching and stable characteristics Ability to operate under wide variety of critical environments Low noise level and small size Drawback Tunnel diodes cannot replace rectifier diodes, because tunnel diode is very leaky in reverse bias. Tunnel diode based oscillator Tank circuits oscillate but “die out” due to the internal resistance (Positive Resistance). If a tunnel diode is properly biased to operate in the negative resistance region the “negative resistance” provided by the tunnel diode can overcomes the loses due to the positive resistance and maintains the oscillations Tunnel diode oscillator Class Notes by. K.Elampari, Associate Professor of Physics, S.T.Hindu college, Nagercoil 9 Transferred Electron Devices (TED) - Gunn Diode Ridely and Watkins (1961) theoretically discussed the possibility of negative resistance effect in semiconductors. In 1962 Hilsum discussed the possibility of obtaining a voltage controlled bulk negative conductance in GaAs called as Transferred Electron Mechanism. In 1963 Gunn observed micro wave oscillations in GaAs sample. Gunn Effect Gunn observed that, above some critical voltage (corresponding to electric field of 2K-4K V/cm) the current passing through n-type GaAs material becomes a periodic fluctuating function of time (oscillation) . He also observed that, the frequency of oscillation is determined mainly by the material not by the external circuit. The period of oscillation is inversely proportional to the specimen length and is equal to the transit time of electrons between the electrodes. The Gunn Effect was successfully explained by the Two-Valley Model theory. Figure Ososcillating current after V>Vth I Vth V Class Notes by. K.Elampari, Associate Professor of Physics, S.T.Hindu college, Nagercoil 10 Gunn Diode Gunn Diode is also known as Transfer Electron Device. Even though it is called as a diode it does not contain PN junction. It is a Bulk device, and because of its two electrodes it is called as a diode. Thus the operation of a Gunn diode is based upon the bulk properties of the material and not on the properties of PN junction. Construction and Working Symbol and Equivalent Circuit Gunn diodes are fabricated from a single piece of n-type semiconductor. The most common materials are Gallium Arsenide, (GaAs) and Indium Phosphide (InP). Class Notes by. K.Elampari, Associate Professor of Physics, S.T.Hindu college, Nagercoil 11 The device is simply an n-type bar with n+ contacts. It is necessary to use n-type material because the transferred electron effect is only applicable to electrons and not to holes. The most common method of manufacturing a Gunn diode is to grow and epitaxial layer on a degenerate n+ substrate. The active region is very thin and its thickness is between a few microns and a few hundred microns. This active layer has a low doping level between 1014 /cm3 and 1016 /cm3 - this is considerably less than that used for the top and bottom areas of the device.