The normal operation, a varactor diode is always reverse biased. The capacitance of varactor diode is found as :

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1 Varactor Diode A junction diode which acts as a variable capacitor under changing reverse bias is known as a varactor diode. When a pn junction is formed, depletion layer is created in the junction area. Since there are no charge carriers within the depletion zone, the zone acts as an insulator. The p-type material with holes (considered positive) as majority carriers and n-type material with electrons ( ve charge) as majority carriers act as charged plates. Thus the diode may be considered as a capacitor with n-region and p-region forming oppositely charged plates and with depletion zone between them acting as a dielectric. This is illustrated in Fig. (i). A varactor diode is specially constructed to have high capacitance under reverse bias. Fig. (ii) shows the symbol of varactor diode. The values of capacitance of varactor diodes are in the picofarad (10 12 F) range. The normal operation, a varactor diode is always reverse biased. The capacitance of varactor diode is found as : C T = Total capacitance of the junction 11

2 ε = Permittivity of the semiconductor material A = Cross-sectional area of the junction W d = Width of the depletion layer When reverse voltage across a varactor diode is increased, the width W d of the depletion layer increases. Therefore, the total junction capacitance C T of the junction decreases. On the other hand, if the reverse voltage across the diode is lowered, the width W d of the depletion layer decreases. Consequently, the total junction capacitance C T increases. A forward biased varactor diode would serve no useful purpose Fig. below shows the curve between reverse bias voltage V R across varactor diode and total junction capacitance C T. Note that C T can be changed simply by changing the voltage V R. For this reason, a varactor diode is sometimes called voltagecontrolled capacitor. tunnel diode A tunnel diode or Esaki diode is a type of semiconductor diode which is capable of very fast operation, well into the microwave frequency region, by using quantum mechanical effects. 22

3 Tunnel Diodes (Esaki Diode) Tunnel diode is the p-n junction device that exhibits negative resistance. That means when the voltage is increased the current through it decreases. Regular p-n Diode Tunnel Diode Tunnel Diode principles Concept of Electron Tunneling 33

4 For thick barrier, both Newtonian and Quantum mechanics say that the electrons cannot cross the barrier. It can only pass the barrier if it has more energy than the barrier height. For thin barrier, Newtonian mechanics still says that the electrons cannot cross the barrier. However, Quantum mechanics says that the electron wave nature will allow it to tunnel through the barrier. Newtonian Mechanics Quantum Mechanics 44

5 Tunnel Diode Operation Under Forward Bias Step 1: At zero bias there is no current flow Step 2: A small forward bias is applied. Potential barrier is still very high no noticeable injection and forward current through the junction. Step 3: With a larger voltage the energy of the majority of electrons in the n-region is equal to that of the empty states (holes) in the valence band of p-region; this will produce maximum tunneling current 55

6 Step 4: As the forward bias continues to increase, the number of electrons in the n side that are directly opposite to the empty states in the valence band (in terms of their energy) decrease. Therefore decrease in the tunneling current will start. Step 5: As more forward voltage is applied, the tunneling current drops to zero. But the regular diode forward current due to electron hole injection increases due to lower potential barrier. 66

7 Step 6: With further voltage increase, the tunnel diode I-V characteristic is similar to that of a regular p-n diode. Under Reverse Bias In this case the, electrons in the valence band of the p side tunnel directly towards the empty states present in the conduction band of the n side creating large tunneling current which increases with the application of reverse voltage. The TD reverse I-V is similar to the Zener diode with nearly zero breakdown voltage. 77

8 Tunnel Diode I-V The total current I in a tunnel diode is given by ( ) ( ) I excess : is an additional tunneling current related to parasitic tunneling via impurities. This current usually determines the minimum (valley) current, I v. R v and V ex are the empirical parameters; in high-quality diodes, R v >> R 0. V ex = 1..5 V 88

9 Typically, m = 1.3; V 0 = V R 0 is the TD resistance in the ohmic region The maximum negative differential resistance NDR can be found as The peak voltage V p Photovoltaic cell The reverse biased pn junction photodiode operates in the photoconductive mode. If the illuminated diode is used without external bias, a measurable forward voltage appears between the p and n regions. This is called photovoltaic effect. The solar cells convert radiation from the sun directly into electrical energy. In practice, the open-circuit voltage of the silicon photovoltaic cells is about V. Their efficiency is about 15 %. 99

10 Definition of laser A laser is a device that generates light by a process called stimulated emission. The acronym LASER stands for Light Amplification by Stimulated Emission of Radiation Semiconducting lasers are multilayer semiconductor devices that generates a coherent beam of monochromatic light by laser action. A coherent beam resulted which all of the photons are in phase. Laser Operating Principles Two things are required to operate a laser: (1) gain medium that can amplify the electromagnetic radiation propagating inside it and provide the spontaneous emission noise input and (2) a feedback mechanism that can confine the electromagnetic field through the well-defined optical modes. the gain medium for a semiconductor laser consists of a semiconductor material. A simple, practical and most commonly used method employs current injection through the use of a forward-biased p-n junction. Such semiconductor lasers are referred to as injection lasers or laser diodes. The excited atoms eventually return to their normal ground state and emit light in the process. Light emission can occur through two fundamental processes known as spontaneous emission and stimulated emission. 1. Spontaneous emission represents the case of an electron in the conduction band recombining spontaneously with a hole (missing electron) in the valence band to generate a photon. 2. Absorption stimulates the generation of an electron in the conduction band while leaving a hole in the valence band. 110

11 3. Stimulated emission is stimulating the recombination of an electron a hole and simultaneously generating a new photon. This is the all-important positive gain mechanism that is necessary for lasers to operate. Producing intense laser beam or amplification of light through stimulated emission requires higher rate of stimulated emission than spontaneous emission and self-absorption, which is only possible f or N 2 > N 1 even though E 2 >E 1 (opposite to the Boltzmann statistics). It means that one will have to create the condition of population inversion In a semiconductor laser, population inversion is accomplished by injecting electrons into the material to fill the lower energy states of the conduction band. Background Physics Consider the stimulated emission as shown previously. Stimulated emission is the basis of the laser action. The two photons that have been produced can then generate more photons, and the 4 generated can generate 16 etc etc which could result in a cascade of intense monochromatic radiation. Population Inversion Therefore we must have a mechanism where N 2 > N 1 This is called population inversion Population inversion can be created by introducing a so call metastable centre where electrons can piled up to achieve a situation where more N 2 than N 1 The process of attaining a population inversion is called pumping and the objective is to obtain a non-thermal equilibrium. It is not possible to achieve population inversion with a 2-state system. If the radiation flux is made very large the probability of stimulated emission and absorption can be made far exceed the rate of spontaneous emission. But in 2-state system, the best we can get is N1 = N2. To create population inversion, a 3-state system is required. The system is pumped with radiation of energy E 31 then atoms in state 3 relax to state 2 non radiatively. The electrons from E 2 will now jump to E 1 to give out radiation

12 Laser diode Forward electrical bias across the laser diode causes holes and electrons to be "injected" from opposite sides of the p-n junction into the depletion region. When an electron and a hole are present in the same region, they may recombine or "annihilate" with the result being spontaneous emission The difference between the photon-emitting semiconductor laser and conventional phonon-emitting (non-light-emitting) semiconductor junction diodes lies in the use of a different type of semiconductor, one whose physical and atomic structure confers the possibility for photon emission. These are the "direct bandgap" semiconductors. Laser Diode Principle Consider a p-n junction In order to design a laser diode, the p-n junction must be heavily doped. In other word, the p and n materials must be degenerately doped By degenerated doping, the Fermi level of the n-side will lies in the conduction band whereas the Fermi level in the p-region will lie in the valance band. Diode Laser Operation 12 12

13 P-n junction must be degenerately doped. Fermi level in valance band (p) and conduction band (n). No bias, built n potential; evo barrier to stop electron and holes movement Forward bias, ev> Eg Built in potential diminished to zero Electrons and holes can diffuse to the space charge layer Population Inversion in Diode Laser E Fn -E fp = ev ev > Eg ev = forward bias voltage Fwd Diode current pumping injection pumping There is therefore a population inversion between energies near EC and near EV around the junction. This only achieved when degenerately doped p-n junction is forward bias with energy > E gap 13 13

14 The Lasing Action The population inversion region is a layer along the junction also call inversion layer or active region Now consider a photon with E = Eg Obviously this photon cannot excite electrons from EV since there is NO electrons there However the photon can stimulate electron to fall down from CB to VB. Therefore, the incoming photon stimulates emission than absorption The active region is then said to have optical gain since the incoming photon has the ability to cause emission rather than being absorbed. Pumping Mechanism in Laser Diode It is obvious that the population inversion between energies near EC and those near EV occurs by injection of large charge carrier across the junction by forward biasing the junction. Therefore the pumping mechanism is forward diode current Injection pumping Laser Diode Characteristics Nanosecond & even picosecond response time (GHz BW) Spectral width of the order of nm or less High output power (tens of mw) Narrow beam (good coupling to single mode fibers) Laser diodes have three distinct radiation modes namely, longitudinal, lateral and transverse modes. In laser diodes, end mirrors provide strong optical feedback in longitudinal direction, so by roughening the edges and cleaving the facets, the radiation can be achieved in longitudinal direction rather than lateral direction