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Learn about the theory, construction, characteristics, and applications of varactor diodes in the field of electronics and telecommunications engineering.
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ShriSantGajananMaharaj College of EngineeringDepartment of Electronics & Telecommunications Engineering Electronic Devices & Components Unit – V Special Purpose Diodes Varactor Diode and Schottky Diode
Topics under Unit-V Theory, Construction, Characteristics and Applications of Tunnel, Varactor, Schottky Diode Opto-Devices: LED, Photodiode, LCD
SMD Varactor Diode In a varactor diode, if reverse voltage is increased, depletion region width increases and capacitance decreases
Varactor Diode Depletion Region Width Depletion region width itself depends upon applied reverse voltage VR
Varactor Diode Characteristics In a varactor diode, if reverse voltage is increased, depletion region width increases and capacitance decreases
Application of Varactor diode These are used in radio receivers to vary frequency and tune to different radio stations
Application of Varactor diode These are used in radio receivers to vary frequency and tune to different radio stations
Application of Varactor diode In FM Radio Transmitter, FM signal is generated with help of varactor diode
Application of Varactor diode Voltage Controlled Oscillator
Varactor Diode Working Principle - A reversed-biased diode has two conducting regions separated by an insulating depletion region • this structure resembles a capacitor • variations in the reverse-bias voltage change the width of the depletion layer and hence the capacitance • this produces a voltage-dependent capacitor • these are used in applications such as automatic tuning circuits
The Name “Varactor” means: variable reactor (or reactance), also called “Varicap” meaning variable capacitance. Both names: varactor and varicap are the same form of semiconductor or a P-N Junction Varactor or Varicap takes advantage of the fact that the capacitance of the diode PN junction varies with the applied reverse bias voltage. This differs from other diodes, such as rectifying diodes and switching diodes, which use the rectifying effect of the PN junction, or current regulation diodes, which take advantage of zener breakdown or avalanche breakdown. A Varactor provides an electrically controllable capacitance, which can be used in tuned circuits. It is small and inexpensive, which makes its use advantageous in many applications. Its disadvantages compared to a manual mechanical variable capacitor are a lower Q, nonlinearity, lower voltage rating and a more limited range. Varactor Diode Working Principle
Varactor Diode • Operate in reverse bias • Dielectric is acting as capacitor • C capacitance, A=plate area, d=thickness
Varactor Diode Capacitance To understand how a varactor or varicap diode works, we need to know “what a capacitor is” and what can change its capacitance. A parallel plate capacitor consists of two plates with an insulating dielectric medium between them. Plates Each of area “A” + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Dielectric medium (Air) The amount of charge that can be stored depends on the area (A) of the plates and the distance (d) between them. d Capacitance depends on area “A”, Dielectric constant of medium & distance “d” between Plates. Sio2 P+ N Metal p - - + + + + - - - + + + + - - - - + + + + - - - + + + + - - - - + + + + Wp Wn W In P-N junction, depletion layer on heavily doped P-side is very small Compared to that on lowly doped N-side (# Charges on both side is equal). Silicon is dielectric medium here. Junction Capacitance is inversely proportional to “W = Wn+Wp , and charges in depletion.
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - Varactor Diode Space Charge - - - - + + - - - - + + - - - - + + - - - - + + - - - - + + - - - - + + P N qND x -qNA Distribution of NA and ND in junction (impurity profile) determines Capacitance variation
Varactor Diode Characteristics V-I Characteristics similar to ordinary diode
Varactor Diode Capacitance Reverse biased PN junction is a capacitor.
Varactor Diode Capacitance Internal structure of a varicap In the figure it can be seen cross-section of a varactor with the depletion layer formed at the p-n jn.
Varactor Diode Characteristics A P-N junction has a junction capacitance that is a function of the voltage across the junction. An electric field in the depletion layer is set up by the ionized donors and acceptors. A higher reverse bias widens the depletion layer, uncovering more fixed charge and raising the junction potential. The capacitance decreases as the reverse bias increases, according to the relation C = Co/(1 + VR/Vo)n Where Cois junction capacitance at reference reverse voltage Vo is the forward voltage of the diode. The exponent ‘n’ depends on how the doping density of the semiconductors depend on distance away from the junction.
An Example of Tuning with Varactor Diode • It is connected across an inductor to from an LC resonant circuit • Tuning Ratio (TR) • Max to min capacitance ratio • Example: 1N5148 • Applications • Tuning circuits in TV to set the resonant frequency • Bandpass filter (R+(L||C))
An Example of Tuning with Varactor Diode VRmin = 2.9 VRmax = 29 • Hence C2.9 = 55 pF C29 = 17 pF fmin = 0.679 MHz fmax = 1.22 MHz
Varactor Diode Doping Profile Zero bias (i.e under no applied voltage bias) the depletion width of a P-N junction depends on carrier doping in the n- and p- regions at the junction. If doping is heavy, the depletion width will be small to give high zero bias capacitance. For low doping, depletion with is large and the junction will exhibits low capacitance value at zero applied bias. Commonly employed profiles are: 1. Linearly graded junction: in which carrier concentration varies linearly with distance away from the junction. It may be on both side or only one side. Generally one side junction are used. 2. Hyper abrupt p-n junction employs very heavy doping on one side of the junction.In this case depletion region width on heavy doping side is negligible compared to lowly doped side.
Varactor Diode Doping Profile For abrupt doping profile n=0.5. For linear graded doping, n=0.33
P-N Junction Doping Geometry and Varactor Diode • Voltage-controlled capacitance of a pn junction can be used in tuning stage of a radio or TV receiver. • CJ (VR)–n , where n = 1/2 for an abrupt pn junction. However, n can be made higher than 1/2 by suitably changing the doping profile. NA or ND Linearly graded abrupt Hyper abrupt x
Varactor Diode Doping Profile For a graded junction (linear variation), n = 0.33. For an abrupt junction (constant doping density), n = 0.5. If the density jumps abruptly at the junction, then decreases (called hyperabrupt), n can be made as high as n = 2. The doping on one side of the junction is heavy, and the depletion layer is predominately extends on the other side only.
Step or Abrupt junction Impurity distribution in this type of p-n junction is shown in the Figure. This structure is formed by diffusing p- type impurity in n- type epitaxial silicon wafer. So concentration of p- type impurity varies with distance from the surface while in n-type region the doping is constant and is that of epi- layer. Shape or slope of p- profile depends on time and temperature of p- diffusion (i.e determined by technology)
Hyper Abrupt p-n junction: In aHyper abrupt junction first n- type impurity is introduced into n- epilayer followed by p-type diffusion. Thus in this case carrier concentration varies with distance on both side of the junction. This structure is employed to fabricate high capacitance ratio Varactors for tuning applications
Applications of Varactor Diode Frequency Multipliers - used in applications where it’s difficult to generate microwave signals. Producing relatively high power outputs at frequencies up to 100GHz.
Applications of Varactor Diode Frequency Tripler fo = 3fi
Applications of Varactor Diode Parametric Amplifiers. - named for the time-varying parameter, or value of capacitance, associated with the operation. Since the underlying principle of operation is based on reactance, the parametric amplifier is sometimes called a REACTANCE AMPLIFIER.
Applications of Varactor Diode • Tuning- Since the frequency can be made to vary they are used as electronic tuning devices in tuners for television, mobiles. • Other Applications: They are used in PLL, voltage controlled oscillators, harmonic generation, electronic tuning devices in tuners for television, mobiles, parametric amplification, AM radios, voltage-variable tuning, frequency multipliers, etc.
Varactor Diode as VCO 1 GHz Voltage Controlled Oscillator
Voltage Controlled Oscillator In an electronic circuit, a capacitor is replaced with the varactor diode, but it is necessary to also ensure that the tune voltage, i.e. the voltage used to set the capacitance of the diode can be inserted into the circuit, and that no voltages such as bias voltages from the circuit itself can affect the varactor diode. Voltage controlled oscillator using a varactor diode In this circuit D1 is the varactor diode that is used to enable the oscillator to be tuned. C1 prevents the reverse bias for the varactor or varicap diode being shorted to ground through the inductor, and R1 is a series isolating resistor through which the varactor diode tuning voltage or bias is applied.
Schottky Diode 1N5819 Schottky Diode
Power Schottky Diode This Schottky diode has forward voltage of only 30 V and forward current of 30 A. Used for AC to DC conversion [Rectifier Circuits]
Internal Structure of Schottky Diode It is Metal-Semiconductor contact diode
