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What is Varactor diode and its symbol

Varactor diode is a semiconductor device which exploits property of semiconductor's depletion region capacitance changing with applied voltage. It is also known as varicap diode or tuning diode. It is useful in various electronic applications such as tuning circuits, VCOs (Voltage Controlled Oscillators), frequency multiplier circuits, frequency synthesizers etc.

Varactor diode operates in reverse biased whose operation depends upon transition capacitance. Here P and N regions have high concentration of majority carriers and hence will have low resistance areas. Here space charge region or depletion region caused due to depletion of majority carriers act as dielectric. The P and N regions act as plates of capacitor. The depletion region mentioned acts as insulating dielectric layer.

The reverse biased P-N junction possesses junction capacitance due to depletion region formed at the interface of P-type and N-type materials. It is often referred as "transition capacitance" or "depletion capacitance". It is expressed as follows:
C_T = (ε*A) / W .....equation-1
Where in,
ε = Permittivity of semiconductor
A = Area of P-N junction
W = Width of space charge region
Figure-1 depicts varactor diode symbol and varactor diode equivalent circuit.

Transition capacitance in terms of reverse bias voltage (V_R) is expressed as below.
C_T = K / (V_K + V_R )ⁿ .....equation-2
Where in ,
K = constant depending on semiconductor material
V_K = diode knee voltage or barrier potential
V_R = reverse bias
n = 1/2(for alloyed junction) or 1/3(for diffused junction)

Varactor diode tuning circuit

It is a circuit that uses the voltage-dependent capacitance of a varactor diode to adjust the frequency or characteristics of another circuit such as an oscillator, filter or resonant circuit.

Figure-2 depicts varactor diode tuning circuit. In this circuit two diodes D1 and D2 provide total variable capacitances in a parallel resonant circuit. Here Vc is the variable DC control voltage which controls the reverse bias and consecutively capacitance of the diodes. L mentioned in the circuit is inductance in henrys.

Resonant frequency of the zener tuning circuit is expressed as follows:
fo = 1 / [2* π * (L*C_T)^0.5 ] .... equation-3
Where in,
C_T = C1*C2/(C1+C2),
Here C1 and C2 are max. and min. values of diode capacitances

Varactor diode as frequency multiplier

Diode frequency multipliers can be typically classified as Schottky barrier diode or varactor diode type. In the varactor diode type multiplier a nonlinear reactive element is used. Varactor reactive type multipliers have high potential conversion efficiency. It needs very low drive power levels but exhibit narrower bandwidth and high sensitivity to operating conditions. It also exhibit stability problems sometimes.

Varactor diode frequency multipliers generate very little amplitude and phase noise. The only noise source is the thermal noise of the series resistance of the varactor diode and the circuit loss resistances. The power capability of a varactor multiplier is limited by breakdown of the device. The varactor diode has a parasitic resistance in series, which dissipates power.

A varactor frequency multiplier is capable of higher efficiency and higher power than a resistive type frequency multiplier. Figure-3 depicts lumped element based frequency doubler and tripler. It is possible to dvelop frequency doubler using microstrip elements and varactor diode.

The varactor diode is connected in parallel with a capacitor in an LC tank oscillator as shown. By altering the DC bias voltage on the varactor diode, its capacitance changes, leading to a shift in the oscillator's resonant frequency.

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