How does varactor capacitance vary with voltage?

C(V) = C0 / (1 + V/Vbi)^gamma. C0 = zero-bias capacitance. V = reverse bias voltage. Vbi = built-in potential (~0.7V Si, ~1.3V GaAs). Gamma (grading coefficient): abrupt junction = 0.5 (C varies as V^-0.5). Linear grading = 0.33. Hyperabrupt = 1.0-2.0 (C varies more steeply, giving wider tuning range and more linear VCO tuning). Tuning ratio = C_max/C_min: abrupt junction: ~3:1 over 0-20V. Hyperabrupt: ~10:1 over 0-20V. For VCO: frequency tuning ratio = sqrt(C_max/C_min). A 4:1 capacitance ratio gives 2:1 frequency tuning (one octave).

What limits varactor performance?

Q factor: Q = 1/(2*pi*f*C*Rs) where Rs is the series resistance. Higher Q = lower loss = lower phase noise in VCOs. Silicon varactors: Q = 50-200 at 1 GHz. GaAs varactors: Q = 200-500 at 1 GHz (lower Rs due to higher mobility). Q decreases with frequency (Q proportional to 1/f). At 10 GHz, Si Q drops to 5-20. This is why GaAs varactors dominate at microwave frequencies. Power handling: large RF signals modulate the varactor bias, causing AM-to-PM conversion and phase noise degradation. Keep RF swing << DC bias. Rule: RF voltage < 10% of DC bias for linear operation. Temperature: C-V curve shifts with temperature (~200 ppm/C for Si). Compensated by temperature-stable bias circuits.

What are varactor alternatives?

Digitally-tuned capacitors (DTC): switched capacitor banks using FET switches. Discrete tuning steps. High Q (>100 at 1 GHz). Used in antenna tuning and reconfigurable filters. MEMS varactors: mechanically tunable capacitor. Very high Q (>200). Slow tuning speed (microseconds). Used in reconfigurable antennas and tunable filters. BST (barium strontium titanate): ferroelectric material with voltage-dependent permittivity. Capacitance tuning ratio 3:1. Very fast (nanoseconds). Used in tunable filters. MOS varactors: on-chip varactors in CMOS. Low Q but integrated. Used in on-chip VCOs where external varactors are not practical. Each alternative trades off tuning speed, Q, tuning ratio, integration, and cost differently.

Semiconductor Devices

Varactor

Q: What are varactor alternatives?

Digitally-tuned capacitors (DTC): switched capacitor banks using FET switches. Discrete tuning steps. High Q (>100 at 1 GHz). Used in antenna tuning and reconfigurable filters. MEMS varactors: mechanically tunable capacitor. Very high Q (>200). Slow tuning speed (microseconds). Used in reconfigurable antennas and tunable filters. BST (barium strontium titanate): ferroelectric material with voltage-dependent permittivity. Capacitance tuning ratio 3:1. Very fast (nanoseconds). Used in tunable filters. MOS varactors: on-chip varactors in CMOS. Low Q but integrated. Used in on-chip VCOs where external varactors are not practical. Each alternative trades off tuning speed, Q, tuning ratio, integration, and cost differently.

/vah-rak-ter/ variable-reactance

Reverse-biased diode: C(V) = C₀/(1+V/V_bi)^γ. Abrupt: γ=0.5, tuning 3:1. Hyperabrupt: γ=1-2, tuning 10:1. VCO freq ratio = √(C_max/C_min). Q: Si 50-200, GaAs 200-500 at 1 GHz. RF swing <10% of DC bias for linearity. Used for VCO tuning, voltage-controlled filters, phase shifters, frequency multipliers.

Si Q: 50-200 @1 GHz

GaAs Q: 200-500 @1 GHz

Tuning: 2:1 to 10:1

Understanding Varactors

The varactor is the key to electronic frequency tuning in RF systems. Every VCO contains at least one varactor that sets the oscillation frequency based on the tuning voltage from a PLL. The varactor's C-V curve, Q factor, and linearity directly determine the VCO's tuning range, phase noise, and tuning sensitivity. At microwave frequencies, GaAs varactors are preferred for their higher Q, while at lower frequencies, silicon hyperabrupt varactors provide wider tuning ranges.

Varactor Parameters

Varactor (voltage-variable capacitor):
C(V) = C_j0/(1+V_R/V_bi)^γ
γ = 0.5 (abrupt junction), γ ≈ 1 (hyperabrupt)
V_bi ≈ 0.7V (Si), 1.3V (GaAs)

Tuning ratio:
TR = C_max/C_min = (1+V_max/V_bi)^γ
Typical: TR = 2–10 depending on technology

Quality factor:
Q = 1/(ωC×R_s), decreases with frequency

Varactor Technology Comparison

Technology	Q @1 GHz	Tuning Ratio	Voltage	Speed	Application
Si abrupt	50-150	3:1	0-30V	ns	Low-cost VCOs
Si hyperabrupt	50-100	8-10:1	0-20V	ns	Wideband VCOs
GaAs	200-500	3-5:1	0-15V	ns	Low phase noise
BST	50-100	3:1	0-20V	ns	Tunable filters
MEMS	200+	4:1	5-50V	μs	Reconfigurable ant

Key Equations

Noise Figure cascade (Friis):
NF_total = NF₁ + (NF₂−1)/G₁ + (NF₃−1)/(G₁G₂)

Gain (dB):
G = 10log(P_out/P_in) = 20log(V_out/V_in)

IP3 & dynamic range:
SFDR = 2/3(IIP3 − NF − 10log(kTB)) dB

Comparison

Type	C range	Q @1GHz	Tuning ratio	Application
Abrupt Si	1–20 pF	50–200	2–4	VCO
Hyperabrupt Si	1–20 pF	30–100	6–10	Wideband VCO
GaAs	0.5–5 pF	200–2000	2–4	Low-loss tuning
BST (ferroelectric)	1–10 pF	50–200	3–5	Tunable filter
MEMS	0.5–5 pF	100–500	3–6	Reconfigurable

Common Questions

Frequently Asked Questions

C-V curve?

C = C0/(1+V/Vbi)^gamma. Higher gamma = steeper C change = wider tuning. Hyperabrupt (gamma=1-2) gives 10:1 tuning. Abrupt (gamma=0.5) gives 3:1. VCO freq ratio = sqrt of C ratio.

Limits?

Q drops with frequency (Q = 1/(2*pi*f*C*Rs)). GaAs has higher Q (lower Rs). RF must be << DC bias (10% rule). Temperature shifts C-V curve (200 ppm/C). Low Q = high VCO phase noise.

Alternatives?

DTC: switched caps, discrete steps, high Q. MEMS: high Q, slow. BST: ferroelectric, fast, moderate Q. MOS: on-chip, low Q. Each trades tuning speed, Q, ratio, and integration.

Related Terms

← Transmission Line VSWR →