Transistor Junction Capacitance

In a Bipolar Junction Transistor (BJT), three main junction capacitances exist:

1. Base-Emitter Junction Capacitance (CBE):

• Origin: Depletion region formed at the p-n junction when forward-biased (emitter voltage higher than base voltage).

• Typical values: 10 pF - 100 pF (depending on transistor size and doping levels).

• Voltage variation: Increases with increasing forward bias voltage (due to narrowing of the depletion region).

• Biasing condition: More significant in active and saturation regions as the junction is forward-biased.

• Effects:

  • Limits high-frequency performance by coupling input and output signals, causing phase shifts and reducing gain.

  • Creates feedback in amplifier circuits, potentially leading to instability.

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2. Base-Collector Junction Capacitance (CBC):

• Origin: Depletion region formed at the p-n junction when reverse-biased (collector voltage higher than base voltage).

• Typical values: 1 pF - 10 pF (smaller than CBE due to wider depletion region).

• Voltage variation: Decreases with increasing reverse bias voltage (due to widening of the depletion region).

• Biasing condition: More significant in active and cutoff regions as the junction is reverse-biased.

• Effects:

  • Less impactful than CBE due to its smaller value and reverse biasing.

  • Can contribute to high-frequency roll-off and feedback in certain circuit configurations.

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3. Collector-Substrate Junction Capacitance (CCS):

• Origin: Depletion region formed at the p-n junction between the collector and substrate (usually grounded).

• Typical values: 0.1 pF - 1 pF (smallest due to deep isolation of the collector).

• Voltage variation: Similar to CBC, decreases with increasing reverse bias voltage.

• Biasing condition: Most relevant in saturation region when significant collector current flows.

• Effects:

  • Usually negligible in most applications due to its small value.

  • Can contribute to leakage current and substrate noise in high-impedance circuits.

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Overall effects of junction capacitances:

• Limit the operating frequency of BJT circuits by introducing unwanted signal coupling and reducing gain.

• Create feedback paths that can lead to instability or ringing in amplifiers.

• Affect input and output impedance at high frequencies.

Minimizing the impact of junction capacitances:

• Choose BJTs with smaller intrinsic capacitances.

• Operate BJTs at lower bias currents.

• Operate BJTs at higher voltages.

• Use bypass capacitors to shunt unwanted high-frequency signals.

• Employ circuit design techniques to compensate for parasitic capacitances.

Forward Bias Charge Storage

Diffusion capacitance (CD)

Diffusion capacitance occurs in a forward biased p-n junction diode. Diffusion capacitance is also sometimes referred as storage capacitance. It is denoted as CD.

In a forward biased p-n junction, diffusion capacitance is much larger than the transition capacitance. Hence, diffusion capacitance is considered in a forward biased junction.

The diffusion capacitance occurs due to stored charge of minority electrons and minority holes near the depletion region.

When forward bias voltage is applied to the p-n junction diode, electrons (majority carriers) in the n-region will move into the p-region and recombines with the holes. In the similar way, holes in the p-region will move into the n-region and recombines with electrons. As a result, the width of depletion region decreases.

The electrons (majority carriers) which cross the depletion region and enter into the p-region will become minority carriers of the p-region similarly; the holes (majority carriers) which cross the depletion region and enter into the n-region will become minority carriers of the n-region.

A large number of charge carriers, which try to move into another region will be accumulated near the depletion region before they recombine with the majority carriers. As a result, a large amount of charge is stored at both sides of the depletion region.

The accumulation of holes in the n-region and electrons in the p-region is separated by a very thin depletion region or depletion layer. This depletion region acts like dielectric or insulator of the capacitor and charge stored at both sides of the depletion layer acts like conducting plates of the capacitor.

Diffusion capacitance is directly proportional to the electric current or applied voltage. If large electric current flows through the diode, a large amount of charge is accumulated near the depletion layer. As a result, large diffusion capacitance occurs.

In the similar way, if small electric current flows through the junction, only a small amount of charge is accumulated near the depletion layer. As a result, small diffusion capacitance occurs.

With increasing forward bias the width of depletion region decreases. the resistance of the junction decreases and the diffusion capacitance increases.