PN Junction Diode V-I Characteristics, Resistance & Switching Times

Posted on Mar 1, 2026 in Industrial Electronics and Automation Engineering

V-I Characteristics of a PN Junction Diode

1. V-I Characteristics of a PN Junction Diode?

The voltage-current (V-I) characteristics of a PN junction diode describe the relationship between the voltage applied across the diode and the resulting current that flows through it. This relationship is highly nonlinear and is typically divided into three regions: forward bias, reverse bias, and breakdown.

Forward Bias Region

Forward Bias Region: In the forward bias region, the positive terminal of the voltage source is connected to the p-type material, and the negative terminal is connected to the n-type material.

Reverse Bias Region

Reverse Bias Region: In the reverse bias region, the positive terminal of the voltage source is connected to the n-type material, and the negative terminal is connected to the p-type material.

Leakage Current (IR): A very small current that flows due to minority carriers. This current is almost constant and independent of the applied reverse voltage until the breakdown voltage is reached.

Reverse Saturation Current: The reverse current remains approximately constant and is equal to the saturation current.

Breakdown Region

Breakdown Region: If the reverse voltage is increased beyond a certain point, the diode will enter the breakdown region.

Breakdown Voltage (VB): The voltage at which the diode undergoes breakdown and a significant current starts to flow in the reverse direction.

V-I Characteristic Curve

V-I Characteristic Curve: The V-I characteristic curve of a PN junction diode can be summarized as follows:

  1. Forward Bias (V > 0): Current increases exponentially with voltage after the threshold voltage.
  2. Reverse Bias (V < 0): Current remains very small and approximately constant (leakage current) until the breakdown voltage is reached.
  3. Breakdown (V < -VB): Current increases rapidly in the reverse direction.

Static and Dynamic Resistance

2. EXPLAIN STATIC AND DYNAMIC RESISTANCE?

Static Resistance

Static Resistance: Static resistance (also known as DC resistance) is the ratio of the voltage across the diode to the current flowing through it at a specific operating point. It is calculated using the coordinates of a point on the diode’s V-I characteristic curve.

Rstatic = VD / ID

· VD: Voltage across the diode.

· ID: Current through the diode.

Characteristics of Static Resistance:

  • Non-linear: Since the V-I characteristic of a diode is non-linear, the static resistance changes with the operating point.
  • Forward Bias: In the forward bias region, as the forward voltage increases, the current increases exponentially, making the static resistance relatively small.
  • Reverse Bias: In the reverse bias region, the current is very small, resulting in a very high static resistance.

Dynamic Resistance

Dynamic Resistance: Dynamic resistance (also known as AC resistance or small-signal resistance) is the resistance of the diode to small changes in voltage around a particular operating point. It is the inverse of the slope of the V-I characteristics curve at that point.

rd = (dV / dI) |VQ, I Q

· rd: Dynamic resistance.

· VQ: Quiescent (operating point) voltage.

· IQ: Quiescent (operating point) current.

Characteristics of Dynamic Resistance:

  • Small-Signal Analysis: Used in AC analysis and for small variations in the input signal around the quiescent point.
  • Calculation: Can be approximated by the thermal voltage (VT) divided by the quiescent current (IQ) for a forward-biased diode.

rd ≈ VT / IQ where VT is the thermal voltage (~26 mV at room temperature).

Comparison and Practical Examples

Comparison and Practical Examples:

Forward Bias Region: In the forward bias region, the diode current increases exponentially with voltage. Thus:

  • Static Resistance decreases as the forward current increases.
  • Dynamic Resistance is much lower than static resistance and decreases rapidly as the current increases.

Example:

. If VD = 0.7 V and ID = 1 mA, then Rstatic = 0.7 V / 1 mA = 700 Ω.

. For a small change around this operating point, if IQ = 1 mA, then rd = 26 mV / 1 mA = 26 Ω.

Reverse Bias Region: In the reverse bias region, the current is very small (leakage current), and:

  • Static Resistance is very high due to the minimal current.
  • Dynamic Resistance is also very high but not typically used in reverse bias analysis unless near breakdown.

Switching Times of a Diode

3. EXPLAIN SWITCHING TIMES OF DIODE?

Switching Times of a Diode

A. Switching Times of a Diode: The switching time of a diode refers to the time it takes for the diode to transition between its conducting (on) and non-conducting (off) states. This is crucial in high-speed electronic circuits. The switching process is characterized by two main parameters: the reverse recovery time and the forward recovery time.

Reverse Recovery Time (trr)

Reverse Recovery Time (trr): The reverse recovery time is the time taken for a diode to switch from conducting in the forward direction to blocking in the reverse direction.

· This time consists of two parts:

  1. Storage Time (ts): When the forward current is suddenly reversed, the diode continues to conduct in the reverse direction due to the stored charge in the depletion region and the semiconductor material. The storage time is the duration during which the diode still conducts a significant reverse current.
  2. Transition Time (tt): After the storage time, the reverse current decreases exponentially to its steady-state value (reverse leakage current).

Factors Affecting Switching Times

Factors Affecting Switching Times:

  1. Diode Type: Different types of diodes (e.g., Schottky, standard PN junction, fast recovery) have different switching characteristics. Schottky diodes, for instance, have very short reverse recovery times because they do not store significant charge in the depletion region.
  2. Doping Levels: Higher doping levels in the semiconductor material can reduce the reverse recovery time by decreasing the amount of stored charge.
  3. Junction Capacitance: The junction capacitance, which depends on the physical structure and doping profile, affects the switching speed. Lower capacitance typically results in faster switching.
  4. Operating Temperature: Higher temperatures can increase the switching times due to increased carrier lifetimes and diffusion times.

Switching Time Analysis

Switching Time Analysis:

• Forward to Reverse Transition:

  • When the forward current is suddenly switched to a reverse voltage, the diode initially conducts a high reverse current due to the stored charge.
  • The total reverse recovery time is the sum of the storage time and the transition time.

• Reverse to Forward Transition:

  • When the reverse voltage is suddenly switched to a forward voltage, there is an initial delay before the diode starts conducting.
  • The forward recovery time is the period during which the diode builds up the necessary charge carrier concentration to conduct fully.