Knee Voltage and Crystal Diodes Explained

The knee voltage, also known as the cut-in voltage or threshold voltage, is a crucial parameter for understanding the operation of a crystal diode (or semiconductor diode). It represents the minimum forward-bias voltage required across the diode terminals for the diode to begin conducting a significant amount of current in the forward direction. Without reaching this voltage, the diode essentially remains in an 'off' state.

Barrier Potential and Diode Conduction

To understand the knee voltage, it's essential to first grasp the concept of barrier potential. Inside a semiconductor diode, a depletion region forms at the p-n junction. This region has an inherent electric field that opposes the flow of majority charge carriers (electrons from the N-side and holes from the P-side) across the junction. This electric field creates a potential difference known as the barrier potential (or built-in potential barrier or diffusion potential).

• For commonly used silicon (Si) diodes, the typical barrier potential is approximately \(0.7\,V\).
• For germanium (Ge) diodes, the typical barrier potential is approximately \(0.3\,V\).

When a diode is forward biased, an external voltage is applied across the diode in a direction that opposes this internal barrier potential. As the applied forward voltage increases, it gradually reduces the width of the depletion region and weakens the effective barrier. When the applied forward voltage becomes approximately equal to the barrier potential, the barrier is effectively overcome. At this point, majority carriers can easily flow across the junction, and current starts to flow significantly through the diode. This specific voltage at which substantial current begins to flow is defined as the knee voltage.

Why Knee Voltage Equals Barrier Potential

The relationship between knee voltage and barrier potential is direct: the knee voltage is approximately equal to the barrier potential. This is because for a crystal diode to conduct effectively in the forward direction, the external applied voltage must be sufficient to neutralize or overcome the inherent potential barrier formed at the p-n junction. Until this voltage (the barrier potential) is reached, the diode's resistance is very high, allowing only a negligible leakage current to flow. Once the applied forward voltage crosses this threshold, the diode's resistance drops sharply, and the forward current increases exponentially with small increases in voltage.

Analyzing Other Options

• Reverse Voltage: This refers to the voltage applied across the diode when it is reverse biased. In reverse bias, the external voltage adds to the barrier potential, widening the depletion region and preventing significant current flow (except for a very small reverse saturation current) until the breakdown voltage is reached. The knee voltage is a forward-bias characteristic, entirely distinct from reverse voltage.
• Forward Voltage: While knee voltage is a specific point on the forward voltage axis, "forward voltage" is a general term for any voltage applied across the diode in the forward direction. The knee voltage is the particular forward voltage at which the diode's conduction becomes substantial, marking the 'knee' of the V-I characteristic curve.
• Breakdown Voltage: This is a characteristic of a diode under reverse bias. When the reverse voltage applied across the diode exceeds the breakdown voltage, the diode begins to conduct a large current in the reverse direction due to phenomena like avalanche breakdown or Zener breakdown. This destructive process is not related to the knee voltage, which describes the onset of forward conduction.

Therefore, the knee voltage of a crystal diode is approximately equal to its barrier potential.