Q. In a semiconductor diode, the cut-in voltage is the Voltage
- Up to which the current is zero
- Up to which the current is very small
- At which the current is 12 % of the maximum diode-rated current
- At which the depletion layer is formed
Answer:
The correct option is (2)
In a semiconductor diode, the cut-in voltage is the voltage up to which the current is very small. Once the voltage across the diode exceeds a certain threshold known as the cut-in voltage, the diode starts conducting heavily.
Explanation
The cut-in voltage of a semiconductor diode, often referred to as the “threshold voltage” or “turn-on voltage,” is the minimum voltage at which the diode starts to conduct significant current. Below this voltage, the diode essentially acts as an open circuit and allows very little current to flow. The reason for this behavior lies in the physical properties and structure of the diode. To get insights into the concept, let us discuss the semiconductor diode’s structure, depletion region, built-in voltage, and role of cut-in voltage.
Semiconductor Diode Structure
A semiconductor diode is typically made from a p-n junction, which is created by doping two adjacent regions of a semiconductor crystal differently—one as a p-type and the other as an n-type. The p-type region has abundant holes (positive charge carriers), while the n-type region has excess electrons (negative charge carriers).
Depletion Region and Built-in Voltage
When these two regions are joined, electrons from the n-type region recombine with holes in the p-type region near the junction, creating a depletion region. This region lacks free-charge carriers and acts as a barrier to current flow. Additionally, this recombination leads to a built-in electric field that opposes the movement of electrons from the n-type region to the p-type region and holes in the opposite direction.
Role of Cut-in Voltage
When an external voltage is applied to the diode in the forward-bias direction (positive on the p-type side and negative on the n-type side), it starts to reduce the built-in electric field of the depletion region. The cut-in voltage is the forward bias voltage at which the electric field is sufficiently weakened, allowing the charge carriers to cross the junction efficiently. At this point, the depletion region narrows significantly, and the diode begins to conduct current effectively. Thus, the current through the diode is very small up to the cut-in voltage, and beyond the cut-in voltage, the diode current exponentially increases when the external voltage is increased. The diode equation is,
The below graph shows the magnitude of the diode current before and after the cut-in voltage of the diode.
Material Dependence
The value of the cut-in voltage depends on the materials used to make the diode. The typical cut-in voltage for silicon diodes is around 0.6 to 0.7 volts, while for germanium diodes, it’s about 0.3 volts. This difference arises from these materials’ different energy band gaps, which dictate how much energy is needed to move the carriers across the junction.
From above, it is clear that in a semiconductor diode, the cut-in voltage is the voltage up to which the current is very small. When the external voltage exceeds the cut-in voltage, the diode current increases exponentially with the applied voltage.
Q.1 The cut-in voltage for the silicon diode is approximately?
Q.2 Why does a diode only conduct in one direction?
Q.3 Diffusion Capacitance of PN Junction Diode________