A semiconductor device is connected in series circuit with a battery and resistance. A current is found to pass through the circuit. If the polarity of the battery is reversed, the current chops at almost zero. The device may be:
A p-n junction
Understanding Semiconductor Device Behavior in Circuits
The question describes a semiconductor device connected in a series circuit with a battery and a resistor. A key observation is that current flows through the circuit when the battery has one polarity, but the current drops to almost zero when the battery's polarity is reversed. This behavior is a strong indicator of a device that allows current to flow preferentially in one direction while blocking it in the opposite direction. Such a device acts like a one-way valve for electrical current.
Analyzing the Options for the Semiconductor Device
Let's examine why the other options do not fit the description:
- A p-type semiconductor: A p-type semiconductor is formed by doping a semiconductor material (like silicon or germanium) with acceptor impurities, resulting in an excess of holes as majority carriers. When connected in a circuit, current can flow in either direction depending on the polarity of the voltage, although the conductivity is typically higher than intrinsic semiconductors. It does not inherently block current flow in one specific direction when polarity is reversed in the way described.
- An intrinsic semiconductor: An intrinsic semiconductor is a pure semiconductor material with no significant doping. It has a relatively low conductivity because the number of free electrons and holes is small and equal. It does not exhibit directional current blocking behavior.
- An n-type semiconductor: An n-type semiconductor is formed by doping a semiconductor material with donor impurities, resulting in an excess of free electrons as majority carriers. Like a p-type semiconductor, it conducts current in either direction when connected in a circuit and does not block current flow in one direction upon polarity reversal in the way described.
Why a p-n Junction Fits the Description
A p-n junction is formed by joining a p-type semiconductor and an n-type semiconductor. This structure creates a region called the depletion zone at the interface. The behavior of a p-n junction depends heavily on the polarity of the applied voltage:
- Forward Bias: When the positive terminal of the battery is connected to the p-side and the negative terminal to the n-side, the p-n junction is said to be in forward bias. The applied voltage reduces the width of the depletion zone, allowing majority carriers (holes from the p-side and electrons from the n-side) to cross the junction. This leads to a significant current flow through the device.
- Reverse Bias: When the negative terminal of the battery is connected to the p-side and the positive terminal to the n-side, the p-n junction is in reverse bias. The applied voltage increases the width of the depletion zone, effectively creating a barrier for the majority carriers. Only a very small leakage current flows across the junction, mainly due to minority carriers. This current is typically negligible compared to the forward current.
The description in the question — current flowing in one direction and being almost zero when the polarity is reversed — perfectly matches the behavior of a p-n junction diode under forward and reverse bias conditions. Therefore, the device is most likely a p-n junction.
Conclusion
Based on the distinct characteristic of allowing current flow in one direction and blocking it in the reverse direction, the semiconductor device described is a p-n junction.
| Semiconductor Type | Behavior in Circuit | Directional Current Flow |
|---|---|---|
| p-type semiconductor | Conducts in both directions (ohmic like) | No significant directional blocking |
| n-type semiconductor | Conducts in both directions (ohmic like) | No significant directional blocking |
| Intrinsic semiconductor | Low conductivity in both directions | No directional blocking |
| p-n junction (diode) | Conducts significantly in one direction (forward bias), blocks in the other (reverse bias) | Strong directional blocking/conduction |
Revision Table: Semiconductor Device Properties
| Device/Material | Key Feature | Current-Voltage Characteristic | Rectifying Property |
|---|---|---|---|
| Intrinsic Semiconductor | Pure semiconductor | Linear (at low fields), relatively low conductivity | No |
| p-type Semiconductor | Doped with acceptors (holes majority) | Linear (ohmic-like) | No |
| n-type Semiconductor | Doped with donors (electrons majority) | Linear (ohmic-like) | No |
| p-n Junction | Junction of p-type and n-type | Non-linear (exponential in forward bias, low current in reverse bias) | Yes (Rectifies AC into DC) |
Additional Information: p-n Junction Diodes and Rectification
A p-n junction device is commonly known as a semiconductor diode. The ability of a p-n junction to allow current flow in one direction (forward bias) and block it in the reverse direction (reverse bias) is called rectification. This property is fundamental to many electronic applications, such as converting alternating current (AC) into direct current (DC) in power supplies. The nearly zero current in reverse bias is not absolute zero; a very small leakage current exists due to the diffusion of minority carriers and thermal generation. If the reverse voltage becomes very high, breakdown can occur (Zener or avalanche breakdown), leading to a sudden increase in reverse current, but this happens at a specific reverse breakdown voltage, much higher than typical operating voltages.
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