Transistor Fundamentals: EB Junction Biasing

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is a fundamental building block of modern electronic circuits. A bipolar junction transistor (BJT) typically has three terminals: Emitter (E), Base (B), and Collector (C), and two junctions: the Emitter-Base (EB) junction and the Collector-Base (CB) junction.

Transistor Operating Regions and Current Flow

For a transistor to operate effectively, especially as an amplifier (in the active region), its junctions must be biased correctly:

• The Emitter-Base (EB) junction is typically forward-biased. This allows majority carriers (electrons in NPN, holes in PNP) to be injected from the emitter into the base. This injection creates the emitter current ($\text{I}_{\text{E}}$) and subsequently the base current ($\text{I}_{\text{B}}$).
• The Collector-Base (CB) junction is typically reverse-biased. This reverse bias helps to sweep the minority carriers (which were injected from the emitter and diffused through the base) into the collector, forming the collector current ($\text{I}_{\text{C}}$).

The collector current ($\text{I}_{\text{C}}$) is primarily dependent on the emitter current ($\text{I}_{\text{E}}$) and the base current ($\text{I}_{\text{B}}$). In an NPN transistor, for example, electrons flow from the emitter to the base (forward-biased EB) and then a large portion of these electrons are collected by the collector (reverse-biased CB).

Collector Current with Reverse Biased EB Junction

The question asks what happens to the collector current when the EB junction of a transistor is reverse biased. Let's analyze this specific condition:

• Reverse Biasing the EB Junction: When the Emitter-Base (EB) junction is reverse-biased, it behaves like a reverse-biased PN diode. In a reverse-biased diode, the depletion region widens, and there is very little flow of majority charge carriers.
• Impact on Emitter and Base Current: If the EB junction is reverse-biased, the injection of majority carriers from the emitter into the base is effectively stopped. This means that the emitter current ($\text{I}_{\text{E}}$) becomes practically zero (or very close to zero, with only a small leakage current). Since the base current ($\text{I}_{\text{B}}$) is a small fraction of the emitter current (i.e., $\text{I}_{\text{B}} = \text{I}_{\text{E}} - \text{I}_{\text{C}}$), and the collector current ($\text{I}_{\text{C}}$) is proportional to the base current ($\text{I}_{\text{C}} = \beta \text{I}_{\text{B}}$), if $\text{I}_{\text{E}}$ is zero, then $\text{I}_{\text{B}}$ will also be zero.
• Impact on Collector Current: Since the primary source of carriers for the collector current originates from the emitter, if there's no significant emitter current, there will be no significant collector current due to transistor action. Therefore, the collector current will be practically zero. Any remaining current would be a very small leakage current, typically ignored in ideal analysis.

Analyzing the Options

• Zero: This aligns with our understanding. When the EB junction is reverse-biased, the transistor effectively turns off, preventing the flow of current from emitter to collector.
• very high: This is incorrect. Reverse biasing a junction *prevents* large current flow; it does not cause it to be very high.
• equal to saturation current: Saturation occurs when *both* the EB and CB junctions are forward-biased, leading to the maximum possible collector current for a given collector resistor. This is the opposite of having a reverse-biased EB junction.
• Reversed: While leakage currents might flow in the reverse direction, the main collector current, which is due to transistor action, does not "reverse" in this scenario. It simply ceases to flow.

Therefore, when the EB junction of a transistor is reverse-biased, the collector current becomes practically zero because the transistor is effectively turned off.