Understanding Transistor Biasing and Collector Current

A transistor is a semiconductor device used for switching or amplifying electronic signals. It has three terminals: emitter, base, and collector, and two junctions: the emitter-base (E-B) junction and the collector-base (C-B) junction.

The operation of a transistor heavily depends on how these two junctions are biased (forward-biased or reverse-biased). Different biasing conditions lead to different modes of operation:

• Active Region: E-B junction is forward-biased, C-B junction is reverse-biased. Used for amplification.
• Cutoff Region: E-B junction is reverse-biased, C-B junction is reverse-biased. Transistor is off.
• Saturation Region: E-B junction is forward-biased, C-B junction is forward-biased. Transistor is fully on (acts like a closed switch).
• Reverse Active Region: E-B junction is reverse-biased, C-B junction is forward-biased. Transistor operates in reverse mode (rarely used).

Analyzing the Emitter-Base Junction Reverse Bias Condition

The question specifically asks about the scenario when the emitter-base junction of a transistor is reverse-biased. Let's consider what happens under this condition for both NPN and PNP transistors.

NPN Transistor with Emitter-Base Reverse Bias

In an NPN transistor, the emitter is heavily doped with N-type material (excess electrons), the base is lightly doped P-type material (excess holes), and the collector is moderately doped N-type material.

When the emitter-base junction is reverse-biased, a positive voltage is applied to the N-type emitter relative to the P-type base (or a negative voltage to the base relative to the emitter). This voltage pushes the majority carriers (electrons from the emitter and holes from the base) away from the junction, widening the depletion region.

Normally, in the active region, electrons from the emitter are injected into the base (this forms the emitter current, $I_E$). With reverse bias on the E-B junction, this injection of electrons from the emitter into the base is effectively stopped. Since the main collector current ($I_C$) is formed by these injected electrons that diffuse through the base and are collected by the collector, stopping the emitter current flow means the collector current also stops (or becomes very small, consisting only of leakage current).

Mathematically, the collector current in the active region is approximately given by:

\(I_C \approx \alpha I_E\)

where \(\alpha\) is the common base current gain (typically close to 1). If $I_E$ is nearly zero due to reverse bias on the E-B junction, then $I_C$ will also be nearly zero.

PNP Transistor with Emitter-Base Reverse Bias

In a PNP transistor, the emitter is P-type (excess holes), the base is N-type (excess electrons), and the collector is P-type.

When the emitter-base junction is reverse-biased, a negative voltage is applied to the P-type emitter relative to the N-type base (or a positive voltage to the base relative to the emitter). This voltage pushes the majority carriers (holes from the emitter and electrons from the base) away from the junction, widening the depletion region.

Normally, in the active region, holes from the emitter are injected into the base (forming $I_E$). With reverse bias on the E-B junction, this injection of holes from the emitter into the base is effectively stopped. Similar to the NPN case, since the main collector current ($I_C$) is formed by these injected holes that diffuse through the base and are collected by the collector, stopping the emitter current flow means the collector current also stops (or becomes very small leakage current).

Again, the relationship $I_C \approx \alpha I_E$ holds. If $I_E$ is nearly zero, $I_C$ is also nearly zero.

Conclusion on Emitter-Base Reverse Bias

In both NPN and PNP transistors, reverse biasing the emitter-base junction stops the injection of majority carriers from the emitter into the base. This action prevents the primary mechanism by which collector current flows. Therefore, the main collector current essentially stops, regardless of whether it is a PNP or an NPN transistor.

This biasing condition (E-B reverse bias) typically places the transistor in the cutoff region, where the transistor acts like an open switch, and ideally, no current flows between the collector and the emitter.

Let's examine the given options based on this understanding:

• Option 1: decreases for PNP transistor - While it decreases significantly, saying it just "decreases" might not fully capture that it essentially stops. More importantly, the same effect happens for NPN.
• Option 2: increases for NPN transistor - This is incorrect. Reverse bias on E-B stops current flow, it doesn't increase it.
• Option 3: stops, irrespective of PNP or NPN - This accurately describes the outcome. The main collector current flow mechanism is halted, leading to negligible collector current.
• Option 4: reverses, irrespective of PNP or NPN - This is incorrect. Reversing the collector current would imply current flows in the opposite direction, which does not happen under this specific E-B reverse bias condition. A reverse active mode (E-B reverse, C-B forward) does allow current, but the primary E-C current direction isn't typically described as "reversed" in the sense of normal operation. And even then, it requires C-B to be forward biased, which is not specified here.

Therefore, when the emitter-base junction is reverse-biased, the collector current effectively stops for both PNP and NPN transistors, placing the transistor in the cutoff region.

Revision Table: Transistor Biasing States

Additional Information: Leakage Current

While we say the collector current "stops" when the emitter-base junction is reverse-biased, in reality, there is a very small current that still flows. This is called leakage current or reverse saturation current ($I_{CBO}$). This current is due to the minority carriers that are thermally generated within the semiconductor material and are swept across the reverse-biased junctions by the electric field.

For typical junction transistors, this leakage current is usually in the range of nanoamperes (\(nA\)) or microamperes (\(\mu A\)), which is significantly smaller than the normal operating collector current (which can be in milliamperes (\(mA\)) or amperes (\(A\))). For most practical purposes, this small leakage current is negligible, and we can consider the collector current to have stopped when the transistor is in the cutoff region due to a reverse-biased emitter-base junction.

This cutoff state is essential for using the transistor as a switch in digital circuits, where it represents the "off" state.