Additional: Transistors (Basic Introduction)

Introduction to Bipolar Junction Transistors (BJTs - NPN, PNP)

While diodes (formed by a single p-n junction) are essential components, they primarily act as one-way switches or rectifiers. To perform more complex functions like amplification and switching in electronic circuits, devices with more terminals and controllable current flow are needed. The transistor is one such fundamental semiconductor device. It is the cornerstone of modern electronics and is used in virtually every electronic device.

The most common types of transistors are Bipolar Junction Transistors (BJTs) and Field-Effect Transistors (FETs). Here, we'll focus on a basic introduction to BJTs.

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Bipolar Junction Transistors (BJTs)

A Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device formed by joining three alternately doped semiconductor regions. It has two p-n junctions. The term "bipolar" refers to the fact that both electrons and holes are involved as charge carriers in the operation of the transistor.

A BJT has three terminals:

1. Emitter (E): This region is heavily doped and primarily emits majority charge carriers into the base.
2. Base (B): This is the middle region, which is very thin and lightly doped. It controls the flow of charge carriers from the emitter to the collector.
3. Collector (C): This region is moderately doped and larger in size compared to the emitter. It collects the charge carriers from the base region.

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Types of BJTs: NPN and PNP

Based on the arrangement of the p and n type semiconductor layers, BJTs are of two types:

1. NPN Transistor: This type consists of a p-type semiconductor layer sandwiched between two n-type semiconductor layers.

   Structure and symbol of an NPN transistor. The arrow indicates conventional current direction in the emitter.

   In an NPN transistor, the majority carriers in the emitter and collector are electrons, and in the base, they are holes. The arrow in the symbol always points from the p-region (Base) to the n-region (Emitter), indicating the direction of conventional current flow when the base-emitter junction is forward biased.
2. PNP Transistor: This type consists of an n-type semiconductor layer sandwiched between two p-type semiconductor layers.

   Structure and symbol of a PNP transistor. The arrow indicates conventional current direction in the emitter.

   In a PNP transistor, the majority carriers in the emitter and collector are holes, and in the base, they are electrons. The arrow in the symbol points from the p-region (Emitter) to the n-region (Base).

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Working Principle (Basic)

The operation of a BJT involves biasing its two p-n junctions: the emitter-base (EB) junction and the collector-base (CB) junction. For the most common mode of operation (active region, used for amplification), the EB junction is forward-biased, and the CB junction is reverse-biased.

Consider an NPN transistor with the EB junction forward-biased and the CB junction reverse-biased.

Biasing and charge carrier flow in an NPN transistor (active region).

1. Emitter Current ($I_E$): Due to the forward bias across the EB junction, a large number of majority carriers from the emitter (electrons in NPN) are injected into the base region. This constitutes the emitter current.
2. Base Current ($I_B$): The base region is very thin and lightly doped. Most of the electrons entering the base from the emitter diffuse across the thin base region towards the collector. A small fraction of these electrons (typically less than 5%) recombine with the holes (majority carriers in the base for NPN) in the base region. This small flow of electrons out of the base terminal (or holes into the base terminal in conventional current direction) constitutes the base current. The base current is typically very small compared to the emitter current.
3. Collector Current ($I_C$): The remaining large fraction of electrons (typically more than 95%) that diffuse across the thin base region reach the CB junction. The CB junction is reverse-biased, and its electric field sweeps these electrons from the base into the collector region. This constitutes the collector current.

The total emitter current is the sum of the base current and the collector current:

$ I_E = I_B + I_C $

In essence, a small base current ($I_B$) controls a much larger collector current ($I_C$). The ratio of the collector current to the base current is called the current gain of the transistor, usually denoted by $\beta$ or $h_{fe}$.

$ I_C = \beta I_B $

Values of $\beta$ can range from around 50 to several hundreds. This control property is fundamental to using a transistor as an amplifier or a switch.

PNP transistors work similarly, but the roles of electrons and holes are interchanged, and the polarities of the biasing voltages are reversed. In a PNP transistor, holes are the majority carriers in the emitter and collector, injected from the emitter into the base, and a small fraction of them recombine with electrons (majority carriers in the base). The remaining holes are swept into the collector by the reverse-biased CB junction. The currents are flows of holes, and voltage polarities are opposite to NPN.

The invention of the transistor in 1947 by Bardeen, Brattain, and Shockley at Bell Labs revolutionised electronics, leading to miniaturisation, increased reliability, and the development of integrated circuits and modern computing.

Transistor as a Switch

One of the key applications of a transistor is as an electronic switch. Unlike mechanical switches, transistor switches have no moving parts, can operate at very high speeds, and can be controlled by electrical signals. A BJT can be used as a switch by operating it in two distinct regions: the cut-off region and the saturation region.

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Transistor as an Open Switch (Cut-off Region)

To use a transistor as an open switch (OFF state), we operate it in the cut-off region. This is achieved by biasing both the emitter-base (EB) junction and the collector-base (CB) junction in reverse bias.

NPN transistor in cut-off region (OFF state). Both junctions are reverse biased.

When both junctions are reverse-biased, the depletion regions at both junctions are wide, and the potential barriers are high. This prevents the significant flow of majority carriers across either junction. Only a very small reverse saturation current (due to minority carriers) flows, which is typically negligible.

In this state, the collector current ($I_C$) is approximately zero. If the transistor is connected in a circuit with a load (e.g., an LED or a motor) in the collector circuit, the lack of collector current means the load is effectively disconnected from the power supply (like an open switch).

A small or zero input signal at the base can be used to turn the transistor OFF (put it in cut-off).

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Transistor as a Closed Switch (Saturation Region)

To use a transistor as a closed switch (ON state), we operate it in the saturation region. This is typically achieved by biasing both the emitter-base (EB) junction and the collector-base (CB) junction in forward bias.

NPN transistor in saturation region (ON state). Both junctions are forward biased.

When both junctions are forward-biased, the depletion regions are very thin, and the barriers are low. This allows for a large flow of majority carriers across both junctions. The collector current is no longer controlled by the base current through the linear relationship $I_C = \beta I_B$. Instead, the collector current becomes limited mainly by the external circuit components (like the load resistance and the supply voltage). The transistor acts like a low-resistance path between the collector and emitter terminals.

In this state, a significant collector current flows, and the voltage drop between the collector and emitter ($V_{CE}$) is very small (ideally close to zero, typically a few tenths of a volt). If the transistor is connected in a circuit with a load, the large collector current flows through the load, effectively connecting it to the power supply (like a closed switch).

A sufficiently large input signal (current) at the base can be used to turn the transistor ON (put it in saturation).

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Switching Operation Summary

By controlling the base current (or base voltage), we can switch the transistor between the cut-off state (OFF switch, $I_C \approx 0$, $V_{CE} \approx$ supply voltage) and the saturation state (ON switch, $I_C$ is large and limited by load, $V_{CE}$ is small).

This switching capability is fundamental to digital electronics, where transistors are used to implement logic gates and digital circuits. A transistor in cut-off can represent the binary state 0 (LOW output voltage or no current flow), and a transistor in saturation can represent the binary state 1 (HIGH output voltage or current flow).

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Transistor as an Amplifier (qualitative)

Another major application of transistors, particularly BJTs, is as an amplifier. An amplifier is a circuit that increases the amplitude or power of an electrical signal. This is essential in many electronic systems, from audio equipment (boosting sound signals) to communication systems (amplifying radio signals). A transistor can amplify a signal because a small change in the base current (or base voltage) can cause a much larger change in the collector current.

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Principle of Amplification

To use a BJT as an amplifier, it is biased to operate in the active region. In the active region, the emitter-base (EB) junction is forward-biased, and the collector-base (CB) junction is reverse-biased. This biasing allows the transistor to control a large collector current using a small base current.

Recall the relationship in the active region: $I_C = \beta I_B$, where $\beta$ is the current gain. Since $\beta$ is typically large (e.g., 100), a small change in $I_B$ causes a change in $I_C$ that is $\beta$ times larger.

Simplified concept of a transistor amplifier. A small input signal controls a larger output current.

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How it Works (Qualitative)

Consider a simple amplifier circuit where a small AC input signal (e.g., an audio signal) is applied to the base-emitter junction, causing a small variation in the base current ($i_b$). This input signal is usually superimposed on a steady DC bias voltage to ensure the transistor remains in the active region.

Due to the transistor's current gain ($\beta$), this small varying base current $i_b$ causes a larger varying collector current $i_c = \beta i_b$.

This larger varying collector current flows through a load resistor ($R_C$) connected in the collector circuit. The voltage drop across the load resistor is $v_{out} = i_c R_C$.

So, a small input voltage signal (which causes $i_b$) results in a larger output voltage signal across $R_C$ (due to $i_c$). The output voltage signal is an amplified version of the input signal.

The voltage gain of the amplifier circuit is the ratio of the change in output voltage to the change in input voltage. The power gain is the ratio of the output signal power to the input signal power.

For amplification to be linear (i.e., the output is a faithful, scaled-up copy of the input signal without distortion), the transistor must be operated within its active region. Proper DC biasing is necessary to set the quiescent (operating) point in the middle of the active region.

In summary, the transistor's ability to control a large current flow between the collector and emitter using a small current or voltage signal at the base allows it to act as a highly effective amplifier for weak electrical signals. This property is fundamental to the operation of numerous electronic circuits, from audio amplifiers to radio transmitters and receivers.

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