Junction transistor, transistor action

Introduction

The bipolar junction transistor (BJT) is a three-terminal semiconductor device with two p/n junctions that can amplify or magnify a signal. BJTs are available in a variety of sizes and forms. The charge carriers in a junction transistor are both electrons and holes. The base of a transistor is made thin and lightly doped, which implies that the density of the majority carriers in the base is lower than the density of the majority carriers in a transistor’s emitter and collector. Emitted current flows into the collector, which in turn receives it. The base acts as a conduit for communication between the emitter and the collector. The conventional current is indicated by the arrow in a junction transistor. The emitter arrow in an n-p-n transistor points away from the base, whereas the emitter arrow in a p-n-p transistor points towards the base. When a junction transistor is utilized in a circuit, the base-emitter junction is forward biased and the base-collector junction is reverse biased.

Types of Junction transistors

• NPN Transistor– A n-doped emitter and an n-doped collector are connected to a p-type semiconductor base in an NPN transistor. Because electron mobility is easier than electron-hole mobility, NPN transistors are the most often utilized bipolar transistors. Electrons make up the majority of the charge carriers in an n-p-n transistor, while holes make up the minority. A big quantity of current flows from the emitter to the collector whereas only a tiny amount of current flows via the base terminal. The bulk of charge carriers in the emitter are repulsed towards the base due to the transistor’s forward biasing. In the base region, electron-hole recombination is extremely rare, with the majority of electrons traveling to the collector region instead.
• PNP Transistor– Emitter and collector of a PNP transistor are made up of a p-doped semiconductor, whereas the base is n-doped. These transistors use holes as majority carriers, whereas electrons are used as minority carriers in these devices. The emitter of a PNP transistor is biased forward, while the collector is biased backward.

Theory and modeling

To understand BJTs, think of them as two diodes (P–N junctions) that are connected to each other. Two diodes sharing an N- and P-type cathode region in a PNP BJT and two diodes sharing a P-type anode region in an NPN BJT are analogous. Using wires to connect two diodes does not create a BJT because the minority carriers cannot travel through the wire. BJTs work by allowing the base current to regulate an amplified output from the collector, which both types of BJTs do. An excellent switch can be made by using the base input of the BJT. The BJT is also a good amplifier since it can increase the strength of a weak signal by around 100 times. BJT networks can be utilized to create powerful amplifiers for a wide range of purposes.

History of the Transistor

It is a semiconductor device with three terminals that can be connected to an electrical circuit. Commonly, the third terminal acts as a switch to regulate how much electricity flows between the two other endpoints. As in a radio receiver, this is for amplification, as in digital circuits, or for fast switching. Triodes, also known as (thermionic) valves, were much larger in size and required a lot more power to operate than transistors. In 1947, Bell Labs in Murray Hill, New Jersey successfully demonstrated the first transistor. Founded by American Telephone and Telegraph, Bell Labs is the company’s research department (AT&T). Shockley, Bardeen, and Brattain are the three inventors that have been attributed with the transistor’s creation. The transistor’s introduction is frequently regarded as one of the most significant technological developments of all time.

Field-effect Transistors

Using an electric field, the field-effect transistor (FET) may control the flow of current in a semiconductor. The source, the gate, and the drain are the three terminals of a FET. Changing the gate voltage varies conductivity between the drain and source of a FET, which in turn affects how much current flows through the device. They’re sometimes referred to as unipolar transistors because of their single-carrier nature. In other words, FETs use either electrons or holes as charge carriers, but not both. Field effect transistors come in a wide variety of shapes and sizes. At low frequencies, the input impedance of field effect transistors is typically very high. The MOSFET is the most common field-effect transistor (metal-oxide-semiconductor field-effect transistor).

Applications of BJTs

• In logic circuits, bipolar junction transistors (BJTs) are employed.
• Amplification is the primary function of this device.
• It’s a multivibrator, after all.
• Clipping circuits employ it for wave shaping.
• Detection or demodulation is used.
• Also known as a modulator.
• Timer and time delay circuits use this component.
• As an electronic switch, it’s a common occurrence.
• Switching circuits rely on it.

Voltage, current and charge control

Both the base–emitter current (current control) and the base–emitter voltage (voltage control) can be used to influence collector–emitter current (voltage control). Views of the base–emitter junction’s current–voltage relationship are linked by curve, which is typical of a p–n junction (diode). The concentration gradient of minority carriers in the base area explains collector current. Surplus minority carriers influence ambipolar transport rates because of low-level injection (in which excess majority carriers outnumber excess minority carriers), which results in ambipolar transport rates being governed by the excess minority carriers. For example, the Gummel–Poon model of transistor action specifically accounts for the distribution of this charge in order to explain transistor behavior more precisely. With the charge-control approach, it is easy to handle phototransistors, where minority carriers in the base region are formed by absorption and handle the dynamics of turn-off or recovery time that depends on recombining charges. 

Although base charge is not a signal that can be seen at the terminals, current and voltage control views are commonly employed in circuit design and analysis. Because it is nearly linear, the current-control perspective is sometimes employed in analogue circuit design. Basic circuits can be created by assuming that the base–emitter voltage and collector current are nearly constant. It is necessary to use a voltage-control model (such as Ebers–Moll) when designing manufacturing BJT circuits, though. A linearization of the Ebers–Moll model, in which the transistor can be treated as a transconductance, makes the design of circuits like differential amplifiers a linear problem again, hence the voltage-control approach is often favored in these cases. Voltage-controlled current sources are commonly used to represent transistors in translinear circuits, where the exponential I–V curve is critical to the operation of transistors. A designer isn’t normally concerned about the mathematical model’s complexity when performing transistor level circuit analysis with SPICE or an equivalent analogue circuit simulator; nonetheless, a simplified representation of the characteristics allows designs to be constructed in a logical manner.

Conclusion 

Radiation damage to the transistor occurs when it is exposed to ionizing radiation. Recombination centers are built up in the base region due to radiation. The transistor’s gain gradually decreases as a result of a decrease in minority carrier lifetime. Devices have “maximum ratings,” which are fundamentally limited by self-heating, maximum collector and base currents (both continuous/DC ratings and peak), maximum breakdown voltage ratings, beyond which the device may fail or function badly. A failure scenario known as secondary breakdown occurs when high current and natural flaws in the silicon die lead areas of the silicon inside the device to grow disproportionately hotter than the rest. Temperature-dependent electrical resistivity of doped silicon, like that of other semiconductors, has a positive temperature coefficient. Consequently, the hottest portion of the die carries the most current, which causes its conductivity to rise, which then leads it to heat up again until the device fails inside. Once secondary breakdown is activated, the thermal runaway process associated with secondary breakdown begins almost immediately and can cause catastrophic damage to the transistor package. BJT current gain may be permanently reduced if the emitter–base junction is reverse biased into avalanche or Zener mode for a short length of time, as the emitter is smaller than the collector and cannot effectively dissipate considerable power. In low-voltage devices, this is a frequent ESD failure mechanism.