bipolar junction transistors
Bipolar junction Transistors
1. Introduction
The Bipolar Junction Transistor (BJT) is one of the most important solid-state devices. It is a two-junction, three-terminal device, in which the current flow properties of one p-n junction can be modulated by another p-n junction. The structure can be either p-n-p or n-p-n. The three terminals are called emitter, base and collector. The circuit symbols of the two types of BJT are shown in Fig.1. The arrow on the emitter signifies the direction of the current flowing at the emitter terminal. In most of our following discussions we shall focus on the n-p-n transistor, which is the device predominantly used in integrated circuits. However, the analysis will be valid for the p-n-p transistor as well, with appropriate changes in carrier types and directions of currents. Collector (C)
Collector (C)
Base (B)
Base (B)
Emitter (E)
Emitter (E)
(a)
(b)
Fig.1: Circuit symbol of (a) npn and (b) pnp transistor
2. Principle of Operation
In order to understand the interaction of the two p-n junctions in a BJT, let us first consider the structure shown in Fig.2(a) in which a region of p-silicon is sandwiched between two n regions (n1 and n2). Let us assume that the thickness of the intermediate p-layer is much larger than the diffusion length of electrons (Ln). [The diffusion length of electrons is the average distance the electrons can travel before they recombine with holes and is a property of the material] If one of the two junctions (J1) is now forward biased and the other junction (J2) is reverse-biased, practically all the electrons injected by the forward-biased junction into the pregion will recombine before they reach the second junction. [Electrons diffuse from the emitter towards the collector because of the concentration gradient. The electron concentration would be enhanced near a forward biased junction and depleted near a reverse biased junction]. The
electron
1. Introduction
The Bipolar Junction Transistor (BJT) is one of the most important solid-state devices. It is a two-junction, three-terminal device, in which the current flow properties of one p-n junction can be modulated by another p-n junction. The structure can be either p-n-p or n-p-n. The three terminals are called emitter, base and collector. The circuit symbols of the two types of BJT are shown in Fig.1. The arrow on the emitter signifies the direction of the current flowing at the emitter terminal. In most of our following discussions we shall focus on the n-p-n transistor, which is the device predominantly used in integrated circuits. However, the analysis will be valid for the p-n-p transistor as well, with appropriate changes in carrier types and directions of currents. Collector (C)
Collector (C)
Base (B)
Base (B)
Emitter (E)
Emitter (E)
(a)
(b)
Fig.1: Circuit symbol of (a) npn and (b) pnp transistor
2. Principle of Operation
In order to understand the interaction of the two p-n junctions in a BJT, let us first consider the structure shown in Fig.2(a) in which a region of p-silicon is sandwiched between two n regions (n1 and n2). Let us assume that the thickness of the intermediate p-layer is much larger than the diffusion length of electrons (Ln). [The diffusion length of electrons is the average distance the electrons can travel before they recombine with holes and is a property of the material] If one of the two junctions (J1) is now forward biased and the other junction (J2) is reverse-biased, practically all the electrons injected by the forward-biased junction into the pregion will recombine before they reach the second junction. [Electrons diffuse from the emitter towards the collector because of the concentration gradient. The electron concentration would be enhanced near a forward biased junction and depleted near a reverse biased junction]. The
electron