Basic Electronics Engineering : Basics of Semiconductor diodes and transistors and characteristics

INTRODUCTION TO PN JUNCTION DIODE

Majority carrier electrons in the n-region will begin diffusing into the p-region and majority carrier holes in the p-region will be diffusing into the n-region. If we assume there are no external excitation to the semiconductor, then this diffusion process cannot continue indefinitely. As electrons diffuse from the n-region, positively charged donor atoms are left behind. Similarly, as holes diffuse from the p-region, they uncover negatively charged acceptor atoms. The un-neutralized ions in the neighborhood of the junction are referred to as uncovered charges. The general shape of the charge density ‘ρ’ depends upon how the diode is doped. Since the region of the junction is depleted of mobile charges, it is called depletion region, the space-charge region, or the transition region.

Figure 1

The net positive and negative charges in ‘n’ and ‘p’ regions induce an electric fields in the region near the metallurgical junction, in the direction from the positive to the negative charge, or from the n to the p region.

Density gradients still exist in the exist in the majority carrier concentrations at each edge of the space charge region and producing a “diffusion force” that acts on the majority carriers as shown in Figure 1. The electric field in the space charge region produces another force on the electrons and holes which in the opposite direction to the diffusion force for each type of particle. In thermal equilibrium, the diffusion force and the field force exactly balance each other.

Symbol: 

The Arrow mark on the symbol denotes, the direction of forward current.

Volt-Ampere Characteristics of a P-n Junction Diode

Below figure indicates the characteristic curve consisting of three distinct regions:

 PN Junction Characteristics

NOTE:

When PN junction is reverse biased, the reverse voltage must be always less than breakdown voltage of device, otherwise the normal diode will be damaged.

Diode Resistance

The static resistance of a diode is defined as the ratio V/I of the voltage to the current. At any point on the volt-ampere characteristic of the diode the resistance R_f is equal to the reciprocal of the scope of a line joining the operating point to the origin.

BASIC STRUCTURE OF BJT

The bipolar transistor has three separately doped regions and two p-n junctions. Below Figure  shows the basic structure of an n-p-n bipolar transistor and p-n-p bipolar transistor, along with the circuit symbols. The three terminal connections are called the emitter base and collector. The width of the base region is small compared to the minority carrier diffusion length. Generally, the sequence of length of the three regions are W_B < W_E < W_C.

Figure: Simplified block diagram of (a) n-p-n (b) p-n-p and circuit symbol of (c) n-p-n and (d) p-n-p bipolar transistors

There are few specific features that differentiate one region from the other region. These are:

(a) Emitter Region

• A region which supplies or emits majority carriers, for example in p-n-p transistor emitter will supply holes and in n-p-n transistor it supplies electrons.
• Emitter is heavily doped, so that it can emit large number of carriers.
• Impurities are added in the ratio 1:10³.

(b) Collector Region

• A region which receives or collects majority carriers coming from emitter
• It is moderately doped and largest in size.
• Large collector will help in rapid transfer of heat to the surroundings.
• In a transistor, collector junction develops large amount of heat because it operates at higher current and higher voltage
• If collector is lightly doped its conductivity will decrease, which is undesired,
• If collector is heavily doped, breakdown voltage of collector junction will decrease which is undesired. Therefore, collector is moderately doped so that it has better conductivity and collector junction has higher breakdown voltage.

(c) Base Region

• A region through which majority carriers travel from emitter to collector.
• Base is lightly doped and small in size, i.e. it has narrow width.
• The number of electron-hole recombination inside base will be reduced by keeping it narrow and by doping it lightly.
• Impurities are added in ratio 1:10⁸.

The three terminal connections are called the emitter (E), base (B), and collector (C). The width of the base region is small compared to the minority carrier diffusion length. The (++) and (+) notation indicates the relative magnitudes of the impurity doping concentrations normally used in the moderately doped. The emitter region has the largest doping concentration; the collector region has the smallest.

Typical Doping Concentrations for BJT

Figure 2 shows an idealized impurity doping profile in an n-p-n bipolar transistor for the case when each region is uniformly doped. Typical impurity doping concentrations in the emitter, base, and collector may be in the order of 10¹⁹, 10¹⁷ and 10¹⁵ cm^-3, respectively.

Figure : Idealized doping profile of a uniformly doped n-p-n Bipolar Transistor

Depletion Region

A transistor with its three terminals emitter, base and collector left open is called an unbiased transistor, or an open-circuited transistor. The diffusion of the majority carries takes place across the junction due to the concentration gradient of charge carries. Due to the diffusion of the majority carries across the two junctions, depletion layer are formed at both emitter and collector junction as shown in figure .

Following are some important points about depletion region.

• The depletion layer around the emitter junction penetrates more into the base region and less into the emitter region. This is because the emitter is heavily doped as compared to the base.
• The depletion layer around the collector junction penetrates more into the base region and less into the collector region. This is because the collector is heavily doped, whereas the base is lightly doped.
• The depletion layer penetrates more into the collector region and less into the emitter region. Therefore, the depletion layer formed at collector junction is larger that depletion layer formed at emitter junction.

NOTE: An unbiased transistor is not useful for any practical purpose because the conduction of current across its junction is very small.

Modes of Operation

The transistor consists of two p-n junctions, the emitter-base junction (EBJ) and the collector-base junction (CBJ). Depending on the bias condition (forward or reverse) of each of these junctions, different modes of operation of BJT are obtained, as shown in Table .

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