What is Transistor? Working of npn and pnp transistors.

Transistor was first invented in 1948 by J. Bardeen and W.H. Brattain of Bell Telephone Laboratories U.S.A. Though transistor is only slightly more than 40 years old, yet it is fast replacing old electronic equipment (vaccum tubes like triode and pentode) in almost all applications because of its innumerable merits viz longer life, higher efficiency, more mechanical strength, light in weight, smaller in size, smaller power consumption etc.

It is also known as Bipolar Junction Transistor (BJT) since its operation depends upon the interaction of both the majority and minority carriers. This device has now become more or less heart of most of the electronic applications. 

TRANSISTOR 

A semiconductor device consisting of two pn junctions formed by sandwitching either p-type or n-type semiconductor between a pair of opposite types is known as a transistor.

Accordingly, there are two types of transistors, namely: 

(ⅰ) npn transistor (ⅱ) pnp transistor

(ⅰ) npn Transistor : A transistor in which two blocks of n-type semiconductor are separated by a thin layer of p-type semiconductor is known as npn transistor.

(ⅱ) pnp Transistor : A transistor in which two blocks of p-type semiconductors are separated by a thin layer of n-type semiconductor is known as pnp transistor.

In both the types of transistors, the following points are worth nothing :

(ⅰ) There are two pn junctions connected back to back. In other words, a transistor may be regarded as two crystal diodes connected back to back. The diode on the left is called the emitter-base diode or emitter diode. Whereas, the diode simply on the right is called the collector-base diode or the collector diode.

(ⅱ) Transistor has three terminals, taken from each type of semiconductor.

(ⅲ) The middle section is made of a thin layer. This is the most important factor in the function of a transistor.

TRANSISTOR TERMINALS 

Every transistor has three terminals called emitter, collector and base.

Emitter : The section on one side of the transistor that supplies a large number of majority carriers is called emitter. The emitter is always forward biased w.r.t. base so that it can supply a large number of majority carriers to its junction with the base. Since emitter is to supply or inject a large amount of majority carriers into the base, it is heavily doped but moderate in size. 

Collector : The section on the other side of the transistor that collects the major portion of the majority carriers supplied by the emitter is called collector. The collector-base junction is always reverse biased. Its main function is to remove majority carriers (or charges) from its junction with base. The collector is moderately doped but larger is size so that it can collect most of the majority carriers supplied by the emitter.

Base : The middle section which forms two pn junctions between emitter and collector is called base. The base forms two circuits, one input circuit with emitter and the other output circuit with collector. The base-emitter junction is forward biased, providing low resistance for the emitter circuit. The base-collector junction is reversed biased, offering high resistance path to the collector circuit. The base is lightly doped and very thin so that it can pass on most of the majority carriers supplied by the emitter to the collector. 

UNBIASED TRANSISTOR 

When a transistor is unbiased, two pn junctions are formed. The free electrons diffuse across the junctions forming two depletion layers. For each of these depletion layers, the barrier potential is nearly 0.7 V at 25°C for a silicon transistor or 0.3 V for a germanium transistor. The width of the two depletion layers will be different as the three regions are doped at different levels. The more heavily doped a region is, the greater is the concentration of ions near the junction. This means that the depletion layer penetrates slightly into the emitter region (heavily doped) but deeply into the base (lightly doped). Therefore, the depletion  layer formed at the emitter junction is small.

However, at the collector junction, the depletion layer penetrates deep into the base (lighty doped) and quite a deep into the collector (moderately doped). Therefore, the depletion layer formed at collector junction is larger compartively.

This summarises that the depletion layer at the emitter junction is small and the depletion layer at the collector junction is large.

WORKING OF npn TRANSISTOR  

The emitter-base junction is forward biased while collector-base junction is reverse biased. The forward biased voltage V_EB is quite small, whereas, reverse biased voltage V_CB is considerably  high.  

As the emitter-base junction is forward biased, a large number of electrons (majority carriers) in the emitter (n-type region) are pushed towards the base. This constitutes the emitter current I_E. When these electrons enter the p-type material (base), they tend to combine with holes. Since the base is lightly doped and very thin, only a few electrons (less than 5%) combine with holes to constitute base current I_B. The remaining electrons (more than 95%) diffuse across the thin base region  and reach the collector space charge layer. These electrons then come under the influence of the positively biased n-region and are attracted or collected by the collector. This constitutes collector current I_C.

Thus, it is seen that almost the entire emitter current flows into the collector circuit. However, to be more precise, the emitter current is the sum of collector current and base current i.e.

I_E =I_C+I_B

WORKING OF pnp TRANSISTOR

The emitter- base junction is forward biased while collector-base junction is reverse biased. The forward biased voltage V_EB is quite small, whereas, the reverse biased voltage V_CB is considerably high. As the emitter-base junction is forward biased, a large number of holes (majority carriers) in the emitier (p-type semiconductor) are pushed towards the base. This constitutes the emitter current I_E. When these holes enter the n-type material (base), they tend to combine with electrons.

Since the base is lightly doped and very thin, only a few holes (less than 5%) combine with electrons to constitute base current I_B. The remaining holes (more than 95%) diffuse across the thin base region and reach the collector space charge layer. These holes then come under the influence of the negatively biased p-region and are attracted or collected by the collector. This constitutes collector current I_C. Thus, it is seen that almost the entire emitter current flows into the collector circuit. However, to be more precise, the emitter current is the sum of collector current and base current i.e.  

I_E = I_B + I_C

In the above explanation, it may be noted that inside the transistor, the current is constituted by the change of position of the holes. Whereas, outside the transistor i.e. in the leads of the circuit, the current is constituted by the flow of electrons.