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A Transistor

Objectives

  • Understand the working of a transistor
  • Learn to use transistors in circuits

Let’s begin!

A transistor is a three-terminal semiconductor that is used for amplification or switching electronic signals. A transistor is a semiconductor, meaning that sometimes it conducts electricity, and sometimes it doesn’t.  Its internal resistance varies, depending on the power that you apply to its base. There can be divided into PNP and NPN transistors. The NPN type is a sandwich with P-type silicon in the middle, and the PNP type is a sandwich with N-type silicon in the middle.

Transistor types

The most common type of transistor we use in circuits is the Bipolar Junction Transistor.

• All bipolar transistors have three connections: Collector, Base, and Emitter, abbreviated as C, B, and E respectively.
• NPN transistors are activated by positive voltage on the base relative to the emitter.
• PNP transistors are activated by negative voltage on the base relative to the emitter.

Using transistors in Circuits

fig A

You can think of a bipolar transistor as if it contains a little button inside. When the button is pressed, it allows a large current to flow. To press the button, you inject a much smaller current into the base by applying a small voltage to the base. In an NPN transistor, the control voltage is positive. In a PNP transistor, the control voltage is negative.

• Never apply a power supply directly across a transistor. You can burn it out with too much current.
• Protect a transistor with a resistor, in the same way, you would protect an LED.
• Avoid reversing the connection of a transistor between positive and negative voltages.

If you forget which wire is which, some multimeters have a function that will identify emitter, collector, and base for you.

Basic circuit representation of a transistor is given in fig A.

Working of Transistor

Now suppose we use three layers of silicon in our sandwich instead of two. We can either make a p-n-p sandwich (with a slice of n-type silicon as the filling between two slices of p-type) or an n-p-n sandwich (with the p-type in between the two slabs of n-type). If we join electrical contacts to all three layers of the sandwich, we can make a component that will either amplify a current or switch it on or off—in other words, a transistor. Let’s see how it works in the case of an n-p-n transistor.

So we know what we’re talking about, let’s give names to the three electrical contacts. We’ll call the two contacts joined to the two pieces of n-type silicon the emitter and the collector, and the contact joined to the p-type silicon we’ll call the base. When no current is flowing in the transistor, we know the p-type silicon is short of electrons (shown here by the little plus signs, representing positive charges) and the two pieces of n-type silicon have extra electrons (shown by the little minus signs, representing negative charges).

Another way of looking at this is to say that while the n-type has a surplus of electrons, the p-type has holes where electrons should be. Normally, the holes in the base act like a barrier, preventing any significant current flow from the emitter to the collector while the transistor is in its “off” state.

A transistor works when the electrons and the holes start moving across the two junctions between the n-type and p-type silicon.

Let’s connect the transistor up to some power. Suppose we attach a small positive voltage to the base, make the emitter negatively charged, and make the collector positively charged. Electrons are pulled from the emitter into the base—and then from the base into the collector. And the transistor switches to its “on” state.

The small current that we turn on at the base makes a big current flow between the emitter and the collector. By turning a small input current into a large output current, the transistor acts like an amplifier. But it also acts like a switch at the same time. When there is no current to the base, little or no current flows between the collector and the emitter. Turn on the base current and a big current flows. So the base current switches the whole transistor on and off. Technically, this type of transistor is called bipolarbecause two different kinds (or “polarities”) of electrical charge (negative electrons and positive holes) are involved in making the current flow.

We can also understand a transistor by thinking of it like a pair of diodes. With the base positive and the emitter negative, the base-emitter junction is like a forward-biased diode, with electrons moving in one direction across the junction (from left to right in the diagram) and holes going the opposite way (from right to left). The base-collector junction is like a reverse-biased diode. The positive voltage of the collector pulls most of the electrons through and into the outside circuit (though some electrons do recombine with holes in the base).

Working of a Field Effect Transistor

The FET used in many circuits constructed from discrete components in areas from RF technology to power control and switching to general amplification.

However the major use for the field effect transistor, FET is within integrated circuits. In this application FET circuits are able to consume only very small levels of power, and this enables the huge very large scale integrated circuits to operate. If bipolar technology was used the power consumption would be orders of magnitude greater and the power generated far too large to accommodate within a single integrated circuit.

The Junction FET is a voltage controlled device. In other words, voltages appearing on the gate, control the operation of the device.

Both N-channel and P-channel devices operate in similar ways, although the charge carriers are inverted, i.e. electrons in one and holes in the other. The case for the N-channel device will be described as this is the more commonly type used.

The thickness of this layer varies in accordance with the magnitude of the reverse bias on the junction. In other words when there is a small reverse bias the depletion layer only extends a small way into the channel and there is a large area to conduct current. When a large negative bias is placed on the gate, the depletion layer increases, extending further into the channel, reducing there area over which current can be conducted. With increasing bias the depletion layer will eventually increase to the degree that it extends right across the channel, and the channel is said to be cut off.

When a current flows in the channel the situation becomes slightly different. With no gate voltage electrons in the channel (assuming an n-type channel) will be attracted by the positive potential on the drain, and will flow towards it enabling a current to flow within the device, and hence within the external circuit. The magnitude of the current is dependent upon a number of factors and included the cross sectional area of the channel, its length and conductivity (i.e. the number of free electrons in the material) and the voltage applied.

From this it can be seen that the channel acts as a resistor, and there will be a voltage drop along its length. As a result of this it means that the p-n junction becomes progressively more reverse biased as the drain is approached. Consequently the depletion layer takes becomes thicker nearer the drain as shown. As the reverse bias on the gate is increased a point is reached where the channel is almost closed off by the depletion layer. However the channel never completely closes. The reason for this is that the electrostatic forces between the electrons cause them to spread out, giving a counter effect to the increase in thickness of the depletion layer. After a certain point the field around the electrons flowing in the channel successfully opposes any further increase in the depletion layer. The voltage at which the depletion layer reaches its maximum is called the pinch off voltage.

 

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