<< Chapter < Page Chapter >> Page >


The diode is the simplest semiconductor device, made up of a p-type semiconductor and an n-type semiconductor in contact. It can conduct in only one direction, but it cannot control the size of an electric current. Transistors are more complicated electronic components which can control the size of the electric current flowing through them.

This enables them to be used in amplifiers. A small signal from a microphone or a radio antenna can be used to control the transistor. In response, the transistor will then increase and decrease a much larger current which flows through the speakers.

Interesting fact

One of the earliest popular uses of transistors was in cheap and portable radios. Before that, radios were much more expensive and contained glass valves which were fragile and needed replacing. In some parts of the world you can still hear people talking about their `transistor' — they mean their portable radio.

You can also use a small current to turn the transistor on and off. The transistor then controls a more complicated or powerful current through other components. When a transistor is used in this way it is said to be in switching mode as it is acting as a remotely controlled switch. As we shall see in the final sections of this chapter, switching circuits can be used in a computer to process and store digital information. A computer would not work without the millions (or billions) of transistors in it.

There are two main types of transistor - bipolar transistors (NPN or PNP), and field effect transistors (FETs). Both use doped semiconductors, but in different ways. You are mainly required to know about field effect transistors (FETs), however we have to give a brief description of bipolar transistors so that you see the difference.

Bipolar transistors

Bipolar transistors are made of a doped semiconductor `sandwich'. In an NPN transistor, a very thin layer of p-type semiconductor is in between two thicker layers of n-type semiconductor. This is shown in [link] . Similarly an PNP transistor consists of a very thin n-type layer in between two thicker layers of p-type semiconductor.

An NPN transistor. This is a type of bipolar transistor.

In an NPN transistor a small current of electrons flows from the emitter (E) to the base (B). Simultaneously, a much larger current of electrons flows from the emitter (E) to the collector (C). If you lower the number of electrons able to leave the transistor at the base (B), the transistor automatically reduces the number of electrons flowing from emitter (E) to collector (C). Similarly, if you increase the current of electrons flowing out of the base (B), the transistor automatically also increases the current of electrons flowing from emitter (E) to collector (C). The transistor is designed so that the current of electrons from emitter to collector ( I E C ) is proportional to the current of electrons from emitter to base ( I E B ). The constant of proportionality is known as the current gain β . So I E C = β I E B .

How does it do it? The answer comes from our work with diodes. Electrons arriving at the emitter (n-type semiconductor) will naturally flow through into the central p-type since the base-emitter junction is forward biased. However if none of these electrons are removed from the base, the electrons flowing into the base from the emitter will fill all of the available `holes'. Accordingly, a large depletion band will be set up. This will act as an insulator preventing current flow into the collector as well. On the other hand, if the base is connected to a positive voltage, a small number of electrons will be removed by the base connection. This will prevent the `holes' in the base becoming filled up, and no depletion band will form. While some electrons from the emitter leave via the base connection, the bulk of them flow straight on to the collector. You may wonder how the electrons get from the base into the collector (it seems to be reverse biased). The answer is complicated, but the important fact is that the p-type layer is extremely thin. As long as there is no depletion layer, the bulk of the electrons will have no difficulty passing straight from the n-type emitter into the n-type collector. A more satisfactory answer can be given to a university student once band theory has been explained.

Questions & Answers

what is the difference between momentum and a change in momentum?
Chavonne Reply
How to name a branched molecule from right or left?
Vadin Reply
What's Free Fall
Senzo Reply
Free Fall means there is no acting force on that object.
only gravitational force
no external force acting on an object
by only force of gravite
but gravitational force
Do polymers form restrictedly only if compound is saturated, only?
milani Reply
what is a free fall?
Beyanca Reply
is when The Only Force acting On an Object is Gravitational Force
Thats right
then Why ask when you Know the answer?
She's just helping those who forgot it...bro
guys I need help on Getting ready for a last minute test
Kenelioe Reply
what help you need
we'll I'm in grade 12 so we doing this topic about upac thing
on What?
the organic molecule section
you should also look at structural isomers. Its crucial that they might add that one. also try and write down the structural formula of all the given compounds on the table
hi guys i can explain everything in terms of physics and chemistry
Neil Reply
guys 2mrrow I'm writing a test in chemistry I need help
Hi guys. Can anyone please tell me what a functional group is?
a functional group depends on how many bonds there are between carbon atoms, if there are single bonds all the way it's an alkane, if there is a presence of at least one double bond it's a Alkene and if there's at least one triple bond it's an alkyne.
which quantity is measured in Watt?
Saara Reply
explain for me absai
is Power Not Work work is Measured In Joules it has Energy
what is galvanic cell ?
Oratile Reply
is an electrochemical cell that derives electrical energy from spontaneous redox reactions taking place within the cell.
how to understand alkane names
Sboniso Reply
suffix Ane in the IUPAC NAMING
elaborate a little more plz
Alkanes are Saturated Meaning they don't have Multiple Bonds between Carbon Atoms and When you are Naming THEM THEY MUST END WITH (-ANE ) GET ME?
What are hydrocarbons
Phelo Reply
are organic Compounds with only Carbon and Hydrogen Atoms
whats the difference between aldehydes and ketones
Angel Reply
definition of chain parent
Lea Reply
what is wave?
Agness Reply
when butane burns in an excess of oxygen ,the products are
CO2 and H2O
Carbon dioxide and Water...
what is hydrocarbon
Ntswaki Reply
Hydrocarbon is a compostion of mainy carbon and hydrogen
hydrocarbon compound made up of carbon and hydrogen only

Get the best Siyavula textbooks: gr... course in your pocket!

Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Siyavula textbooks: grade 12 physical science' conversation and receive update notifications?