Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 20.1 Field-Effect Transistors  Introduction  An Overview of Field-Effect Transistors  Insulated-Gate Field-Effect Transistors  Junction-Gate Field-Effect Transistors  FET Characteristics  Summary of FET Characteristics  FET Amplifiers  Other FET Applications Chapter 20

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 20.2 Introduction  Field-effect transistors (FETs) are probably the simplest form of transistor –widely used in both analogue and digital applications –they are characterised by a very high input resistance and small physical size, and they can be used to form circuits with a low power consumption –they are widely used in very large-scale integration –two basic forms:  insulated gate FETs  junction gate FETs 20.1

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 20.3 An Overview of Field-Effect Transistors  Many forms, but basic operation is the same –a voltage on a control input produces an electric field that affects the current between two other terminals –when considering amplifiers we looked at a circuit using a ‘control device’ –a FET is a suitable control device 20.2

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 20.4  Notation –FETs are 3 terminal devices  drain (d)  source (s)  gate(g) –the gate is the control input –diagram illustrates the notation used for labelling voltages and currents

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 20.5 Insulated-Gate Field-Effect Transistors  Such devices are sometimes called IGFETs (insulated-gate field-effect transistors) or sometimes MOSFETs (metal oxide semiconductor field-effect transistors)  Digital circuits constructed using these devices are usually described as using MOS technology  Here we will describe them as MOSFETs 20.3

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 20.6  Construction –two polarities: n-channel and p-channel

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 20.7  Operation –gate volt controls the thickness of the channel –consider an n-channel device  making the gate more positive attracts electrons to the gate and makes the gate region thicker – reducing the resistance of the channel. The channel is said to be enhanced  making the gate more negative repels electrons from the gate and makes the gate region thinner – increasing the resistance of the channel. The channel is said to be depleted

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 20.8 –the effect of varying the gate voltage

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 20.9 –gates as described above are termed Depletion- Enhancement MOSFETs or simply DE MOSFETs –some MOSFETs are constructed so that in the absence of any gate voltage there is no channel  such devices can be operated in an enhancement mode, but not in a depletion mode (since there is no channel to deplete)  these are called Enhancement MOSFETs –both forms of MOSFET are available as either n-channel or p-channel devices

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  MOSFET circuit symbols

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT Junction-Gate Field-Effect Transistors  Sometimes known as a JUGFET  Here we will use another common name – the JFET  Here the insulated gate of a MOSFET is replaced with a reverse-biased pn junction  Since the gate junction is always reverse-biased no current flows into the gate and it acts as if it were insulated 20.4

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  Construction –two polarities: n-channel and p-channel

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  Operation –the reverse-biased gate junction produced a depletion layer in the region of the channel –the gate volt controls the thickness of the depletion layer and hence the thickness of the channel –consider an n-channel device  the gate will always be negative with respect to the source to keep the junction between the gate and the channel reverse- biased  making the gate more negative increases the thickness of the depletion layer, reducing the width of the channel – increasing the resistance of the channel.

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT –the effect of varying the gate voltage

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  JFET circuit symbols

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT FET Characteristics  While MOSFETs and JFETs operate in different ways, their characteristics are quite similar  Input characteristics –in both MOSFETs and JFETs the gate is effectively insulated from the remainder of the device  Output characteristics –consider n-channel devices –usually the drain is more positive than the source –the drain voltage affects the thickness of the channel 20.5

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT 20.17

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  FET output characteristics

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  Transfer characteristics –similar shape for all forms of FET – but with a different offset –not a linear response, but over a small region might be considered to approximate a linear response

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  Normal operating ranges for FETs

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  When operating about its operating point we can describe the transfer characteristic by the change in output that is caused by a certain change in the input –this corresponds to the slope of the earlier curves –this quantity has units of current/voltage, which is the reciprocal of resistance (this is conductance) –since this quantity described the transfer characteristics it is called the transconductance, g m Note:

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  Small-signal equivalent circuit of a FET –models the behaviour of the device for small variations of the input about the operating point

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT Summary of FET Characteristics  FETS have three terminals: drain, source and gate  The gate is the control input  Two polarities of device: n-channel and p-channel  Two main forms of FET: MOSFET and JFET  In each case the drain current is controlled by the voltage applied to the gate with respect to the source  Behaviour is characterised by the transconductance  The operating point differs between devices 20.6

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  FET circuit symbols:

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT FET Amplifiers  A simple DE MOSFET amplifier –R G is used to ‘bias’ the gate at its correct operating point (which for a DE MOSFET is 0 V) –C is a coupling capacitor and is used to couple the AC signal while preventing externals circuits from affecting the bias –this is an AC-coupled amplifier 20.7

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  AC-coupled amplifier –input resistance – equal to R G –output resistance – approximately equal to R D –gain – approximately –g m R D (the minus sign shows that this is an inverting amplifier) –C produced a low-frequency cut-off at a frequency f c given by where R is the input resistance of the amplifier (which in this case is equal to R G )

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  Negative feedback amplifier –reduces problems of variability of active components –voltage across R s is proportional to drain current, which is directly proportional to the output voltage –this voltage is subtracted from input voltage to gate –hence negative feedback

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  Source follower –similar to earlier circuit, but output is now taken from the source –feedback causes the source to follow the input voltage –produces a unity-gain amplifier –also called a source follower

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT Other FET Applications  A voltage controlled attenuator –for small drain-to-source voltages FETs resemble voltage-controlled resistors –the gate voltage V G is used to control this resistance and hence the gain of the potential divider –used, for example, in automatic gain control in radio receivers 20.8

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  A FET as an analogue switch

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT  A FET as a logical switch

Storey: Electrical & Electronic Systems © Pearson Education Limited 2004 OHT Key Points  FETs are widely used in both analogue and digital circuits  They have high input resistance and small physical size  There are two basic forms of FET: MOSFETs and JFETs  MOSFETs may be divided into DE and Enhancement types  In each case the gate voltage controls the current from the drain to the source  The characteristics of the various forms of FET are similar except that they require different bias voltages  The use of coupling capacitors prevents the amplification of DC and produced AC amplifiers  FETs can be used to produce various forms of amplifier and a range of other circuit applications