Tutorial 6 - Transistors and MOSFETs

Learning Objectives:

• describe the use of an npn junction transistor as a switch;

• describe the use of an n-channel (enhancement mode) MOSFET as a switch;

• compare the advantages and disadvantages of a MOSFET and a junction transistor when they are both used as switches.

Bipolar Junction Transistors Transistors are solid state electronic switches.  Solid state means that there are no moving parts, and the switching action is regulated by the arrangement of the semiconductor materials.  A transistor has three terminals:

• Collector – takes current form the top rail (wire connected to the positive of the power supply).

• Emitter – delivers current to the load.

• Base – this is connected to the sensor part of the circuit.  The increase in voltage at the base turns the transistor on.

Click on the button to find out how a transistor works.

 Question 1 Complete the gap-fill exercise Current in a transistor

If the base emitter voltage is higher than about 0.5 - 0.6 V, the transistor turns on.  We say that it is forward-biased.  Current flows through an npn transistor from two sources:

• the base to the emitter, the base-emitter current (Ibe);

• the collector to the emitter, the collector-emitter current (Ice). For a pnp transistor, the emitter current splits to the emitter-base current and the emitter-collector current. If the voltage is reversed on a transistor, no current will flow.  We call this reverse bias. Notice here that

• the current coming out of the emitter (Ie) = 500 mA;

• the current going through the base (Ibe) = 5 mA;

• the current coming into the collector (Ice) = 495 mA

It doesn't take a genius to see that:

5 mA + 495 mA = 500 mA

Ie = Ibe + Ice

This is consistent with Kirchhoff's First Law.

We can measure the current gain of a transistor, which is the ratio of the collector-emitter current to the base-emitter current.

Gain = Ice ÷ Ibe

Gain is just a number; it has no units.  A gain of 40 tells us that the collector-emitter current is 40 times the base-emitter current.  So if the base-emitter current is 10 mA, the collector-emitter current is 400 mA = 0.4 A.  In catalogues, you may see gain as hfe.

 Question 2 What is the current gain of the transistor shown above?

Here is a typical circuit involving a transistor.  It is a light operated switch that uses a relay to turn on a mains-powered bulb. 1. When the LDR is in the dark, its resistance is high.

2. The LDR is part of a potential divider.

3. Therefore there is a large voltage across the LDR.

4. There is a voltage drop across the base of the transistor so that the base is at 0.7 V.

5. The resistor R3 limits the current to the base.

6. The transistor is turned on.

7. It can allow a big enough current to flow to turn the relay on.

8. The reverse biased diode D1 is there to protect the transistor from high voltage spikes that can occur when the relay turns off.  These could wreck the transistor.

 Question 3 A transistor circuit is shown below: V out = 7.0 V.  Calculate: (a) The voltage across the load (1 k) resistor. (b) The current through the load resistor (c) The base - emitter current, assuming that V be = 0.5 V (d) The current gain of the transistor

Characteristic Graphs for the Bipolar Transistor

Semiconductor transistors need a base-emitter voltage of about 0.6 V before they switch on, about the same as a diode.  We can plot a voltage current graph.  The gradient gives the conductance of the transistor. The voltage current characteristic sometimes called the transconductance.

We can measure the collector-emitter current and plot it against the base-emitter current: There is a linear region which gives the gain.  The gain is the gradient.  At very low currents, the graph is not quite linear, but we can ignore this.  At large values of base-emitter current, there is saturation.  We call this graph the transfer characteristic.  The transfer characteristic is specific for the kind of transistor.  In some texts, you may see that the transfer characteristic is a curve.  In which case, you need to take a tangent to find the gradient at a particular point.

The gain is referred to in some texts as hfe (h-parameter, forward bias, common emitter).  If the transistor is in a DC circuit, the gain parameter is written as hFE.  There is a subtle difference, but practical electronic engineers ignore it.  The circuits we are looking at are in the common emitter arrangement.  It is possible to have common base, and common collector arrangements.

The theory of bipolar transistors is very complex, and is beyond the scope of these notes.

MOSFETs

The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a Voltage controlled device.    This means that a voltage at the gate means that a current flows from the drain to the source.  For more about the MOSFET, press the button.

There are three terminals:

• gate - connected to the input device.

• drain - connected to the positive, since electrons drain away to the positive.

• source - the source of the electrons  We can use MOSFETs where we have a source of voltage that can provide very little current.  This circuit is a touch sensor: The general characteristics for a MOSFET are:

• The input resistance is very high, about 1012 W.

• The output resistance is about the same as a bipolar transistor.  The actual value depends on the type.  For a signal MOSFET it would be in the range 10 to 50 kW, while in a power MOSFET it would be somewhat lower.

The transconductance characteristic of a MOSFET is shown here. From this graph, the MOSFET turns on with a gate-source voltage of 2 V.  At 2.77 V the MOSFET is saturated and cannot carry a bigger current.

• The drain source voltage is fixed.

• From a threshold voltage of 2.0 V, the drain current increases linearly with the gate source voltage.

• The gain of any FET is measured using its transconductance.

Gain = DID

DVGS

[DID – change in drain current caused by the change DVGS.  Units are milliamps per volt (mA/V), or milliseimens (mS)*]

Transconductance can be measured by working out the gradient of the graph.

Here the MOSFET is used as a switch.  There are advantages and disadvantages when compared to the bipolar transistor as a switch:

 Advantages of a MOSFET Disadvantages Switching time is about 10 times faster than a bipolar transistor Higher resistance than a bipolar transistor Very much smaller switching current Can be destroyed by high voltages, especially static electricity Less affected by temperature

Darlington Pair

In this experiment a MOSFET is compared with a bipolar transistor to switch on a motor. The motor ran when the sensor (two 4 mm plugs) was put in the coffee.

But when a bipolar transistor was used, there was no effect.  However when a power transistor was added to the output of the bipolar transistor, the motor did run.  The arrangement on the right is called a Darlington pair.  On prototype board, it looks like this. Summary Transistor Current controlled device. Switches on when base-emitter voltage is 0.7 V Requires a base current of about 1 – 10 mA. MOSFET Voltage controlled device Input resistance is very high (1012 W). Very sensitive to static electricity.
 Links How transistors work (St Andrews University) Measuring transistor properties History of the transistor Video Tutorial