MOSFET

In previous sections we saw how transistors can be used to amplify signals. Modern digital logic gates use transistors as switches. Figure 11.3 illustrates an inverter logic gate, built from a single MOSFET and a resistor.

Figure 11.3a shows the actual circuit. The input to this circuit is the voltage, V_in , applied to the gate of the MOSFET. The output is the voltage, Vout , which appears at the connection between the MOSFET's drain and the resistor. The gate controls the ease with which current can flow from the transistor's source to drain. When V_in = 0 Volts there's no channel between the transistor's source and drain, so no current can get through the transistor. When we increase V_in we will form a channel and allow some current through the transistor.

We can represent this behaviour in two ways. Firstly, we can use the picture shown in 11.3b, where the transistor has been replaced by a second resistor, R_ds, whose value depends upon the gate (input) voltage. 

When Vin = 0 V the transistor won't pass any current — i.e. it behaves like a very high resistance. The circuit then behaves just as if we'd removed the transistor altogether and replaced it with an ‘open’ switch as illustrated in figure 13.3c. As a result, a low input voltage (logic ‘0’) makes the output appear as if it's only connected to the +5 Volts and we see an output of around 5 Volts (logic ‘1’).

The way in which R_ds varies with V_in depends upon the details of how the MOSFET was made. This determines how big a gate voltage is needed to create a source-drain channel large enough to pass a lot of current. Here we'll assume that the MOSFET was built so that it can only pass a lot of current when V_in is above about +2·5 Volts.

Using this kind of MOSFET we find that, when V_in is around 5 Volts, the transistor can pass current very easily — i.e. it behaves like a low resistance, i.e. R_ds = about 0 Ohms. In effect, this means the circuit behaves just as if we'd replaced the transistor with a ‘closed’ switch as shown in figure 13.3d. The output is now almost directly connected to zero volts, so V_out is approximately 0 Volts. As a result the truth table for the inverter is:

The exact resistance of the MOSFET isn't very important. Provided that current finds it almost impossible to get through the transistor when V_in is low (about 0 Volts) and very easy when V_in is high (about 5 Volts) we can analyse what the circuit is doing by regarding the transistor as a switch which is open or closed by the input voltage. We can use this same idea of ‘transistors as switches’ to analyse most modern digital electronics. In this case we can say that, “input high means output low, input low means output high”. The gate therefore acts as a logical inverter. A “1” in produces a “0” out and vice versa. The truth table for this kind of gate is shown above. Using A to represent its input and B to represent its output we can say it performs the logical function,

or “B equals not-A.” Note that logical inversion is indicated by putting a ‘bar’ over the symbol. Some people call this null-A, but electronics engineers tend to regard this term as a bit too fancy! Using it tells them that either:

a) you are a philosopher, not an engineer!
b) you read SF books written by van Vogt!!

By using this ‘transistors as switches’ approach we can now analyse some other basic digital logic gates.