Java CMOS basic gate demonstration

This page demonstrates how CMOS transistors and basic gates work. It is intended for our computer science undergraduate students. The applets were written as a test and working demonstration for Java/Hotjava.

Java Beta-1 version

The applets use different colors to indicate voltage levels. They will only display correctly on a color display and with Netscape 2.0b (Java-enabled).
The individual applets can also be run using the SUN appletviewer.

All comments, hints and bug reports are welcome: Please contact Further Java demonstrations on our server (ever wanted to learn ballroom dancing on the Net?) can be found here.

Basic CMOS technology

In CMOS (Complementary Metal-Oxide Semiconductor) technology, both N-type and P-type transistors are used to realize logic functions. Today, CMOS technology is the dominant semiconductor technology for microprocessors, memories and application specific integrated circuits (ASICs). The main advantage of CMOS over NMOS and bipolar technology is the much smaller power dissipation. Unlike NMOS or bipolar circuits, a CMOS circuit has almost no static power dissipation. Power is only dissipated, in case the circuit actually switches. This allows to integrate many more CMOS gates on an IC than in NMOS or bipolar technology, resulting in much better performance.

The following applets demonstrate the N-type and P-type transistors used in CMOS technology, the basic CMOS inverter, NAND and NOR gates, and finally a AOI32 complex gate.

The first applet illustrates the function of both N-type and P-type MOS transistors.

Click on the source and gate contacts of the transistors to toggle the corresponding voltage levels and watch the resulting output value on the drain contacts. The applet uses colors to display the different voltages.

Note that the N-type transistor is conducting when its input is '1', while the P-type transistor is conducting when its input is '0'. The applet displays the channel of a conducting transistor as a rectangle filled with the color of its source voltage. The channel of a nonconducting transistor is shown as rectangle outline in black.

The CMOS Inverter

The most important CMOS gate is the CMOS inverter. It consists of only two transistors, a pair of one N-type and one P-type transistor. The applet demonstrates how the inverter works.

Voltage levels are shown in colors as above: a logical '1' corresponding to electrical level VCC is shown in red, a logical '0' (corresponding to 0V or GND) in blue.

Again, a floating wire (not connected to either VCC or GND) is shown in orange. Because of parasitic effects, the voltage level on such wire may reach some undefined voltage between VCC and GND after some time. A floating wire will cause problems, when its voltage is around VCC/2, because a gate voltage around VCC/2 on either N-type and P-type transistors implies that the transistor is conducting. The applet illustrates why this is a serious problem: When both transistors are conducting, there is a direct path from VCC to GND, and this implies a short-circuit condition (shown in light green), which dissipates much energy and may destroy the device.

Click anywhere in the applet to toggle the input voltage for the inverter from GND to VCC to Z (unknown) to GND.

If the input voltage is '1' (VCC) the P-type transistor on top is nonconducting, but the N-type transistor is conducting and provides a path from GND to the output Y. The output level therefore is '0'. On the other hand, if the input level is '0', the P-type transistor is conducting and provides a path from VCC to the output Y, so that the output level is '1', while the N-type transistor is blocked.

If the input is floating, both transistors may be conducting and a short-circuit condition is possible:

Power consumption of the CMOS inverter

The previous discussion of the CMOS inverter shows why CMOS logic has (almost) no static power dissipation: If the gate voltage is either '1' or '0' there is no conducting path from VCC to GND, and there is no static current through the inverter. In normal operation, the short-circuit condition shown in the applet above arises only during the very short interval, when the gate voltage is switched. Typical switching times for the gate are around 2 ns, and the static current dissipation occurs only during a fraction of this time (while the input voltage is near VCC/2). All other basic CMOS gates have almost no static power dissipation as well.

But there is a dynamic current dissipation in CMOS gates. The applet below illustrates this effect for the CMOS inverter. The gate of a MOS transistor forms a small capacitor. Typical values for the gate capacity are of order Cg = 10 fF. If the input of the inverter is connected to VCC at time t1, this capacitor is charged (Charge Q = Cg * VCC). If the input is connected to GND at time t2 it is discharged. The net effect of this is a very small current of I = dQ/dt = (Cg * VCC)/(t2-t1).

However, due to several reasons the total current drawn by a big CMOS chip, for example a microprocessor, can be quite large:

The total current resulting from the short-circuit currents during switching is difficult to estimate. But the current resulting from switching the input capacitance alone is quite large in the example:
I ~ #gates * (Ctotal*VCC) / dt = (1% * 1.000.000) * (1pF * 3.3V) / 5ns = 6.6 A

On the other hand, the quiescent current in typical static CMOS ICs is very small. For example, an 2K*8 bit CMOS SRAM dissipates only 1 uA when idle.

The next applet illustrates the current dissipation in the CMOS inverter. If the input voltage stays at '1' or '0', either the N-type or the P-type transistor in nonconducting, and there is no current through the inverter.

The power consumption of static CMOS logic is therefore directly proportional to switching frequency.

The basic NAND and NOR gates

The following three applets demonstrate the basic 2-input NAND and NOR gates, and a 3-input NAND gate. As in all static CMOS gates, each input is connected to the gates of a pair of N-type and P-type transistor.

Usage of the applets: The applets are similar to the Inverter applet. Wires with logical '1' (VCC) are again shown in red, wires with logical '0' (GND) in blue. Unknown floating values are shown in orange color, a short-circuit is shown in green.

To simplify the applet, only '1' and '0' levels are allowed as input values. Obviously, a short-circuit may occur in these gates as well, if the input voltages are floating near VCC/2 (because both N-type and P-type will be conducting, thereby providing direct paths from VCC to GND).

The NOR gate with 2 inputs:

The 2-input NOR gate is the simplest CMOS gate to illustrate the name complementary MOS: The P-type transistors are connected in series between VCC and the output Y, while the N-type transistors are connected in parallel between GND and the output Y. That is, the N-type and the P-type parts of the CMOS gate are complementary (in respect to topology, and therefore function).

Only if both inputs A and B are '0' (corresponding to GND), there is a conducting path from VCC to the output (output level '1'). A input combination with A or B '1' blocks the path from VCC to the output, but opens a path from GND to the output (so that the output level is '0').

Watch the voltage level between the two P-type transistors. If both are nonconducting, the voltage level is unknown (floating). However, as that wire is not connected to any MOS-transistor gate, there is no problem:

The NAND gate with 2 inputs:

In the NAND gate the P-type transistors are connected in parallel between VCC and the output Y, while the N-type transistors are connected in series from GND to the output Y.

The NAND gate with 3 inputs:

The generalization of the 2-input NOR and NAND gates is obvious. As an example, the next applet shows a NAND gate with 3-inputs. As for the 2-input NAND, all (three) P-type transistors are connected in parallel between VCC and the output Y, while all N-type transistors are connected in series.

Again, the wires connecting the N-type transistors may have floating voltage levels when the transistors are nonconducting. That is no problem, because these wires are not connected to any transistor gate.

NOR gates with three and more inputs are constructed correspondingly - all P-type transistors are connected in series and the N-type transistors are connected in parallel between GND and the output Y.

However, the series connection of transistors implies longer propagation delay (especially for P-type transistors) and a voltage drop across the transistors. Therefore, NAND gates for actual CMOS cell libraries are usually limited to 4-inputs (4 N-type transistors in series) and NOR gates to 3-inputs (3 P-type transistors in series).

NAND and NOR gates with more inputs are realized as a combination of simpler gates with up to 3 (4) inputs.

Complex gates

As in NMOS technology, there are certain logic functions that can be realized very efficiently by CMOS gates. These gates are called complex gates because they realize a complex logic function - although the gates themselves are rather simple.

Typical example for complex gates found in almost all cell libraries are combinations of AND-OR-INVERT and OR-AND-INVERT gates.
For example, the gate for the logic function Y = !((A ^ B) | (C ^ D ^ E)) - that is, the NOT of the OR of two ANDs - is typically called an AOI32 gate. This gate is shown in the next applet. It needs 10 transistors only (5 pairs of N-type and P-type transistors, one pair for each input).

To simplify the circuit schematic, the input wires are not drawn completely. Rather, all input wires are broken into two pieces, connected to the corresponding N-type and P-type transistors. For example, to toggle the input voltage for input A, you can click both near the N-type or the P-type transistor gate connected to input A.

Note that again the N-type and P-type paths in this gate are complementary. P-type transistors connected in series correspond to N-type transistors connected in parallel, and vice versa.

The generalization to other complex gates should be obvious. Again, gates with more than three transistors connected in series are not used. Most cell libraries contain gates from AOI21 and OAI21 up to aOI33 and OAI33. Try to construct one of these on paper and understand how it works!

The CMOS transmission gate

The first applet on this page demonstrated the switching behaviour of N-type and P-type transistors. However, while both N-type and P-type transistors indeed have a very large resistance between source and drain when switched off, a detailed analysis reveals that the resistance between source and drain depends on the source and drain voltages when switched on. Especially, there is a voltage drop across a conducting N-type transistor when the source voltage is near VCC, and a voltage drop across a conducting P-type transistor when its source voltage is near GND. (Note that thhis poses no problem in the static CMOS gates, where all source contacts of N-type transistors are connected to GND and all source contactes of P-type transistors are connected to VCC.)

Therefore, the use of single N-type or P-type transistors as switches is limited to circuits, where the voltage drop across the conducting transistors is not critical. A series connection of transistors used as switches is usually not possible in digital circuits.

But a combination of N-type and P-type transistors allows to realize efficient switches in CMOS technology. The circuit consists of one N-type and one P-type transistor connected in parallel and controlled by inverted gate voltages. This circuit, called a transmission gate (T-gate) is demonstrated in the following applet:

If the gate voltage of the N-type transistor is '0', the P-type transistors has a gate voltage of '1' and both transistors are nonconducting. On the other hand, if the gate voltage of the N-type transistor is '1' and the gate voltage of the P-type transistor is '0', both transistors are conducting. If the source voltage is near VCC, there is a voltage drop across the N-type transistor but (almost) no voltage drop across the P-type transistor. If the source voltage is near GND, the N-type transistor has (almost) no voltage drop. Because of the symmetry of standard MOS transistors, there is no reason to differantiate between source and drain in a T-gate. The contacts are therefore usually called 'L' (left) and 'R' (right).

The CMOS D-latch with T-gates

In CMOS technology, T-gates allow efficient realizations of several important logical functions. Perhaps the most important use is demonstrated in the next applet. A standard D-latch (level controlled flipflop) can be build from four 2-input NAND gates. Therefore, 16 transistors are needed for one D-latch. (Try to construct this circuit on paper - the simple D-latch circuit is one of our pet examination problems!)

The applets shows how a D-latch can be realized using only 8 transistors (2 inverters and 2 T-gates), if both the clock and the inverted clock signal are available - a savings of 50% of transistors and therefore chip-area. If the inverted clock is not available, an additional inverter is needed to provide the control signal for the two T-gates, still with a savings of 10 transistors versus 16.

The function of the T-gate D-latch is very easy to understand. If the C input is '1', the input T-gate is conducting, and the data input value is connected to the first inverter input, which generates !D. The second inverter generates !!D = D, that is, the output Q equals D (the latch is transparent).

If the C input is switched to '0', the first T-gate is blocked while the feedback T-gate (whose R contact is connected to Q, which still equals D) is opened. Therefore, the circuit feeds its Q signal back to the first inverter (!Q) and to the second inverter !!Q=Q. That is, the value of Q is stored until the C input becomes '1' again.

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last change: 29.07.95, Norman Hendrich