Electrical components

The following sections take a closer look at various types of semiconductor devices, including diodes, light-emitting diodes (LEDs), bipolar transistors and field-effect transistors. They cover the electrical behaviour, structure, key parameters and diverse applications of these devices.

1 Diode

The diode is the semiconductor component with the simplest structure, consisting of only one pn junction. This results in two terminals: the anode at the p-doped region and the cathode at the n-doped region. As already described in section 1.4, the pn junction allows current to flow in only one direction, which is why diodes are often used for rectification and voltage stabilisation.

1.1 Electrical behaviour

The electrical behaviour of diodes is essentially described by their characteristic curve, which is highly non-linear. This characteristic curve represents the current flowing through the diode as a function of the externally applied voltage. Characteristic areas of the characteristic curve are the forward, reverse and breakdown areas.

• Forward bias range: Operation in the forward bias range occurs when a positive voltage is applied in the direction of the diode, i.e. when the electrical potential at the anode is greater than that at the cathode (range to the right of the y-axis, see Figure 2). At a voltage above the barrier voltage (\(U_\mathrm {S}\)), the diode current increases approximately exponentially. The barrier voltage is a material-specific voltage that is required at least to reduce the space charge zone sufficiently to allow current to flow. For silicon diodes, this is typically between \(0.6\) and \({0.7}\,\mathrm {V}\).
• Blocking range: Occurs when the applied voltage is negative (range to the left of the y-axis). Hardly any current flows in this range because the resistance of the diode is very high.
• Breakdown region: At a voltage below the breakdown voltage (\(U_\mathrm {BR}\)), a breakdown occurs and a very high current flows abruptly. If the diode is not specifically designed for breakdown, the high current leads to overload and consequently to destruction of the component.

The equivalent circuit diagram of a silicon diode helps to better understand the behaviour of the diode in different operating states. It simplifies the real diode by modelling its essential characteristics.

Components of the equivalent circuit diagram:

• Ideal diode (D): Conducts in the forward direction without voltage loss and completely blocks in the reverse direction.
• Lock voltage/threshold voltage (\(U_\mathrm {S}\)/ \(U_\mathrm {T0}\)): This voltage is the value at which a measurable current flows through the real diode. In the equivalent circuit diagram, this is represented by an ideal voltage source.
• Differential resistance (\(r_\mathrm {D}\)): The differential resistance (\(r_\mathrm {D}\)) describes the resistance of the diode in the forward region. It is defined as the change in voltage (\(\Delta u_\mathrm {D}\)) divided by the change in current (\(\Delta i_\mathrm {D}\)) and models the non-linearity of the diode after reaching the gate voltage.

\begin {equation} r_D = \frac {\Delta u_\mathrm {D}}{\Delta i_\mathrm {D}} \label {Formel 1} \end {equation}

Determining the operating point of a diode is an important step in circuit design, as it defines the stable operating state of the diode, taking into account the applied voltage and forward current. It is important to consider the maximum power dissipation of the diode to ensure that it does not overheat and become damaged. Both graphical and mathematical methods can be used to determine the operating point. The graphical method is based on the representation of the current-voltage characteristic curve of the diode and the load line of the circuit. The operating point is the intersection of these two lines. Both the operating point can be determined by the specified resistance, and the resistance can be determined based on the desired operating point. In both cases, the operating point should be below the asymptote for the maximum power dissipation of the diode.

The mathematical method is based on solving equations that describe the forward current of the diode and the voltage drop across the load. The operating point is determined by equating these two expressions. It is important to take into account the maximum power dissipation of the diode to ensure that it is operated within its specifications.

1.2 Structure

As already described in the introduction, the diode usually consists of a single pn junction. In addition to the simple silicon diode discussed so far, there are numerous other types of diodes. In this section, the structure discussed so far is compared with that of a Schottky diode. This is very common and does not require a pn junction. The pn diode consists of two semiconductor regions, namely the p-doped (positively charged) and the n-doped (negatively charged) region, which meet at a common interface. The p-side is called the anode and the n-side is called the cathode.

Typically, the pn junction consists of silicon or germanium. The electrons from the n-side recombine with the holes from the p-side at the interface, which leads to the formation of a space charge zone. This space charge zone forms the barrier for current flow in the reverse direction. A Schottky diode consists of a metal-semiconductor junction instead of a pn junction. During manufacture, no different dopants need to be introduced. Applying a suitable metal layer is sufficient. The semiconductor material is typically n-doped. The junction between the metal and the semiconductor forms a Schottky barrier that blocks the flow of current. With a suitable combination of materials, a space charge region can form at the interface, similar to a silicon diode.These diodes are optimised for fast switching operations and a low voltage drop in the forward direction.

1.3 Special diodes

In addition to the diodes mentioned above, there are a number of other variants that have different properties due to their design and enable corresponding applications. The following table provides an overview of typical diodes.

1.4 Applications/Basic circuits

This section introduces typical applications and basic circuits of diodes. Topics covered include single-phase rectifiers, bridge rectifiers, and series and parallel connections of diodes. Single-phase rectifiers and bridge rectifiers are essential circuits for converting alternating current into direct current.

Single-phase rectifier
A single-phase rectifier is a simple circuit for converting alternating current into direct current. This circuit typically consists of a single diode connected in series with the load. The main function of the single-phase rectifier is to allow only the positive half-waves of the alternating current to pass through, while blocking the negative half-waves.
During the positive half-wave of the alternating current, the potential at the anode is higher than that at the cathode, causing the diode to conduct in the forward direction. This allows current to flow through the diode and the connected load, resulting in a positive voltage across the load. During the negative half-wave of the alternating current, the potential at the anode is lower than that at the cathode, causing the diode to operate in the reverse direction and block the flow of current. As a result, the potential at the anode is lower than that at the cathode, causing the diode to operate in the reverse direction and block the flow of current. During the negative half-wave of the alternating current, the potential at the anode is lower than that at the cathode, so the diode operates in reverse direction and blocks the flow of current. As a result, no voltage is applied to the load. The following figure shows an example of the input voltage and the resulting voltage at the load. The difference between the two voltages (\(\Delta u\)) corresponds to the voltage drop across the diode. In the example, the voltage drop is approximated by the barrier voltage of a silicon diode.

The half-wave rectifier has the advantage of a simple circuit and low cost, making it suitable for basic applications where simple rectification is sufficient. However, this circuit also has significant disadvantages. Since only the positive half-waves of the alternating current are used, efficiency is low and the output voltage has a strong ripple, which is referred to as ripple voltage. This ripple can be reduced by additional filter and smoothing circuits. A capacitor may be sufficient to generate a more stable DC voltage.

Bridge rectifier
A bridge rectifier is a widely used circuit for converting alternating current into direct current. This circuit consists of four diodes arranged in a bridge configuration. The bridge rectifier utilises both half-waves of the alternating current, resulting in more efficient rectification than with half-wave rectifiers.

During each half-wave of the alternating current, two of the four diodes conduct and form a path for the current to flow through the load. In the positive half-wave, two diodes conduct the current in one direction (Figure 8 left), and in the negative half-wave, the other two diodes conduct the current in the same direction through the load (Figure 8 right).

The resulting voltages, as a consequence of the two current paths shown previously, are illustrated below. This behaviour of the bridge rectifier means that the voltage across the load always has the same polarity, which produces a rectified output voltage.

The output voltage has significantly shorter interruptions compared to a half-wave rectifier, resulting in a more stable and smoother DC voltage. However, the bridge rectifier requires more diodes than a half-wave rectifier, leading to higher costs and a greater voltage drop.

Series and parallel connection
Series and parallel connection of diodes are fundamental methods for adjusting the electrical characteristics of circuits. These configurations are often used to meet the voltage and current requirements of diodes in various applications.

• Series connection: In a series connection, several diodes are connected in sequence. The maximum permissible total voltage is the sum of the reverse voltages of the individual diodes. This makes it possible to block higher voltages than with a single diode. This configuration is often used in high-voltage applications.
• Parallel connection: In a parallel connection, several diodes are connected in parallel, with the anodes and cathodes connected to each other. This configuration increases the maximum current carrying capacity, as the current is divided between the diodes. This method is often used to distribute the current and reduce the load on individual diodes.

By combining diodes in series or parallel circuits, the electrical properties of the overall circuit can be specifically adjusted to meet specific requirements.

2 Bipolar transistor

The bipolar junction transistor (BJT) is an essential component of many electronic circuits. It consists of three layers of semiconductor materials with alternating doping: the emitter (E), the base (B) and the collector (C). The functioning of a bipolar transistor is based on the control of the current flow between the collector and emitter by the base current. Depending on the order of the doping, a distinction is made between npn and pnp transistors. The following figure shows a simplified cross-section, the representation using diodes according to the pn junctions and the respective circuit symbol.

2.1 Electrical behaviour

The following section examines the electrical behaviour of the more commonly used NPN transistor based on the differently doped semiconductor layers and the associated band model. The behaviour can also be applied to the PNP transistor accordingly. Depending on the structure, two PN junctions are formed between the three layers. The blocking behaviour of the two diodes in the unconnected state can be seen both in the formed space charge zone (SCZ) and in the high energy difference between the bands in the different areas. The different heights of the composite band model can be explained by the Fermi levels of the individual areas. In the p region, the Fermi level is lower than in the n region. However, since the Fermi level is constant across the individual regions, this results in the steps shown in the band model.

If a positive voltage is applied between the collector and emitter, the pn junction between the base and emitter is in the forward direction, but the junction between the base and collector is in the reverse direction, which is why no current flow is possible. The charge carriers provided by the voltage source reduce the RLZ between the base and emitter. Conversely, the electrons flowing away from the collector side increase the associated RLZ. In addition, the following figure shows the strong shift in the bands due to the applied voltage.

Applying an additional positive voltage between the base and emitter completely breaks down the RLZ in the direction of the emitter. The applied voltage corresponds to the barrier voltage of the pn junction. As a result, electrons can pass from the emitter to the base. These electrons are located immediately in front of the RLZ of the collector-base junction, which is in the forward direction. Consequently, the electrons are accelerated by the electric field formed and can completely traverse the semiconductor. The potential difference between the base and collector, which the electrons can easily traverse, can also be seen in the band model.

The following figure shows the relevant currents and voltages for npn and pnp transistors. The voltages are usually referenced to the emitter potential and the currents are plotted in the direction of the transistors. In the case of the npn transistor, this results in a negative value for the emitter current, which represents the sum of the two partial currents.

To describe the electrical behaviour, the collector and emitter currents as well as the two aforementioned voltages are considered. This results in four couplings: the currents as a consequence of the corresponding voltages and, in each case, between the two currents and the two voltages. First, the input characteristic curve, the base current \(I_\mathrm {B}\) as a result of the base-emitter voltage \(U_\mathrm {BE}\), is considered. As can be seen in the following figur, the curve corresponds to that of a diode characteristic curve. The characteristic curve shown is representative of a defined collector-emitter voltage \(U_\mathrm {CE}\). If this voltage varies, the curve is stretched or compressed. This is also referred to as the input characteristic curve field. As already known from the diode characteristic curve, the differential resistance \(r_\mathrm {BE}\) can be determined from the slope at the operating point.

\begin {equation} r_\mathrm {BE} = \frac {\Delta u_\mathrm {BE}}{\Delta i_\mathrm {B}} \end {equation}

In the output characteristic curve, the collector current \(I_\mathrm {C}\) is considered as a result of the collector-emitter voltage \(U_\mathrm {CE}\). In the active range, the influence of \(U_\mathrm {CE}\) on \(I_\mathrm {C}\) is negligible and a linear relationship can be assumed. In real components, the curve is flatter than shown in the illustration. Depending on the base current, the output characteristic field shown (see Figure 16) results. The differential resistance \(r_\mathrm {CE}\) can be determined using the slope of the characteristic curve.

\begin {equation} r_\mathrm {CE} = \frac {\Delta u_\mathrm {CE}}{\Delta i_\mathrm {C}} \end {equation}

Below the active region, at low values of \(U_\mathrm {CE}\), both diodes are switched in the forward direction and the transistor goes into saturation with a decreasing collector current. If the characteristic curves are extended into the negative region to the left of the y-axis, they intersect at a point on the x-axis. The underlying dependency is referred to as the Early effect and the voltage as the Early voltage (\(U_\mathrm {Early}\)).

Another relationship considers the effect of the control current \(I_\mathrm {B}\) on \(I_\mathrm {C}\). The representation is therefore referred to as the current control characteristic field. This results in the parameter direct current gain \(B\) and the differential current gain factor \(\beta \) at the operating point. \begin {equation} B = \frac {I_\mathrm {C}}{I_\mathrm {B}} \end {equation} \begin {equation} \beta = \frac {\Delta i_\mathrm {C}}{\Delta i_\mathrm {B}} \end {equation}

In the last case, the feedback characteristic field, the relationship between \(U_\mathrm {BE}\) and \(U_\mathrm {CE}\) is considered. The couplings depend on the set base current. Analogous to the description for current, the differential DC voltage gain \(D\) at the operating point can be considered for voltages. \begin {equation} D = \frac {\Delta u_\mathrm {BE}}{\Delta u_\mathrm {CE}} \end {equation}

Combining the four characteristic curves results in the following four-quadrant characteristic curve (see Figure 17). The representation of all couplings makes it possible, for example, to graphically transfer a desired operating point at the output to the input and to determine the parameters required for this.

The differential variables used for mathematical description at the operating point are also called small-signal parameters. Using the small-signal parameters, the following equations can be set up and represented in an equivalent circuit diagram. This can be used for small-signal operation up to approx. 1 MHz. \begin {equation} u_\mathrm {BE} = r_\mathrm {BE} \cdot i_\mathrm {B} + D \cdot u_\mathrm {CE} \end {equation} \begin {equation} i_\mathrm {C} = \beta \cdot i_\mathrm {B} + \frac {u_\mathrm {CE}}{r_\mathrm {CE}} \end {equation}

Since D is typically very small, the voltage source can usually be neglected.

2.2 Operating point and small-signal behaviour

The characteristic curves shown above can be used to determine the operating point and the necessary parameters. To do this, let us consider the circuit shown in Figure 19.

The supply voltage \(U_\mathrm {V}\) is \(20\,\mathrm {V}\). Half the supply voltage should be applied across the resistor \(R_C\), which represents an ohmic load, and a current of \(10\,\mathrm {mA}\) should flow.

In the first step, the resistance line can be plotted in the output characteristic field, starting from the operating voltage. The second necessary point represents the desired operating point A. Starting from this point, the operating point can be transferred to the other quadrants.

The following operating points and parameters result in the respective quadrants. \begin {flalign*} &\text {Output (at: $I_\mathrm {B}={20}\,{\mathrm {\mu A}}$):}&&\\ &&U_\mathrm {CE} &= {10}\,\mathrm {V}\\ &&I_\mathrm {C} &= {10}\,\mathrm {mA} \\ &&R_\mathrm {C} &= \frac {U_\mathrm {RC}}{I_\mathrm {RC}} = \frac {U_\mathrm {V}/2}{I_\mathrm {C}} = \frac {{10}\,\mathrm {V}}{{10}\,\mathrm {mA}} = {1000}\,{\Omega }\\ &\text {Current control:}&&\\ &&I_\mathrm {C} &= {10}\,\mathrm {mA} \\ &&I_\mathrm {B} &= {25}\,\mathrm {\mu A} \\ &&B&=\frac {I_\mathrm {C}}{I_\mathrm {B}}=\frac {{10}\,\mathrm {mA}}{{25}\,\mathrm {\mu A}}=400 \\ &\text {Entrance:}&&\\ &&I_\mathrm {B} &= {25}\,\mathrm {\mu A} \\ &&U_\mathrm {BE} &= {0,72}\,\mathrm {V} \\ &&R_\mathrm {V} &= \frac {U_\mathrm {RV}}{I_\mathrm {RV}} = \frac {U_\mathrm {V} - U_\mathrm {BE}}{I_\mathrm {B}} = \mathrm {\frac {{20}\,{V} - {0,72}\,{V}}{{25}\,{\mu A}}} ={771,2}\,\mathrm {k \Omega }\\ &\text {Feedback:}&&\\ &&U_\mathrm {BE} &= {0,72}\,\mathrm {V} \\ &&U_\mathrm {CE} &= {10}\,\mathrm {V} \\ &&D &= \frac {U_\mathrm {BE}}{U_\mathrm {CE}} = \mathrm {\frac {0,72\,V}{10\,V}} = {0,072} \end {flalign*}

In the event of fluctuations in the base current, whether due to an analogue input signal or interference, the resulting operating points or ranges can also be represented using the four-quadrant characteristic curve field. The behaviour can be seen in the following figure. An increase in the base-emitter voltage (\(U_\mathrm {BE}\)) also causes an increase in the base current (\(I_\mathrm {B}\)). In accordance with the coupling via the current control characteristic curve, the collector current (\(I_\mathrm {C}\)) also increases; only the collector-emitter voltage (\(U_\mathrm {CE}\)) decreases in this example.

2.3 Structure

Until now, the bipolar transistor has been depicted as stacked according to the left side of Figure 22, with the differently doped regions lying directly on top of each other and the base in the middle. This simplified representation cannot be directly implemented in reality, as the different dopings are introduced one after the other into a surface. Accordingly, the lower layers cannot be directly electrically contacted as shown on the left. A typical implementation can be seen on the right in Figure 22. In an n-doped semiconductor, a p-doped area is subsequently introduced for the base. Within this p-well, further dopant atoms are introduced, creating a smaller n-well for the emitter. The desired layer structure with n-p-n can thus be seen below the emitter region. The different regions can be electrically connected by means of conductive connections on the surface.

2.4 Application/Basic circuits

Basic circuits
With bipolar transistors, a distinction is made between three basic circuits: emitter, collector and base circuits. The naming is based on the reference potential of the input and output voltage.

The most common use is the emitter circuit shown above, in which the transistor is used as an inverting amplifier. The inverting behaviour refers to a phase shift of -180° of the output signal relative to the input signal. The following figure shows the corresponding equivalent circuits; the resulting properties are summarised in Table 2. The differential resistance \(r_\mathrm {CE}\) is parallel to the current source \(\beta \cdot i_\mathrm {B}\). Since it is very large, it is not taken into account in the simplified consideration.

Transistor as a switch
If small electrical loads need to be switched quickly and without contact, a bipolar transistor can be used. This assumes two different states: conductive and non-conductive in the collector-emitter path. In the conducting state, the C-E path has low resistance and represents a closed switch; in the blocking state, the opposite is true. The state can be controlled via the base-emitter path, resulting in an emitter circuit. The following figure shows the corresponding circuit and the voltage curves.

Darlington transistor
If the necessary current amplification of a single transistor is too low, it is possible to use a Darlington transistor or a Darlington circuit. This is based on two transistors, whereby the emitter of the first transistor feeds the base of the second transistor. This is based on two transistors, with the emitter of the first transistor feeding the base of the second transistor. Approximately, the resulting current amplification is the product of the individual amplifications:

\begin {equation} B \approx B_1 \cdot B_2 \end {equation}

3 Field-effect transistor

In addition to the bipolar transistor, there is another widely used type of transistor known as the field-effect transistor (FET). Its operating principle differs fundamentally from that of the bipolar transistor. While in a bipolar transistor a pn junction is set to the conductive state by a control terminal, in a field-effect transistor an electric field influences the distribution of free charge carriers in the semiconductor. This changes the resistance in the component. In FETs, the three terminals are called gate (G), source (S) and drain (D). The gate, also known as the control electrode, which influences the electric field, can be designed in different way, resulting in different types of transistors. The following figure shows an overview of the different types of transistors, including bipolar transistors. FETs are mainly divided into three types: junction field-effect transistors (JFETs), self-conducting MOSFETs (enrichment type) and self-blocking MOSFETs (depletion type). The suffix MOS stands for metal-oxide-semiconductor, the original layer structure of the control electrode.

3.1 Electrical behaviour

The general electrical behaviour of FETs differs greatly from that of the bipolar transistors discussed so far, in which a base current controls the load current. This requires less power than a bipolar transistor. In FETs, depending on their design, the resistance of the current path is controlled by a voltage applied to the gate and the resulting electric field. The current flow is thus controlled without power.

The following section takes a closer look at the functioning of a self-blocking n-channel MOSFET as an example. The necessary voltages are not shown in relation to the source, but to the fourth connection, bulk (B). In transistors, this connection is usually already connected to the source, which is why they can be equated. For the sake of understanding and the resulting electric fields, it is listed separately.

1. Without applied gate voltage (\(U_\mathrm {GS} ={0}\,{\mathrm {V}}\))
If there is no voltage between the gate and source (\(U_\mathrm {GS}\)), space charge zones, also known as depletion zones, form at the pn junctions. There are no free charge carriers in the channel area between the drain and source, which is why no current flow is possible.

2. Positive gate voltage (\(U_\mathrm {GS} > U_\mathrm {th}\))
If a positive voltage greater than the threshold voltage(\(U_\mathrm {th}\)) is applied between the gate and source, the electric field pulls electrons from the p-doped substrate into the vicinity of the gate oxide, leading to the formation of a conductive channel. The so-called inversion zone contains an enrichment of free charge carriers with the opposite sign to the charge carriers that primarily predominate in the semiconductor layer.

3. Electric current (\(U_\mathrm {DS} >{0}\,\mathrm {V}\))
Once an n-channel has been formed, a voltage can be applied between the drain and source (\(U_\mathrm {DS}\)) to generate a current flow. The electrons move from the source to the drain, creating a current flow (\(I_\mathrm {D}\)). At low \(U_\mathrm {DS}\), the MOSFET is in the linear region, where the current flow \(I_\mathrm {D}\) is proportional to \(U_\mathrm {DS}\). The behaviour is similar to that of a resistor. At higher \(U_\mathrm {DS}\), the MOSFET reaches the saturation region, in which \(I_\mathrm {D}\) becomes largely independent of further increases in \(U_\mathrm {DS}\). The current flow is primarily controlled by \(U_\mathrm {GS}\).

Saturation is caused by the fact that, at high voltage, the free charge carriers are displaced from the channel and the channel is constricted. The charge carriers can still pass through this area, but no further increase in \(I_\mathrm {D}\) is possible due to \(U_\mathrm {DS}\).

After a detailed examination of the self-locking n-channel MOSFET, the following table provides a summary of other types of field-effect transistors. The overview shows the input characteristic curve with \(I_\mathrm {D}\) as a function of \(U_\mathrm {GS}\) and the output characteristic field with \(I_\mathrm {D}\) as a function of \(U_\mathrm {DS}\) at different values of \(U_\mathrm {GS}\) are shown. The individual curves differ in the signs of the currents and voltages, depending on the respective doping. In addition, the different types differ in the size of the threshold voltage \(U_\mathrm {th}\).

Type

Symbole

Input characteristic curve

Output characteristiccurve field

n-channel JFET

p-channel JFET

n-channel MOSFET
self-locking

p-channel MOSFET
self-locking

n-channel MOSFET
self-directed

p-channel MOSFET
self-directed

3.2 Structure

The previous section showed a schematic cross-section of an n-channel MOSFET. For many applications and logic circuits, p-channel MOSFETs are also required. Both complementary variants can be realised using the same starting material, which is then referred to as CMOS technology (Complementary Metal-Oxide-Semiconductor). In the first step of the manufacturing process, local n-doping must be carried out for the p-channel MOSFET (PMOS); the rest of the structure corresponds to that of an n-channel MOSFET (NMOS) with complementary doping. Although the M in MOSFET originally stands for metal, for manufacturing reasons the gate material today is usually made of conductive polysilicon. The electrodes and the interconnection of the areas and the gate are, for example, made using aluminium structures.

The following figure shows a possible cross-section of an n-channel JFET. In direct comparison to the MOSFET, the structure is significantly simpler due to the absence of the insulation layer on the gate. Although the functioning of a MOSFET was described earlier, the JFET could be manufactured about 15 years earlier due to its simpler structure. The highly doped \(\mathrm {n^+}\) areas serve to improve contact with the n-channel and are not absolutely necessary. This means that only three doped areas and their electrical contacts are essential: the n-channel with drain and source connections, and the p-doped gate above and below the channel.

3.3 Applications/Basic circuits

Controllable resistance
When operating in the linear range, the MOSFET has a variable resistance value that can be controlled electronically. The adjustable value can be used as a control element for more complex electronic circuits. An important advantage of using such a transistor is that the control signal is very well isolated from the resistance terminals. The following figure shows how, in the simplest case, a MOSFET serves as a controllable resistor in a voltage divider, as well as a general representation of the circuit represented by it. It should be noted that the circuit shown has only a very limited range of applications. The reasons for this are the limited linearity of the transistor, its low resistance and the influence of the resistor \(R\) on \(U_\mathrm {DS}\).

Switch
A MOSFET can be operated as a switch by controlling the voltage \(U_\mathrm {GS}\). When switched on, a sufficiently positive voltage (for n-channel MOSFETs) is applied to the gate, creating a conductive channel between the source and drain. This allows current to flow through the transistor and the load connected in series. When switched off, \(U_\mathrm {GS}\) is reduced, which removes the conductive channel and stops the current flow.

Typical applications for MOSFETs as switches are switching power supplies, motor controls, logic circuits and generally as replacements for relays. The operating range is in the saturationrange.

Inverter
As already described in the section on structure, CMOS structures contain both p-channel and n-channel MOS transistors. A typical basic element based on this is an inverter, also known as a NOT gate. The input signal \(E\) is fed to the gates connected in parallel. The potential between the drain-source sections connected in series serves as the output signal. The circuit is shown in Figure 36 on the left. This results in the following two states:

1. Input signal \({0}\,\mathrm {V}\) (low):

• \(Transistor_\mathrm {2}\) (\(T_\mathrm {2}\)) is conductive, since \(U_\mathrm {GS2}\) is negative.
• \(Transistor_\mathrm {1}\) (\(T_\mathrm {1}\)) is blocking, since \(U_\mathrm {GS1}\) is zero.
• The current can flow from the supply voltage through \(T_\mathrm {2}\) to the output, or \({5}\,\mathrm {V}\) is present at Q (high signal)..

2. Input signal \({5}\,\mathrm {V}\) (high):

• \(T_\mathrm {2}\) is blocking, since \(U_\mathrm {GS2}\) is zero.
• \(T_\mathrm {1}\) is conductive, since \(U_\mathrm {GS1}\) is positive.
• The current can flow from the output through \(T_\mathrm {1}\) to ground, or \({0}\,\mathrm {V}\) is applied to Q (low signal).

According to the behaviour described, a discretised output signal results that is inverse to the input signal (see Figure 36 on the right). It should be noted that the desired behaviour only occurs reliably at voltages close to \({0}\,\mathrm {V}\) or the supply voltage. At half the supply voltage, for example, the value of the output signal is not precisely defined and the transistors can be either conducting or blocking, or assume a state in between. In addition, this state should be avoided because the supply voltage is short-circuited and a large current flow is possible.