The simplest electronic device is a diode. It is often called a semiconductor diode, but technically a diode has its own specific electrical characteristics. This is true for all electronic devices. They are defined by unique electrical characteristics, although different constructions, types and applications may be available.
That being said, a semiconductor diode is the most common and basic construction of a diode device.
Understanding Diodes
As an electronic device, a diode is a two-terminal one-way switch. In response to an applied signal, it acts as a closed switch for the polarity of a voltage and an open switch for reverse polarity.
Two important characteristics define an electronic device such as a diode:
1. It is a two-terminal device
2. It allows current conduction in one direction and opposes current conduction in the reverse direction.
As a result, any diode has two unique regions, regardless of its type. One of these is an active region, where an applied voltage polarity allows the diode to conduct current through it. The other is a reverse bias region, where an applied polarity causes the diode to oppose current conduction.
A diode is a simple device, but it has countless applications.
The semiconductor diode
A semiconductor diode is the most basic diode construction. In fact, the diode device concept evolved from the semiconductor diode. All semiconductor devices are constructed by joining intrinsic – extrinsic p-type and extrinsic n-type semiconductor materials.
Both materials are formed on an intrinsic substrate by doping acceptor impurity atoms into the donor p-type impurity atoms in the n-type regions, respectively. This creates a p-n junction.
The pn junction – with pen-type materials on both sides, with respective output terminals (conductors) – is a semiconductor diode.
The intrinsic material subjected to doping to form the p-n junction can be silicon, germanium or gallium arsenide.
A diode, as a simple p-n junction, represents the basic function of all semiconductor devices. The same principles that apply to a semiconductor diode apply to other sophisticated semiconductor devices, regardless of their design, complexity, operation or characteristics.
This is why learning about a semiconductor diode is fundamental to modern electronics.
Diodes in action
In the p-type material of a semiconductor diode, holes are the majority-charged carriers and electrons are the minority-charged carriers. In n-type material, it is reversed. Electrons are the carriers with majority charge and hole are the carriers with minority charge in n-type material.
The minority carriers in both materials represent the contribution from the intrinsic substrate and the majority carriers represent the contribution from the impurity atoms. The concentration of majority carriers is 100,000 times greater than that of minority carriers in both materials.
Furthermore, both can have different doping levels, which does not affect the electrical neutrality of either the materials or the diode.
As mentioned, a diode is a two-terminal device. The conductive end of the p-type material is an anode and the conductive end of the n-type is a cathode terminal.
As a result of its electrical characteristics, a diode has some regions of operation.
- In the active region of its voltage-current characteristics, it allows conventional current conduction from its anode to its cathode.
- In the non-conducting region of its voltage-ampere characteristics, it blocks any conventional current flow from the cathode to the anode.
As a voltage-controlled two-terminal device, a diode has three possible electrical conditions:
1. No external voltage is applied to the diode
2. The anode is at a higher potential than the cathode
3. The cathode is at a higher potential than the anode
The electrical conditions…
No bias applied: In the absence of any external voltage across the diode, there is no current flow through it. Once pen-type materials form a junction, p-type holes diffuse into the n-type material near the junction. This results in a layer of positive ions in the ne-type material around the junction.
Similarly, n-type electrons diffuse into the p-type material near the junction. This results in a layer of negative ions on the PE material around the junction. This forms a depletion region at the intersection, which has no free carrier on either side.
Because the majority carriers are in high concentration – there are almost 100,000 times more minority carriers in both materials – only a few of the majority carriers have enough energy to cross the depletion region (due to heat and light).
To pass through the diode, the holes in the p-type material will attempt to overcome the attractive force of the negative ions on the p-type side of the junction and the repulsive force of the positive ions on the n-type side of the junction. .
To pass through the diode, the n-type electrons must also overcome the attractive force of the positive ions on the n-type side of the junction and the repulsive force of the negative ions on the p-type side of the junction. Only a few majority carriers gain enough kinetic energy to cross this depletion region, which is canceled out by the movement of minority carriers across the junction.
As a result, in the absence of any applied voltage, there is no current in the diode. Therefore, the only way current can flow through the diode is if the majority charge carriers gain enough kinetic energy to cross the junction under the influence of an external electric field.
Forward bias: When an anode is at a higher potential than a cathode, the diode is considered forward biased. Due to the positive potential at the conducting end of the p-type material, the holes in this material are pushed towards the n-type. Similarly, due to the negative potential at the conducting end of the n-type material, the electrons in this type of material are pushed towards the p-type.
As a result, the depletion region begins to shrink. At a given positive voltage difference, known as the cutoff voltage , the depletion region allows an abundance of majority carriers from both sides to flow through the diode. This causes an exponential increase in current through the diode.
As the forward bias voltage increases beyond the activation voltage, many majority carriers gain enough kinetic energy (under the influence of the external voltage) to cross the depletion region.
The current will continue to increase with the directly applied voltage until it reaches a maximum limit, where the diode acts as a conductor. The maximum current through the diode in a forward-biased condition is limited by the concentration of free charge carriers in both materials. The higher the dopant level of both materials, the higher the forward current limit of the diode.
After the forward voltage is removed from the diode, the depletion region slowly recovers and the diode returns to a non-conducting state, as is the case without any applied voltage.
Reverse Bias: When a cathode is at a higher potential than an anode, the diode is considered to be in reverse bias. The negative potential at the conducting end of the p-type material pulls the holes in this material toward its conducting end. Similarly, the positive potential at the conducting end of the n-type pulls the electrons of this material toward its conducting end.
As a result, the depletion region widens and the carriers with the highest charge in both materials have no chance of crossing the depletion region. To cross the diode, this voltage polarity allows minority carriers to contribute across the intrinsic substrate on both sides. An extremely small current (due to minority carriers), known as reverse saturation current ,
flows through the diode. This is called reverse saturation current because it quickly reaches a maximum limit, beyond which it will not change.
Reverse saturation current is typically represented in nano amps or micro amps, except in high power diodes. The actual reverse current is greater than the reverse saturation current because it includes other factors such as leakage currents, temperature sensitivity, junction area, and charge carriers in the depletion region.
In electronic circuits, this is such a small amount of current that it is insignificant compared to the current in the lead wire and other current-active components of a network.
Split region: In a reverse bias condition, the depletion region increases as the reverse voltage increases. Because of the high reverse voltage, at a certain point, the minority carriers gain enough kinetic energy to initiate an ionization process when colliding with the atoms. As a result of ionization, several carriers are released in both materials, capable of passing through the diode. This causes a high avalanche current to flow from the cathode to the anode.
The strong collapse of minority carriers is known as avalanche collapse. The maximum reverse voltage before a strong avalanche current is driven through the diode is called peak reverse voltage (PRV) ap eak inverse voltage (PIV), or ak no voltage .
The feature region beyond the PIV classification is the Zener Region . By increasing the dopant level of pen-type materials, the PIV rating can be brought closer to -5V. Due to the increase in the doping level, another phenomenon, known as Zener Analysis occurs in which the increase in the current level is due to the strong electric fields that disturb the atomic bonds in the doped materials.
A special semiconductor diode heavily doped to have a Zener breakdown in reverse bias condition is a Zener Diode . Zener diodes are used for voltage regulation.
Voltage-current characteristics
A diode has two regions of operation. In a “no bias” condition, there is zero current flowing through it. In forward bias, the diode enters a conducting state. This means it allows a small current to pass through the anode until the activation voltage is reached.
Beyond the activation voltage, the current increases exponentially according to the equation:
eu = eu is *e DV/nVT – EU is
Where…
- I is the current passing through the diode
- L is is the reverse saturation current
- V D is the applied forward bias voltage
- n is the ideal factor, which varies between one and two, depending on the operating conditions and construction of the diode
- V T is thermal voltage
The thermal stress is:
V T =k*T K /q
Where…
- TV is thermal voltage
- k is Boltzmann's constant = 1.38*10 -23 J/K
- TK is the absolute temperature in Kelvin
- q is the charge of the electron = 1.6 * 10 -19 C
In forward bias, the forward current through the diode increases exponentially with the forward bias voltage. The value of thermal stress also increases with temperature. Therefore, as the temperature increases, the forward current decreases, and as the temperature decreases, the forward current increases.
In reverse bias, the reverse saturation current due to the minority carriers is the only current that flows through the diode until the knee voltage is reached. Forward current is in the mA range and increases in tenths of volts from forward bias. The reverse bias voltage is in the tens of volts and the reverse saturation current is typically in the pA or uA.
The cutoff voltage, reverse saturation current, and knee voltage depend on the minority charged carriers, which are contributed by the intrinsic substrate. Therefore, the turn-on voltage, reverse saturation current, and knee voltage depend on the substrate material.
The cut-off voltage for:
- Silicon diodes (Si): 0.7V
- Germanium diodes (Ge): 0.3V
- Gallium arsenide (GaAs) diodes: 1.2V
The reverse saturation current for:
- Silicon diodes (Si): 10pA
- Germanium diodes (Ge): 1uA
- Gallium arsenide (GaAs) diodes: 1pA
The peak inverse voltage of:
- Silicon Diodes (Si): 50V ~ 1kV
- Germanium (Ge) diodes: 100-400V
- Gallium Arsenide Diodes (GaAs): 100V ~ 20KV
DC signal response
When a DC signal is applied to a diode, it operates at a specific point related to its characteristic curve. Current flows through the diode only when the DC signal is applied in positive polarity.
Depending on the operating point, the diode conducts a fixed direct current in the mA range, offering a fixed DC/static resistance.
CA Response
When an AC signal is applied to a diode, its operating point on the characteristic curve continually shifts between the positive and negative peaks of the applied signal.
The current through the diode passes up and down at a quiescent point or Q point. This Q point is useful in determining the instantaneous AC resistance of the diode to the signal. The instantaneous AC resistance is derived by the tangent at point Q of the operating signal. Average AC resistance is determined by the change in voltage to the change in current at the positive and negative peaks of the AC signal.
If the applied signal has lower peak voltage levels, the diode's AC resistance to the signal will be greater. If the applied signal has higher peak voltage levels, the AC resistance of the diode will be lower.
Electrical Characteristics
Some important electrical characteristics of a semiconductor diode are:
- cut-off voltage
- Maximum direct current
- Reverse saturation current
- Reverse current
- PIV classification
- Zener voltage
- DC Resistance
- AC Resistance
- Medium AC resistance
- Transition Capacitance
- Diffusion capacitance
Types of diodes
The semiconductor diode is not the only type of diode available. However, there are several types of semiconductor diodes, each designed to operate in specific characteristic regions or to offer specific physical or electrical properties.
Some examples include power, Zeners, small signals, large signals, light-emitting diodes, and others.
For example, there are many diodes with special constructions, such as laser, Shockley and Shottky diodes, etc. Regardless of construction, operational characteristics or physical properties, the characteristic curve and electrical behavior of all diodes remain similar.
All diodes are two-terminal, voltage-controlled unidirectional switches.