Basics of Diodes (Types and Overview of Diodes)

Basics of Diodes (Types and Overview of Diodes)
Application Note
Basics of Diodes (Types and Overview of Diodes)

Outline:
Diodes are used in a wide range of equipment for various applications such as rectification, reverse-current blocking, and circuit protection. In addition to silicon (Si) pn diodes, various other types of diodes are available, including Schottky barrier diodes (SBDs), transient voltage suppressor (TVS) diodes (also known as ESD protection diodes), and Zener diodes. Toshiba’s product portfolio also includes state-of-the-art silicon carbide (SiC) SBDs fabricated using a compound semiconductor. This application note provides an overview of the classification and operation of diodes.

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Toshiba Electronic Devices & Storage Corporation

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Basics of Diodes (Types and Overview of Diodes)
Application Note
Table of Contents
Outline:................................................................................................................... 1 Table of Contents ..................................................................................................... 2 1. Types of diodes .................................................................................................... 5
1.1. Classification of diodes according to their applications and characteristics .....................5 2. Overview of various types of diodes ........................................................................ 6
2.1. pn diodes............................................................................................................6 2.1.1. What is a pn junction? ............................................................................................ 6 2.1.2. pn junction in an unbiased state ............................................................................... 7 2.1.3. pn junction in the forward-biased state ..................................................................... 8 2.1.4. pn junction in the reverse-biased state......................................................................9 2.1.5. Electric characteristics of pn diodes......................................................................... 10 2.1.6. PIN diode ............................................................................................................ 11
2.2. Schottky barrier diodes ....................................................................................... 13 2.2.1. Schottky junction ................................................................................................. 13 2.2.2. JBS structure ....................................................................................................... 17
2.3. SiC Schottky barrier diodes.................................................................................. 18 2.3.1. What is SiC? ........................................................................................................ 18 2.3.2. SBD with an improved JBS structure ....................................................................... 20
2.4. Zener diodes ..................................................................................................... 22 2.4.1. Zener breakdown and avalanche breakdown............................................................ 22 2.4.2. Forward characteristics of Zener diodes................................................................... 24 2.4.3. Reverse characteristics of Zener diodes ................................................................... 24
RESTRICTIONS ON PRODUCT USE ........................................................................ 27

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Basics of Diodes (Types and Overview of Diodes)
Application Note
List of Figures
Figure 1.1 Types of diodes...........................................................................................5 Figure 1.2 Classification of diodes according to their internal connections .....................6 Figure 2.1 Fermi level of intrinsic semi- conductors ............................................................6 Figure 2.2 Fermi level of p-type semi- conductors ..............................................................6 Figure 2.3 Fermi level of n-type semi- conductors ..............................................................6 Figure 2.4 P-type and n-type semi- conductors before they are joined together .....................7 Figure 2.5 P-type and n-type semi- conductors after they are joined together ........................7 Figure 2.6 Band diagram of a pn junction .........................................................................7 Figure 2.7 Band diagram of a pn junction in an unbiased state.............................................8 Figure 2.8 Band diagram of a pn junction in the forward-biased state ...................................9 Figure 2.9 Band diagram of a pn junction in the reverse-biased state....................................9 Figure 2.10 Current-voltage curve of a pn diode .............................................................. 10 Figure 2.11 Example of a pn diode’s reverse current-vs-ambient temperature curve ............. 11 Figure 2.12 Structure of a PIN diode .............................................................................. 11 Figure 2.13 Increasing the breakdown voltage of a PIN diode ............................................ 12 Figure 2.14 Change in the dopant concentration of the n- layer due to conductivity modulation13 Figure 2.15 Example of a structure of a Schottky barrier diode........................................... 13 Figure 2.16 Band diagram of a metal ............................................................................. 14 Figure 2.17 Band diagram of an n-type semiconductor ................................................... 14 Figure 2.18 Band diagram of a Schottky junction in an unbiased state ................................ 15 Figure 2.19 Band diagram of a Schottky junction in the forward-biased state ....................... 15 Figure 2.20 Band diagram of a Schottky junction in the reverse-biased state ....................... 16 Figure 2.21 JBS structure ............................................................................................. 17 Figure 2.22 Formation of depletion layers by the application of a reverse bias across the Schottky
junction................................................................................................................ 17 Figure 2.23 Expansion of depletion layers by an increase in reverse bias across the Schottky
junction................................................................................................................ 17 Figure 2.24 Comparison of the reverse current characteristics of SBDs with typical and JBS
structures............................................................................................................. 18 Figure 2.25 Bandgap of Si ............................................................................................ 19 Figure 2.26 Bandgap of SiC .......................................................................................... 19

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Basics of Diodes (Types and Overview of Diodes)
Application Note
Figure 2.27 Comparison of electric fields in Si and SiC SBDs .............................................. 20 Figure 2.28 Example of a waveform of a charging current flowing to a capacitor for full-wave
rectification........................................................................................................... 20 Figure 2.29 Cross-sectional view of an SBD with an improved JBS structure ........................ 21 Figure 2.30 Forward current-vs-forward voltage curves of a typical SBD, a pn diode, and an SBD
with an improved JBS structure ............................................................................... 21 Figure 2.31 Zener breakdown ....................................................................................... 22 Figure 2.32 Avalanche breakdown ................................................................................. 23 Figure 2.33 Example of Zener voltage temperature coefficient vs Zener voltage ................... 24 Figure 2.34 Example of IZ-VZ curves at different Zener voltages ......................................... 25
List of Tables
Table 2-1 Physical properties of typical semiconductor materials ........................................ 19

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Basics of Diodes (Types and Overview of Diodes)
Application Note
1. Types of diodes
1.1. Classification of diodes according to their applications and characteristics
Different types of diodes are available for various applications. Diodes are broadly classified into two types: pn diodes that are based on the pn junction formed by p-type and n-type semiconductors and metal-semiconductor diodes (commonly known as Schottky barrier diodes or SBDs for short) that are formed by the junction of a metal with either an n-type or p-type semiconductor. Figure 1.1 shows the classification of diodes according to their applications and characteristics.

Figure 1.1 Types of diodes

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Basics of Diodes (Types and Overview of Diodes)
Application Note
Figure 1.2 shows the classification of diodes according to their internal connections.

Figure 1.2 Classification of diodes according to their internal connections
2. Overview of various types of diodes
2.1. pn diodes 2.1.1. What is a pn junction?
A pn junction is an interface between two types of semiconductor materials, p-type and n-type, maintaining their orderly crystal lattice arrangements. These semiconductors have different Fermi levels. The Fermi level of the p-type region is close to the highest energy level of a valence band whereas the Fermi level of the n-type region is close to the lowest energy level of a conduction band.

Figure 2.1 Fermi level of intrinsic semiconductors

Figure 2.2 Fermi level of p-type semiconductors

Figure 2.3 Fermi level of n-type semiconductors

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Basics of Diodes (Types and Overview of Diodes)
Application Note
2.1.2. pn junction in an unbiased state Electrons in the n-type semiconductor close to the pn junction diffuse into the p-type semiconductor and recombine with holes there. Conversely, holes in the p-type semiconductor diffuse into the n-type semiconductor and recombine with electrons there. The recombination of carriers (free electrons and holes) causes a region where no carriers exist to be formed as shown in Figure 2.5. This region is called a depletion layer.

Figure 2.4 P-type and n-type semiconductors before they are joined together

Figure 2.5 P-type and n-type semiconductors after they are joined together

When p-type and n-type semiconductors are joined together, carrier diffusion causes the Fermi level of the p-type semiconductor to move upwards and that of the n-type semiconductor to move downwards, resulting in the formation of new Fermi levels as shown in Figure 2.6. The decrease in the Fermi level in the conduction band is equal to the work function of the p-type and n-type regions. As a result, a diffusion potential (qVD) appears between them.

Diffusion potential

Figure 2.6 Band diagram of a pn junction

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Basics of Diodes (Types and Overview of Diodes)
Application Note
Figure 2.7 shows the electron and hole densities in the p-type and n-type semiconductors when the pn junction is unbiased. Electrons in the n-type semiconductor that have energy higher than Ecp can move into the p-type semiconductor. Likewise, holes in the p-type semiconductor that have energy lower than Evn can move into the n-type semiconductor. At this time, the electron densities in the n-type and p-type semiconductors (nn and np) become almost equal. When nn≈np, the movement of electrons stops, reaching an equilibrium state. The hole densities in the n-type and p-type semiconductor (pn and pp) have the same relationship.
Density of holes in the n-type semiconductor Density of holes in the p-type semiconductor

Figure 2.7 Band diagram of a pn junction in an unbiased state
2.1.3. pn junction in the forward-biased state When a forward bias of qVB is applied across the pn junction (with the p-type semiconductor being more positive than the n-type semiconductor), its diffusion potential decreases from qVD in the unbiased state to q(VD-VB). As a result, the density of electrons in the n-type semiconductor that have energy higher than Ecp becomes higher than the electron density in the p-type semiconductor, causing a huge amount of electrons to move from the n-type semiconductor to the p-type semiconductor. At the same time, a multitude of holes in the p-type semiconductor diffuse into the n-type semiconductor. Therefore, electric current flows.

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Basics of Diodes (Types and Overview of Diodes)
Application Note
Density of holes in the n-type semiconductor Density of holes in the p-type semiconductor
Figure 2.8 Band diagram of a pn junction in the forward-biased state 2.1.4. pn junction in the reverse-biased state When a reverse bias of qVR is applied across the pn junction (with the p-type semiconductor being more negative than the n-type semiconductor, the p-type semiconductor is applied negatively and the n-type semiconductor is applied positively), the diffusion potential increases from qVD in the unbiased state to q(VD+VR). As a result, the density of electrons in the n-type semiconductor that have energy higher than Ecp becomes almost zero. Likewise, holes in the p-type semiconductor that can diffuse into the n-type semiconductor almost disappear, causing the current flow to stop.
Density of holes in the n-type semiconductor Density of holes in the p-type semiconductor

Figure 2.9 Band diagram of a pn junction in the reverse-biased state

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Basics of Diodes (Types and Overview of Diodes)
Application Note
2.1.5. Electric characteristics of pn diodes
In the case of a diode formed by a pn junction, the forward current (IF) is expressed by Equation 2-1. This equation applies to the low-current region, but not to the high-current region because the internal resistance of a diode causes a voltage drop in the high-current region.

𝑞∙𝑉𝐹
𝐼𝐹 = 𝐼𝑆 ∙ (𝑒 𝐾∙𝑇 − 1)

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IF

: Forward current

(A)

IS

: Saturation current

(A)

q

: Electron charge (1.602×10–19)

(C)

VF

: Forward voltage

(V)

K

: Boltzmann constant (1.381×10-23)

(J･K-1)

T

: Junction temperature (in Kelvin)

(K)

(2-1)

0.6 to 0.7V
Figure 2.10 Current-voltage curve of a pn diode
Figure 2.10 shows the current-voltage curve of a pn diode. When a forward bias of 0.6 to 0.7 V is applied across a Si diode, its forward current rises suddenly. The forward voltage (VF) of the Si diode has a temperature coefficient of roughly -2 mV/°C. However, the temperature coefficient decreases in the high-current region because a voltage drop due to internal resistance has a positive temperature coefficient. The current that flows through a diode when it is reverse-biased is called reverse current (IR)*3 or saturation current (IS). IR is approximated by Equation 2-2: *3 Also called leakage current

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