Chapter 4
Schottky Rectifiers
A Schottky rectifier is formed by making an electrically nonlinear contact between a metal and the miconductor drift region. The Schottky rectifier is an attractive unipolar device for power electronic applications due to its relatively low on-state voltage drop and its fast switching behavior. It has been widely ud in power supply circuits with low operating voltages due to the availability of excellent devices bad upon silicon technology. In the ca of silicon, the maximum breakdown voltage of Schottky rectifiers has been limited by the increa in the resistance of the drift region. Commercially available devices are generally rated at breakdown voltages of less than 100 V. Novel silicon structures that utilize the charge-coupling concept have allowed extending the breakdown voltage to the 200 V range.1,2 Many applications described in Chap. 1 require fast switching rectifiers with low on-state voltage drop that can also support over 500 V. The much lower resistance of the drift region for silicon carbide enables development of such Schottky rectifiers with very high breakdown voltages.3 The devices not only offer fast switching speed but also eliminate the large rever recovery current obrved in high-voltage silicon P-i-N rectifiers. This reduces switching loss not only in the rectifier but also in the IGBTs ud within the power circuits.4 In this chapter, the basic structure of the power S悲开头的成语
chottky rectifier is first introduced to define its constituent elements. This chapter then provides a discussion of the basic principles of operation of the metal–miconductor contact. The current transport mechanisms that are pertinent to power devices are elucidated for both the forward and rever mode of operation. In the first quadrant of operation, the thermionic emission process is dominant for power Schottky rectifiers. In the third quadrant of operation, the influence of Schottky barrier lowering has a strong impact on the leakage current for silicon devices. In the ca of silicon carbide devices, the influence of tunneling current must also be taken into account when performing the analysis of the rever leakage current.
女王翻译B.J. Baliga, Fundamentals of Power Semiconductor Devices, doi: 10.1007/978-0-387-47314-7_4,
© Springer Science + Business Media, LLC 2008
168FUNDAMENTALS OF POWER SEMICONDUCTOR DEVICES
The tradeoff between reducing power dissipation in the on-state and the off-state for Schottky rectifiers is also analyzed in this chapter. This tradeoff requires taking into account the maximum operating temperature for the application. The power dissipation in the Schottky rectifier is shown to depend upon the barrier height as well as the duty cycle.
受到惊吓怎么办4.1 Power Schottky Rectifier Structure
The basic one-dimensional structure of the metal–miconductor or Schottky rectifier structure is shown in Fig. 4.1 together with electric field profile under rever bias operation. The applied voltage is supported by the drift region with a triangular electric field distribution if the drift region doping is uniform. The maximum electric field occurs at the metal contact. The device undergoes breakdown when this field becomes equal to the critical electric field for the miconductor.
Schottky rectifier by the transport of electrons over the metal–miconductor contact and through the drift region as well as the substrate. The on-state voltage drop is determined by the voltage drop across the metal–miconductor interface and the ohmic voltage drop in the resistance of the drift region, the substrate, and its ohmic contact.
At typical on-state operating current density levels, the current transport is dominated by majority carriers. Conquently, there is insignificant minority carrier stored charge within the drift region in the power Schottky rectifier. This enables switching the Schottky rectifier from the on-state to the rever-blocking off-state in a rapid manner by establishing a depletion region within the drift
Schottky Rectifiers
169 region. The fast switching capability of the Schottky rectifier enables operation at high frequencies with low power loss, making this device popular for high frequency switch-mode power supply applications. With the advent of high-voltage Schottky rectifiers bad upon silicon carbide, they are expected to be utilized in motor control applications as well.
金典证券4.2 Metal–Semiconductor Contact
筛子玩法Nonlinear current transport across a metal–miconductor contact has been known for a long time. The potential barrier responsible for this behavior was ascribed to the prence of a stable space-charge layer by Walter Schottky in 1938. In this ction, the principles for the formation of a rectifying contact between a metal and barrier height between the metal and the miconductor to their fundamental properties.
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shown in Fig. 4.2 when they are isolated from each other. In general, the position of the Fermi level in the metal and the miconductor will have different energy values. In the example shown in the fig
ure, the Fermi level in the miconductor lies above the Fermi level for the metal. The work function for the metal (ΦM ) is defined as the energy required to move an electron from the Fermi level position in the metal (E FM ) to a state of rest in free space outside the surface of the metal. In the same manner, the work function for the miconductor (ΦS ) is defined as the energy required to move an electron from the Fermi level position in the miconductor (E FS ) to a state of rest in free space outside the surface of the mi-conductor. Since no electrons are located at the Fermi level position in the miconductor, it is uful to define an electron affinity for the miconductor (χS )
an N-type miconductor region are described. This enables relating the Schottky The energy band diagram for a metal and an N-type miconductor is
170 FUNDAMENTALS OF POWER SEMICONDUCTOR DEVICES
南昌美食as the energy required to move an electron from the bottom of the conduction band in the miconductor (E C ) to a state of rest in free space outside the surface of the miconductor. The work function and electron affinity for the miconductor are related by
S S C FS ().E E Φχ=+− (4.1)车开头的成语
The potential difference between the Fermi level in the miconductor and the Fermi level in the metal is called the contact potential (V C ) which is given by
C FS FM M S M S C FS ()().qV E E E E ΦΦΦχ=−=−=−+− (4.2)
When an electrical connection is provided between the metal and the miconductor, electrons are transferred from the miconductor to the metal due
Schottky Rectifiers
171 to their greater energy until thermal equilibrium is established. This transfer of electrons creates a negative charge in the metal and a positive charge within a depletion region formed at the miconductor surface. The resulting band structure is illustrated in Fig. 4.3 for the ca of a paration d between the metal and the miconductor surfaces. When the metal and the micond
uctor surfaces are brought into contact by reducing the paration d to zero, the band structure for the metal–miconductor contact is obtained as illustrated in Fig. 4.4. The entire contact potential is now supported within the depletion region formed at the surface of the miconductor. This voltage is therefore also referred to as the built-in potential (V bi ) of the metal–miconductor contact.
The Schottky barrier height (ΦBN ) is related to the built-in potential by
BN bi C FS ().qV E E Φ=+−
(4.3) Another uful relationship for obtaining the Schottky barrier height is BN M S ,ΦΦχ=− (4.4)
becau the properties for the materials are known. The built-in potential creates 0W =
(4.5)
4.3 Forward Conduction Current flow across the metal–miconductor junction can be produced by the shift in the energy band structure as illustrated in Fig. 4.
5. Current flow across the interface then occurs mainly due to majority carriers – electrons for the ca of an four basic process 5 that are schematically shown in the figure:
1. The transport of electrons from the miconductor into the metal over the
potential barrier (referred to as the thermionic emission current )
2. The transport of electrons by quantum mechanical tunneling through the
potential barrier (referred to as the tunneling current )
3. The transport of electrons and holes into the depletion region followed by
their recombination (referred to as the recombination current )
4. The transport of holes from the metal into the neutral region of the
miconductor followed by recombination (referred to as the minority carrier current )
In the ca of power rectifiers, the doping concentration in the miconductor must be relatively low to support the rever bias (or blocking) voltage. This spreads the depletion region over a substantia
l distance. Conquently, the potential barrier is a zero-bias depletion region within the miconductor who width is given by
N-type miconductor. The current transport across the contact can take place via application of a negative bias to the N-type miconductor region. This produces a
