Schottky-Diode

A Schottky diode, also called hot carrier diode, a special diode, which no pn junction (semiconductor - semiconductor interface ), but a ( blocking ) metal -semiconductor junction is in the electronics has. This interface between metal and semiconductor is referred to as a Schottky contact and in line with the emerging potential barrier as a Schottky barrier. As the pn junction has rectifying character of the Schottky diode. Schottky diodes in the material composition (for example, doping of the semiconductor and the work function of the metal ) is chosen so that in the boundary area in the semiconductor forms a depletion region. Thus, the Schottky contact from the metal-semiconductor junctions in other conditions, such as the ohmic contact, which shows the behavior of an ohmic resistance is different.

History

Named is the Schottky diode after the German physicist Walter Schottky, who developed the 1938 model of the metal - semiconductor contact. The rectifying properties were first observed in 1874 by Ferdinand Braun. Initially, the metal-semiconductor transitions from point contacts were realized with a sharpened metal wire on a semiconductor surface passed ( tip diode). They were used mid-20th century, especially in the then usual detector receivers. The first Schottky diode, then called the crystal detectors, however, turned out to be very unreliable. The point contact was therefore replaced by a thin metal film, which is still the case with today commercially available Schottky barrier diodes.

Schottky diodes in the electronics

As a "fast" diodes are Schottky diodes suitable for high frequency applications up to the microwave range, which is mainly due to their low saturation capacities. Therefore, they are also often used as protection diodes for voltage reduction of induced voltages ( free wheeling diode ) or used as a rectifier diodes in switching power supplies, where it allows switching frequencies up to about 1 MHz. Also for detector circuits are well suited as a demodulator.

As they also have a smaller voltage drop in forward direction (about 400 mV ) than a conventional silicon pn junction, may, when they are connected in parallel to the collector- base junction of a silicon bipolar transistor, and prevent saturation of the transistor, and thus allow substantially faster switching of the transistor in the off state. This was used primarily from the spread of powerful MOSFETs with fast switches, such as in switching power supplies, but also for the realization of faster TTL logic circuits ( digital technology ), for example, in the ranks of 74 (A) S and 74 (A ) LS.

The inherent disadvantage of the Schottky diode, the higher leakage currents in comparison to the p-n diode, and the case structure for higher blocking voltage rapidly increasing conduction losses.

As a semiconductor material is usually silicon for voltages up to 250 V, sometimes also GaAs, SiC used for voltages of 300 V, 600 V and 1200 V, or SiGe.

SiC Schottky diodes provide in power electronics over the conventional Si-PIN diode has a number of advantages. Since they have almost no forward and especially backward Erholverhalten, they come very close to the ideal diode. When used as Kommutierungspartner for IGBT transistors, a considerable reduction in the switching losses in the diode itself, but also in the transistor is possible because of the need to take no reverse recovery current - upon reconnection. The allowable junction temperatures are higher than in silicon.

Function

It is now the function of a Schottky diode with n-doped semiconductor material treated ( the standard design ) based on the band model by the potential energy of the electrons is plotted as a function of position. In a simplified approach is often assumed that a metal are joined together, without the electronic structure changes through the metal-semiconductor bonding in the solid state of metal and semiconductor (left ) and a semiconductor (of right). Assuming that the work function of the metal is greater than the electron affinity of the semiconductor, - which is at the most of the metal - semiconductor combination to be used for Schottky diodes, met - that is generated at the interface between the Fermi level of the metal and the conduction band bottom edge of the semiconductor, a potential step of height.

However, in reality, the surfaces of metals and semiconductors are strongly altered by the binding and the actual amount of potential step or Schottky barrier is mainly determined by the metal-semiconductor bond, but also by process parameters such as the cleaning of the surface and hardly any of the dependent work function of the metal. For n -Si Schottky barrier usually lies between 0.5 and 0.9 eV.

The Fermi energy of the unperturbed WF ( n-type ) semiconductor is (except in degenerate semiconductors ) just below the conduction band. Upon contact metal / semiconductor it comes to balance the charge, the Fermi energies of the two partners of the same in itself, it is then only a common Fermi energy WF (x, t) = const in thermodynamic equilibrium. Due to the different work functions of the two partners, there is Ladungsinfluenz on the two surfaces. At the metal surface electrons, which flow from the semiconductor surface and thus generate positive impurities in semiconductors accumulate. The result is a potential hill and a " bending " of the bands of the semiconductor. About the band bending, electrons can leave the semiconductor, a situation called a depletion zone (English depletion zone ) in which the potential energy of the electrons in the conduction band ( majority carriers ) is high.

The electrons in the semiconductor, as stated, a higher energy level than the electrons in the metal. They are also called " hot carriers ". Derived from this, stirs the term " hot carrier diode " ( German: hot carrier diode ) ago.

Now, a positive voltage is applied (negative pole of the n-type semiconductor), electrons are driven out of the semiconductor material in the depletion layer and the potential barrier is reduced. Electrons can then flow from the semiconductor into the metal ( " forward direction ", Eng. Forward bias). If one the other hand, a negative voltage (which is not too large), the electrons are drawn even more strongly in the direction of the semiconductor, the thickness of the depletion zone increases ( " reverse ", engl. Reverse bias). It is only a very small current because a few electrons of the metal overcome the barrier by thermal excitation or " tunnel" through the barrier can ( quantum mechanical tunnel effect). Too high a voltage in the reverse direction, however, it comes to break.

In the Schottky junction, the minority carriers do not contribute to charge transport. Since the electrons ( majority carriers ) rapidly follow the electric field, the Schottky barrier diode, especially the transition from the forward to the reverse mode is substantially faster than normal semiconductor diode based on a pn junction. Schottky diode made ​​of silicon are switching frequencies greater than 10 GHz, it is possible of GaAs or of InP even of more than 100 GHz.

Ohmic contact

Not all metal-semiconductor contact has a rectifying effect. Since the thickness of the depletion region is inversely proportional to the square root of the density of the donor atoms, at very high doping of the semiconductor, the barrier is so thin that it can be neglected and the contact behaves like a small ohmic resistance. Also, by alloy formation (formation of silicides at the border) can be the Schottky junction to an ohmic contact. Ohmic contacts are required in order to contact the semiconductor chip (die ) and the metallic lead wires can.

Swell

• Ludwig Bergmann, Clemens Schaefer, Wilhelm Raith textbook of experimental physics. Solids. Vol 6, deGruyter, 1992, ISBN 3,110,126,052th