Schottky potential barrier

Schottky barrier refers to a metal-semiconductor contact with rectification characteristics, just like a diode has rectification characteristics. It is a rectifying region formed on the metal-semiconductor boundary.

Schottky barrier refers to a barrier with a large barrier height (i.e. φ bn or φ BP >; & gtKT), and metal-semiconductor contact with doping concentration lower than conduction band or valence band (Shi Min, Physics and Technology of Semiconductor Devices, 2nd Edition, 7. 1.2).

Schottky barrier formed by metal and n-type semiconductor is shown in figure 1. Metal-semiconductor as a whole has the same Fermi level in thermal balance. The biggest difference between Schottky barrier and PN interface is that its interface voltage is lower and the width of the depletion region at the metal end is relatively thin (almost non-existent). From semiconductors to metals, electrons need to overcome potential barriers; From metals to semiconductors, electrons are blocked by barriers. When a forward bias is applied, the barrier on the semiconductor side drops; On the contrary, when a reverse bias is applied, the barrier on the semiconductor side increases. So that the metal-semiconductor contact has rectification effect (but not all metal-semiconductor contacts have rectification effect). If the work function of metal is greater than that of semiconductor for N-type semiconductor, but less than that of semiconductor for P-type semiconductor, and the impurity concentration of semiconductor is not less than 10 19/cm3, ohmic contact will occur, and tunneling effect will be caused due to high impurity concentration, so that the barrier will not be rectified. Not all metal-semiconductor junctions have rectification characteristics, and metal-semiconductor junctions without rectification characteristics are called ohmic contacts. The rectification property depends on the work function of metal, the energy gap of inherent semiconductor and the doping type and concentration of semiconductor. When designing semiconductor devices, it is necessary to be familiar with Schottky effect to ensure that Schottky barrier will not be accidentally generated where ohmic contact is needed. When the semiconductor is uniformly doped, the width of the space charge layer of the Schottky barrier is consistent with the width of the depletion layer of the unilateral abrupt pn junction.

superiority

Because the interface voltage of Schottky barrier is low, it can be used for devices that need to be close to ideal diodes. In circuit design, they are also used together with general diodes and transistors, and their main function is to protect other devices on the circuit by using their low interface voltage.

However, compared with other semiconductor devices, Schottky devices have not been widely used.

ingredient

Schottky diode, Schottky barrier itself as a device is Schottky diode.

Schottky barrier carbon nanotube field effect transistor FET: The contact between metal and carbon nanotube is not ideal, so stacking fault leads to Schottky barrier, and we can use this barrier to make Schottky diodes or transistors and so on.

Actual use

(1) valence band electrons;

(2) Free electrons or holes;

(3) Electrons existing in impurity energy levels.

The available electrons of solar cells are mainly valence band electrons. The absorption of light is determined by the energy transition of valence band electrons to the conduction band, which is called intrinsic or intrinsic absorption.

The basis of energy conversion of solar cells is the photovoltaic effect of junctions. When light irradiates the pn junction, an electron-hole pair is generated. Carriers generated near the junction inside the semiconductor are not recombined and reach the space charge region. Attracted by the built-in electric field, electrons flow into the N region and holes flow into the P region, resulting in excess electrons stored in the N region and excess holes in the P region. They form a photogenerated electric field in the opposite direction to the potential barrier near the pn junction. The photovoltaic field not only partially cancels the barrier electric field, but also makes the P region positively charged, the N region negatively charged, and the thin layer between the N region and the P region generates electromotive force, which is the photovoltaic effect. At this time, if the external circuit is short-circuited, a photocurrent proportional to the incident light energy will flow in the external circuit, which is called short-circuit current. On the other hand, if the two ends of the pn junction are open, the Fermi level in the N region is higher than that in the P region, and a potential difference voc will be generated between the two Fermi levels. This value can be measured and is called open circuit voltage. Because the junction is forward biased at this time, the short-circuit photocurrent is equal to the forward current of the diode, so the value of voc can be determined.

Schottky potential barrier

Energy conversion process of solar cells

Solar cells are devices that directly convert solar energy into electric energy. Its basic structure consists of semiconductor pn junction. In addition, heterojunction, Schottky barrier and so on can also obtain better photoelectric conversion efficiency. This section takes the most common silicon pn junction solar cell as an example to observe the process of converting light energy into electric energy in detail.

Firstly, the characteristics observed outside when the solar cell is working are studied. When the sun shines on this solar cell, a photocurrent iph will flow in the opposite direction to the dark current.

When the solar cell is connected to the load R and irradiated by sunlight, the current im and voltage vm on the load will be determined by the intersection of the illuminated current-voltage characteristic curve and the straight line represented by v=-ir. At this time, there is a *gong* consumption rate of pout=ri2m on the load, which clearly shows that photoelectric energy conversion is going on. By adjusting the size of the load, the maximum output power ratio can be obtained at an optimal working point. The ratio of output * work * rate (electric energy) to input * work * rate (light energy) is called the energy conversion efficiency of solar cells.

Let's turn our attention to the inside of solar cells and study the energy conversion process in detail. The solar cell is composed of silicon pn junction, and the surface and back surface form ohmic contact without rectification characteristics. Assuming that there is no other resistance component except the load resistance R in the circuit, when there is H ν (EV) (H ν >; Eg, eg is the forbidden band width of silicon) When photons of energy irradiate solar cells, electron-hole pairs are generated. Because the energy of photons is greater than the band gap of silicon, electrons are excited to an energy level higher than the bottom of the conduction band. For p-type silicon, the minority carrier concentration np is very small (generally less than 105/cm), and the energy level of the conduction band is almost empty, so the electrons immediately fall to the bottom of the conduction band. At this time, electrons and holes transfer the total excess energy of hν-eg(ev) to the lattice in the form of phonons (lattice vibration). Some electrons falling to the bottom of the conduction band diffuse to the surface or junction, and some recombine and disappear inside or on the surface of the semiconductor. However, some carriers reaching the junction are accelerated by the built-in electric field at the junction and flow into N-type silicon. In N-type silicon, because electrons are the majority carriers, the inflowing electrons propagate in the order of dielectric relaxation time, and at the same time, in order to satisfy the condition of electric neutrality of carriers in N-type silicon, the same number of electrons as the inflowing electrons flow out from the electrode connected to N-type silicon. At this time, the energy lost by electrons is equivalent to the potential height of the space charge region and the potential difference between the conduction band bottom and Fermi level. Let n electrons flow into the load resistance per cubic centimeter per second, and the voltage across the load resistance is v=qnr=ir. Since there is no power supply in the circuit, the voltage v=ir is actually applied to the junction of the solar cell, that is, the junction is positively biased. Once the junction is forward biased, the diode current id=i0[exp(qv/nkt)- 1] is in the opposite direction to the photocurrent iph formed by carriers generated by photo-excitation, then the current value flowing into the load resistance is that an electron loses the energy of qv on the load resistance, which is equal to the photon energy hν being converted into electric energy QV. Electrons flowing through the load resistor reach the surface electrode of P-type silicon and become redundant carriers in P-type silicon, and then recombine with the swept holes to form photocurrent.

? Characteristics of Schottky MC7808ABTG

Manufacturer: on semiconductor

Series:

Packaging? : pipe fittings

Part status: for sale

Output Configuration: Positive

Output Type: Fixed

Number of voltage regulators: 1

Voltage input (max. ):35V

Voltage output (minimum/fixed): 8V

Voltage output (maximum):-

Pressure drop (maximum): 2V @ 1A (standard)