Week 6: Application of the Schottky barrier diode

  The most important application of the organic schottky barrier diode is rectifiers. A rectifier is an electrical device that converts Alternating Current (AC), which periodically reverses direction, to Direct Current (DC), which flows in only one direction.

                                      Figure 1:  Half wave rectification of an AC signal

  Besides the rectifier, organic schottky diode could be used as a voltage clamping as well. The definition of clamping circuit is to clamp a waveform to a specific DC level.

Figure 2: An idealized circuit of clamped capacitor circuit

  For example, the input AC signal is shown below as Figure 3. In the circuit, firstly, the capacitor will be full charged, so Vc will become +6 volts. In this case, the lowest output voltage will “clamped” to zero. When 4V is applied, the output voltage becomes 4+6=10V. And the output voltage will appear as Figure 4.

Figure 3: The input AC signal

Figure 4: The output of the clamped circuit

Reference:

http://whites.sdsmt.edu/classes/ee320/notes/320Lecture9.pdf

http://en.wikipedia.org/wiki/Schottky_barrier_diode

Week 5: Temperature variation

  By 4 weeks, we had calculated several important parameters representing the performance of a specified organic schottky diode assuming that the diode was at the temperature of 300K. However, the performance of the diode could be changed in different temperature.  Therefore, our supervisor gave us sorts of data that represents the current against the voltage applied to the diode made up of a specified organic material in different temperature.

 At first, we plotted the graph of the exponential of current against voltage applied as shown below:

Figure 1: The volt-ampere characteristics of Schottky barrier diode

  In the graph, each curve represents characteristic of current  against voltage at a determined temperature. From the graph, exponential region of each curve can be found. Then we choose two points in exponential region of each curve to calculate the To, Tc and γ of the diode at different temperature (T). The graphs of To, Tc and γ against temperature T are shown below:

Figure 2: The change of To at different temperature

Figure 3: The change of Tc at different temperature

Figure 4: The change of γ at different temperature

  After ignored some error points in the graphs, we found that with the temperature approaching 300K from 188K, To and γ became smaller and Tc became larger. This shows that with the temperature becoming higher, the performance of the diode is better. We suggested that the improvement of performance at higher temperature may cause by the higher mobility of the carriers.

WEEK 4: Doing more calculations

We learnt that the distribution of the states is a very important property of the organic semiconductor which is defined as Tc. 

Tc is given as:

Ideality factor γ is also a very important parameter in diodes which attributed to the interface properties between the metal and semiconductor. 

The ideality factor is given as:

Another essential parameter in disordered organic semiconductor is the Meyer- Neldel Energy (discussed in earlier lecture) which is given as: 

  T0 defines the distribution of the carriers and T is equal to room temperature which is 300K. As calculated before, To can be the values shown below,

Therefore, Tc, γand MNE is calculated:

Week 3: Meeting supervisor, calculating and understanding

 Today we met our supervisor at her office, and she gave us a review of the structure of organic Schottky carrier diode and a graph of the relationship between voltage V and current I. Figure 1 and figure 2 show the details about the relationship between voltage and the logarithm of current with organic semiconductor of Schottky barrier diode. Also it is necessary to do some related calculations to learn more about this kind of  materials.

Figure 1: The relationship between current and voltage

 For the first graph,the x-axis represents the voltage in volts, and the y-axis represents the logarithm of the current in amperes. It can be seen that this graph can be divided into two parts, there is a decrease line on the left of the y-axis which at on current condition, and there is a increase line on the right side when it is at the off current condition.
The second picture describes the change of exponential current when adding the reverse voltage bias to the terminals of organic Schottky barrier diode. It was supposed to be a smooth curve in ideal condition, however the data given by the supervisor shows that this organic Schottky barrier diode is not perfect. It is clear that the value of exponential current increasing slowly when the value of reverse voltages bias is between -1 to 0. This may because the interface of the material is not perfect to stop the electrons cross it quickly.
.
 Finally, we should calculate the value of To according to the equation:

Figure 2: The relationship between current and reverse voltage bias

  The plot shown in the picture 3 was quite successful. According to the given data and the picture, it can be seen that the diode is not ideal. There is a part of folding line instead of the smooth exponential line. On the whole, the plot is quite similar to the ideal graph shown by the supervisor.

 Figure 3. The relationship between current density and voltage

  As a result, different students got different answers. However, because of choosing the inappropriate variables, all of the students calculate the wrong value of To , about 10 times of the right range of answer. Then it was calculated again, there is a table shown the correct results of the To. Different region got the different value of To.

Table 1: The values of To

Week 2: More information about organic schottky diodes

  After the first meeting with our supervisor, we are required to do more reading in order to have a deeper understanding of our project. The below text is some basic notes about organic schottky diodes.

  The above symbol is the schottky diode; it is defined as a semiconductor diode with a low forward voltage drop and a very fast switching action which are caused by Schottky barrier. The anode is metal and cathode is semiconductor (p-type). Here are the forward bias I-V characteristics of a pn junction diode and schottky barrier diode.

  The current in a pn junction diode is controlled by the diffusion of minority carriers. The current in a Schottky diode results from the flow of majority carriers over the potential barrier at the metallurgical junction. This means that there is no minority carrier storage in the Schottky diode, so the switching time from a forward bias to a reverse bias is very short compared to that of a pn junction diode. The storage time, ts, for a Schottky diode is essentially zero. In addition, the reverse-saturation current Is for a Schottky diode is larger than that of a pn junction diode for comparable device areas. This property means that it takes less forward bias voltage to induce a particular current compared to a pn junction diode. We can also determine that the Schottky diode has a smaller turn-on voltage than the pn junction diode.

  Moreover, we also find some basic information about organic semiconductor. We know that an organic semiconductor is an organic material semiconductor properties. There are mainly two differences between organic and inorganic semiconductor. The first is the different shapes of crystals. In terms of organic semiconductor, it is Van der Waals bonded crystal while inorganic is covalently bonded crystals. In addition, their materials are also different. Organic semiconductor is polymer based organic material while inorganic one is silicon based inorganic material.

  When talking about advantages and applications. The organic semiconductor is cheaper, lighter and more flexible than inorganic semiconductor. But it may have high resistance and shorter lifetimes. Finally, organic semiconductor could be used in the display system, like organic light emitting diodes (OLED), it can also be used as RFID (Organic Nano-Radio Frequency Identification Devices) and solar cells.

  In addition, we borrow a book named <<Metal-Semiconductor Schottky Barrier Junctions and Their Applications>> which edited by B.L. Sharma, With the help of this book, we had a better understand about energy band diagram and metal-semiconductor contact.

  Firstly, we searched some information about potential barrier: We know that it is formed when a metal is contacted with a semiconductor and arised from the separation of charges at the metal-semiconductor interface. Secondly, we explored the energy band diagram about p-type.

  when before contact, there is no charges at the surface so that there is no band bending.

When the semiconductor and metal contact, the valance band and conduction band were bent as shown below. In this case, there is no current flowing in the metal-semiconductor contact, and the built-in potential  Vi is formed. 

On forward bias, the energy level diagram is shown below.

The final figure is how the energy changes on reverse bias.

Reference:

http://en.wikipedia.org/wiki/Organic_semiconductor

Week 1: Reading about Organic Schottky diode

The Organic Schottky diode is a ‘two-terminal’ device with Schottky and reference contacts. The basic structure of it is shown below:

From this figure, Organic semiconductor is fabricated between Au and Al, which known as Ohmic contact and Schottky contact respectively. Negative or positive voltage applied to Al can result in different performance of the diode.

The Organic Schottky diode is economical due to volume production and various raw materials.