Sspd_chapter 3_solid state diode physics.

SSPD_Chapter 3_Solid State Diode Physics.

Today we are living in knowledge intensive society which is going through the Third Information Revolution. In this Information Age we are critically dependent upon Solid State Devices for generating, processing and transmitting information and information is nothing but signal. Hence Solid State Devices are performing the following signal processing:

1. Reception/detection of signal. This signal can be very weak and heavily corrupted by noise and interferences.
2. Amplification of Signal through Signal Amplifiers.
3. Signal Processing for restoring the integrity of the signal.
4. Information carried by the signal needs to be stored in the memory. There are compact discs and magnetic tapes.Gradually these are being supplanted by Solid State Memories. They are cheap and are becoming faster.
5. Through Solid State Sensors, environmental parameters are being sensed and converted into electrical/optical signals which are subsequently processed and utilized for different applications.
6. Solid State Displays and Flat Panel Displays utilizing Solid State Devices are increasingly taking the place of the conventional displays.

One of the Solid State Devices which has been in use since early 20 ^th century is PN Junction Diode. It has been used for envelope detection of Amplitude Modulated Signals in Radio Broadcasts.

Subsequently it has been used in power conditioning circuits for generating DC Voltages from AC Voltages.

It has found applications in Frequency Modulation Circuits as Varactor Diodes.

It has found applications in MicroWave Generation and Amplification as Esaki or Tunnel Diodes.

It has found applications as Schottky Diodes in Scottky Clamped TTL Circuits for speeding up the switching speed of TTL Gates.

PN Junction can be modified to behave like ohmic contacts instead of rectifying contacts.

PN Juntions are extensively used as Photo-Voltaic Cells or as Solar Cells.

3.1. Fabrication of PN Junction Diode.

A PN Junction diode is fabricated by diffusing N-type dopent(Phosphorous) into P-Type Wafer with Boron Doping at an uniform concentration of 10 ¹⁵ /cc corresponding to a resistivity of 10 Ω-cm. The diffusion is carried out in Diffusion Furnace at a high temperature of 1000°C to 1300°C with a tolerance of ±0.5°C. The diffusion furnace is shown in Figure 6.14 in Chapter 6.

There can be two kinds of diffusion shown in Figure 3.1:

Pre-depostion- It is a short period constant-source diffusion with a shallow junction. The Doping Profile is Complementary Error Function and the Junction is a one-sided step-junction or abrupt junction.

Limited Source Diffusion- It is a long drive-in diffusion [erfc(z)] with a deep junction. The doping profile is Gaussian and the junction is linearly graded junction as shown in Figure 3.1.

.Both these two kinds of junctions have their unique electrical parameters which we will see in the subsequent section.