Junction Diode Handout PDF

# NNAMDI AZIKIWE UNIVERSITY, AWKA ## JUNCTION DIODES **Introduction** A pn junction or pin diode consists of a semiconductor specimen having two regions, a region of p-type material, a region of n-type material, and in between a region of transition usually from p-type material to n-type; this r...

# NNAMDI AZIKIWE UNIVERSITY, AWKA ## JUNCTION DIODES **Introduction** A pn junction or pin diode consists of a semiconductor specimen having two regions, a region of p-type material, a region of n-type material, and in between a region of transition usually from p-type material to n-type; this region is known as junction. This transition is usually very thin, of the order of 10⁻⁶ to 10⁻⁴ cm in thickness, depending on the fabrication method. The punch throughs are popularly used in applications such as rectifiers, clippers, comparators, diode gates, etc. **FABRICATION OF PN DIODES:** It entails the basic methods of forming pn diodes. Depending on the method of fabrication, pn diodes may be put in the following broad categories: - Grown junction type - Alloy type or fused junction type - Diffused junction type - Epitaxial planer type ### Grown junction diode: The diodes were very popular in the early days of development. The basic grown semiconductor devices: Fig 1.1 illustrate the dimension of semiconductor devices. A semiconductor crystal of n-type is formed by first growing a semiconductor crystal of n-type from a melt of germanium or silicon. A little later, the impurity is changed from n-type to p-type by adding to the melt impurity of p-type sufficient large quantity. Thus, a continuous crystal is formed with n-type and partly p-type with a junction in between. Connecting leads are fused to the external surfaces of p and n materials forming large area nonrectifying contacts. Fig 1.1 gives a two dimensional view, while Fig 1.1 gives the circuit symbol used for a general pn diode irrespective of the function of operation. <br> | Semiconductor | Junction | Junction | P-type | |---|---|---|---| | N | | | | | P | | | | | Lead | | | | | Lead | | | | | | Two dimensional view | | | | | Three dimensional view | | | | | Circuit view | | | _Fig 1.1. Grown junction diode._ <br> ### Alloyed (or Fuse of Junction) Diode: A convenient method of making p-n junction is alloyed type. It involves melting a thin pellet of metal containing n-type atoms on an n-type semiconductor wafer. Fig 1.2(a) shows the general arrangement. Here, a thin dot of p-type indium is attached on a thin wafer of n-type germanium. Theses and it is baked for a short time at a temperature of ~500 deg C - which is high temperature of indium but below the melting point of indium. <br> ### Diffused Junction Diode: Purity diffusion process is the most common technique for forming p-n junction for large scale production. It employs either solid or gaseous diffusion. This process takes more time and more precisely but it is relatively cheaper and more precisely controllable. It uses impurity particles from a region of high concentration drift into a region of lesser concentration. However, this random motion of impurity atom is rather limited except at high temperatures. Accordingly diffusion of impurity into a semiconductor, typically silicon, is done at temperatures of about 1000°C. At such a high temperature several atoms in the semiconductor move out of their lattice sites, leaving vacancies for impurity atoms to move in. **Solid Diffusion:** The process starts with putting p-type impurity (say, indium) on an n-type substrate and heating the two until the impurity atom diffuses a short distance into n-type substrate and a short distance into the substrate to form p-type layer - Fig 1.3. **Gaseous Diffusion:** In the gaseous diffusion process, an n-type material is heated in a chamber containing a high concentration of an acceptor impurity in vapour form (Fig 1.4). Some of the acceptor atoms are diffused (or absorbed) into the n-type substrate to form the p-type layer. <br> ### Epitaxial Planar Diode: Creating a p-n junction by depositing a part of the N-type material during the diffusion process (the remainder being covered by a thin coating of SiO₂); the size of the p-region can be controlled. Finally, metal contacts are electroplated on the surface of each region for connecting the leads. The diffusion technique enables simultaneous fabrication of many hundreds of diodes on one small disc of semiconductor material. That is why it is the most frequently used technique not only for the manufacture of semiconductor diodes but also for the manufacture of transistors and integrated circuits. <br> | N Substrate | P | |---|---| | Solid diffusion | Diode format from solid diffusion | _Fig 1.3. Solid diffusion process of producing p-n diode._ <br> | 0.3mm | 1.5mm | |---|---| | n-Type silicon region | | | | Alumincum metallization | | | p-type | | | SI0₂ | _Fig 1.3 Physical structure of diffused p-n junction diode._ <br> ### Biasing of a p-n Diode **Forward bias:** Fig 1.6 shows a p-n junction with forward bias. V_f is a positive terminal of battery connected to the p-side and the negative to the n-side. The atomic voltage drop across the body of the device is zero under ideal conditions. In a forward biased diode, the height of the potential energy barrier of the junction gets lowered by the magnitude of V_f. This distributes the initial equilibrium between the forces tending to cause diffusion of majority carriers across the junction and the opposing influence. Hence majority carriers flow across the junction from the p-region to the n-region while the electrons cross the junction from the n-region to the p-region. Flow of both types of carriers causes conventional electric current from p-region to n-region and these components get added. **Reverse bias:** Fig 1.7 shows a p-n diode with reverse bias i.e. with positive terminal connected to the n-side and the negative terminal connected to the p-side. The pn junction is now biased with holes in the n-region to move away from the junction (while they are still in the region). The change in density of the positive charge on the p-side and the region of the negative charge density on the n-side increases the width of the depletion layer by an amount: qV. The increased barrier height serves to reduce the flows of majority carriers to the other side. However, the increased barrier height does not completely stop the flow of minority carriers. Since these minority carriers are very few in n-region and p-region, the barrier becomes a huge obstacle for them to move under reverse bias condition. Actually, however, a small number of electrons are continuously generated throughout the crystal due to thermal energy. Hence, a very small current does flow in the reverse direction from n-region to p-region across the junction. This small reverse current is called saturation current and is denoted by I_0. The magnitude of I_0 is really about µA in a Ge diode and about nA in a Si diode. This reverse saturation current increases with the increase in temperature. <br> ### Photo Diode: It is basically a reverse biased pn junction embedded in a plastic capsule or enclosed in a black capsule with clear plastic cap. It is generally illuminated (Fig. 1.12) from one surface of the device through the unpsmiled or clear region, remaining surfaces are painted black. Light falls on one surface of the device through the unspsmiled or clear regions, remaining surfaces are painted black. The light shows the basic photodiode. The dimensions of the device is extremely small with dimensions of a few millimetres. A lens is used to concentrate the light on the junction area. Fig 1.12 gives the circuit symbol. <br> ### Bipolar Junction transistor A bipolar junction transistor or BJT is a three terminal semiconductor device which is widely used in amplification and switching. It is also called a bipolar junction transistor (BJT) because it uses both electron (n-type) and hole (p-type) carriers in operation. A BJT is basically a three region, two junction semiconductor device. The three regions are called emitter, base and collector. These regions are usually made of silicon or germanium. An npn transistor consists of a thin layer of p-type silicon sandwiched between two thick layers of n-type silicon. The p-type layer is called the base while the other two n-type layers are called the emitter and collector. An npn transistor consists of a thin layer of p-type silicon sandwiched between two thick layers of n-type silicon. The p-type layer is called the base while the other two n-type layers are called the emitter and collector. If the p-type silicon layer is sandwiched between two thicker layers of p-type silicon, it is called a pnp transistor. <br> ### LEDs Light-emitting diode (LED) is a forwardbiased p-n junction device. It uses gallium arsenide (GaAs) It uses gallium arsenide (GaAs), gallium phosphide (GaP), or a combination of the two. LED is the mechanism involved in the depletion layer of a forward-biased diode primarily as recombination of electrons and holes within the region centered at the junction. A definite amount of energy is required to break a covalent bond, form an electron-hole pair and create the light emission. In terms of the band diagram, it is energy required to transfer an electron from valence band to conduction band. When recombination takes place, an election and a hole are restored to their original place (i.e. valence band) and the process is accompanied by the release of energy equal to the energy required initially in the formation of the electron-hole pair. It is referred to as radiative recombination. In arsenide phosphide (GaAsP), gallium phosphide (GaP) and gallium arsenide (GaAs) larger fraction of the released energy gets given up in the form of photon, and diode becomes visible light source. Such a recombination of electrons and holes results in radiative recombination, and the process of giving off light by applying forward bias is referred to as electroluminescence.