Semiconductor Physics PDF

Minor basket 3 Introduction to Semiconductor physics explores the underlying principles that govern the behavior of materials like silicon and Semiconductor Physics gallium arsenide. Understanding the unique electronic properties of semiconductors is fundamental to the...

Minor basket 3 Introduction to Semiconductor physics explores the underlying principles that govern the behavior of materials like silicon and Semiconductor Physics gallium arsenide. Understanding the unique electronic properties of semiconductors is fundamental to the design and development of modern electronics, from transistors to integrated circuits. Semiconductor Fundamentals of Electronic Physics Optics Communication and Electronics  N-Type conductivity ❖ Many new free electrons are produced by the addition of pentavalent impurity ❖ Thermal energy of room temperature still generates a few hole-electron pairs. However, the number of free electrons provided by the pentavalent impurity far exceeds the number of holes ❖ When p.d. is applied across the n-type semiconductor, the free electrons (donated by impurity) in the crystal will be directed towards the positive terminal, constituting electric current.  P-Type conductivity ❖ The addition of trivalent impurity has produced a large number of holes. ❖ there are a few conduction band electrons due to thermal energy associated with room temperature. But the holes far outnumber the conduction band electrons. ❖ When p.d. is applied to the p-type semiconductor, the holes (donated by the impurity) are shifted from one co-valent bond to another. As the holes are positively charged🡪hole current  Charge on n-type and p-type Semiconductors n-type semiconductor, current conduction is due to excess of electrons p-type semiconductor, conduction is by holes DOES IT MEAN N TYPE SEMICONDUCTOR HAVE NET NEGATIVE AND P TYPE MATERIALS HAVE NET POSITIVE CHARGE!! “excess electrons” refers to an excess with regard to the number of electrons needed to fill the co-valent bonds in the semiconductor crystal n-type as well as p-type semiconductor is electrically neutral  Majority and Minority Carriers ❖ N type- excess electrons, P-type- excess holes ❖ Thermal energy of room temperature still generates a few hole-electron pairs. ❖ n-type material has a large number of free electrons (MAJORITY CARRIERS) and a small number of holes (MINORITY CARRIERS) ❖ p-type material has a large number of holes (MAJORITY CARRIERS) and a small number of electrons (MINORITY CARRIERS)  pn Junction ❖ When a p-type semiconductor is suitably joined to n-type semiconductor, the contact surface is called pn junction Formation of pn junction ❖ Just pasting P with n!!!– No..It need special fabrication method ❖ One common method of making pn junction is called alloying Small block of indium (trivalent impurity) is placed on an n-type germanium slab The system is then heated to a temperature of about 500ºC. The indium and some of the germanium melt to form a small puddle of molten germanium-indium mixture The temperature is then lowered and puddle begins to solidify. Atoms of indium impurity will be suitably adjusted in the germanium slab to form a single crystal Remaining molten mixture becomes increasingly rich in indium.  Properties of pn Junction 1. free electrons near the junction in the n region begin to diffuse across the junction into the p region where they combine with holes near the junction 2. n region loses free electrons as they diffuse into the junction. This creates a layer of positive charges near the junction 3. As the electrons move across the junction, the p region loses holes as the electrons and holes combine. The result is that there is a layer of negative charges near the junction These two layers of positive and negative charges form the depletion region, depleted (i.e. emptied) of charge carries  Properties of pn Junction 1. Once pn junction is formed and depletion layer created, the diffusion of free electrons stops. 2. depletion region acts as a barrier to the further movement of free electrons across the junction. 3. There exists a potential difference across the depletion layer and is called barrier potential (V0). 4. For silicon, V0 = 0.7 V ; 5. For germanium, V0 = 0.3 V Properties of pn Junction D.C. Voltage Across pn Junction (Biasing)-Forward, Reverse Forward Biasing When external d.c. voltage applied to the junction is in such a direction that it cancels the potential barrier, thus permitting current flow positive terminal of the battery to p-type and negative terminal to n-type (i) The potential barrier is reduced and at some forward voltage (0.1 to 0.3 V), it is eliminated altogether. (ii) The junction offers low resistance (called forward resistance, Rf) to current flow. (iii) Current flows in the circuit due to the establishment of low resistance path. The magnitude of current depends upon the applied forward voltage. D.C. Voltage Across pn Junction (Biasing)-Forward, Reverse Reverse Biasing positive terminal of the battery to n-type and negative terminal to p-type The potential barrier is increased. The junction offers very high resistance (called reverse resistance, Rr) to current flow. No current flows in the circuit due to the establishment of high resistance path. FORWARD BIAS- CURRENT FLOWS REVERSE BIAS- NO CURRENT FLOWS D.C. Voltage Across pn Junction (Biasing)-Forward, Reverse The mechanism of current flow in a forward biased pn junction The free electrons from the negative terminal continue to pour into the n-region while the free electrons in the n-region move towards the junction. The electrons travel through the n-region as free-electrons i.e. current in n-region is by free electrons. When these electrons reach the junction, they combine with holes and become valence electrons. The electrons travel through p-region as valence electrons i.e. current in the p-region is by holes. When these valence electrons reach the left end of crystal, they flow into the positive terminal of the battery THANK YOU