Transcript FED Unit-3
Transient and AC condition Change in behavior of a PN junction as a function of time Lily Gupta Assistant Prof.(ECE) Reverse recovery transient PN junction may be used as an electrical switch. When a forward biased is suddenly reverse biased,the current does not drop to zero instantly but takes a finite time to drop to zero. This time is known as turn off time or reverse recovery time • • • The forward PN junction diode is suddenly reverse biased at time t=0. An ideal diode should change its condition from on to off instantly but a practical diode does not do the same The reason is that the minority carrier under forward biased condition are not apposed by the potential barrier and they constitute the same minority carrier in reverse direction. The region from 0-t1 is known as storage time. Voltage as a function of time The diode is conducting in the forward direction and the input voltage Vin =Vf Let the voltage be reversed abruptly at time t=0 to a value –Vr During the period 0-t1-storage time i.e the voltage changes from Vf to 0 At the instant t1,the process of sweeping up of excess minority carriers has completed. The diode voltage now reverse and falls to Vr Expression for storage time Metal semiconductor junction Metal semiconductor junctions are those junctions which are fabricated by forming a compatible contact between a metal and a semiconductor It may be rectifying contact type or non rectifying contact type(ohmic contact) 13 •Many of the properties of pn junctions can be realized by forming an appropriate metal-semiconductor rectifying contact (Schottky contact) – Simple to fabricate – Switching speed is much higher than that of p-n junction diodes •Metal-Semiconductor junctions are also used as ohmiccontact to carry current into and out of the semiconductor device 14 Ideal MS contacts Assumptions - Ideal MS contacts M and S are in intimate contact, on atomic scale No oxides or charges at the interface No intermixing at the interface MS contacts Vacuum 15 level, E0 - corresponds to energy of free electrons. The difference between vacuum level and Fermilevel is called workfunction, of materials. Workfunction, M is an invariant property of metal. It is the minimum energy required to free up electrons from metal. (3.66 eV for Mg, 5.15eV for Ni etc.) The semiconductor workfunction, s, depends on the doping. Electron affinity:the energy difference between the conduction band edge and the vacuum level Schottky barrier on n type semiconductor • • • There will be flow of electrons from the conduction band of semiconductor to the conduction band of metal till the thermal equilbrium is established The transfer of an electron in a semiconductor results in an upward bending of energy band in it. The charges on a semiconductor and metal established an electric field from semiconductor to metal Schottky barrier on p type semiconductor Electrons flows from the metal into a semiconductor until the fermi level is same throughout Rectifying contacts If we apply forward biased to the schottky barrier the potential barrier is reduced due to which more electrons in the conduction band of semiconductor diffuse across the depletion region to the metal. If we apply reverse bias.This increases the contact potential and reduces the electron flow from the semiconductor to metal. ideal MS contacts 24 n-type p-type M > S rectifying ohmic M < S ohmic rectifying Ohmic contact Figure above shows n type semiconductor with metal having smaller work function. After contact is made the fermi level allign at equilbrium by transfering electrons from the metal to semiconductor. The negative charge of electron that accumulates in semiconductor is essentially a surface charge similarly positive charge accumulate across the metal. There is no depletion region across the contact The above figure is a p type metal with larger work function. In this case too the positive charge that accumulates in a semiconductor is a surface charge and the negative charge that accumulates in a metal is also a surface charge. SCHOTTKY DIODE The charge storage problem in a pn junction diode can be eliminated or minimized in a schottky diode . This diode is also called as hot carrier diode or ESBAR(epitaxial Schottky barrier). Construction of a schottky diode It is a metal semiconductor junction diode without depletion layer. On one side of the junction a metal (like gold silver,platinum or tungsten )is used and on the other side n or p type semiconductor is used. Schottky diodes operates only with a majority carrier. A layer of metal is deposited on a thin epitaxial layer of n type silicon. The metal film forms a positive electrode(anode) and the semiconductor is the cathode. The cathode structure has an ohmic contact i.e no rectifying characteristic. For a heavily doped n region ,such a contact is necessary. There are no minority carriers and thus no reverse leakage current . The schottky diode is very fast switching diode. Working of a schottky diode Electrons in different materials have different absolute potential energy. N type s.c electrons have higher potential energy as compared to electrons of metals. When the two brought in contact,there is flow of electrons in both directions and the flux of electrons from the semiconductor into the metal is much larger due to high absolute potential energy. As a result the metal will become negatively charged and the semiconductor will become positively charged Varactor diode Depletion region in a P-N junction forms a barrier which seperates the positive and negative charges on each side of the junction P side of the depletion layer has negative immobile ions and n side has positive immobile ions. Thus a pn junction possesses junction capacitance. Also the width of the depletion region