1. Current-Voltage - characteristics of a diode. A graph drawn by taking Voltage along the x-axis and the current along the 4- axis is called characterstic curve . 1. Forward bias characterstics :- It's a graph connecting current and voltage of the diode in forward bias. > The Forward voltage is increased from zero gradually and the corresponding current is noted, using milliammeter MA. Voltage noted in voltmeter. V. (millammeter in Series and voltmeter in parallel). I (Yes) Ge mA r f + + Si + HHH Forward bias (volt) V1 Explanation of the Graph:- (1) Forward characteristics is not a streight line. That means diode doesn't obey ohms law (ii) No current flows through the circuit hill the applied P-d becomes equal to the internal potential barrier. This is because the electrons are repelled back by the potential barrier
(III) ds the forward voltage is increased the current starts flowing when the applied Voltage just exceeds the potential barrer This Value of the forward voltage at which a small current begins to flow jor called the cut in voltage or break point voltage (iv) When the voltage is again increased the current starts to increase rapidly This voltage at which the current crises rapidly is called the knee - voltage for silicon if is 0.7V and for Ge , it is 0.3 V. This voltage cs called Fring /threshold voltage because there us an exponential rue in current : > Explanation for the increase in current:- with the increase in forward voltage, the K.E of the electrons also increases. They knock out es is from the covalent bond and the current is increased This heat the crystal Due to increase in temperature current also increases This effect is cumulative and explains the steeper part of the curve. 2. Reverse blas characteristics :- > The reverse voltage VR is gradually increased, in equal steps and the corresponding currents IR are noted in the microammeter MA. reverse bias (VR) si MA Ge reverse V current IR (HA) +
Explanation of the Graph :- with the increase in voltage VR the current inventory reaches a maximum, then remains constant till Q voltage called Lener voltage is reached This current is called reverse saturated current or leakage current which is very small and is due to the minority carriers Once the reverse voltage attains the Lener voltage, the p-n junction breakdown and there is sudden rise in the reverse current The Zeney a Voltage varies from I or 2 volts to several hundred volts depending on the doping When the voltage reaches the zener voltage the electrons get sufficient K.E. The high energetic electrons, by collision, break covalent bonds and more es are again produced. Thus a chain reaction takes Place and avalanche of es is produced Hence reverse current increases and the function breaks down. This is called avalanche break-down. Forward and Reverse Resistance :- The resistance offered by a pn junction diode when it is forward biased is called forward resistance It is different in dc $ a.c. (1) DC. forward resistance (or static resistance) :- opposchon offered by the pr junchion dcode to the dc is called dc forward resistance.
forward resestance N d.c voltage across diode (D.C) d.c current If R = OA B f OB A Yf 11) A.C forward resistance :- opposition offered by the pn function to the changing => forward current is called a.c forward resistance. the forward resestance = change in forward voltage If (AC) change in forward current F Here R very small E = DVS AC f = D OI' OF ( I 2 to 30-2) AC f resestance in vertical A BC Vf region of the charattash Ideal diode :- is very small, much high when ideal diode is forward cat low current levels biaged it has zero resertance and acts as perfect conductor When reserse biaged it has in finite resistance and so it acts as perfect unsulator I I q Vp Io diode current D = S OKT e VD diode Voltage CROL cn text) 9 charge n dentity factor KB 1.38x10-3 B T Temp