PN Junction Diodes

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BASIC ELECTRONICS JUNCTION DIODES Prof. S.Kal

P – N Junction Diodes:

2 P – N Junction Diodes The semiconductor diode is formed by simply bringing p- and n- type semiconductors without disturbing the crystalline continuity across the junction Simplified physical structure of junction diode Cross – section schematic S. Kal, IIT-Kharagpur

P – N Junction Diodes:

3 Holes diffuse from p to n, while electron diffuse from n to p A negatively charged layer of un-neutralised acceptor ions is formed in the p-side and a positively charged layer of un-neutralised donor ions is formed in the n-side. It creates a built-in potential, v bi across the junction. In the thin layer, there cannot be any mobile carrier due to the internal built-in electric field . It is called a depletion layer Built-in field opposes further flow of electron and holes across the junction. Thus, in equilibrium, the net electron and hole currents are zero respectively. P – N Junction Diodes S. Kal, IIT-Kharagpur

Forward – Bias Condition :

4 Forward – Bias Condition A forward-bias or “on” condition is established by applying the positive potential to the p-type material and the negative potential to the n-type material. S. Kal, IIT-Kharagpur

Forward – Bias Condition :

5 Application of forward bias (V) will “pressure” electrons in the n-type material and holes in the p-type material to recombine with ions near the junction and reduce the depletion layer width. Electrons in n-type material and holes in p-type “see” a reduced barrier at the junction and strong attraction at the other side of the junction. As V increases, depletion region continues to shrink until a flood of majority carriers can pass through the junction resulting in an exponential rise in current. I S is minority carrier current and is negligible ( ~  A ) in comparison to I majority ( mA ) Forward – Bias Condition S. Kal, IIT-Kharagpur

Reverse – Bias Condition :

6 A reverse-bias or “off” condition is established when +ve terminal of the external potential is connected to the n-type material and the –ve terminal is connected to the p-type material. Reverse – Bias Condition S. Kal, IIT-Kharagpur

Reverse – Bias Condition :

7 Reverse – Bias Condition The number of uncovered +ve ions in the depletion region of the n-type material will increase due to the large number of “free” electrons drawn to the +ve potential of the applied voltage. Similarly, the number of uncovered –ve ions will increase in the p-type material. The net effect is widening of depletion region – this will establish a great barrier for the majority carrier to overcome and effectively I majority becomes zero. The minority carriers at both sides of the junction will cross the barrier easily resulting in minority carrier current and is known as reverse saturation current ( represented by I s ). The term “saturation” comes from the fact that it reaches its maximum level quickly and does not change with V. S. Kal, IIT-Kharagpur

Current-voltage Characteristics of a PN junction Diode:

8 V DO (= V  , V T )  Cut – in voltage (upto which the I F remains negligible) For Ge, V DO  0.2 V – 0.3 V For Si, V DO  0.6 V – 0.7 V V DO decreases with temperature at a rate –2.5 mV/ 0 C Current-voltage Characteristics of a PN junction Diode S. Kal, IIT-Kharagpur

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9 Diode current equation is I D = I S [ {exp(qV d / k T ) –1} ]  = ideality factor  1 or I D = I S [ {exp(V d / V T ) –1} ] V T = (k T /q) = thermal voltage = 0.026V at 300K I S = reverse saturation current I S is contributed by minority carries and increases with increasing temperature and decreasing band gap. Roughly I S doubles for every 10 o C rise of temperature A simple diode circuit S. Kal, IIT-Kharagpur

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10 S. Kal, IIT-Kharagpur

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11 Graphical analysis of the circuit S. Kal, IIT-Kharagpur

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12 The ideal diode: (a) Diode Circuit Symbol (b) i – v Characteristic S. Kal, IIT-Kharagpur

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13 Approximating the diode forward characteristics with two straight lines Current-Voltage Drop Model of a p-n Junction Diode S. Kal, IIT-Kharagpur

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14 Piecewise – linear model of the diode forward characteristic and its equivalent circuit representation Piecewise-linear Model of p-n Junction Diode S. Kal, IIT-Kharagpur

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