Unit 12: Semiconductor devices. Diode. P-N Junction in equilibrium. - PowerPoint PPT Presentation

Unit 12: Semiconductor devices. Diode.  P-N Junction in equilibrium. Diode.  Diode bias. Forward and reverse bias.  Diode current-voltage characteristics. Models.  Applications.

PN junction in equilibrium At room temperature, holes on Electrons Holes p area go by diffusion towards n area, and e - from n area cross P N to p area (majority carriers diffusion currents). On junction area, holes and e - are recombined, appearing a narrow depletion area (without e - and holes) with a charge J pdif Diffusion Currents 300 K 0 K density due to the ions of the J ndif of majority carriers impurities, negative on p area V 0 X p X n � � � � and positive on n area. E So, an electric field appears, flowing drift currents of minority carriers (e - from p to n area, and holes from n to p area), J pdrift Drift Currents of canceling diffusion currents. minority carriers J ndrift

PN junction in equilibrium. Not biased junction. V 0 p n � � � � E x n x p � � J J drift dif � � J J 0 + = drift dif On not biased p-n junction (or in equilibrium), diffusion currents of majority carriers are cancelled by drift currents of minority carriers. A P-N junction is a DIODE

V 0 p Xn n Xp � � � � E PN junction in equilibrium. Features. p p0 ≈ ≈ N A n n0 ≈ ≈ N D ≈ ≈ ≈ ≈ Charge carriers density distribution. n p0 p n0 qND ρ X p X n 0 + Charge distribution - -qNA

V 0 Xn Xp � � � � E PN junction in equilibrium. Features. E X p X n Electric field on pn junction Drop of potential known as V Built-in potencial V 0 V 0 = 0.7 V for Si diodes X p V 0 = 0.3 V for Ge diodes X n at 20 ºC

Diode bias. Forward bias V’ p n � � � � E � � � � � � � � � � J J J des J dif drift dif � � � � J < V 0 V D I V D creates an electric field opposite to the field on the depletion area, being lower E total and the drop of potential on the junction: V´=V 0 -V D . So, diffusion majority current is increased, and drift minority current is decreased.

Diode bias. Forward bias. � � � � p n E � � J J drift dif � J V D >V 0 If V D >V 0 , diffusion and drift currents have same direction and current can be higher. There isn’t opposition for flowing of current.

Diode bias. Reverse bias. V’ p n � � � � E � � � � � � � � � � J J J des J dif drift dif � � � � J I 0 <<<< V R V R creates an electric field reinforcing the field in the depletion area, increasing the drop of potential: V´=V 0 +V R . Depletion area enlarges. So, the diffusion current of majoritary carriers decreases (holes from p to n area and e- from n to p ), and the drift current of minoritary carriers should increase (e - from p to n area and holes from n to p ). But there are only few minoritary carriers availables (generated by thermic generation), and this current I 0 is very small, and it’s called REVERSE SATURATION CURRENT .

Diode current-voltage characteristic. I 0 : Reverse saturation current V u : Diode forward voltage drop Symbol for diode: 0,15 0,15 m I (mA) I (mA) p area n area Anode Cathode 0,05 0,05 I o I o I 0 < µ A V u -0,05 -0,05 -70 -70 -20 -20 30 30 80 80 V (mV) V (mV) Inverse of the slope (m) on high voltage region is the internal resistance of the diode (r d =1/m)

Models of diode Behaviour of diode can be modeled with three approachings: 1 st approaching. Ideal diode: Not taken in account neither V u nor r d . Forward biased, the diode is a short-circuit. Reverse biased, the I diode is an open circuit. R R ε ε ε I I I = R V voltage-current characteristic for a diode in 1 st approaching R R ε ε I=0 I

Models of diode 2 nd approaching. Simplified model: Only taken in account diode forward voltage drop : V u = 0.3 V for Ge diode I V u = 0.7 V for Si diode R=1k Ω R=1k Ω V 0 = 6V V 0 = 6V V u =0.7 V I I V V u voltage-current characteristic for a diode in 2 nd approaching V V 6 0.7 − − 0 u I 5.3mA = = = R 1k

Models of diode 3 d approaching. Linear diode: I 1/r d Taken in account both diode forward voltage drop as diode internal resistance . R=1k Ω V V u V u =0,7 V I ε = 6V voltage-current characteristic r d =25 Ω for a diode in 3 d approaching R=1k Ω ε V 6 0.7 − − u I I 5.2mA = = = ε = 6V r d = 25 Ω R 1000 25 + V u =0.7 V

Models of diode Three models for junction diode I Ideal diode (1 st approaching) V I V u Simplified model (2 nd approaching) V V u I r d r d V u Linear model (3 d approaching) V V u

Reverse recovery time of diode I Should be… t polarización polarización Reverse biased Forward biased directa inversa I Is..… t t ri tiempo de recuperación inverso Reverse recovery time

Application: Diode as rectifier output Half-wave rectifier: U U ~ input t t output Full-wave rectifier: ~ input U Routput output output t ~ input

Applications: logician circuits V=10 V 1 Logic AND and OR logic gates V= 0 V 0 Logic “AND” gate with diodes “OR” gate with diodes R s A V s A V s B B R R s R >>>R s R 10 V V A V B V S V A V B V S 0 (0) 0 (0) 0,7 (0) 0 (0) 0 (0) 0 (0) 0 (0) 10 (1) 0,7 (0) 10 (1) 0 (0) 9,3 (1) 10 (1) 0 (0) 0,7 (0) 0 (0) 10 (1) 9,3 (1) 10 (1) 10 (1) 10 (1) 10 (1) 10 (1) 9,3 (1)