Previous 10 Years (2009-18) 1 Mark Que.&Sol Subject: Electronic Devices Year: 2011 By- AKHILESH PANDEY
About Me BTech. in Electronics & Communication GATE 2017 AIR 18 (EC) 5 Years Teaching Experience Youtuber (channel- STUDENT MODULATOR)
Q.1 Drift current in the semiconductors depends upon (A) only the electric field (B) only the carrier concentration gradient (C) both the electric field and the carrier concentration (D) both the electric field and the carrier concentration gradient
Sol. 1 In a semiconductor, drift current density is defined as (drift current due to electrons) (drift current due to holes) In E ne oE pe ,B where E applied electric field intensity Conductivity n concentration of electrons e Electronic charge Hm, p Mobility of electron/hole
Diffusion current density is defined as where Dn, D - e - Electronic charge dn dx dp dx Diffusion coefficient of electrons/holes Electron concentration gradient Hole concentration gradient Hence, drift current depends on electric field E, while diffusion current depends on carrier concentration gradient.
Q.2 A Zener diode, when used in voltage stabilization circuits, is biased in (A) reverse bias region below the breakdown voltage (B) reverse breakdown region (C) forward bias region (D) forward bias constant current mode
Sol.2 V-I characteristic of zener diode: In forward bias, it works same as the normal diode. In reverse bias, it breakdowns at a lower voltage, called zener breakdown voltage Vz. > After breakdown (reverse bias), voltage is fixed Vz and current increases.
Sol.2 For the Zener diode connected in a voltage stabilization circuit, Iz current through zener diode Vz - voltage across zener diode V Source voltage R - Load resistance Here, after zener breakdown (reverse bias) Vz is fixed and Iz increase with source voltage. Thus, the voltage across load is fixed at V%, independent of the applied source voltage V Thus, Zener diode connected in reverse bias for voltage stabilization.
Q.3 A silicon PN junction is forward biased with a constant current at room temperature. When the temperature is increased by 10 C, the forward bias voltage across the PN junction (A) increases by 60 mV (B) decreases by 60 mV (C) increases by 25 mV (D) decreases by 25 mV
Sol.3 Effect of temperature on V-I characteristics: Case I: Voltage across diode is fixed and temperature is increased Reverse saturation current through a PN junction increases approximately 7% per C for both silicon and germanium. For every 10 C temperature increase, reverse saturation current becomes double (since(1.07)10 2) Case II: Current across diode is fixed and temperature is increased If the temperature is increased, then in order to maintain a constant current value through PN junction, dv 2.5 mW / C dT Hence, for 10 C temperature rise, the voltage decreases by 25 mV
B.Tech. in EC, GATE 2017 AIR 18, YouTuber (channel name - STUDENT MODULATOR), 5 years Teaching Experience