JNTU B.Tech II ELECTRONIC DEVICES AND CIRCUITS Previous Question Paper (August/September - 2022 - R15)

The diode current equation, also known as the Shockley diode equation, is given by:

Where I is the diode current, I_s is the reverse saturation current, V is the applied voltage, η is the ideality factor (1 for Ge, 2 for Si), and V_T is the thermal voltage (V_T = kT/q ≈ 26 mV at room temperature).

Static (DC) Resistance: Defined as the ratio of DC voltage to DC current at a specific operating point on the V-I characteristic curve. It is calculated as R_dc = V/I. It represents the resistance offered by the diode to the DC supply.

Dynamic (AC) Resistance: Defined as the reciprocal of the slope of the V-I characteristic curve at a given operating point. It is calculated by differentiating the diode current equation with respect to voltage: r_ac = dV/dI = ηV_T / I. It represents the resistance offered by the diode to small signal variations around the operating point.

The V-I characteristics of a P-N junction diode describe the relationship between the current flowing through the diode and the voltage applied across it, divided into two regions:

• Forward Bias: When the P-region is connected to the positive terminal and N-region to the negative terminal, the potential barrier is reduced. Initially, current is negligible until the knee voltage (approx. 0.3V for Ge, 0.7V for Si) is reached. Beyond this, the current increases exponentially with a small increase in voltage.
• Reverse Bias: When the P-region is connected to the negative terminal and N-region to the positive terminal, the potential barrier increases. Only a very small, constant reverse saturation current (I_s) flows due to minority carriers. This current remains nearly independent of the applied voltage until the breakdown voltage is reached.
• Breakdown Region: If the reverse voltage exceeds a critical limit, the diode enters the breakdown region (Zener or Avalanche breakdown), where the current increases sharply, potentially damaging the device if not limited by external resistance.