Diode IV Characteristics

The current-voltage (IV) characteristic of a PN junction diode is a graph that summarizes the complete electrical behavior of the device across all operating regions. It shows how current responds to applied voltage for both forward and reverse bias conditions. This single curve captures the asymmetric, nonlinear nature of the diode and is the starting point for analyzing any diode circuit. GATE and university exams frequently test the ability to read, interpret, and calculate from this curve.

Core Concept: Four Regions of the IV Curve

The diode IV characteristic displays distinct regions including forward bias exponential behavior, reverse saturation current, and breakdown voltage limits.

The diode IV curve can be divided into four distinct regions. In the forward bias region, applied voltage reduces the built-in barrier and current increases exponentially. The curve is nearly flat (very small current) until the applied voltage reaches the cut-in voltage (also called threshold or knee voltage), which is approximately 0.6-0.7 V for silicon and 0.2-0.3 V for germanium. Beyond this, current rises sharply.

In the reverse bias region, the current is nearly constant at -IS regardless of how much reverse voltage is applied. This is the reverse saturation current, arising only from minority carrier drift. It is typically in the nanoampere range for silicon at room temperature. The IV curve appears as a nearly horizontal line just below the voltage axis.

When reverse voltage exceeds the breakdown voltage VBR, current increases very steeply in the negative direction. This is the breakdown region. For Zener diodes, this is the intentional operating region. For rectifier diodes, operation in breakdown is destructive unless current is externally limited.

Mathematical Expression

The complete IV relationship is given by the Shockley diode equation:

I = IS * (e^(V/nVT) - 1) For forward bias where V >> VT, the -1 is negligible and I = IS * e^(V/nVT). For reverse bias where V is negative and |V| >> VT, e^(V/VT) approaches zero and I = -IS. The curve is asymmetric: large positive current in forward bias, tiny negative current in reverse bias.

The slope of the IV curve in the forward bias region at any operating point is the dynamic resistance rd:

rd = dV/dI = nVT / I = 26/I (ohms, with I in mA at room temperature) This shows that dynamic resistance decreases as the diode current increases. At higher current, the diode presents lower AC resistance.

Practical Understanding

In circuit analysis, the diode IV curve is used to find the operating point or Q-point (quiescent point) using a graphical method called load-line analysis. The load line is drawn on the IV curve by connecting the supply voltage on the V-axis to the short-circuit current (VS/RS) on the I-axis. The intersection of load line and IV curve gives the actual operating current and voltage.

The IV curve shifts significantly with temperature. An increase in temperature causes the cut-in voltage to decrease by approximately 2 mV per degree Celsius, and IS to increase. This means a diode conducts more easily at higher temperatures for the same applied voltage, which can lead to thermal runaway in power circuits if not properly controlled.

Given:
IS = 10^-12 A
n = 1
VT = 0.026 V
V1 = 0.6 V, V2 = 0.66 V

Why this formula applies:
Comparing current at two forward voltages shows exponential sensitivity.

Formula:
I = IS * e^(V / VT)

At V1 = 0.6 V:
I1 = 1e-12 * e^(0.6/0.026)
   = 1e-12 * e^23.08
   = 1e-12 * 1.06e10
   = 10.6 mA

At V2 = 0.66 V:
I2 = 1e-12 * e^(0.66/0.026)
   = 1e-12 * e^25.38
   = 1e-12 * 9.56e10

Final Answer:
I2 = 95.6 mA (approximately 9x increase for 60mV rise in voltage)
Dynamic resistance at I1: rd = 26mV / 10.6mA = 2.45 ohms

Exam Tip: In diode load-line problems, the load line connects (VS, 0) on voltage axis to (0, VS/RS) on current axis. The Q-point is the intersection. Also remember: for a 60 mV increase in forward voltage, current increases by approximately 10 times (factor of e^(60/26) which is about 10). This is the decade rule frequently tested in GATE.

Quick Revision

• IV curve has 3 regions: forward bias (exponential rise), reverse bias (flat at -IS), and breakdown (sharp negative current rise).
• Shockley equation: I = IS*(e^(V/nVT) - 1).
• Cut-in voltage: ~0.7 V (Si), ~0.3 V (Ge). Below this, current negligible.
• Dynamic resistance: rd = nVT/I = 26/I (mA) at room temp.
• Q-point found using load-line analysis: intersection of load line and IV curve.
• Temperature effect: cut-in voltage decreases ~2 mV/°C; IS doubles per 10°C rise.
• Trap: Doubling reverse voltage does NOT double reverse current in ideal model -- IS is constant.