Physics · Ch 9 — Semiconductor Electronics
Reverse Characteristics
Reverse Characteristics
The reverse V-I characteristic uses an identical circuit but with the diode connected in reverse bias: the p-region to the negative terminal and n-region to the positive terminal of the DC supply, with a microammeter in the circuit since reverse currents are far smaller than forward currents. A graph of the reverse voltage against the resulting current -- now only microamps, roughly 20 A for silicon and 50 A for germanium -- is the reverse characteristic: this small current, almost independent of the applied reverse voltage, is the reverse saturation current carried by minority carriers (the leakage current). The reverse voltage can only be raised up to a rated value before the diode enters the breakdown region (developed fully in 9.3.5). Plotting the forward and reverse characteristics together on one set of axes -- forward voltage and current in the first quadrant, reverse voltage and current in the third -- gives the single combined curve most textbooks use to summarise a diode's complete behaviour in one picture. …
What this figure shows. Panel (a) shows the reverse-bias test circuit: a microammeter (A) in series with the diode and a variable resistor R across a 0-15 V DC supply, oriented so the diode is reverse biased, with a voltmeter V reading the reverse voltage. Panel (b) plots reverse voltage on the negative x-axis against reverse current (A) on the negative y-axis: the curve is nearly flat at a small, roughly constant current (marked '50 A Germanium' and '20 A Silicon') until it reaches the 'Reverse breakdown voltage' marker, beyond which the current would rise sharply (break …
What this figure shows. A single set of axes overlays both regimes: the first-quadrant portion plots forward voltage (V) against forward current (mA), curving sharply upward past the knee exactly as in Figure 9.14(b); the third-quadrant portion plots reverse voltage against reverse current (A), staying essentially flat exactly as in Figure 9.15(b). Seeing both regimes on one graph makes their vastly different current scales (milliamps versus microamps) and shapes (steep exponential versus nearly flat) immedi …