ElectronicsI_L3 - Even More Forward Bias If we know the...

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Even More Forward Bias • If we know the voltage and current for a diode at any point on it’s I-V curve we can find any other operating point: • Alternatively in Log base 10: • Here we can see that for a decade change in current, the voltage drop across the diode will change by 2.3nV T , for a diode with n=1 this is approximately 60mV at room temp. • If the temp increases, then the voltage drop will increase for a fixed current. () T nV V V e I I 1 2 2 1 = 1 2 1 2 ln I I nV V V T = 1 2 1 2 log 3 . 2 I I nV V V T = Log base 10
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Cut-in Voltage • With the exponential dependence of the current on the applied voltage, the I-V curve in forward bias appears to start off with zero current near zero bias and then suddenly increase substantially. • Of course there is a current near zero bias, but it is extremely small (pA or nA) • The cut-in voltage is typically around 0.5V. This voltage drop will have consequences for circuits using diodes as we shall see later. • A diode that is “fully conducting” is typically operating with biases of 0.6 to 0.8 Volts.
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Reverse Bias and Breakdown The reverse bias region is much simpler than the forward bias region. – If we go back to the diode equation, we can see that negative values for the applied bias result in an effectively constant current: – Perhaps you can see where the term saturation current came from, in reverse bias the current appears to saturate. – In reality though, the current in reverse bias is dominated by leakage currents, due to imperfections in the diodes. These leakage currents are about 1000x larger. Just like the forward bias region, even larger negative biases lead to breakdown. This is not covered in this model * . The voltage that this occurs at is called the breakdown voltage V ZK where the Z stands for Zener (we’ll talk about that later) and the K stands for “knee” meaning the point where the slope changes dramatically on the I-V curve. * It’s worth mentioning here a general issue. Just about every equation you see is an approximation. You should always keep in mind what its limits are! = 1 T nV v S e I i 0 S I i
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Modeling a Diode Though accurate, the exponential model is also the most difficult to use because of the added complexity of calculating logarithms. Let’s look at a simple circuit with a diode and resistor to see just how bad it can get: – We’ll assume the voltage across the diode V DD is greater than 0.5V so we can use the simplified exponential model. – The other equation that governs the circuit is the Kirchhoff loop equation Assuming we know the diode parameters, we have two equations and two unknowns. But solutions are made challenging by the
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ElectronicsI_L3 - Even More Forward Bias If we know the...

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