ECE 440 Lecture 29 : Introduction to the BJT-I Class Outline: Narrow-Base Diode BJT Fundamentals BJT Amplification
Things you should know when you leave Key Questions How does the narrow-base diode multiply current? How can a transistor function as both a switch and an amplifier? What are the main characteristics of a BJT? What are the main mechanisms for current flow?
Narrow- Base Diodes In the final few weeks of the class, we will discuss the operation of the bipolar junction transistor (BJT) Our plan will be to build up our knowledge about related devices and then discuss the operation of the bipolar junction transistor. To this end, we begin with the narrow-base diode. The narrow-base diode is comprised of: p + -n diode. Heavily doped n + contact. Typically, the neutral portion of the lightly doped region is much less than the average hole diffusion length away.
Narrow- Base Diodes Do you remember how it works?
Narrow- Base Diodes How does current flows through the narrow-base diode? we will need to solve the diffusion equation But we can simplify it Most of the minority injected holes will diffuse across the n-type base. I(x = 0) ~ I(x = x n ). Make the straight line approximation for the variation of the excess carrier profile
Narrow- Base Diodes What is the main contribution to the current in a narrow-base diode? It is the minority hole diffusion. Let s write down the hole current density in the n-region: But we ve used the straight line approximation so we can simplify this equation But the current here is much larger than in a normal p+-n diode at the same voltage. This is because l replaces L p in the denominator of the diode equation since l << L p. Theoretically speaking, all excess holes recombine immediately at x n = l. Since the excess concentration is ~ 0 this is far below the normally expected concentration in a normal p + -n diode.
Narrow- Base Diodes The much larger change in hole concentration across the neutral n-region sets up a large diffusion current. We already know that only a few holes will recombine within the n-region. Each time this happens another electron must flow in from the n+ contact to preserve the charge neutrality. Or mathematically This sets up an electron current flowing into the n-region at x n = l compensates for the small decrease in hole diffusion current due to recombination The difference in the brackets is small as l << Lp so the hole diffusion current remains roughly constant up to the n + contact.
Narrow- Base Diodes Visually we can describe the operation in the following manner x > l inside the n+ contact the entire current is carried by majority electrons.
Narrow- Base Diodes What does the straight line approximation tell allow us to do? How does this simplify the device analysis? Assuming a constant cross-section, we can solve the diffusion equation exactly using linear combinations of exponentials (as before) So, the hole diffusion current in the n-region is given by And we already know the boundary conditions. And the current injected at the boundary becomes: Use small angle expansion:
Narrow- Base Diodes What is the hole current at x n = l at the n + contact? Which reduces to The difference between these two hole currents will represent the electron current flowing into the base from the n + contact.
BJT Fundamentals Before we proceed with our discussion of the BJT, we should discuss transistor operation in general What is a transistor? Three terminal device. Current flowing through two terminals can be controlled by the application of voltages to the third terminal. Remember the inverter
BJT Fundamentals Consider the following two terminal device characteristics Bias the device with a simple resistor and battery Which gives the following I-V curve We want to know the current and voltage for our element from the loop equation about our simple circuit But here we have two unknowns, so we need another equation so we use the figure on the right.
BJT Fundamentals Now we would like to add a third terminal to the circuit which will control the transfer characteristics Our new circuit With new I-V
BJT Fundamentals Now we may begin our analysis of the Bipolar Junction Transistor (BJT) Start by remembering the p-n junction in reverse bias The reverse saturation current depends on the rate at which minority carriers are generated. Independent of bias. Increase EHP generation (use light with energy greater than the band gap) and we can increase the saturation current.
BJT Fundamentals We already know a good hole injector from our analysis of pn junctions What happens if we now add a second p-layer on the other side of the n-layer to make a p+-n-p junction and reverse bias this second p-n junction? What should the I-V characteristics look like for this configuration of layers and potentials?
BJT Fundamentals The structure we have described is a bipolar junction transistor I-V
BJT Fundamentals There are many processes at work, let s try to summarize them
BJT Fundamentals Let s make a few observations about the operation of a BJT If properly biased, I E flows into the emitter and I C flows out of the collector. The base current, I B, should be small since I E ~ I C. What makes up the base current, I B?