Semiconductor Device Physics Lecture 9 Dr Ing Erwin

Semiconductor Device Physics Lecture 9 Dr. -Ing. Erwin Sitompul President University http: //zitompul. wordpress. com President University Erwin Sitompul SDP 9/1

Chapter 9 Optoelectronic Diodes Photodiodes Reverse current due to carriers swept by the E-field Electron-hole pair generation due to light President University Erwin Sitompul SDP 9/2

Chapter 9 Optoelectronic Diodes I–V Characteristics and Spectral Response Open circuit voltage voc Upper limit ~ highest wavelength ~ lowest frequency ~ lowest energy Short circuit current isc President University Erwin Sitompul SDP 9/3

Chapter 9 Optoelectronic Diodes p-i-n Photodiodes p-i-n : positive–intrinsic– negative W ≈ Wi-region Þ most carriers are generated in the depletion Þ faster response time (~10 GHz operation) President University Reverse biased • current arises mostly in the totally • • depleted i-region, not in quasineutral region as in pn diode generated carriers do not need to diffuse into the depletion region before they are swept by the E-field enhanced frequency response Erwin Sitompul SDP 9/4

Chapter 9 Optoelectronic Diodes Forward bias Increasing EG Light Emitting Diodes (LEDs) n LEDs are typically made of compound semiconductors (direct semiconductors with band-to-band recombination) n It releases energy by dissipating light / emitting photon President University Erwin Sitompul SDP 9/5

Chapter 10 BJT Fundamentals Bipolar Junction Transistors (BJTs) n Over the past decades, the higher layout density and lowpower advantage of CMOS (Complementary Metal–Oxide– Semiconductor) has eroded away the BJT’s dominance in integrated-circuit products. n Higher circuit density better system performance n BJTs are still preferred in some digital-circuit and analog-circuit applications because of their high speed and superior gain n Faster circuit speed (+) n Larger power dissipation (–) • Transistor: current flowing between two terminals is controlled by a third terminal President University Erwin Sitompul SDP 9/6

Chapter 10 BJT Fundamentals Introduction n There are two types of BJT: pnp and npn. n The convention used in the textbook does not follow IEEE convention, where currents flowing into a terminal is defined as positive. n We will follow the normal convention: . . . President University Erwin Sitompul SDP 9/7

Chapter 10 BJT Fundamentals Circuit Configurations Common-Emitter I–V Characteristics Most popular configuration Active Mode Saturation Mode IC < b. IB President University In active mode, bdc is the common emitter dc current gain Erwin Sitompul SDP 9/8

Chapter 10 BJT Fundamentals Modes of Operation n Common-Emitter Output Characteristics Mode E-B Junction C-B Junction Saturation forward bias Active/Forward forward bias reverse bias Inverted reverse bias forward bias Cutoff reverse bias President University Erwin Sitompul SDP 9/9

Chapter 10 BJT Fundamentals BJT Electrostatics n Under equilibrium and normal operating conditions, the BJT may be viewed electrostatically as two independent pn junctions. W : quasineutral base width President University Erwin Sitompul SDP 9/10

Chapter 10 BJT Fundamentals BJT Electrostatics n Electrostatic potential, V(x) n Electric field, E(x) n Charge density, ρ(x) President University Erwin Sitompul SDP 9/11

Chapter 10 BJT Fundamentals BJT Design n Important features of a good transistor: n Injected minority carriers do not recombine in the neutral base region short base, W << Lp for pnp transistor n Emitter current is comprised almost entirely of carriers injected into the base rather than carriers injected into the emitter must be doped heavier than the base pnp BJT, active mode President University Erwin Sitompul SDP 9/12

Chapter 10 BJT Fundamentals Base Current (Active Bias) n The base current consists of majority carriers (electrons) supplied for: 1. Recombination of injected minority carriers in the base 2. Injection of carriers into the emitter 3. Reverse saturation current in collector junction 4. Recombination in the base-emitter depletion region EMITTER COLLECTOR BASE 1 4 p-type President University 2 n-type Erwin Sitompul 3 p-type SDP 9/13

Chapter 10 BJT Fundamentals BJT Performance Parameters (pnp) IEn ICp IEp n Emitter Efficiency § Decrease Negligible compared to holes injected from emitter n Base Transport Factor § Decrease 5 relative to 1 and 2 to increase efficiency 1 relative to 2 to increase transport factor Common base dc current gain: President University Erwin Sitompul SDP 9/14

Chapter 10 BJT Fundamentals Collector Current (Active Bias) n The collector current is comprised of: n Holes injected from emitter, which do not recombine in the base 2 3 n Reverse saturation current of collector junction ICB 0 : collector current when IE = 0 Common emitter dc current gain: President University Erwin Sitompul SDP 9/15

Chapter 11 BJT Static Characteristics Notation (pnp BJT) Minority carrier constants President University Erwin Sitompul SDP 9/16

Chapter 11 BJT Static Characteristics Emitter Region n Diffusion equation: n Boundary conditions: President University Erwin Sitompul SDP 9/17

Chapter 11 BJT Static Characteristics Base Region n Diffusion equation: n Boundary conditions: President University Erwin Sitompul SDP 9/18

Chapter 11 BJT Static Characteristics Collector Region n Diffusion equation: n Boundary conditions: President University Erwin Sitompul SDP 9/19

Chapter 11 BJT Static Characteristics Ideal Transistor Analysis n Solve the minority-carrier diffusion equation in each quasineutral region to obtain excess minority-carrier profiles n Each region has different set of boundary conditions n Evaluate minority-carrier diffusion currents at edges of depletion regions n Add hole and electron components together terminal currents is obtained IC IE IB President University Erwin Sitompul SDP 9/20

Chapter 11 BJT Static Characteristics Emitter Region Solution n Diffusion equation: n General solution: n Boundary conditions: n Solution President University Erwin Sitompul SDP 9/21

Chapter 11 BJT Static Characteristics Collector Region Solution n Diffusion equation: n General solution: n Boundary conditions: n Solution President University Erwin Sitompul SDP 9/22

Chapter 11 BJT Static Characteristics Base Region Solution n Diffusion equation: n General solution: n Boundary conditions: n Solution President University Erwin Sitompul SDP 9/23

Chapter 11 BJT Static Characteristics Base Region Solution n Since n We can write as President University Erwin Sitompul SDP 9/24

Chapter 11 BJT Static Characteristics Base Region Solution n Since President University Erwin Sitompul SDP 9/25

Chapter 11 BJT Static Characteristics Terminal Currents n Since n Then President University Erwin Sitompul SDP 9/26

Chapter 11 BJT Static Characteristics Simplified Relationships n To achieve high current gain, a typical BJT will be constructed so that W << LB. n Using the limit value Due to VEB n We will have Due to VCB President University Erwin Sitompul SDP 9/27

Chapter 11 BJT Static Characteristics Performance Parameters n For specific condition of n “Active Mode”: emitter junction is forward biased and collector junction is reverse biased n W << LB, n. E 0/p. B 0 = NB/NE President University Erwin Sitompul SDP 9/28

Chapter 6 pn Junction Diodes: I-V Characteristics Homework 7 n 1. (10. 17) n 2. (10. 14) Consider a silicon pnp bipolar transistor at T = 300 K with uniform dopings of NE = 5× 1018 cm– 3, NB = 1017 cm– 3, and NC = 5× 1015 cm– 3. Let DB = 10 cm 2/s, x. B = 0. 7 μm, and assume x. B << LB. The transistor is operating in saturation with JP = 165 A/cm 2 and VEB = 0. 75 V. Determine: (a) VCB, (b) VEC(sat), (c) the number/cm 2 of excess minority carrier holes in the base, and (d) the number/cm 2 of excess minority carrier electrons in the long collector, take LC = 35 μm. Problem 10. 4, Pierret’s “Semiconductor Device Fundamentals”. n Deadline: 07. 04. 2011, at 07: 30 am. President University Erwin Sitompul SDP 9/29