17 Principle of Reverse Recovery Characteristics of Diodes
Entering the plum rain season, the rain never stops from morning to night, just like an old lady's endless dripping.
Annoying, but not lethal.
Traffic also slowed down in the drizzle, making the journey long and boring.
Uh, how can it be boring?
——Because the driver changed from the dry-humored Xiao Ma Ge to the taciturn Xu Zhiqian.
Moreover, Ni Yue found that his cousin was not the same as before. She spoke very little recently. She only laughed when she was helping customers take a bath. She was silent once she got in the car.
Ni Yue slumped into her seat like a cooked prawn, her eardrums throbbing with the creaking of wipers scraping against the windshield. She felt uncomfortable, anxious to dry out the damp silence, so she asked while scrolling through her phone, "Have you seen a Hong Kong movie called 'Under the Day'?"
"No." The two teammates said in unison.
As soon as he finished speaking, Xu Zhiqian glanced at Zhang Chenling in the rearview mirror and saw that the latter was staring out the window with an expressionless face, even her peripheral vision was cold.
His Adam's apple rolled unconsciously, and he asked Ni Yue in a hoarse voice, "What are you talking about?"
"It tells the story of the dark side of a private welfare home in Hong Kong that specializes in taking in the elderly, the weak, the sick and the disabled."
"What's the shady deal?" Zhang Chenling turned her head and showed interest.
"The director of that orphanage is incredibly perverted, repeatedly sexually assaulting mentally disabled girls. The staff are also incredibly evil and twisted, often abusing and beating people with disabilities. There's a particularly shocking scene in the movie where a staff member, in broad daylight, ties an elderly man with dementia to a chair on the rooftop and showers him with a water gun!"
As Ni Yue spoke, she glanced down at her phone. She happened to see this clip on Xiaohongshu just now, so she mentioned this movie.
"Huh?" Zhang Chenling was baffled. "Isn't this a crime?"
"Absolutely!" Xu Zhiqian agreed.
"It's a crime!" Ni Yue responded. "And these plots are all based on true stories! But in reality, the director who sexually assaulted a mentally disabled girl was acquitted in the first trial due to insufficient evidence, and the reporter who exposed these scandals was accused of meddling in other people's affairs, and blamed for the closure of the institution, leaving the elderly, the weak, and the disabled without a place to live... In short, this movie deeply reflects the differences in human nature. Unfortunately, it was only released in mainland China for a few days before being withdrawn, and I haven't seen it yet."
"I'll look for resources online and send them to you!" Xu Zhiqian became motivated.
"Forget it, I'd better not watch this kind of movie. It will make me upset!" Ni Yue regretted it.
Women are so fickle, Xu Zhiqian pulled the corner of his mouth helplessly.
Ni Yue sighed again, "If even a developed city like Hong Kong can experience such appalling things, wouldn't it be even worse in less developed areas? Is it so difficult for the elderly, the weak, the sick, and the disabled to maintain their dignity and just take a clean shower?"
"It's difficult!" The two teammates said in unison again.
…
The three of them were driving to the home of Aunt Xu, who suffered from depression. After talking about movies, Zhang Chenling naturally introduced Aunt Xu's situation to Xu Zhiqian.
Xu Zhiqian said, "To be honest, I know Professor Xu!"
"Oh?" The two sisters were surprised.
"You all have the surname Xu, so you're not relatives, are you?" Ni Yue joked with a smile.
"When I was in college, I really wanted to be related to her!" Xu Zhiqian opened up for once. "You know, when I opened that Excel spreadsheet, the first thing I saw was Professor Xu's name!"
Xu Huaiyun, a top academic figure in the field of electronic engineering and a star professor at Shanghai Jiao Tong University, her name is always so eye-catching and dazzling.
Although he graduated from university many years ago, Xu Zhiqian still remembers how difficult it was to get a spot in Professor Xu's elective course, Advanced Circuit Analysis.
That year, just as a sophomore, he set his alarm for 5:00 AM to grab a class. He woke up, didn't even wash his face, and rushed to the school computer lab, only to find more than a dozen people already waiting in line, half of them eager to take Professor Xu's class. When the doors finally opened at 6:00, he tripped and fell, missing out on the few spots available for electives.
Xu Zhiqian was unwilling to give up. He wanted to take this course because he really wanted to learn it, not just to get credits. So, with such a thief's intention, he just hid in the corner of the lecture hall and took the class.
According to the school regulations at the time, attending classes for free was not allowed, so at first he kept a low profile for fear of being discovered, but later he could not control himself and asked questions and spoke in class, and had a direct discussion with Professor Xu on academic issues.
As they communicated more and more, Professor Xu naturally remembered her face. Until one day, she couldn't help but ask him, "Hey student, what's your name?"
…
Xu Zhiqian's plain narration, like boiled water, made the two sisters deeply involved in it. Hearing this, they held their breath at the same time, feeling as if they had been pulled out of the class.
"Oh!" He perfectly imitated Professor Xu's reaction when he heard his name. "Are we from the same family? But why is your name not on my list?..."
From then on, Professor Xu often called his name in class and asked him to answer questions, and he naturally became the top auditor.
However, he thought, it has been so many years, Professor Xu must have forgotten him a long time ago!
"Xu Zhiqian? Why are you here to help me bathe?"
Unexpectedly, Professor Xu recognized Xu Zhiqian the moment he saw him. The latter froze in his tracks, hesitating for a moment before nodding and greeting him, "Hello, Professor Xu!"
Travel back to college days in a second.
The perceptive Zhang Chenling sensed Xu Zhiqian's embarrassment.
At the same time, she was surprised to find that a gleam of light flashed in Aunt Xu's originally empty and dull eyes.
That light was probably a chemical reaction produced by the collision of several complex emotions:
There is the surprise of reunion after a long separation, the preference that teachers have for their students, especially the good students, and perhaps, some misunderstanding... plus, the feeling of being disappointed in their students?
Professor Xu would never have dreamed that the clever and smart boy who was an auditor in the classroom back then would now be helping people wash baths with his face covered in dirt!
…
After slowly immersing Professor Xu's body in the water, Xu Zhiqian quietly moved to the top of her head, intending to help her wash her hair, avoiding excessive eye contact.
"Hey, come and sit here!" Unexpectedly, she looked for Xu Zhiqian with her eyes raised, asking him to sit next to her.
She, who almost never expressed her opinion during the bathing assistance process in the past, made a request for the division of labor in bathing assistance for the first time.
"Then let me wash your hair!" Ni Yue was considerate and walked over to push the embarrassed Xu Zhiqian to the side of the bathtub.
He had no choice but to sit down on the small stool and picked up the water scoop.
As usual, Zhang Chenling was responsible for scrubbing the body.
Professor Xu stared at Xu Zhiqian without blinking.
Feeling that he was being scrutinized, his hair stood on end, and he cautiously stole a glance at her.
Unexpectedly, their eyes met and they were successfully paired.
Professor Xu, who usually would just stare at the ceiling and say "thank you" and "whatever", immediately asked him in a tone that was both gentle and stern: "Xu Zhiqian, do you still remember the principle of the reverse recovery characteristics of diodes that I taught in class?"
"Huh?" He was stunned. He raised his hand out of habit and used his index finger to support the bottom edge of his frameless glasses. "I remember... I should remember."
"I can't quite remember, can you help me recall it?"
“!!!”
Sweat broke out on his forehead. After a moment's thought, he replied, "The reverse recovery characteristic must be explained from the structure of the diode. The essence of a diode is a PN junction..."
A few minutes later...
"The answer is correct, 100 points!" Aunt Xu gave the affirmation in an unusually loud voice.
It’s not easy, Xu Zhiqian breathed a sigh of relief.
He never thought that the oral exam of questions and answers had just begun.
"Do you remember how I explained the differences in the working principles of transistors and MOSFETs?" Professor Xu asked again.
"Of course I remember. I was once stuck on this question in class when you asked me that!"
"Well, answer it again. You can't get stuck again this time!"
"We need to analyze it from three perspectives: cutoff, amplification, and saturation..."
Professor Xu asked questions one after another.
Xu Zhiqian racked his brains, his face flushed red, and beads of sweat on his forehead became more and more frequent. Later, they condensed together and flowed down his cheeks and neck, soaking a large area of the collar of his T-shirt.
Zhang Chenling and Ni Yue were forced to audit a university electronics class, but they couldn't understand a single word, as if they were listening to Martian language.
By the end of the wash, Ni Yue was drowsy and his eyelids were almost sticking together.
Zhang Chenling was also very sleepy and regretted not buying a cup of coffee at the entrance of the alley before getting on the bus.
It was not until the conversation between the master and the disciple turned from advanced academic discussions to personal topics that the two sisters woke up from their chaotic state.
Lovers of gossip, they both pricked up their ears at the same time and heard Professor Xu ask Xu Zhiqian: "...How many years have you graduated? How old are you this year?"
He answered truthfully, "I graduated thirteen years ago, and I'm thirty-six this year."
"Huh, my zodiac year! ...So, do you still remember a little girl named Lin Meiyi in my class? She always sat next to you in class?"
"...I don't remember!" He said he didn't remember, but his face turned red and white.
Professor Xu rolled his eyes at him and muttered, "She was a very good student back then, but I haven't seen her since graduation... You kids, don't you guys come back to school often?"
Xu Zhiqian laughed awkwardly at himself: "Other alumni who come back to school usually go back to donate money or recruit people. I'm not doing well, and I don't have anything to do with either. How can I go back and embarrass myself?"
"So, tell me what's going on with you? How can you come and help me bathe?"
Xu Zhiqian hated himself for speaking without thinking and putting himself in danger.
He didn't want to lie, but also didn't want to lose face, so he had to write in a simple and elegant manner: "At the end of last year, our company adjusted its business, and I took a break for a while. Later, I thought being a bath assistant would be interesting, so I found Mr. Zhang..."
The words "Mr. Zhang" hit him, and he felt a little hot on his lips. He looked embarrassed and looked away, "Of course, the overall environment isn't very good right now!"
"So you are the one who destroys the environment?" Professor Xu's sarcasm hit the nail on the head.
Zhang Chenling smiled secretly. Aunt Xu was quite smart. It was obvious that she was just depressed before and not confused.
Xu Zhiqian was dumbfounded by the question.
Finally, Professor Xu's gaze fell on the white hair at his temples, and his tone became more tender. "...So, are you married? Do you have children?"
Asked the key question.
Xu Zhiqian, however, hesitated, seemingly unwilling to answer. Aunt Xu's daughter, seeing this, came to his rescue, "Oh, Mom, are you checking the household registration? This is Xiao Xu's private matter. Can we not pry into it?"
Professor Xu curled his lips, muttered a few words, and stopped asking questions.
Zhang Chenling pretended to glance at Xu Zhiqian casually and saw that he was wiping the sweat off his face and neck with his sleeves, looking extremely embarrassed.
The two people's eyes met briefly, and a smile as if he had survived a disaster suddenly appeared on his face.
After the bath, the three of them had almost finished packing up and were about to say goodbye to the mother and daughter when Professor Xu suddenly remembered something and gestured with her hands to direct her daughter, "Go to the study and get my latest book!"
Her daughter was startled, then turned and walked out of the room.
"Bring me another pen, I want to sign for Xiao Xu!" Professor Xu added in a raised voice.
Soon, her daughter came back with a thick book and a pen.
Professor Xu leaned against the headboard, holding the book in one hand and the pen in the other hand, tremblingly, signing the title page of the book.
Xu Zhiqian took Professor Xu's new work as if it were a treasure, but his first feeling when holding it in his hand was that something was not right. The book was in 16-mo format and very thick, but the production was not fine, even a bit perfunctory. The light yellow cover had a very poor texture, the paper was thin, and it felt a bit wrinkled, just like a pirated book.
He wondered, maybe the publishing house is out of money and has started to cut costs?
The title of the book is simple and direct as always:
The Latest Theoretical Research on Semiconductor Physics and Radio Frequency Circuits - Xu Huaiyun
I opened the cover and saw the blue-black ink stains on the title page were still wet:
The road ahead is long and arduous, but if you keep going, you will reach your destination. If you keep going, you will have a promising future.
—Xu Huaiyun to his beloved disciple Xu Zhiqian, June 25, 2020
Although the handwriting is sloppy, one can still see that the strokes are strong and powerful.
He flipped back a few pages and couldn't wait to read the main text.
Huh?
The more I read, the more puzzled I became. The content of this book is good, but the typesetting and proofreading are very different from Professor Xu's previous publications.
He quickly flipped through the pages again, and suddenly, as if he understood something, he looked up at Aunt Xu's daughter.
She just nodded to him silently, with tears in her eyes.
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